The Sky-Watcher USA Star Adventurer multi-purpose multi-purpose mount is perfect for anyone — Milky Way photographers, eclipse chasers and budding astrophotographers. It’s the ideal night-and-day, grab-and-go package.
Star Adventurer Photo package Only $319
Compact and portable — weighing only 2.5 pounds — this versatile mount is also powerful. Its quality construction, utilizing precision all-metal gearing, delivers an impressive 11-pound 11-pound payload capacit y. The Star Adventurer converts easily from a tracking photo mount to a graband-go EQ astronomical mount. Allowing you to spend more time doing what you love and less time setting up. The Star Adventurer features:
• Multiple preprogrammed speeds perfect for time-lapse photography, wide angle astrophotography and astronomical tracking • Tracking selectable between multiple rates, sidereal, solar and lunar • Built-in polar scope with illuminator • DSLR interface for automatic shutter control • Built-in auto-guiding interface • Long battery life — up to 72 hours • External Mini USB power support . 1 l 2 l 0 ’ 7 u o 1 - y 0 d 2 n . a e k c o i t o o l n ! t w e t i u n o f o h t r u k i o n w s i ’ t h e t g I . u n s o a y h t c t c u a o d h t o t r c p w e r w j o b u u o n s n k g o s u n n t i c e e i r e L p r . g s d w h n e t a n n o s n a m o g i t n w a i e s f c u i f i e t c r x e e ’ n p e e S w h . t t A a r h S e v U t o r d e e i c t h i f c t t o o a n e r W - e o y v a m k h S y g n 8 a i e 1 m e 0 s 2 u o e © Y b
• Compatible with 1/4-20 and 3/8 inch camera tripod • Comes in two packages: Astro pack (i ncludes Dec L bracket ) Photo pack (includes ball head adapter)
Optional accessories Ball head adapter - $15
Dec L bracket - $40 Latitude base - $65
Counterweight kit - $30
Photographer: Carlos Guana Camera: Canon 5D IV Lens: Rokinon 14mm 2.8 Mount: Star Adventurer
For information on all of our products and services, or to find an authorized Sky-Watcher USA dealer near you, just visit www.skywatcherusa.com. Don’t forget to follow us on Facebook, Twitter and Instagram!
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Pull Back the Curtain on the Unseen Universe For a few hundred thousand years, we used our eyes as our primary astronomical tool. But all that changed in the 1930s when a young engineer named Karl Jansky detected radiation below the visible part of the spectrum emanating from an astronomical object—and radio astronomy was born. Radio Astronomy: Observing the Invisible Universe takes Universe takes you on a thrilling journey through astounding discoveries and a virtual tour of the world’s most powerful radio telescopes with Felix J. Lockman, Ph.D., of the Green Bank Observatory as your guide. But perhaps the most astounding of all radio astronomy discoveries is this: The dominant molecular structures in interstellar space are based on carbon. That is not what scientists had expected. We have always labeled these molecules “organic” because life on Earth is carbon based. Now we know the chemistry of the entire Milky Way is organic, not just our home planet, and it is likely that any extraterrestrial galactic life would be related to us, at least on the molecular level. Will we find other organic life forms out there? Radio astronomers don’t know. But they’re certainly working on it.
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MARCH 2018 VOL. 46, N O. 3
H C E T L A C � L P J / A S A N
ON THE COVER The Cassini-Huygens mission produced an amazing new understanding of Saturn and its moons.
CONTENTS
20 COLUMNS Strange Universe 8
FEATURES
BOB BERMAN
20 COVER STORY Cassini unveils Saturn Tis intrepid spacecraf spent 13 years studying the ringed planet, transorming transorming our view o this captivating world. LIZ KRUESI
38 StarDome and Path of the Planets RICHARD TALCOTT; ILLUSTRATIONS BY ROEN KELLY
Brown dwar jets.
Usually known or its rings, the Saturn system is also home to some o our solar system’s most intriguing moons.
46 A detailed look inside Cassini
FRANCIS REDDY
Te spacecraf’s 12 instruments showed Saturn and its amily in unprecedented detail.
36 Sky This Month
RICHARD TALCOTT
Mercury at its evening best. MARTIN RATCLIFFE AND ALISTER LING
wo massive telescopes in the Lone Star State monitor 450 suns in the hopes o �nding other worlds. ROBERT REEVES
48 72 minutes on Titan In 2005, the Huygens probe pierced the moon’s shroud to reveal a surprisingly Earth-like world. KOREY HAYNES
• M A R C H
For Your Consideration 18 JEFF HESTE R
Binocular Universe 67 Observing Basics 68
Over decades, the observatory’s powerhouse instrument charted a new course in planetary imaging.
Snapshot 7 Astro News 10
KLAUS BRASCH
64 Astronom Astr onomyy tests Celestron’s CGX mount I you’re ready or the next level o telescope mounts, this may be the one or you. TOM TRUSOCK
GLENN CHAPLE
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IN EVERY EV ERY ISSUE From the Editor 6 Astro Letters 9 New Products 66 Advertiser Index 69 Reader Gallery 70 Breakthrough 74 (ISSN 0091-6358, USPS 531-350) is Astronomy (ISSN
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60 The photographic legacy of Lowell’s Great Refractor
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55 In pursuit of exoplanets
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FROM THE EDITOR BY DAVID J. EICH ER Editor David J. Eicher Art Director LuAnn Williams Williams Belter EDITORIAL
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Requiem for a spacecraft
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aunched in 1997, the Cassini-Huygens spacecraft sped toward Saturn and its moons for nearly seven years, entering orbit orbit in 2004. La st September, September, Cassini ended its journey by plummeting into the saturnian at mosphere, burning up. Those 13 years in between gave us some of the most incredible planetary science in recent memory. With Cassini in mind, this issue carries a special theme of giving you everything you need to know about this historic mission. Four special stories deliver the goods: Liz Kruesi reports on the major scientific fi ndings of the Cassini spacecraft at Saturn itself; Frank Reddy describes the incredible findings the mission made at Saturn’s moons; Korey Haynes details the Huygens lander’s
extraordinary touchdown on Saturn’s big moon Titan; and Rich Talcott gives us an “exploded” view of the spacecraft and its retinue of instruments. The mission’s list of scientific achievements is stunning. The spacecraft tested general relativity (and Einstein won again). It discovered seven new saturnian moons. It revealed Titan to be a world saturated with methane lakes. The mission detected and imaged waterrich geysers and plumes casting skyward from the surface of Enceladus. It imaged and studied the planet’s rings and spokes in far greater detail than ever before. It studied Sat urn’s great atmospheric storms and hurricanes with a distinct eyewall. The craft focused on the strange hexagon at Saturn’s north pole. It
studied the weird moons Phoebe and Hyperion. The list goes on and on and on. Rarely in recent times have I seen such attachment to a space probe. That happened with New Horizons as well, no doubt. But when Cassini took its final plunge last fall, Twitter lit up with science types who were downright depressed, and some even to the point of sending alarming, morose tweets. I hope this special issue will only boost your feelings about Cassini. This mission was one of the greatest we’ve seen in recent times. Now we can bask in the details of the science while we hope funding for future missions regains a more healthy composure.
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EVERYTHING YOU NEED TO KNOW ABOUT THE UNIVERSE THIS MONTH . . .
HOT BYTES BYTE S >> TRENDING TO THE TOP
BIG FUTURE
COLD COMFORT
LET IT SHINE
As the Sun ages, it will expand. ALMA has found a red giant star the size our Sun is expected to reach at the end of its life.
The James Webb Space Telescope has completed cryogenic testing, moving it a step closer to launch.
The Zwicky Transient Facility at Palomar Observatory in California saw first light November 14.
S E I R O T A V R E S B O L A C I T P O H C E T L A C ; A S A N ; S G N I M M E L V . W / � O A R N / J O A N / O S E � A M L A : T F E L M O R F P O T ; I S S / H C E T L A C � L P J / A S A N
The south pole of Saturn’s moon Enceladus contains a geyser basin that spews water vapor into space at velocities up to 1,360 mph (about 2,190 km/h). The moon is heated internally through tides, radioactivity, and chemistry, and it contains a salty, subsurface ocean that feeds the periodic jets.
SNAPSHOT
What Cassini taught us During the spacecraft’s exploration of the Saturn system, we saw eerie, otherworldly sights, and also a glimpse of ourselves.
The Cassini-Huygens mission, which came to an end in late 2017, 2017, rewrote t he book on Saturn and its moons. Arguably the loveliest planet, at least from an observational standpoint, Saturn is now better understood by orders of magnitude, thanks to the spacecraft that visited it for 14 years beginning in 2004. There’s no doubt that Cassini took us to some weird new heights. The closest ever examination of Saturn’s rings, storms on the planet, lakes on Titan,
flybys of incredibly strange moons like Hyperion and Phoebe, and close-ups of many otherworldly surfaces are just a few of the countless milestones. But Cassini also told us a little something about ourselves. The spacecraft imaged watery jets emanating from the moon Enceladus. The strange moon has a subsurface, salty ocean that could contain an enormous amount of water. And maybe it contains some sort of microbial life.
As we look out into the universe, we find that in many places, what seems completely strange and alien also holds connections to what we know best. The chemistry of the cosmos is uniform throughout. The presence of water on Enceladus should remind us that we are likely not so incredibly special, that life probably exists in countless places in this vast cosmos we call home. — David J. Eicher
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STRANGEUNIVERSE BY BOB BERMAN
Lessons learned Spectacular sky events stand out in years past — and in years to come.
T
he Great American Eclipse is long over. But there’s always a “next time” for super celestial events — and lessons to learn. I recall the big events that lay ahead when I completed school in the ’60s. I vividly remember the sky spectacles that beckoned, dreamlike, in the d istant future. I ached to see them. Comet Halley’s return in 1986. The longest upcoming totality in 1991. The expected Leonid meteor storm in 1999. The transits of Venus in 2004 and 2012. The mysterious eclipse of Epsilon Aurigae in 2009. The Great American E clipse in i n 2017. 2017. I’ve not been entirely lucky. When I took a group to the equator to optimally see Halley, its tail fell off. Earth and the famous comet were on opposite sides of the Sun during its February 1986 perihelion, and the comet’s tail disappeared just when it was supposed to be best. It was the worst Halley apparition since the days of the Roman Empire. We’ll do much better in 2061. The 1991 totality was wonderful, and yet, looking back, the unusual disparity between the lunar and solar disk sizes made prominences harder to see. As for the Leonids, well, a storm like we had in 1966 and 1833 did not materialize. No one saw 80 meteors per second this time around. But we still got a heck of a nice display in the wee hours of November 18, 2001: five brilliant green meteors per minute with trains. The first Venus transit was yummy, though clouds blocked the second. And in 2009,
Epsilon Aurigae (the nearest little star to Capella) lost half its light. In those days, I was perilously riding my bike through Asia. At night, I’d stare at Epsilon. It was amazing to see it so dim, a three-yearlong event that occurs every 27 years. Researchers at the time announced they’d finally found the source of the dimming: a huge orbiting cloud of particles, each about the size of a piece of gravel. Weird! A few eclipse chasers make their totalities a matter of pride. They add up their cumulative shadow time. They scored big during the 1991 totality, when they picked up 6½ minutes in one shot. We can hope that science won’t someday learn t hat the Moon’s shadow gives you Alzheimer’s or something.
Comet Halley’s February 1986 apparition was disappointing, as it lacked a long, trailing tail by the time the comet reached perihelion. NASA
but this ti me I bought lots of 14s for the group because I thought the high position of the Sun and the resulting i ncreased brightness called for a da rker filter. But as a test, when guests stepped off their coach we asked each to look at the Sun using a 12 and then a 14, and decide which they preferred. Both are equally safe. I’m glad I’ d brought along a bunch of 12s — without exception, everyone preferred the brighter image through the 12 to the fainter Sun through the 14. A few said that havi ng a
It keeps going. Our lives, marked by dramatic events in the sky. Farfetched? Well, Well, in India they believe eclipses are “unhealthy,” “unhealthy,” and most still hide from them. I saw this firsthand in 1980, as citizens huddled behind shuttered windows. As for last August’s American totality, I loved watching as our guests were swept away by the magic, and in the process I learned two new things. First, I noticed that some respectable news sources like The New York Times wrongly urged observers to only use use shade 14 welder’s goggles. I’ve preferred preferred shade 12 during a ll my totalities going back to 1970,
12 was very important to t hem. So for the next two totalities (both in southern South America), with the Sun fairly low both times, we’re definitely sticking with 12s. My other lesson involved binoculars. My long-held advice has always been to mostly watch totality naked eye and use binoculars as an adjunct. But this time around, using the best of my three image-stabilized Canons (the obscenely expensive 10x42 L), I have to say that the rock steadiness and clarity made the pink nuclear flames so “present,” I felt I was closely hovering above the
BROWSE THE “STR ANGE UNIVERSE” ARCHIVE AT www.Astronomy.com/Berman.
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MARCH ARCH A S T R O N O M Y • M
2018
prominences. I gasped. It delivered the most exquisite eclipse views of my entire li fe as a dog — not a sundog, but a dog that can’t help chasing these things like a f ire engine. So now now I’ll bark even louder about the value of good binocul ars as a vital totality totalit y tool. My new top five “wonders to come” for the next two decades? Some are very unusual. First the winter solstice hy percon junction of 2020, when Saturn Saturn will appear as close to Jupiter as some of its moons! Then the 2024 four-minute U.S. total ity, followed by the longest total eclipse until t he 22nd century, under virtually guaranteed clear skies on the Nile River in 2027. Then the Friday the 13th Apophis visit — that’s April 2029, when the 3rd-magnitude, Empire State Building-sized asteroid will come just a tenth of the Moon’s distance and glide across the sky, visible to the naked eye. Then in 2036, Epsilon Aurigae’s next weird eclipse occurs a s it is covered by that gravel cloud, or whatever its companion may be. It keeps going. Our lives, marked by dramatic events in the sky. Join me and Pulse of the Planet ’s Jim Metzner Metzner in my my new podcast, Astounding Universe , at http://astoundinguniverse.com.
ASTROLETTERS CCD challenge accepted I often look for chal lenges when it comes to CCD imaging. One of the main places I go to is my monthly Astronomy monthly Astronomy magazine. magazine. In the November issue, I read “Fall into autumn galaxies” by Stephen James O’Meara, which covered my favorite sub ject: gala xies. The T he challenge here was to image NGC 7814, which looks great in the magazine but through a 130mm scope? I said, “Why not?” So I took ten 15-minute exposures using my Starlight Xpress SX694C camera, and this is the result. Thank you, Mr. O’Meara. — Eugene Faulkner, Whiting, NJ R E N K L U A F E N E G U E
Latin pronunciations In the October issue, there was a fascinating article about pronunciation errors in stars and constellations by Bob Berman. It was a very interesting read, and I too can be very picky about correct pronunciations. However, Mr. Berman made an error when discussing the Latin genitive. I’ve studied Latin for several years now and intend to make it a minor in college. While “ae” is the ending for the Latin genitive singular in the first declension (also used for the dative singular and nominative plural), it is not pronounced “EE.” Latin in the church, such as at a Latin Mass, would pronounce the “ae” ending as “AY” like i n “May” or “weigh.” However, However, I’m sure we’re all interested in using the classical Latin pronunciation, which would be “EYE” as in “eyeball” or “light.” So, Ursae Majoris would be UR-sye mah-JOR-is, not UR-see. However, I do still thank Mr. Berman for reigniting my interest in this topic. (Unfortunately, I will probably continue to pronounce Uranus as yor-AY-nis, but that is really just for the joke material.) — Maggie Bradley, Asheville, NC
Praise for balance Bob Berman’s column has always been my favorite in your magazine. I can hardly see how this will e ver change after reading his December column entitled “Intelligent design?” I’ve never before seen a better, more even-handed and balanced treatment of this subject that you could fit onto a single page. Well said. — Richard S. Wright Jr., Lake Mary, FL
Sky Puppies I enjoyed Tom Trusock’s article in the December issue entitled “How to get kids’ heads in the stars.” I would add one more item to all that Trusock suggested. The Astronomical League offers a multitude of observing programs. One of these programs, Sky Puppies, is designed specifically to help get kids’ heads in the stars. Like its other observing programs, the League offers a certificate and a pin to anyone who successfully completes the program. Unlike other observing programs, membership in the League is NOT required of the Sky Puppies program. There is an age limit in this program. One must be 10 years old or younger. Details on the program can be found at the Astronomical League website, or by going to www.astroleague.org/al/ obsclubs/skypuppy/skypuppy2.htm. — W. Maynard Pittendreigh, Orlando, FL
Two sides of the same coin Bob Berman’s editorial in the December 2017 issue of Astronomy of Astronomy — — “Intelligent design?” — is among the best and clearest I have read on the subject of scientific truth and spiritual beliefs. Science has long accepted the dual nature of man, and even as we enter the age of scientific enlightenment, the more we explain, the more we find unexplainable. I am a creature of this planet, and have been around for 74 years. I have be en fortunate to have done many things and experienced a great deal, and I continue
to wonder at life and its mysteries. I have trained my mind to open new doors as they are discovered but do not close doors behind me. Intelligent design is one of these doors, and I often revisit it. If you consider yourself a scientist, you will undoubtedly become a wrestler in the match between science and spiritual beliefs. This event is held regularly, and the outcomes are always different for each person. I would call your attention to two men who went through this experience and left behind very clear and compelling descriptions of their ideas. Jacob Bronowski’s work, The Ascent of Man, Man , and the essays of Allan Sandage on religion and science will give you an excellent background if and when you start your wrestling match. They were, by all standards, great scientists who left compelling legacies for all of us. I would hope that as you proceed with your mental exercises, you keep the attitude of “we do not know.” As for my personal beliefs, I view science and religious beliefs as two sides of the same coin. In order to spend the coin, both sides need to be present. In order to find truth, you must be able to see both sides. — Donald Craig, Jr., Indianapolis We welcome your comments at Astronomy Letters, P. O. Box 1612,
Waukesha, WI 53187; or email to letters@ astronomy.com. Please include your name, city, state, and country. Letters may be edited for space and clarity.
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ASTRONEWS ASTRO NEWS
IT ANTI�MATTERS. Researchers at JILA in Colorado are exploring whether the electron is more egg-shaped than round, which could explain why there’s more matter than antimatter in the universe.
OUR SOLAR SYSTEM RECEIVES AN INTERSTELLAR VISITOR
JUST PAS SING T HROUGH . 1I/2017 U1
(’Oumuamua), a visitor from another solar system, swung past our Sun in September 2017.. Aside from its strange cigar-shaped 2017 appearance, the asteroid bore striking similarities to those found in our inner solar system. EUROPEAN SOUTHERN OBSERVATORY/M. KORNMESSER
O
n October 19, University of Hawaii astronomer Rob Weryk Weryk noticed a n unusual 20th-magnitude streak in images taken with the 1.8-meter PanSTARRS 1 telescope. After spotting the same object in images taken the previous night, he contacted European Space Agency astronomer Marco Micheli. Sure enough, his telescope in the Canary Islands had caught it, too. “Its motion could not be explained using either a normal solar system asteroid or comet orbit,” Weryk Weryk said. sa id. “When bot h our datasets were fit together, it became clear that the only explanation was a hyperbolic trajectory.” The object was an “interstellar interloper,” interloper,” an asteroid from outside our solar system, swinging around the Sun only once before racing away, never to return. The object is now designated 1I/2017 U1 (’Oumuamua), a Hawaiian name loosely translating to mean the first “scout” or “messenger,” signifying its discovery as the first identified traveler from one solar system to another. It entered the solar system at 57,000 57,000 mph (25.5 ki lometers per second); as it passed through, astronomers raced to observe it. “We had to act quickly,” Olivier Hainaut of the European Southern Observatory in Garching, Germany, said in a press release. “ ’Oumuamua had already passed its closest point to the Sun [in September 2017] 2017] and was heading back into interstellar space.” ’Oumuamua was visible only for 10 days and never reached a magnitude above 19.7. Observations with the European Southern Observatory’s Very Very Large Telescope revealed an oblong, reddish, solid chunk of rock or
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metal nearly a quar ter-mile (400 m) long and 10 times longer than it is wide. It came from the region of sky now occupied by Vega, but that star was in a different position 300,000 years ago when ’Oumuamua would have passed through the region. While its shape is unusual, ’Oumuamua’s color and rotation rate are similar to properties seen in asteroids in our inner solar system. “The most remarkable thing about U1 is that, except for its shape, how familiar and physically unremarkable it is,” said Jayadev Rajagopal of the National Optical Astronomy Observatory. This has led astronomers to liken it to comets and asteroids believed to have been ejected from our solar system during its formation, and is likely the story behind ’Oumuamua’s fate. While ’Oumuamua is the first discovered interstellar asteroid, it’s likely not the first to have passed through our solar system.
Asteroids require careful observation and sensitive instruments to detect, especially if they’re moving fast. But surveys like PanSTARRS and those planned with the upcoming Large Synoptic Survey Telescope should be able to detect other objects like it. Such interstellar objects have been roaming the frigid depths of space for eons, with few evolutionary forces — such as heating or collisions — acting upon them, essentially stored in deep freeze. “They are well-preserved examples of things made in other star systems,” planetary scientist and New Horizons principal investigator Alan Stern told Astronomy told Astronomy . “For that reason, they’re going to be valuable as the population is studied.” With these interstellar objects, astronomers can learn more about other solar systems — and our own. — Robert Naeye, John Wenz, Wenz, Alison Klesman
‘Oumuamua’s path around the Sun
Mercury Venus
Sun
Earth Mars’ orbi t
1I/2017 U1 (’Oumuamua)
QUICK TRIP. ’Oumuamua’s trajectory brought it within 23,400,000 miles (37,600,000 km) of the Sun; it passed
closest to Earth October 14, just days before its discovery. Its path will now take it out of the solar system, never to return. This figure shows the planets and ‘Oumuamua on October 25, 2017. ASTRONOMY : ROEN KELLY, AFTER NASA/JPL�CALTECH
ASTRONEWS ASTRO NEWS
NIGHT SKY THREAT. The adoption of LED lighting led to a 2 percent per year increase in light pollution between 2012 and 2016, according to research published November 22 in Science Advances.
Early results of NASA’s Twins Study
QUICK TAKES
As part of NASA’s Twins Study, astronaut Scott Kelly spent a year in zero gravity on the International Space Station. In the meantime, his identical twin brother, former astronaut Mark Kelly, went about his daily life on Earth. When Scott returned, he was temporarily two inches taller, but his height wasn’t the only thing that changed. According to preliminary results from the study, Scott’s year in space also drastically increased his rate of DNA methylation, the process responsible for turning genes on and off. By regulating gene expression, DNA SIBLING RIVALRY. RIVALRY. Former astronaut Mark Kelly (left) poses with his identical methylation is essential for normal twin brother, astronaut Scott Kelly. As part of NASA’s Twins Study, Scott spent nearly human development, but it is also a year in space, while Mark stayed on Earth. This gave researchers a chance to study believed to play a major role in the the health effects of long-term spaceflight. NASA progression of many diseases, ranging from cancer to cardiovascular disease. Over the last year, NASA has telomeres are associated with fewer “With this study, we’ve seen thoureleased a number of fascinating pre- age-related problems. sands and thousands of genes liminary results from some of the 10 “This study represents one of the change how they are turned on and research projects that make up the most comprehensive views of human turned off,” said Chris Mason, princi Twins Study. For example, researchers biology,” Mason said. “It really sets pal investigator of the Twins Study. were surprised to find that Scott’s the bedrock for understanding “This happens as soon as an astrotelomeres — the protective caps that molecular risks for space travel as naut gets into space, and some of the shield the ends of DNA strands — well as ways to potentially protect activity persists temporarily upon were longer than Mark’s. Previous and fix those genetic changes.” — Jake Parks return to Earth.” research has shown that longer
WHERE DOES WATER FREEZE IN THE SOLAR SYSTEM? FREEZING TEMPERATURES. The
Jupiter 5.2 AU
Ceres
2.8 AU
Water snow line
3 AU Mars
1.5 AU
Y L L E K N E O R :
Y M O N O R T S A
Sun
Earth
Asteroid belt
1 AU 2.2 AU
3.2 AU
Although 71 percent of Earth is covered by oceans, water represents only about 0.02 percent of the planet’s mass.
FAST FACT
snow line, also called the frost line or ice line, is the distance from a star at which the temperature drops enough that a given molecule — such as water — will freeze into a solid. The simplest models of planet formation state that objects (such as planets) that form inside the snow line are smaller and composed of mostly rock; those that form outside the snow line are more massive and include a higher percentage of ices. The location of the snow line depends on the properties of the star itself, as well as the molecule in question, as some molecules freeze at higher or lower temperatures than others. In our forming solar system, the water snow line occurred at about 3 astronomical units (AU; 1 AU equals 93 million miles [150 million kilometers]), or three times the distance between Earth and the Sun. This lies between the orbits of Mars and Jupiter, separating the terrestrial and giant planets in our solar system. However, other young stars have been observed with water snow lines tens of AU in distance. This indicates the location of the snow line may evolve over time, while giant planets close to their stars (inside the snow line) likely migrated there after forming farther out. — A.K.
LIGHTNING STRIKES Lightning on Earth can generate gamma rays that produce antimatter particles called positrons and kick off matter-antimatter annihilation.
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BRIEFER HISTORY The University of Cambridge made Stephen Hawking’s Ph.D. thesis publicly available as part of Open Access Week in October.
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ANCIENT ECLIPSE Historical accounts of a solar eclipse in 1207 B.C., mentioned in the Bible and ancient Egyptian texts, may help better pinpoint the reign of Ramses the Great.
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APOLLO ASTRONAUT PASSES AWAY Gemini XI and Apollo 12 astronaut Richard “Dick” Gordon passed away November 6 at the age of 88.
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ANCIENT SUNS New analyses of star movements identified 29 nearby suns as belonging to some of the first groups of stars formed in the Milky Way.
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TWINKLE, TWINKLE, LITTLE PLANET A “twinkle” every 18 months in the young star EC53 suggests that a newly minted planet has formed around it.
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DRONES FOR MARS NASA, the SETI Institute, the Mars Institute, and sensor manufacturer Fybr are collaborating on drones for use in Mars’ thin atmosphere.
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OLD BATTLE Cornell University astronomers found two massive galaxies merging 13 billion light-years away, only 500 million years after the universe formed.
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RE�FUNDED The fifth generation of the Sloan Digital Sky Survey gained funding from the Alfred P. Sloan Foundation, allowing it to begin continuous observations in 2020.
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WE ARE STARDUST Space dust colliding with organic molecules in our upper atmosphere could fling microbes off our planet — or bring life from other planets or moons to us. — J.W.
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ASTRONEWS ASTRO NEWS
CALM SUN. Astronomers
announced Ross 128b, a potentially habitable planet in the Ross 128 system, November 15. Ross 128 is a calmer M dwarf, reducing the likelihood that it has blown away Ross 128b’s atmosphere.
Positron excess may have dark matter origins
THE WORLD’S 10 LARGEST REFRACTORS OBSERVATORY
DIAMETER
Yerkes Observatory, Williams Bay, Wisconsin
40 inches
Full Moon (to scale)
Roque de los Muchachos Observatory, Canary Canary Islands 38.6 inches Lick Observatory, Mt. Hamilton, California Paris Observatory, France
Geminga
36 inches 32.7 inches
Leibniz Institute Institute for Astrophysics, Astrophysics, Potsdam, Germany Germany 31.5 inches Nice Observatory, France
30.3 inches
Allegheny Observatory, Pittsburgh, Pennsylvania
30 inches
Royal Greenwich Observatory, London, England
28 inches
Rolfsche Refractor, Rathenow, Germany
27.6 27.6 inches
Vienna Observatory, Austria
27 inches
PSR B0656+14
40 UNDERPOWERED. Extended
gamma ray emission from the Geminga and PSR B0656+14 pulsars (yellow and red) cannot account for the positron excess measured in Earth orbit. Instead, a more exotic source is likely responsible. HAWC COLLABORATION; MOON IMAGE: GREGORY H. REVERA; COURTESY MIGUEL MOSTAFA �PENN STATE�
Yerkes Observatory, Williams Bay, Wisconsin Y L L E K N E O R :
Y M O N O R T S A
27 Vienna Observatory, Austria
4 Backyard telescope FIRST GLASS. A refractor telescope uses a lens to collect and focus light. Because lenses are glass and glass can behave like a fluid, any lens larger than 40 inches in diameter will sag under its own weight, rendering it unusable. Also, casting and figuring such behemoths are time-consuming and expensive processes. Because huge refractors are no longer practical, this list probably is the final one. — Michael E. Bakich
The 40-inch Yerkes refractor collects 2.2 times as much light as the 27-inch Vienna refractor and 100 times as much as a 4-inch scope.
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FAST FACT
The European PAMELA satellite first registered an unexpectedly high number of positrons in near-Earth space in 2008. Since then, two competing theories to explain the anomaly have developed. The simple explanation is that the extra positrons — the antimatter counterparts to electrons that have a positive charge — are coming from nearby stellar remnants called pulsars. The more exotic theory is that heavy dark matter particles in our galaxy are mutually annihilating one another whenever they come into close proximity, self-destructing into a cascade of positrons and other particles. Although Occam’s razor suggests the first explanation is more likely, a recent paper published by an international team of scientists November 17 in Science all but excludes it. And if nearby pulsars are not responsible for the excess of positrons, scientists are left to entertain the second option. The study uses observations taken wi th the High-Altitude Water Cherenkov (HAWC) Observatory. HAWC consists of 300 large water tanks in the Mexican state of Puebla to detect high-energy gamma rays, the highestenergy form of light. When a gamma ray strikes an atom in Earth’s upper atmosphere, it creates a shower of secondary particles that rain downward, triggering detections in HAWC’s HAWC’s water tank s. From November 2014 to June 2016, HAWC clearly detected high-energy gamma rays coming from an extended region around two relatively nearby pulsars in the constellation Gemini the Twins. The pulsars, known as Geminga and PSR B 0656+14, 0656+14, are roughly 800 and 900 light-years away, respectively. As each pulsar spins, it throws off positrons and electrons, which, in turn, interact with nearby particles to produce gamma rays.
“The gamma rays that we measure [with HAWC] are a tracer for the electrons and positrons near the [pulsar] source. Using this, we can map out how fast the electrons and positrons are moving away from the source. Knowing the age and the distance of the pulsars, we can figure out if [the positrons] can get here,” says the HAWC principal investigator and U.S. spokesperson, Jordan Goodman of the University of Maryland. The team concluded that the puls ars are not producing anywhere near enough positrons to explain the observed excess. Furthermore, because these two pulsars are among the closest to Earth, it seems clear that pulsars in general cannot account for the anomaly. So if pulsars can’t explain the positron excess, what can? Some theorists have proposed supernova remnants and black hole jets as the culprit s. HAWC has also detected these objects, but “most are too far away and too young to send particles all the way to Earth,” Goodman explains. This leaves dark matter annihilati on as the most likely explanation. This theory has been on the books for many years, and it’s not contradicted by any astronomical observations. If annihilating dark matter is indeed responsible for the positron excess, the particles themselves would have whopping masses of about a thousand protons — about the mass of four uranium atoms. Experiments at the Large Hadron Collider in Switzerland and in underground laboratories around the world have yet to turn up direct evidence for dark matter particles. So although HAWC seems to have ruled out pulsars as the source of the excess positrons, their origin remains a mystery, as does the nature of dark matter. — J.P., R.N.
ASTRONEWS ASTRO NEWS
OVER THE MOON. The Indian Space Research Organisation has nearly completed its second moon probe, Chandrayaan-2, set to launch soon.
LINE UP.
The Mars Reconnaissance Orbiter snapped this image of seasonal recurring slope lineae — dark, narrow streaks cutting through the landscape — on the southern rim of Tivat Crater. Once thought to form due to subsurface water seepage, these features now appear to be dry granular flows. NASA/JPL�CALTECH/UA/USGS
Evidence mounts for a dry Mars Although Mars is cold and dry today, planetary scientists believe it could have been warmer and wetter in the past, and that a significant volume of water may still exist below the surface. In particular, dark streaks on the planet’s surface called recurring slope lineae (RSL) have been cited as evidence of subsurface water flows. These features appear and grow over time during the warmest part of the Red Planet’s summer, then fade away again — behavior
suggestive of seeping water. water. But new evidence from a study conducted by the U.S. Geological Survey (USGS), the University of Arizona, the Planetary Science Institute, and Durham University paints a different picture: RSL are more likely granular flows of dry material that don’t require water to form. Their findi ngs, published November 20 in Nature Geoscience, Geoscience, are based on a study of 151 RSL and state that the ends of RSL look identical
to the slopes of aeolian sand dunes formed by wind found elsewhere on Mars. If RSL were due to water, more liquid would be necessary to form longer features. But the end result of dry flows should look similar regardless of length, which was exactly the finding from the study. “This new understanding of RSL supports other evidence that shows that Mars today is very dry,” said the lead author, Colin Dundas of the USGS, in a press release. — A.K.
10 YEARS
The timeframe within which astronomers expect to detect gravitational waves from a merger of supermassive black holes.
Comets swarm a distant star A study published October 31 in Monthly in Monthly Notices of the Royal Astronomical Society out outlines a big first: the discovery of the first transiting exocomet. Working with Kepler data as part of the Planet Hunters project, citizen scientist Thomas Jacobs spotted the telltale signs of a comet’s tail in the light of the star KIC 35421 3542116, a dim M dwarf. He noticed a 0.1 percent dip in the star’s light occur three times. An MIT team helped confirm that the dip represents a comet’s trail of debris or tail, which spreads over a much wider area than the nucleus. The team and other collaborators spent months trying to figure out what the object was. It resembled a disintegrating planet, but it lacked the same periodicity expected from transiting planet debris, which should be short based on the type Kepler usually spots. After they refined the data further, six total comets were found in the system, leading
MANY TAILS. A group of comets circles a distant
star, leaving behind trails of debris. Citizen scientist Thomas Jacobs discovered such a trail around a star observed with the Kepler telescope. DANIELLEFUTSELAAR
discoverers to believe that the system could be in the middle of a “bombardment era” where large planets fling smaller objects like comets and asteroids into the inner planetary system. Such events could have seeded life on Earth, or even caused extinction-level events in the past. KIC 3542116 may be the smallest star an exocomet has ever been discovered circling. Most exocomets, usually discovered through spectroscopy, have been around A-type stars, which are larger and more massive than the Sun. — J.W.
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ASTRONEWS ASTRO NEWS
DENSE JETS. Magnetic fields are thought to prevent neutron stars from forming jets, but the belief is being challenged by University of Amsterdam astronomers after analyzing data from 27 telescopes.
BRIEFCASE
Earth stops energetic neutrinos
PROBING A PYRAMID
DEEP DOWN.
Physicists used a technique called muonography to discover a large chamber in the Great Pyramid of Giza. The authors of the November 2 Nature paper revealed the void by tracking subatomic particles called muons, which are produced when cosmic rays s trike atoms in the upper atmosphere, creating a cascade of secondary particles that rain down to Earth. Since muons are partially absorbed by stone, the researchers could “image” the hidden chamber by measuring where the flux of muons was higher than expected. Although the purpose of the void remains unknown, Egyptologists hope the discovery will provide insights into h ow the 4,500-year-old pyramid was built.
To detect neutrinos zipping by, the IceCube Neutrino Observatory lowers “strings” equipped with neutrino detectors into deep holes in the Antarctic ice. These detectors have confirmed that energetic neutrinos are more likely than their lower-energy counterparts to interact with Earth.
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STEADY AS SHE GOES
Using over 60 years of observations, astronomers have discovered that microwaves from the Sun during the past five solar minimums have been surprisingly constant, whereas microwaves from the past five maximums have drastically varied. “It is very meaningful to discover a trend extending beyond a single solar cycle,” said Masumi Shimojo, lead author of the study published October 10 in The Astrophysical Journal . “This is an important step in understanding the creation and amplification of solar magnetic fields, which generate sunspots and other s olar activity.”
NSF/B. GUDBJARTSSON
Neutrinos are chargeless, nearly massless particles created in some of the most extreme events imaginable: exploding stars and the mergers of black holes and neutron stars. They are small and fast, interac ting with ver y little in the universe — they typically zip right through most matter, including humans, Earth, and even the instruments used specifically to detect them. But while interactions are rare, they do occur. Now, an international team of researchers associated with the IceCube Neutrino Observatory at the South Pole has brought us one step closer to understanding when those collisions are most likely to happen, confirming current particle physics theories and shedding light on how to study neutrinos further. The work, published November 22 in Nature, hinges on measurements taken of the number of neutrinos that do interact with Earth instead of passing right through. Each neutrino has a cross section for collision. The cross section represents the probability that a particle will collide with another; if you picture a particle as a ball, its diameter would be analogous to its cross section. Particles with larger cross sections are more likely to collide with others. When a neutrino
passes closer to another particle than the extent of this cross section, the two will interact. But because a neutrino’s cross section is small, collisions with other particles don’t occur often. By measuring the rate of neutrino interactions and the types of neutrinos that do interact with Earth, IceCube has confirmed that the size of a neutrino’s cross section is determined by its energy, with higher-energy particles generating a bigger cross section. Such high-energy neutrinos are more likely to be stopped by interactions with Earth — a prediction made by our current Standard Model of physics. This work represent s the firs t study of high-energy neutrinos with energies 1,000 times greater than previously measured. It is a first step toward a greater understanding of not only neutrinos, but the field of particle physics. “IceCube was built to both explore the frontiers of physics and, in doing so, possibly challenge existing perceptions of the nature of universe. This new finding and others yet to come are in that spirit of scientific discovery,” said James Whitmore, a program director with the National Science Foundation’s physics division. — A.K.
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MERGER MADNESS
Scientists confirmed yet another gravitational wave detection November 15, with the discovery of GW170608. GW170608. The merging black holes that produced it were about a billion light-years away and roughly seven and 12 times as massive as the Sun, making this the smallest merger observed by Advanced LIGO so far. During the merger, energy equivalent to about 1 solar mass was released in the form of gravitational waves, leaving behind a lone black hole nearly 18 times as massive as the Sun. — Sun. — J.P.
A S A N , E L B B U H / A S E
MERCURY IN THE EVENING
Secrets of a cosmic snake
15° 15 e 10° d u t i t l A
5° 0°
20
10
7 July 2
12
22
March 5
25
6
Nov. 1 11 27 16
27
17 22
Oct. 22
June 17
West
Azimuth
WORLD AT DUSK. The innermost
planet has a reputation for being elusive because it rarely appears outside of twilight from mid-northern latitudes. This chart plots Mercury’s position 45 minutes after sunset for an observer at 35° north latitude for the planet’s three evening elongations of 2018. Notice that Mercury’s peak altitude doesn’t necessarily coincide with its greatest solar elongation (dates highlighted in yellow). — Richard Talcott
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ASTRONOMY
• M A R C H 2 0 1 8
Mercury’s best evening appearance of the year comes in mid-March, when it reaches a peak altitude of 8.3°.
FAST FACT
Y L L E K N E O R :
Y M O N O R T S A
EARLY DEVELOPMENT. The Cosmic Snake is a gravitationally lensed galaxy sitting behind the massive galaxy cluster MACSJ1206.2–0847. As light from this distant, young galaxy passes near the cluster, it is bent by the cluster’s mass and re-formed into a squiggly, snakelike image. Though warped, the image is also magnified, making the galaxy appear much brighter and its minute details easier to discern. By studying the areas of star formation visible in this image and others like it, astronomers can piece together how stars formed in very early galaxies. Studies of less-detailed images suggested that early galaxies host areas of star formation much more massive than those seen today. But recent work studying the Cosmic Snake f inds that the galaxy’s massive star-forming regions are actually made up of smaller, distinct clumps of star formation, more like those found in nearby galaxies. — A.K.
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15
SECRETSK SECRET SKY Y BY STEPHEN JAMES O’MEARA
What’s “overhead”? Looking straight up may be harder than you think.
R
ecently, I went outside for a walk and casually noticed that the Sun was about one-third of the way up the eastern sky. Or was it? Out of curiosity, I noted the time and later used software to determine that the Sun was actually only 17° high at that time. How could I have been so far off?
Part psychology, part physiology? In Light & Color in the Open Air , Marcel Minnaert helps us to understand why it is common for us to overestimate the heights of objects in the sky. It stems from a psychological effect in which we naturally perceive the hemisphere of sky as a flattened vault that appears about twice as close overhead as it does to the horizon. If you try to estimate the midpoint between these two points, the result will not lie at a height of 45° but generally somewhere between 20° and 30°. “It is very important that unprejudiced observers be found” who must divide “not the angle but the arc” into two equal parts, Minnaert says. The illusion intensifies under cloud cover and lessens on clear and crisp starlit nights. Generalities also come into play. Some of us use the word overhead overhead loosely. This is especially true for those living at mid-northern latitudes, when, say, we see the noonday Sun sailing “high overhead” in summer, or a “midnight”
winter Moon. In reality, the Sun never passes overhead from these latitudes but 20° to 30° from it (depending on the latitude from which we observe).
Part pain in the neck Typically, when looking straight ahead, the eye can see objects about 60° above its center point. So a Sun 60° high will appear at the summit of the eye’s field of view. Tilting the head back to see a 70°-high Sun or Moon only seems to affirm that it is overhead — a location most observers will admit can literally be a “pain in the neck,” so little bother is given to the accuracy of the observation. “Overhead,” in the most casual sense, means an object within 30° of the zenith.
Of voids and optic moths Like Minnaert, I also found that casually estimating the midpoint of the sky during the day was difficult. It was easier on a starlit night for the simple reason that it’s hard to be accurate in a blank sea of sky, but easier when it’s full of stars serving as guideposts. Accuracy in the perceived midpoint increases if the Sun or Moon is near it — especially if you take the average of three observations (similar to what visual Jupiter observers do when making transit estimates). The accuracy also increases dramatically the more you make these observations over the course of days,
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ASTRONOMY
MARCH ARCH 2018 • M
Determining a specific star’s altitude, or even finding the overhead point, is not as easy as you might think. A star-filled sky helps. STEPHEN JAMES O’MEARA
which speaks to Minnaert’s warning of having only unprejudiced observers make the initial observations. observations. For instance, when I made my first observation of a waning daytime Moon midway up the sky from the horizon, I was off by 15°. Later, I refined t he observation to within 3°, and finally to the correct value by taking the average of three observations over an hour. At night, I also began by misjudging a star’s altitude but was able to refine it to an acceptable value. I also found that I misplaced “overhead” when a bright star was within 20° of the zenith. One night, however, however, when no bright sta r was available near that point, I accurately selected a faint star
www.Astronomy.com/OMeara.
to within a couple of degrees of true overhead. The reason? I believe that our eyes are attracted like moths to the brightest object near our invisible target destinations (midpoint or zenit h), drawing ou r attention away. When only faint stars are visible, we spend more time scrutinizing the sky, which sharpens our accuracy. Bright stars may serve well as “immediate” signposts, but they actually lead us astray. As always send your thoughts and comments to
[email protected]. Stephen James O’Meara is a globe-trotting observer who is always looking for the next great celestial event.
ASTRONEWS ASTRO NEWS
GRACEFUL EXIT. NASA ended the Gravity Recovery and Climate
Experiment after 15 years studying gravitational anomalies around Earth.
This star exploded not once, but twice For the first time, astronomers have discovered a star that has gone supernova more than once. This so-called “zombie star” — which reportedly exploded at least twice in the past 60 years — has baffled scientists by challenging many of the existing theories about how massive stars end their lives. “This supernova breaks everything we thought we knew about how they work,” said Iair Arcavi, lead author of a letter published November 9 in Nature, in a press release. “It’s the biggest puzzle I’ve encountered in almost a decade of studying stellar explosions.” The undying st ar, named iPTF14hls, was first discovered in September 2014 by researchers using the Palomar Transient Factory (PTF). Although the supernova initially faded after its 2014 explosion, within a few months it began to mysteriously grow brighter again. Over a subsequent three-year span, iPTF14hls fluctuated between bright and dim at least five separate times. When the astronomers realized iPTF14hls was not an average supernova, they decided
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S A Ñ I V E S O J � A I R A M / A S A N
RINSE AND REPEAT. In
this artist’s concept of a supernova, an expanding shell of dust and gas is blown outward from the star’s dense, white dwarf core. Most stars explode just once, ending their lives for good. However, iPTF14hls appears to have gone supernova twice in just 60 years.
to search through archival data. They were flabbergasted to find that in 1954, another explosion was recorded in the exact same location. Somehow, this star survived its first explosion, waited 60 years, and exploded again. Although researchers are still uncertain about what caused iPTF14hls to go supernova twice, one theory is that the “zombie star” is actually a pulsation pair-instability supernova — a star so massive and
hot that it generates antimatter in its core, causing it to become violently unstable and undergo multiple bright eruptions. “These explosions were only expected to be seen in the early universe and should be extinct today,” said co-author Andy Howell, leader of the Las Cumbres Observatory supernova group. “This is like finding a dinosaur still alive today. If you found one, you would question whether it truly was a dinosaur.” — J.P.
The number of cameras the Mars 2020 rover will carry to the Red Planet. A D I / P S A D / M P U / A T N I / O S S / A A I / M A L / D P U / S P M M A E T S I R I S O R O F S P M / A T T E S O R / A S E
Rosetta spots a cometary dust jet NICE SHOT. On
July 3, 2016, the Rosetta spacecraft caught an eruption of material from Comet 67P/ChuryumovGerasimenko. The plume is rising from the comet’s Imhotep region as it rotates into the light of the Sun, warming the surface and providing Rosetta with a picture-perfect view. Using measurements taken by instruments that captured dust grains from the eruption, European Space Agency researchers found that the plume was filled with more dust than normal. This led them to suspect it was an eruption from deeper inside the comet, rather than J.W. sunlight sublimating surface ice into water vapor. — J.W.
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YOUR R CONSIDERATION FOR YOU BY JEFF HESTER
The mind’s siren call Our craving for certainty can make us blind to true knowledge.
W
e all know the feeling. You’re sitting there trying to figure something out, but it just won’t come together. Frustration and annoyance grow until suddenly you get it — or at lea st you think you do. “Aha!” The relief comes flooding in. Neurologist Robert Burton talks about that feeling in his book On Being Certain: Believing You Are Right Even When You’re Not . Physiologically speaking, our brains crave certainty in the same way a junkie craves a fix. Satisfying those cravings acti vates the neural pathway responsible for pleasure and motivation. An aha moment feels good because it releases a lovely hit of dopamine in the brain. It’s not hard to understand where our addiction to certainty comes from. For our evolutionary ancestors living on the savanna, often the worst possible strategy was to do nothing. The feeling of knowing frees us from paralyzing indecision. It enables us to act. But feeling certain has squat to do with being right, Burton stresses. The feeling of knowing is not even a cognitive process. Rather, certainty is a sensation that need not be associated with any particular thought at all, he explains. In The Logic of Scientific Discovery , Karl Popper argues that the foundation of knowledge is falsifiability. “I know” means that I have worked to discover that an idea is false, but so far have failed. If an idea can withstand that challenge, I
am obliged to keep it, at least for now. But if the idea c an’t take that heat, out the window it goes. Certainty pulls the rug out from under the whole notion of justified justif ied knowledge. Log ically, icall y, once we reject the possibility that we are wrong, our supposed knowledge becomes nothing but illusion. Of course, none of this changes the fact that certainty feels really good. So like any addict, our natural tendency is to do the worst possible thing — we try to score. We seek out information and people that reinforce our certainty, always craving the next hit of dopamine while turning our backs on anything or anyone that might call our certainty into question. Here lies a road paved with confirmation bias, groupthink, and a menagerie of other cognitive errors. Once we embrace without question that deep, heartfelt, compelling sensation of certainty that we so desperately crave, we become blind to reality. We build ourselves a house of cards, believing the whole time that it is made of brick. Knowing something (experiencing the sensation of knowing) and really knowing something (having reasonably justified justif ied belief ) are two completely different things, even if we call them by the same name. The irony is thick enough to cut with a knife! Our brains crave certainty, but if we want real knowledge, certainty is the one thing that we can’t allow ourselves. Which is not to say that the feeling of knowing is always a bad thing! When mistaken for
BROWSE THE “FOR YOUR CONSIDERATION” ARCHIVE AT
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ASTRONOMY
• MARCH 2018
For our evolutionary ancestors like Australopithecus afarensis, quick thinking and the feeling of knowing often made the difference between life and death. JOHN GURCHE/SMITHSONIAN INSTITUTION
justified justif ied knowledge, t he feeling of knowing can lead us down a primrose path. But when recognized for what it is, that sensation can be a valuable guide. In his book Blink: The Power of Thinking Without Thinking , Malcolm Gladwell discusses the way our brains rapidly and subconsciously combine even small amounts of information with our previous knowledge and experience to reach tentative conclusions. Those thoughts, accompanied by a feeling of knowing, enter our conscious minds as intuition. Intuition alone is never a substitute for justified knowledge. But if grounded in justified knowledge and enough relevant experience, intuition can suggest a path. Science is all about justified knowledge, but intuition is vital ly important even here. Scientists often rely on gut feelings to decide what ideas might be worth pursuing, or what approach might be likely to yield good results. The absolutely crucial caveat is that while intuition might be a good place to start, it is only a start. An idea might feel right, but that doesn’t matter to scientists
www.Astronomy.com/Hester.
until it’s put through the wringer. By the w ay, if you don’t look for the flaws in your pretty idea, rest assured that someone else will do it for you! Knowledge is a slippery juxtaposition of philosophical considerations and ages-old neurological imperatives buried deep within our brains. With that comes a practical challenge with profound realworld consequences for each of us. In a complex world where knowledge matters, how do we navigate treacherous waters filled w ith comfortable, comfortable, specious ideas eager to abduct our all-too-willing brains? After decades in the trenches as a scientist, I can share what works for me. I listen to my intuit ion, but I’m gun-shy. When I start feeling too certain about something that’s my cue to get out the sledgehammer and start pounding on my precious idea to see if it breaks. Only then can I talk about what I know k now.. Jeff Hester is a keynote keynote speaker, speaker, coach, and astrophysicist. Follow his thoughts at jeff-hes ter.com.
ASTRONEWS ASTRO NEWS
ODD ONE OUT. NGTS-1b, a planet whose mass is similar to Jupiter’s, was found orbiting a red dwarf star supposedly too small to have formed a planet that big, challenging planet formation theories.
The case of the shrinking white dwarf Astronomers have observed many white dwarfs over the years. But a study published November 13 in Monthly Notices of the Royal Astronomical Society presented the first observational evidence of a contracting white dwarf that has been steadily shrinking for the past 2 million years. “For decades, it has been theoretically clear that young white dwarfs are contracting,” Sergei Popov, an astrophysicist and lead author of the study, said in a press release. “Yet that very phase of contraction has never been obser ved in ‘real time.’ ” The contracting white dwarf is part of a binary system named HD 49798/RX J0648.0– 4418, 4418, located some 2,000 light-years away in the constellation Puppis. Astronomer Sandro Mereghetti, co-author of the study, recently discovered that the white dwarf’s rotational velocity was the fastest ever observed for such a remnant, and has been speeding up over the past
20 years. Mereghetti also found that the white dwarf’s original 13-second spin period — the time it takes to complete one full rotation — was decreasing by about 7 nanoseconds each year. Although a few nanoseconds per year may not seem like much, for an object as massive and compressed as a white dwarf, this corresponds to a significant shift in angular momentum — something that could not be accomplished solely through the accretion of matter. Instead, the researchers demonstrated that the white dwarf’s faster spin could be easily explained if the star were contracting, much like the way a spinning figure skater rotates faster as she pulls in her arms. “Thanks to this discovery, astrophysicists will be able to study and evaluate the evolution patterns of young white dwarfs — and successfully look for similar systems in the galaxy,” Popov said. — J.P.
SPEED SPINNING. In
this artist’s concept, the white dwar (let) in the binary system HD 49798/RX J0648.0–4418 is spinning while surrounded by an accretion disk o matter taken rom its larger companion star (right). Astronomers think the white dwar is spinning aster over time because it is contracting. FRANCESCO MEREGHETTI
Journey to Iceland for an unforgettable view of the aurora borealis! Everyone should experience the astonishing beauty o the aurora borealis once in their lives — and one o the best places on Earth to take in the spectacle is under Iceland’s northern skies.
This unique, new Iceland itinerary features: • 8 nights o viewing just steps away rom your comortable countryside accommodations. • The seldom-visited Great North region and the stunning Westfords peninsula. • Whale watching, ascinating local museums, a superb local guide, and much more. 5 4 3 2 3 P
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This intrepid spacecraft spent 13 years studying the ringed planet, transforming our view of this captivating world. by Liz Kruesi
Saturn’s globe blocked the Sun while Cassini captured this panoramic view showing the planet’s ring system in exquisite detail. The imaging team created this mosaic from 165 separate images taken over a three-hour period. NASA/JPL�CALTECH/SSI
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the evening of September 11, 2017, Griffith Observatory hosted an enthusiastic group of observers. The assembled crowd looked through the 12-inch 12-inch Zeiss refracting telescope, the centerpiece of the venerable public astronomy venue in Los Angeles. They watched as light from Saturn and its largest moon, Titan, passed through the telescope’s optics, where lenses bent and focused it onto their eyes.
The onlookers could see the beautiful rings circling Saturn, t he planet’s yellowish cloud bands, and the orange-tinged dot of the big moon near the planet; what they couldn’t make out was a much smaller, human-made human-made target. On that late summer evening, the Cassini spacecraft was just 75,000 miles (120,000 kilometers) from Titan on its final path toward Saturn. The spacecraft and Titan had enjoyed their “goodbye kiss,” as the astronomers and engineers on the mission called the last gravitational yank that would send the
2004-2017
spacecraft into the planet it had been studying for 13 years. These observers at Griffith were no ordinary members of the public. They were members of Cassini’s Project Science Group, watching their beloved spacecraft on its final journey around the t he giant world. world. “It was a magical evening,” says Cassini’s Project Scientist Linda Spilker. Over the next few days, hundreds of scientists and engineers on the Ca ssini mission team would reminisce about the spacecraft, which had launched from
Cape Canaveral, Florida, nearly 20 years earlier. But their thoughts were not all on the past: Cassini was still collecting data and sending it back to Earth. On September 15, at 3:31 �.�. PDT, PDT, Cassini entered Saturn’s upper atmosphere at a shallow angle. It would travel through the gas for nearly 1½ hours. The team members were gathered at NASA’s NASA’s Jet Propulsion Laboratory in Pasadena, California, where they watched and waited. “The room got quieter and quieter as we got down to those final minutes,” says Spilker.
At 4:55 �.�. PDT, they saw the last signal from Cassini fade away on the screen. The room erupted in applause — not for the end of the mission, but for what the spacecraft and those hundreds of people had achieved. Cassini revealed surprise after surprise at Saturn: an incredibly complex system of moons and moonlets, rings that change structure on hourly timescales, and a beautiful atmosphere atmosphere wracked by huge storms. The 13 years of images and measurements changed humanity’s view of the ringed world. But there’s still more to learn from
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Above: Cassini could probe Saturn’s ring structure by sending radio signals through the rings. In this simulated view of the A ring and the Cassini Division (at left center), red denotes particles 2 inches (5 cm) or more in diameter; green indicates particles less than 2 inches across; and blue signifies particles less than 0.4 inch (1 cm) across. NASA/JPL�CALTECH Right: Vertical structures at the B ring’s outer edge cast long shadows onto the rings two weeks before Saturn’s August 2009 equinox. The structures rise some 1.6 miles (2.5 km) above the rest of the rings, which average about 33 feet (10 m) thick. NASA/JPL�CALTECH/SSI
the last several months of data t hat Cassini collected. Scientists hope those final obser vations will tell them about Saturn’s Saturn’s interior interior — in particular, how it generates a magnetic field and how its mass is distributed.
An extended stay Spacecraft already had visited Saturn three times before Cassini arrived in mid-2004, mid-2004, so scientists had some inkling of what they might find. But as with any new mission — especially one involving a machine with 12 sophisticated instruments that would remain in orbit orbit instead of fly ing past as its predecessors had — Cassini revealed a complex complex planet full ful l of surprises. And t hat’s a good thing. “If Saturn had been exactly as expected, it would have been a lot more boring,” says Spilker. Cassini arrived at Saturn for a primary mission set to last four years. But when mid-2008 mid-2008 came, ca me, the spacecraft continued with its Equinox M ission extension. extension. And in September 2010, the mission began its
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Solstice extension, which ultimately bled into the Grand Finale. This final stage commenced in April 2017 and featured 22 close-in orbits that skimmed just above Saturn’s cloud tops. Because Cassini’s 12 instruments were attached directly to the spacecraft, the entire contraption had to rotate for an instrument to point toward a specific target. That meant multiple instruments couldn’t observe observe the same spot at the same time. Instead, while one looked at a moon, another might observe Saturn’s rings. And that made the la st five months of the mission — the Grand Finale — a work of impressive coordination. Although 22 orbits might sound like a lot, they aren’t much to work with when you have to divide the limited time during those close flybys fly bys among the full instrument lineup.
Rings, rings, and more rings Saturn’s Saturn’s rings are a re the planet’s defining characteristic. From afar, afar, they look like l ike a
vinyl record circling circling the yellow gas giant. But bring a camera close to Saturn, and the smooth disk resolves into belt after belt after belt, with spaces separating them. That was the view revealed by the Voyager flybys in 1980 and 1981, which led scientists to think th ink the rings ri ngs were probably probably made of tiny ice particles that slowly bump into one another as they orbit the planet. Use Cassini’s instruments to watch as the rings filter light from a background star, however, and all of a sudden those belts become far more complex. The particles clump together and form bigger bodies. The gravity of those objects — boulders and minimoons — controls the rings, herding smaller particles and building structures and patterns. And they change quickly, says Larry Esposito, principal investigator on Cassini’s Ultraviolet Imaging Spectrograph, who has studied Saturn’s rings for more than four decades. “Structures develop within hours in the rings.” Planetary scientists have identified identified several different types of structures. Some, which come and go and come back again, are called kittens k ittens — “because they seem to have multiple lives,” says Esposito. Others, called propellers, migrate slightly inward or outward. They are consequences of gravitational interactions between a small moon embedded embedded within the rings and t he ring particles themselves. The moon tries, unsuccessfully, to clear away the particles and create a gap. Bigger moons tend to have more noticeable effects. Prometheus, for example, whose diameter averages 53 miles (86 km), “dips in almost to the edge of the F ring and pulls out streamers,” says Spilker. Several other moons also leave their gravitational imprints. Scientists had long known that Mimas creates the Cassini Division — like the spacecraft, named for the 17th-century Italian-French astronomer Giovanni Cassini — the broadest gap in the rings. But it took data from the Cassini probe to reveal that seven midsized moons combine to keep the outer A ring from dispersing. The rings also contain density waves that show up as va riations in brightness and thickness. After studying t hese patpatterns, scientists, including the University of Idaho’s Matt Hedman, showed that these brightness changes are tied to Saturn’s interior. The researchers used fine-scale density variations in the rings as a seismometer of sorts to learn about how the planet’s interior oscillates, in much the same way
Above left: A disturbance in Saturn’s narrow F ring appeared April 8, 2016. The disorder likely arose when a small body embedded in the ring interacted with material at the ring’s core. The small moon Pandora (lower right) was a mere bystander. NASA/JPL�CALTECH/SSI Above right: Potato-shaped Prometheus (lower left) dips into the F ring’s inner edge once each 15-hour orbit, pulling particles into a streamer. This image captures the moon as it creates a new streamer; the dark streamers at upper right formed during the moon’s previous two incursions. NASA/JPL�CALTECH/SSI Left: Tiny Daphnis orbits in the Keeler Gap near the A ring’s outer edge. Here, the 5-mile-wide (8 km) moon makes waves from the fine particles at the gap’s edge. The waves dissipate quickly, however, as the moon travels toward the image’s right side. NASA/JPL�CALTECH/SSI
that solar astronomers have studied how brightness variations at the Sun’s surface correspond correspond to its inner pulsations. Despite all the incredible ring structu re that Cassini’s cameras a nd spectrometers resolved, scientists still have questions. The biggest one concerns the ring system’s mass. They don’t want to know this mass just for knowledge’s knowledge’s sake. Instead, the mass is linked to the age of the rings and how they formed. This is important because Saturn’s rings are the closest example astronomers have of astrophysical disks — such as the flattened disks of gas and dust out of which solar systems form. “It’s not the same, but it’s analogous,” says Hedman. And this means, “if we don’t understand what’s going on in the
rings, we could be missing things about, say, how the solar system formed.” The processes going on in the rings could give astronomers valuable insights into how planetary systems develop. The Grand Finale data are getting get ting scientists closer than ever to figuring out the rings’ mass. During those fina l months, Cassini flew bet ween the inner rings and Saturn’s upper atmosphere 22 times. Throughout the previous 12½ years of Saturn exploration, the spacecraft stayed outside the rings, and thus it felt the combined pull from Saturn and the rings. “When you are between the rings and Saturn, the rings are pulling in one direction, and Saturn is pull ing in the other ot her,, so you can disentangle t he two effects,”
says Luciano Iess, who is leading C assini’s gravity data analysis. Disentangling the two will not be easy, however. The preliminary analysis, ana lysis, he says, “seems to indicate that the rings d id not form with Saturn.” It will take ta ke more research research to firm up this result, and to find out when and how the rings formed.
Cloudy weather Beneath Saturn’s majestic rings lies the planet’s equally magnif icent cloud tops. Cassini unveiled churning and swirling clouds in the upper atmosphere, and places where warm gases rise up through cooler layers and erupt into long-lasting thunderstorms. Cassini resolved these t hese thunderstorm clusters into minute detail, watching
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Tiny embedded moonlets create “propellers” as they unsuccessfully try to open gaps in the rings. In one of its final images, Cassini captured one such feature just above the Keeler Gap in the outer A ring. NASA/JPL�CALTECH/SSI
The informally named Earhart propeller resides in the A ring just inside t he Keeler Gap (right). Earhart is the attempt of an unseen moonlet to create a ring gap, but the large mass of the surrounding material quickly fills the nascent breach. Dozens of small propellers occupy the so-called propeller belts in the middle of the A ring. The propellers look like double dashes and appear on both sides of the density wave that cuts diagonally across this scene.
NASA/JPL�CALTECH/SSI
NASA/JPL�CALTECH/SSI
them evolve and listening to the radio static from lightning flashes. While normal photos painted pretty pictures of the whirling atmosphere, infrared images let scientists see below the cool cloud tops to warmer regions beneath. “And that’s our secret weapon for how to analyze the depths of Saturn,” says Cassini scientist Kevin Baines. “[It’s] how Saturn was revealed to be not this nice demure place, but this roiling dynamic place.” He and his colleagues watched as clouds in the upper atmosphere blocked heat from below. They also identified vortices and a giant cyclone at each of Saturn’s poles, though only the north pole features a hexagonal jet stream. But one storm stood out from all the others. The Great White Spot erupted unexpectedly December 5, 2010. Earthbased observations of Saturn over the past 140 years had shown that a giant, longlasting storm pops up every 30 years or so, alternating between cloud bands in the northern hemisphere and near the equator. In 1876, one appeared at the equator; in 1903, another developed at mid-northern latitudes; and in 1933, a storm emerged back at the equator. The pattern continued over the decades, and scientists expected the next storm would arrive around 2020 — after Cassini’s reign. But it fortuitously arrived 10 years early, and gifted Cassini scientists with an up-close look at how these giant storms evolve.
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Cassini’s imaging camera first saw the storm December 5, at the same time another instrument heard it — or at least, the radio bursts created by its lightning. A similar phenomenon phenomenon happens on Eart h. If you have ever been in a car listening to an AM radio station during a t hunderstorm, hunderstorm, you probably heard what sounded like static. “That static is not actual ly static,” says William Kurth. “It’s actually radio emissions from the lightning strokes a nd the thunderstorm, and they propagate propagate at the speed of light.” Kurth is t he principal investigator investigator of Cassini’s C assini’s Radio and Plasma Wave Science (RPWS) instrument, which listened in on the Great White Spot’s
high-frequency radio emissions created in lightning strokes. The jet streams in Saturn’s atmosphere carried the t he northern hemisphere storm along its cloud band. By late January 2011, it wrapped around the planet and st retched 9,000 miles (15,000 km) north-south. As the storm progressed, scientists used the imaging instruments and RPWS to view it. In summer 2011, after some 200 days of roiling, swirling, and spreading, t he storm died out and t he atmosphere atmosphere cleared. The region, says Baines, “has been very boring ever since.” Because scientists could watch the great storm evolve with Cassini’s broad array of
Saturn’s north polar hexagon is a meandering jet stream n ear 77° nort h latitude. Ea ch side of the hexagon measures slightly longer than Earth’s equatorial diameter. NASA/JPL�CALTECH/SSI
A giant vortex resides at Saturn’s north pole. The storm, which appears red in this false-color image, spans 1,250 miles (2,000 km) and has winds up to 330 mph (540 km/h). NASA/JPL�CALTECH/SSI
The Great White Spot appears as a multihued snake in this false-color mosaic from February 2011. Yellow and white reveal high, thick clouds associated with thunderstorms; red shows deep clouds with no towering tops; and blue areas are cold spots. NASA/JPL�CALTECH/SSI
instruments, they could piece together a coherent picture of what causes these longlived events. Caltech’s Andrew Ingersoll and his t hen-graduate hen-graduate student Cheng Li put forth the most likely t heory. They say it’s due to a convective competition between water-rich clouds and the lighterweight atmosphere of mostly hydrogen and helium. The heavier, wet clouds can’t rise until the lightweight upper clouds become denser and sink. But this competition is a marathon. “The air above has to cool off, rad iating its heat to space, before its density is greater than that of the hot, wet air below,” said Li in a press release. “This cooling process takes about 30 years, and t hen come the storms.” Once the storm rains out its water content, convection shuts down, and the storm stops.
Magnetic makeup When you think of Saturn, t he ornate rings and cloudy atmosphere likely come to mind first, but no object exists in isolation. So, how does the giant planet affect its surroundings? That’s where Saturn’s magnetic field factors in, and it’s why Cassini brought along instruments to study it and
The Great White Spot erupted in December 2010 and quickly evolved into a massive storm. By the time Cassini captured this image 12 weeks later, Saturn’s jet streams had carried the storm completely around the planet. NASA/JPL�CALTECH/SSI
the region the field controls, called the magnetosphere. Previous observations of Saturn had shown aurorae at the planet’s poles, similar to the northern and southern lights seen in Earth ’s polar regions. Cassini’ Cassini’ss RPWS instrument monitored monitored auroral activity act ivity by detecting the radio waves that aurorae
Although Saturn’s north polar hexagon has lasted for at least 35 years (the Voyager spacecraft first imaged it in the early 1980s), it does change. These natural-color views show the hexagon in June 2013 (left) and April 2017. Scientists thin k an increase i n solar radiation duri ng those four year s caused yellowish smog to form. NASA/JPL�CALTECH/SSI/HAM �CALTECH/SSI/HAMPTON PTON UNIVERSITY
generate, in much the same way it heard radio flashes associated with lightning. “We’ve been able to use the intensity of these radio emissions as a proxy,” says Kurth, to address questions of “how intense are the auroras and is there a lot of activity going on.” RPWS also monitored how Saturn’s magnetosphere and aurorae changed when the Sun delivered a burst of high-energy particles and radiation. But how does Saturn produce its magnetic field? To find out, scientists used Cassini’s magnetometer. magnetometer. This instrument measures the strength and location of the planet’s magnetic field lines, which trace how charged particles travel. Elect rons, for example, have a negative charge, and they always move toward a magnet’s positive pole. Both Saturn and Earth are essentially giant dipole magnets: They have a positive pole and a negative one. Each planet generates its magnetic field deep in its interior. For Earth, researchers have a pretty good idea of how it happens. “You have heat, you have convection convection taking place in the interior, you have rotation in the interior, and
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Enceladus prepares to set behind Saturn’s limb September 13, 2017. This was one of the last images Cassini took of the geologically active moon before the probe crashed into the gas giant September 15. NASA/JPL�CALTECH/SSI
Above: Saturn’s aurora glows blue while the underlying atmosphere appears deep red in this infrared composite image. As on Earth, the aurora arises as Saturn’s magnetic field funnels energetic solar particles to the polar regions. NASA/JPL�CALTECH/UNIVERSITY NASA/JPL�CALTECH/UNIVERSITY OF ARIZONA
Left: Cassini captured the ultraviolet glow from Saturn’s aurora one day before the spacecraft crashed into the planet. The north pole lies at the center of this image, while the bottom faces the Sun. NASA/JPL�CALTECH/UNIVERSITY OF COLORADO/UNIVERSITY OF LIEGE�LPAP
you have flowing electrical electrica l currents,” says Michele Dougherty, principal investigator of Cassini’s magnetometer. “All of those combine to give you the magnetic field that you measure outside the planet.” A key component in understanding Saturn’s Saturn’s magnetic field is the length lengt h of a saturnian day, and this was a major quesquestion scientists hoped Cassini would resolve. This shouldn’t be a dif ficult question, right? It’s just the rotation period. But that’s a much harder problem to solve for gas giant planets than it is for Earth. The cloud tops rotate at different speeds, and t he
thick atmosphere hides the planet’s solid core — assuming it has one. To measure Jupiter’s day, for example, scientists track the magnetic axis and find it wobbles with respect to the planet’s rotation. The magnetic field’s axis and t he rotation axis tilt t ilt relative to each other, and that wobble relates relates directly to how fast the planet’s core is spinning. The problem with Saturn, though, is that the two axes are nearly perfectly aligned. This makes it awfully hard to f ind that wobble. The precise alignment also perplexes researchers because it implies that the
magnetic field should be decaying — and scientists have seen no evidence of a diminishing magnetic field at Saturn. When Cassini flew close to Saturn during the Grand Finale, the magnetometer collected data about t he magnetic field. “We really expected these t hese Grand Finale orbits orbits to clearly measure the tilt, and a nd all we’ve we’ve been able to do so far is put a limit on it,” says Dougherty. The angle between the two axes must be less than 0.06°. 0.06°. The team has had the data for only a couple of months, however, and Dougherty is confident that after she and her colleagues complete their careful and thorough analysis, they’ ll know k now what Saturn’s Saturn’s internal magnetic field is like. The biggest hurdle is accurately calibrating the instrument. The analysis requires absolute precision — the exact location and timing of t he spacecraft’s trajectory, and knowledge of where Cassini was when the instrument collected each bit of data. Researchers have predicted orbits, orbits, positions, and times, but they have to know k now whether Cassini’s actual orbit followed them precisely. For example,
On May 28, 2017, Cassini flew between Saturn’s rings and its cloud tops, capturing the images for this mosaic. Saturn appears in the left foreground, adorned with shadows cast by the rings. The rings themselves emerge from behind the planet’s limb and extend to the right. NASA/JPL�CALTECH/SSI
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Saturn posed for Cassini one last time September 13, 2017. The imaging team assembled this natural-color mosaic from 42 wide-angle images taken through three color filters from about 15° north of the ring plane. NASA/JPL�CALTECH/SSI
During its Grand Finale mission, Cassini captured subtle atmospheric details. In this view, the Sun shines at a low angle near Saturn’s terminator, where day turns to night, and some high clouds cast shadows on lower regions. NASA/JPL�CALTECH/SSI
might there have been a half-second delay because the craft felt more drag from t he atmosphere atmosphere than expected? “It’s a really complicated process,” says Dougherty of the analysis. “It’s like trying to find t hree or four needles in a haystack that’s changing shape and size at the time.”
Mapping gravity’s pull The magnetic field analysis isn’t the only one proving to be extremely complex and requiring precise calibration. Scientists also want to know about Saturn’s interior, and in particular, how the planet’s mass is distributed. To do that, they need to measure the planet’s gravity. That’s not as simple as it might sound. “There is no instrument aboard a satellite which can reveal the gravity field by itself,” says Iess. Instead, scientists passed radio signals between Earth
Cassini crashed into Saturn’s atmosphere September 15, 2017, at the spot marked by the oval. This nighttime infrared view shows heat coming from the planet’s interior in red; the dark regions are silhouetted clouds. NASA/JPL�CALTECH/UNIVERSITY NASA/JPL�CALTECH/UNIVERSITY OF ARIZONA
and Cassini and measured the slight changes in radio frequency. Those changes arose from gravitational tugs of mass pulling on the spacecraft — the more mass, the bigger the tug. So Iess and his colleagues can use those tiny frequency changes to map the distribution of mass within Saturn. Because Cassini skimmed the planet’s cloud tops during its final months, it felt a stronger gravitational pull from those mass d istributions, and was able to sense finer details. Precisely understanding those Grand Finale orbits is crucial crucial to the gravity analysis of Saturn. So far, the team has lear ned that theoretical models of Saturn’s Saturn’s gravity do not match the data. “The gravity field of Saturn is surprising,” says Iess. “We found Saturn has features that can be explained only by differential rotation,” rotation,” meaning some portions or layers of the planet move
at different speeds than others. The researchers still have more orbit trajectories to calibrate, and thus are still st ill months away from a major announcement. Revealing that the t he interior doesn’t doesn’t align with models would be a fitting discovery from a mission that already has found so many surprises at the Saturn system. sy stem. Cassini’s suite of instruments offered the flexibility that allowed scientists to make those discoveries. The mission’s scientists and engineers worked in sync for decades to perform what Spilker calls C assini’s “intricate ballet.” “It’s “It’s for the unknown, un known, the unexpected,” she says. “That’s why you do science.” Contributing Editor Liz Kruesi writes about distant objects from her Earthbound home in Austin, Texas.
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Saturn’s small wonders Usually known for its rings, the Saturn system is also home to some of our solar system’s most intriguing moons. by Francis Reddy
ye candy is not in short supply at Saturn. For visitors who tire of watching the planet’s stormy atmosphere or gazing into the solar system’s most beautiful and complex ring system, there's always the giant satellite Titan to explore. This colossal moon is bigger than Mercury and sports a hazy orange atmosphere denser than Earth’s, producing methane rains that flow across Titan’s icy landscape and pool into vast lakes. But look again. Even Saturn’s small moons display some unusual dy namic relationships. relationships. For instance, Pan and Daphnis dwell in t he Encke and Keeler ring gaps, respectively, where their gravity rumples the ri ng’s ng’s boundary and a nd sweeps away particles to keep the gap clear. There’s Janus and Epimetheus, whose orbital differences are smaller than their diameters, dia meters, so they should collide, but don’t. Instead, these “co-orbiting” moons effectively play leapfrog, swapping orbits over a four-year cycle. Other small satell ites orbit orbit in the gravitational safe zones — called Lagrangian points — of the midsized moons Dione and Tethys. Lagrangian points are locations 60° in front of and behind a larger object’s orbit where a less-massive body can move in an identical stable orbit. Dione travels with Helene and Polydeuces, while Telesto
and Caly pso orbit along with Tethys Tethys — an arrangement thus far unseen among a mong any other moons in the solar system. And this is just for starters. “The Saturn system is full of surprises,” says Paul Schenk, a planetary geologist at the Lunar and Planetary Institute in Houston. There’s There’s a satellite that likely originated in the Kuiper Belt, the storehouse of icy bodies beyond Neptune’s orbit; a piebald moon nearly encircled by an equatorial ridge containing some of the tallest mountains in t he solar system; a spongy-looking tumbling satellite; and a moon that vents its subsurface sea into space, providing scientists scientists with an a n unexpected potential niche for extraterrestrial life.
Above: NASA’s Cassini mission took images as the spacecraft approached (left) and departed (right) Saturn’s moon Phoebe during its only close flyby of the satellite. Cassini passed just 1,285 miles (2,068 km) above the surface on June 11, 2004. Phoebe is thought to be a centaur that might have become a Jupiter-family comet, had Saturn not captured it. NASA/JPL�CALTECH Opposite: Cassini took this Saturn mosaic October 21, 2013. NASA/JPL/SSI
Phoebe was discovered in 1899 and is the fir st satellite found photographically. NASA’s NASA’s Cassini spacecraft made its only close flyby of the distant moon June 11, 11, 2004, about three t hree weeks before slowing to enter orbit around the planet. At only about 130 miles (210 (210 kilometers) across, Phoebe is about one-sixteenth the size of our Moon. Its heavily cratered surface is mostly dark as soot with no signs of resurfacing due to geological activity. But bright cliffs on the rims of the largest craters, as well as bright rays WWW.ASTRONOMY.COM
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Ring king Saturn
Iapetus
Titan
Phoebe
n g i n r e e b o P h
Saturn’s Phoebe ring is the solar system’s largest. The bulk of the material starts 3.7 million miles (5.9 million km) from the planet and extends outward to 10 million miles (16 million km), and possibly farther. The ring is also about 20 times as thick as the planet’s diameter. NASA/JPL�CALTECH
extending from smaller ones, reveal ice beneath a layer of dark material up to about 1,600 feet (490 meters) deep. Ground-based telescopes have detected the presence of frozen water, and Cassini’s instruments showed the presence of frozen carbon dioxide (dry ice) and organic material as well. Phoebe orbits orbits in the opposite direction of Saturn’s spin, on a path that is both more eccentric and more highly inclined than the planet’s inner moons. On the basis of these orbital characteristics, astronomers have long suspected Phoebe of being an interloper ensnared by the planet’s gravity, rather than a native to the Saturn system. Phoebe’s chemical makeup resembles C-type asteroids commonly found in the farthest regions of the main asteroid belt, while its density suggests an ice-rock mixture similar to Neptune’s moon Triton (itself thought to be a captured object) and Pluto. Phoebe could have been a centaur. Centaurs are comet-like bodies that follow planet-crossing orbits between 30
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Jupiter and Neptune. Gravitational interactions with the giant planets will eventually destabilize their orbits. It’s thought centaurs originated in the Kuiper Belt and were perturbed into their current orbits by Neptune. Once they start crossing the orbits of giant planets, centaurs may collide with or be captured by them, be flung out of the solar system, or be rerouted by Jupiter into orbits bringing them much closer to the Sun, where they become Jupiter-family Jupiter-family comets. The idea that Phoebe may have been a centaur before being captured by Saturn is consistent with its surface composition, but it’s difficult to say more with the available observations. In 2009, infrared observations by NASA’s Spitzer Space Telescope showed that Phoebe resides within a supersized, tenuous tenuous ring of ice and dust par ticles that had been previously undetected. “If you could see the ring, it would span the width of two Full Moons’ worth of sky, one on either side of Saturn,” says Anne Verbiscer, an astronomer at the University
of Virginia, Charlottesville, who led the research. A 2015 study using data from NASA’s Wide-Field Infrared Survey Explorer found this king of rings extends even farther, starting 3.7 million miles (5.9 million km) from the planet and reaching at least 10 million miles (16 (16 million mil lion km). Continual small impacts on Phoebe regularly eject dust that maintains the ring, while particles smaller than a few inches gradually migrate inward, likely the source for dark material found on the surfaces of other ot her satellites, especially Iapetus.
In 1671, Italian astronomer Giovanni Domenico Domenico Cassini discovered Iapetus, the next moon of Saturn as we move inward from Phoebe. He noticed that when Iapetus was on one side of Saturn, it was very bright, but on the opposite side, it nearly disappeared from view. He correctly proposed that Iapetus is tidally locked — meaning it always turns the same face to Saturn — just as our Moon
As it approached Iapetus on September 10, 2007, Cassini captured this view, shown in enhanced color. The prominent equatorial ridge is at center left. Inset: This view along the ridge system shows mountains with elevations reaching 6 miles (10 km). In places, the peaks of Iapetus extend twice as high. A fresh impact crater on the distant slope has exposed bright �CALTECH/SPACE CE SCIENCE INSTITUTE subsurface ice. NASA/JPL�CALTECH/SPA
Global color mosaics of Iapetus were assembled from images taken by Cassini during its first decade at Saturn. The colors are enhanced relative to human vision, extending from the ultraviolet into the infrared. Left: The bright trailing hemisphere. Right: The dark leading NASA/JPL�CALTECH/SPACE SCIENCE INSTITUTE/LUNAR AND PLANETARY INSTITUTE hemisphere. NASA/JPL�CALTECH/SPA
WWW.ASTRONOMY.COM
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Daphnis
Atlas
Pan
6 miles (10 kilometers)
is to Eart h, and that the hemisphere centered on the direction of motion is covered covered with a dark material. Planetary scientists later named this dark da rk feature Cassini Regio in his honor, as well as the Cassini mission. At 927 miles (1,492 km) across, Iapetus is the third-largest moon of Saturn, and the planet’s only large satellite in a highly inclined orbit, which carries the moon in the same direction Saturn rotates, unlike Phoebe. Among the moons in the solar system that exhibit tidally locked rotation, Iapetus is by far the most distant from its planet. The moon’s density is only slightly g reater than frozen water, indicating that rock makes up perhaps only one-fourth of its composition. As Iapetus circles Saturn, dust migrating in the opposite direction from the Phoebe ring smacks into Cassini Regio like bugs on a windshield, but the shape of the dark patch can’t be fully explained by this simple accumulation accumulation of material. For one, the dark side of Iapetus is much redder than Phoebe. In 2010, John Spencer at the Southwest Research Institute in Boulder, Boulder, Colorado, and Tilmann Denk at the Free University University of Berlin proposed that the dark deposits, which reflect as litt le sunlight as fresh asphalt, warm the leading hemisphere just enough enough that water ice molecules can sublimate, turning directly to a gas. These molecules migrate from the warmer war mer leading hemisphere hemisphere to the colder tra iling hemisphere, where they freeze onto the surface again. The slow, tidally locked spin of Iapetus produces unusually high daytime temperatures and water ice sublimation rates for a given reflectance, so once this thermal migration begins 32
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(thanks to the initia l deposits from Phoebe), it can be sustained. As the surface ice departs from Cassini Regio, what’s left behind is a residue native to Iapetus mixed in with material from the Phoebe ring. On New Year’s Eve, 2004, the Cassini spacecraft approached Iapetus for the first time, and planetary scientists were enthralled with t he images it returned. The heavily cratered body showed showed numerous large impact basins — the largest in the Saturn system — but its standout feature was a conspicuous ridge running almost exactly along the equator, giving the icy moon the look of a walnut. The ridge extends over 800 miles (1,300 km) and cuts completely across Cassini Regio. In places, it breaks into isolated mountain peaks that may reach 12 miles (19 (19 km) high, rivaling t he giant martian volcano Oly mpus Mons but located on a world five times smaller. There is nothing like it any where else in the solar system, and a completely satisfactory explanation of its origin eludes
Ring moons Atlas, Daphnis, and Pan are shown here in color-enhanced Cassini images at the same scale. Atlas orbits just outside the A ring, the outermost of Saturn’s main rings. Pan orbits in the ring’s Encke Gap, clearing it of particles except for a single ringlet, which the moon maintains. Both Atlas and Pan have equatorial ridges of accreted ring material, making them look like ravioli. Smaller Daphnis orbits within the A ring’s narrow Keeler gap, where its gravitational pull generates ripples along the edges. The combined gravitational influence of Atlas and the moons Pan, Prometheus, Pandora, Epimetheus, Mimas, and Janus help corral particles within the A ring and sculpt its outer edge. NASA/JPL�CALTECH/SPACE SCIENCE INSTITUTE
scientists. Did it erupt or grow from the inside, when Iapetus was young and its interior was still warm? Or is it t he accreted remains of a ring either kicked up by an impact or formed when a small satellite passed too close and was crushed in the tides of Iapetus? A 2014 study led by Erica Lopez Garcia at Brown University University examined exami ned the topography of the ridge and found predominantly triangular slopes, much like what you’d get by slowly pouring sugar into a pile, suggesting an external source. But new models exploring possible internal formation mechanisms continue to appear. “I think more people favor the ring explanation, but the debate is still ongoing,” says Francis Nimmo, a planetary scientist at the University of California, Santa Cruz. Supporting the ring interpretation are equatorial bulges on other moons in the system, especially Atlas, Pan, and Daphnis, which accumulate particles while orbiting in and near Saturn’s A ring. And then there’s Rhea, Saturn’s
Blue streaks on the moon Rhea, enhanced here to highlight color differences, reveal fresh ice re-ex posed when material in Rhea’s ring struck the surface. The streaks form a very narrow band only about 6 miles (10 km) wide, straddling Rhea’s equator, that can be traced over at least 75 percent of the moon’s circumference. These views were created using stereo topography from Cassini imaging data returned in 2008. It remains unclear whether the moon retains a tenuous ring today. PAUL SCHENK, LUNAR AND PLANETARY INSTITUTE
second-largest moon, which may or may not retain a tenuous ring today. “On Rhea, we found very unusual bluish spots along its equator, which are now interpreted as evidence for the re-accretion of a very thin ring of debris surrounding that moon in the not-sodistant past,” says Schenk. Inward of Iapetus lies Hyperion, Saturn’s Saturn’s largest irregularly shaped moon, measuring 224 miles by 165 miles (360 km by 266 km). A 2005 Cassini flyby revealed a bizarre spongelike appearance, a single giant crater surrounded by a profusion of smaller ones, and about half the mean density of pure, solid water ice — one of the lowest-density lowest-density materials in the t he outer solar system. This means Hyperion truly must be spongelike, full of ti ny holes that greatly reduce its overall mass and surface gravity. Hyperion’s high fraction of pores helps it preserve older craters because more recent impacts eject less debris to cover them up. But the moon’s most unusual characteristic is its rotation. Hyperion’s Hyperion’s irregular shape, eccentric orbit, and proximity to Saturn’s big moon Titan create conditions that wrench it out of any kind of stable spin and even prevent it from tidally locking to Saturn. Its rotational period and the direction of its spin axis can change unpredictably over days or weeks as Hyperion tumbles along its orbit. Once believed unique, we now know Hyperion’s chaotic spin is shared by at least two of Pluto’s moons, Nix and Hydra, thanks to large torques generated by Pluto and its largest moon, Charon.
This color-enhanced view of Saturn’s moon Hyperion, imaged by Cassini in 2005, reveals crisp details across the strange, tumbling moon’s surface. Hyperion’s naturally reddish tint was reduced and other colors emphasized to better show subtle color variations across the surface. Hyperion’s low density and low gravity combine to preserve the original shapes of its craters. Impacts tend to compress the porous surface rather than blast it out, and what little ejecta is produced is more likely to leave the moon than cover up older craters. NASA/JPL�CALTECH/SPACE �CALTECH/SPACE SCIENCE INSTITUTE
In 1981, Voyager 2 imaged parts of Enceladus, a midsized moon, at high resolution, olution, revealing troughs, t roughs, scarps, g roups of ridges, and craterless plains plains — all types of terrain indicating internal forces had altered the sur face comparatively comparatively recently
Long, wispy fingers of icy particles extend from the geysers on Enceladus (black dot, center) NASA/JPL/SPACE SCIENCE INSTITUTE and into Saturn’s E ring. NASA/JPL/SPACE
in geological terms. “We knew there t here was something special about Enceladus because it reflects almost all the sunlight striking it. It had markings indicating it had an active geologic life,” says Ed Stone, project scientist for the Voyager missions. Such diversity was surprising for a satellite only 313 miles (504 km) across, leading some to suggest that t hat Enceladus needed an unexpectedly large internal heat source to power these changes. Soon after the f lyby, researchers researchers noticed that Enceladus orbits within Saturn’s Saturn’s broad, diffuse E ring, which extends from about 131,000 miles to 298,000 miles (211,000 (211,000 km k m to 480,000 km) and thickens away from the moon. Ice grains in the E ring have limited survival times because of collisions with highenergy ions trapped in Saturn’s rotating magnetic field. This process, called sputsputtering, whittles away micrometer-sized micrometer-sized ice grains in decades and breaks down smaller particles in just a few years. So for the E ring to exist, a regular regu lar supply of new grains must be ejected from Enceladus. In the early 1980s, several scientists suggested meteorite impacts, WWW.ASTRONOMY.COM
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This enhanced-color mosaic of Enceladus’ southern hemisphere was produced from images taken during Cassini’s first decade at Saturn. The famous tiger stripe fractures, which vent water vapor and ice crystals into space, appear bluish due to large-grained ice exposed on the surface. PAUL SCHENK, LUNAR AND PLANETARY INSTITUTE
Dramatic plumes spray water ice and vapor from locations along the tiger stripes. Alexandria Sulcus, the least active, is on the left; Baghdad and Damascus, the most active, are on the right. NASA/JPL/SPACE SCIENCE INSTITUTE
geysers, or volcanic eruptions as possible ejection mechanisms, but the matter remained speculative. “Prior to Cassini, nobody would have anticipated anticipated that there t here was an ocean beneath the surface of Enceladus,” says Nimmo. “I think seeing actual geysers erupting was one of t he biggest surprises in planetary science in recent memory.” Cassini observations in 2005 revealed Enceladus as one of the most extraordinary bodies in the solar system. Four 34
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warm, roughly parallel trenches at the moon’s south pole, nicknamed “tiger stripes,” erupt plumes of water vapor, hydrogen, hydrogen, and hydrocarbons that regularly strengthen and weaken as Enceladus orbits Saturn. “The eruption mechanism is understood, at least i n outline,” Nimmo explains. “The tiger stripes appear to be water-filled water-filled cracks, connected to an ocean beneath, which open and close every orbit under the influence of tides.” The water exposed to t he vacuum simultaneously
boils and freezes, creating the vapor and ice crystals we see, some of which fall back to paint the landscape as white as freshly fallen fal len snow. snow. Due to mutual gravitational interac i nterac-tions, Enceladus orbits Saturn twice for every orbit of its more distant neighbor Dione. This orbital resonance forces Enceladus into a slightly eccentric orbit where tides heat the moon’s interior. “Tidal heating is slightly st ronger at the poles anyway, so you might get a feedback loop: The polar region warms up, it becomes more deformable, giving rise to more tidal heating and so on,” Nimmo notes. “But why this only happened at the south pole is a mystery.” There is certainly ev idence that other parts of the surface were warmer in the past. Some impact craters look like they have flowed, indicating warm ice near the surface, and it seems fairly clear t hat different parts of Enceladus were heated at different times. “There are even features that look a bit like a ncient ncient tiger stripes, though this is controversial,” Nimmo explains. “It’s hard to tell from observations whether the activity was episodic, but the fact that you see heavily cratered and very lightly cratered areas, but not moderately cratered cratered areas, suggests that activity was not continuous.”
Unusual reddish arcs cut across the surface of Saturn’s ice-rich moon Tethys in this enhanced-color mosaic. The arcs are only a few miles wide but extend over several hundred miles. Among the most oddly colored features on any moons seen by Cassini, their origin remains a mystery. NASA/JPL�CALTECH/SPACE SCIENCE INSTITUTE/PAUL SCHENK, LUNAR AND PLANETARY INSTITUTE
Cassini flew through the plumes and sampled their composition, showing that nearly 98 percent of the gas in the plumes is water vapor, about 1 percent is hydrogen, and the rest is a mixture of other molecules, including carbon dioxide, methane, and ammonia. Phosphorus Phosphorus remains the only element essential to life on Earth that t hat has yet to be found in the Enceladus plumes, but it’s likely present. With the necessary ingredients — a warm subsurface sea, plentiful hydrogen that organisms could potentially potentially harness as a chemical energy source, and geysers con veniently veniently delivering samples into into space — Enceladus is arguably the most likely and exciting target for the search for life.
While it’s hard to top Enceladus, the neighboring midsized moons Dione and Tethys tantalize planetary scientists with as-yet-unexplained features. “On Dione, a magnificent set of tectonic fractures was
observed, some in a very pristine state of preservation,” says Schenk. A pair of unusual walled depressions in the moon’s smooth terrain might be volcanic vents; half of Dione’s surface is covered by what is likely ancien a ncientt volcanic ice deposits; and some propose that Dione may possess a subsurface sea. On Tethys, a huge rift zone named Ithaca Chasma runs nearly three-quarters of the moon’s circumference, and Cassini’s cameras detected a set of mysterious red arcs that appear to have formed very recently. recently. “So while Enceladus has been stealing our attention, attention, these moons have shown intermittent signs of activity, too,” Schenk adds. On September 15, 2017, Cassini plunged into Saturn’s atmosphere, ending its mission. Scientists Scientists will be mining the data Cassini returned on these t hese diverse worlds for decades to come. Even from the perspective of its smaller satellites, Saturn beckons. When will we retur n?
Dione’s trailing hemisphere displays a network of long, deep, steep-sided fractures flanked by bright, icy cliffs. The pattern may be related to Dione’s orbital evolution and tidal stresses over time. Cassini imaged the moon in visible light in 2015. NASA/JPL�CALTECH/SPACE SCIENCE INSTITUTE
Francis Reddy is Reddy is the senior science writer for the Astrophysics Science Division at NASA’s Goddard Space Flight Center in Maryland. WWW.ASTRONOMY.COM
35
SKY THIS THIS MONTH
Visible to the naked eye
and ALISTER LING describe the solar system’s changing landscape as it appears in Earth’s sky. MARTIN RAT RATCLIFFE CLIFFE
Visible with binoculars Visible with a telescope
March 2018: Mercury at its evening best
The MESSENGER spacecraft revealed Mercury’s stunning geology in this false-color image. Even under the optimal viewing conditions in March, however, the planet appears bland through amateur scopes. NASA/JHUAPL/CIW
M
ercury and Venus rule the early evening sky, a pair of bright worlds seemingly tethered to each other for much of March. Uranus joins the party late in the month as it wraps up a fi ne evening appearance, though you’ll need binoculars to see its fainter glow. Not to be outdone, the morning sky features three bright planets — Mars, Jupiter, and Saturn — that grow more prominent by the week. As March begins, Mercury appears deepest in evening twilight. In the Northern Hemisphere, the innermost planet has a deserved reputation for being an elusive target. Its tight orbit around the Sun means it never strays far from our star, so it typically appears in twilight either shortly after sunset or before sunrise. 36
ASTRONOMY
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But two factors make its appearance this month stand out. First, the ecliptic — the apparent path of the Sun across the sky that the planets follow closely — makes a steep angle to the western horizon after sunset. Mercury’s elonelongation from the Sun thus translates mostly into altitude. But the bigger reason for Mercury’s easy visibility in March is its proximity to Venus. Earth’s closest planetary neighbor shines brilliantly at mag nitude –3.9 and stands out in the western sky after sundown. Locate it, and finding nearby Mercury will be a snap. The two remain in a single binocular field during March’s first three weeks. You can start sta rt searching for the planets early this month. On the 1st, Venus hangs 5° above the horizon a half-hour
after sunset and Mercury appears 1.8° to its lower right. Mercury glows at magnitude –1.3, –1.3, about 10 times di mmer than its companion, but shows up nicely through binoculars and should be visible to the naked eye in good conditions. The two planets climb slowly away from the Sun during the next two weeks. Mercury orbits our star along a faster track, however, however, and gains altitude more quickly. The two come closest to each other March 3, when Mercury lies 1.1° to Venus’ right. Turn a telescope on the pair and Venus sports a 10"-diameter disk that appears nearly full. Mercury appears 5.6" across with an 87-percent-lit phase. Mercury passes 1.4° due north (to the upper right) of Venus on March 5. Five days later, Venus stands 7° above the western horizon 30 minutes after sunset and Mercury appears 3° higher. hig her. A telescope reveals essentially no changes to Venus’ size and shape, but Mercury now shows a disk that
measures 6.5" across and is 64 percent illuminated. The innermost planet reaches its peak at greatest elongation March 15. It then lies 18° east of the Sun and stands 12° high in the west 30 minutes after sundown. (It’s still 6° high in a darker sky a half-hour later.) Although it has dimmed to magnitude –0.4, that’s brighter than any other early evening object except for Venus and the night sky’s brightest star, Sirius. A telescope shows Mercury’s 7.3"-diameter disk, which appears slightly less than half-lit. After greatest elongation, the innermost planet sinks lower with each passing day. It passes due north of Venus again March 17, 17, this time at a distance dista nce of 4°. Be ready to capture some twilight shots on the 18th, when a crescent Moon stands 4° to Venus’ left and Mercury appears the same distance dis tance to Venus’ Venus’ upper right. The trio stands 10° high 30 minutes after sunset.
The innermost planet’s exceptional show PISCES
PEGASUS
Mercury Venus CETUS
5°
March 15, 30 minutes after sunset Looking west Use brilliant Venus as a guide to locating Mercury as it reaches its peak altitude for the year in mid-March. ALL ILLUSTRATIONS: ASTRONOMY : ROEN KELLY
RISINGMOON
A First Quarter Moon eureka moment Towering Towering turrets, chains of pointy peaks, and a parade of rugged crater walls highlight any First Quarter Moon. For the best views, look along the terminator — the dividing line between day and night that cuts the Moon in half. The evening of March 24 provides splendid views as the rising Sun casts long shadows. North of the lunar equator, Montes Apenninus (Apennine Mountains) lies west of Mare Serenitatis (Sea of Serenity). This mountain arc forms the southeastsoutheastern edge of the giant Mare Imbrium. The massive impact that created this basin threw out an incredible amount of material. Later on, lava welled up from bel ow and buried much of it. But some of this excavated material remains on view, most prominently on the southeastern side of the Apennine chain.
Mercury fades quickly as it falls toward the Sun. The planet becomes a challenge in twilight by the 23rd, when it glows at 2nd magnitude. Mercury will pass between the Sun and Earth on April 1, setting up a poor morning appearance in late April. Meanwhile, Venus continues to climb away from the Sun. Its ascent sets up a close conjunction conjunction w ith Uranus on March 28. From North America that evening, the two lie a mere 4' apart, their closest since the same date in 2003. Uranus glows at 6th magnitude, however, however, and will w ill be hard to see in tw ilight. Track Venus through binoculars or a telescope as twilight darkens and watch for the more distant planet to pop into view. You’ll have an easier time catching Uranus in early March when it stands 25° high in the west once twilight fades to darkness. Look for it 2.3° due west of 4th-magnitude Omicron (ο) Piscium in the southeastern corner of Pisces the Fish. — Continued on page 42
Archimedes
Inside the mountainous arc lies the attractive circular form of Archimedes. Acting like a canvas, the crater’s smooth floor displays a remarkable series of saw-toothed shadows cast by the rim. Lunar cartographers named this 52-mile-wide crater after the famed Greek mathematician and physicist of the third century B.C. After looking at this striking feature, take a moment to enjoy the smaller craters Aristillus and Autolycus nearby. You can heighten your experience by watching carefully during the evening hours. The long daggers of darkness knifing westward onto Archimedes’ floor retreat with each passing hour. By March 25, the much shorter shadows are merely pointy, and they disappear in a couple more nights.
Aristillus
Autolycus Archimedes
s n u i n n n p e A s t e n o M
N E
At First Quarter phase, jagged peaks along the rim of Archimedes cast pointed shadows across its smooth floor. CONSOLIDATED LUNAR ATLAS /UA/LPL; INSET: NASA/GSFC/ASU
METEORWATCH The zodiacal light’s soft glow The Sun lights up solar system dust on Moon-free March evenings. The pyramidshaped glow seems to point to the Pleiades star cluster. DEREK MELLOTT
Dusty debris sets the evening sky aglow March is a lean month for meteor observers. No major showers occur, and the only minor one (the Gamma Normids) resides deep in the southern sky. Still, dark skies always offer a chance to see sporadic meteors. The dust par ticles that gi ve rise to meteors are debris from
asteroid collisions and comets passing through the inner solar system. This dust concentrates along the ecliptic, the plane in which the planets orbit. When the ecliptic angles steeply to the western horizon after sunset, as it does every March, the dust appears as a pyramidal
glow to the naked eye. To see this so-called zodiacal light, plan to observe sometime between March 3 and 18, when the Moon is out of the evening sky. Then, find a site far from the city, wait for twilight to fade away, and search for the soft glow.
OBSERVING Mercury puts on its best evening appearance of 2018 during HIGHLIGHT the first three weeks of March. WWW.ASTRONOMY.COM
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N
STAR DOME How to use this map: This map portrays the sky as seen near 35° north latitude. Located inside the border are the cardinal directions and their intermediate points. To find stars, hold the map overhead and orient it so one of the labels matches NE the direction you’re facing. The stars above the map’s horizon now match what’s in the sky.
A I
H E R C U L E S
R D A C O
R O N I M A S R U
P C N
The all-sky map shows how the sky looks at:
B O Ö T E S
10 P.M. March 1 10 P.M. March 15 9 P.M. March 31
M 5 1
Planets are shown at midmonth
B C E O R E M N A I C E S
A r c t u r u s
E
M 6 4
N G P
D M e 6 n 6 e b o l a
V I R G O
STAR MAGNITUDES
Sirius 0.0 1.0 2.0
3.0 4.0 5.0
S p i c a
M 1 0 4 C O R V U S
C R A T E R
s i r a l o P
S I L A D R A P O L
M 8 1
M z i a r
C A N E S V E N A T I C I
S U E H P E C
M 8 2 a l l
M A U J R O S R A
X N Y L
L E O M I N O R
r o t s a C
x u l l
L E O
M44 P o
M 6 5 R e g u l u s
S E X T A N S
C A N N C CER
y o n I S P r o c A N R C I N O M
H Y D R A
O E R C N O M O 7 M 4
S N I R A C J O M A
STAR COLORS A star’s color depends on its surface temperature.
• • • • • •
The hottest stars shine blue Slightly cooler stars appear white Intermediate stars (like the Sun) glow yellow
PYXIS
SE
A d
A N T L I A
P U P
Lower-temperature stars appear orange
38
ASTRONOMY • MARCH 2018
P I S
2 4 7 7 N G C
The coolest stars glow red Fainter stars can’t excite our eyes’ color receptors, so they appear white unless you use optical aid to gather more light
I N I M E G
V E L A
S
Moon phases in the calendar vary in size due to the distance from Earth and are shown at 0h Universal Time. Note:
MARCH 2018 SUN.
MON.
TUES.
WED.
THURS.
FRI.
SAT.
1
2
3
8
9
10
MAP SYMBOLS
Open cluster A D E M O R D N A
P O I S S A C
Globular cluster Diffuse nebula 4
5
6
7
Planetary nebula
NW
Galaxy 11
9 68 C G N
S l U o l E g S A R E P a C
M U L U G N A I R T
A G I R U A
1 M
n a r a b e d l A
s e d a y H
18
25
S E I R A
14
15
16
17
t h
P a
n S u h e t o f
N O I R O s e u e l g t e e B
2 4 M
l e i g R
s i u i r S
1
S U T E C
S U R U A T
27
21
22
28
S U N A D I R E
S U P E L
SW
23
29
The Moon passes 0.9° north of Regulus, 1 A.M. EST
18
Full Moon occurs at 7:51 P.M. EST
30
24
Y B S N O I T A R T S U L L I
31
The Moon passes 8° south of Mercury, 2 P.M. EDT The Moon passes 4° south of Venus, 3 P.M. EDT
4
Neptune is in conjunction with the Sun, 9 A.M. EST
19
The Moon passes 5° south of Uranus, noon EDT
5
Mercury passes 1.4° north of Venus, 1 P.M. EST
20
Vernal equinox occurs at 12:15 P.M. EDT
7
The Moon passes 4° north of Jupiter, 2 A.M. EST
9
Jupiter is stationar stationary, y, 5 A.M. EST
W
Dwarf planet Ceres is stationary, 5 P.M. EDT 22
Mercury is stationary, 1 P.M. EDT The Moon passes 0.9° north of Aldebaran, 7 P.M. EDT
The Moon passes 4° north of Mars, 8 P.M. EST
24
10
The Moon passes 2° north of Saturn, 9 P.M. EST
26
11
The Moon is at apogee (251,455 miles from Earth), 5:14 A.M. EDT
The Moon is at perigee (229,352 miles from Earth), 1:17 P.M. EDT
28
The Moon passes 1.0° north of Regulus, 10 A.M. EDT
15
A M B U L C O
26
20
Last Quarter Moon occurs at 6:20 A.M. EST
1 M 4
a r a
19
Y M O N O R T S A
Calendar of events
c ) i c p t i l ( e c
s e d a i e l P
3 M 5 3 M
13
3 3 M
4 88 C G N M A C
6 8 3 3 M M
12
Y L L E K N E O R :
17
Mercury is at greatest eastern elongation (18°), 11 11 A.M. EDT New Moon occurs at 9:12 A.M. EDT Mercury passes 4° north of Venus, 9 P.M. EDT
First Quarter Moon occurs at 11:35 A.M. EDT
SPECIAL OBSERVING DATE 28 Venus passes 0.07° north of Uranus in evening twilight. 31
Full Moon occurs at 8:37 A.M. EDT
BEGINNERS: WATCH A VIDEO ABOUT HOW TO READ A STAR CHART AT www.Astronomy.com/starchart. WWW.ASTRONOMY.COM
39
PATH OF THE PLANETS The planets in March 2018 CA S
DR A
Objects visible before dawn
UM �
L AC
CV N
HE R
CY G
LM � BO Ö
LY R C� B
CO M
VU L
LE O
PE G
PSC
t h o f P a t
SE R
AQ L S u n
OP H
SE R
VI R
Celestial equator Neptune
AQ R
) c ) i c p t i i p
l l ( e c S u n e h f t h
L IB CA P
CR V
a ta Ves t Ve
Pluto SC L
18
C� A
LU P
SC O
VE L CE N
GR U Dawn
Moon phases
19
AN T
Mars
SG R MI C
PH E
17
16
Midnight
15
14
13
12
11
10
9
8
7
6
5
4
3
2
31
The planets in the sky
HYA
CR T
Jupiter
Saturn
SE X
h o t h P a
SC T
CE T
P�A
30
Uranus Mars
S E N
Venus
Ceres
Neptune
10"
Jupiter
Planets
ME RC UR Y
VE N US
M AR S
C ER ES
JU P ITE R
S ATU RN
U R AN US
NE P T UN E
P LUT O
Date
M ar ar ch ch 15
M ar ar ch ch 15
M ar ar ch ch 15
M ar ar ch ch 15
M ar ar ch ch 15
M ar ar ch ch 15
M ar ar ch ch 15
M ar ar ch ch 15
M ar ar ch ch 15
Magnitude
– 0.4
–3.9
0.6
7.6
–2. 3
0.5
5.9
8 .0
14. 3
Angular size
7. 3"
10. 2"
7.4"
0.7 "
4 0 .7 "
16 .2"
3.4"
2. 2"
0.1"
Illumination
47%
9 6%
8 8%
98%
99%
10 0%
10 0 %
10 0 %
10 0%
Distance (AU) from Earth
0.927
1.630
1. 268
1.815
4.838
10.2 57
20.729
30.922
33.956
Distance (AU) from Sun
0. 314
0.724
1. 554
2.561
5.420
10.0 6 6
19. 895
29.9 4 4
33. 523
0 h3 h37. 4m 4m
17 h5 h5 2. 2. 2m 2m
8 h4 h42 .3 .3 m
15 h2 h2 3. 3. 4m 4m
18 h3 h3 4. 4. 4m 4m
1h 38 38 .5 .5 m
2 3h 3h 02 02 .8 .8 m
19 h2 h2 9. 9. 2m 2m
2 °52'
–23°23'
31°57 '
–17 °19 '
–22°19 '
9 °4 0 '
–7 ° 0 6'
–21°30 '
Right ascension (2000.0) Declination (2000.0)
40
Pluto
Saturn
ASTRONOMY
•
0 h4 h42 .4 .4 m 6°4 4'
MARCH 2018
1
29
These illustrations show the size, phase, and orientation of each planet and the two brightest dwarf planets at 0h UT for the dates in the data table at bottom. South is at the top to match the view through a telescope.
Mercury
W
n
o o h e M
SG E
EQ U
28
This map unfolds the entire night sky from sunset (at right) until sunrise (at left). Arrows and colored dots show motions and locations of solar system objects during the month.
Jupiter’s moons
CA S
Objects visible in the evening
LY N
L AC
AN D
PE R
AU R
TR I GE M
Ceres
PE G AR I Mercury appears bright in the evening sky in mid-March
CN C I r r i is s TAU
Dots display positions of Galilean satellites at 4 A. M. EDT on the date shown. South is at the top to match S the view E through a W N telescope.
Io Europa
Ganymede Callisto
Uranus PS C CM I
V e n u s
OR I
1
S u n
Io
2
MO N
3
P a l l a s
AQ R
CE T
4
CM A
Jupiter
5 LE P
P�A
PY X PU P
6
F OR
ER I
CO L
SC L
7 CA E PH E
8
Early evening
9
To locate the Moon in the sky, draw a line from the phase shown for the day straight up to the curved blue line. Note: Moons vary in size due to the distance from Earth and are shown at 0h Universal Time.
Ganymede Callisto
10 11
27
26
25
24
23
22
21
20
19
18
17
16
12 13
Europa
14
Mercury Greatest eastern elongation is March 15
15 16 17
Ceres 18
Venus
Earth Vernal equinox is March 20
19 20
Mars
21 22 23
Jupiter
24 25 26
The planets in their orbits Arrows show the inner planets’ monthly motions and dots depict the outer planets’ positions at midmonth from high above their orbits.
Uranus
28
Jupiter Saturn
Y L L E K N E O R :
27
Neptune Solar conjunction is March 4
Pluto
Y M O N O R T S A
29 30
Y B S N O I T A R T S U L L I
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— Continued from page 37 The Galilean satellites take sides
WHEN TO VIEW THE PLANETS EVENING SKY
MIDNIGHT
Mercury (west)
Jupiter (southeast)
Venus (west)
S
March 25, 5:00 A.M. EDT
Jupiter
MORNING SKY
Mars (south)
W
Jupiter (south)
Uranus (west)
Saturn (southeast) Neptune (east)
Once Uranus sets, the sky remains devoid of planets until Jupiter rises shortly before midnight. The giant planet lies against the backdrop of Libra the Scales, but it is far brighter than any of that constellation’s constellation’s stars. Jupiter Jupiter shines at magnitude –2.2 in early March and at magnitude –2.4 by month’s end. If you target the planet through a telescope, wait until an hour or two before dawn. This is when it appears highest in the south and its light passes through less of Earth’s image-distorting atmosphere. atmosphere. You’ll be rewarded with sharper views of fine details in the jovian cloud tops.
Jupiter’s Jupiter’s equatorial d iameter grows from 39" to 43" during March, plenty big enough that any scope can deliver exquisite views. The most prominent features are the two dark belts that straddle a brighter zone coinciding with the t he equator. equator. More subtle aspects abound on t he periphery of both belts. Also keep an eye out for the Great Red Spot. This massive storm lies near the southern edge of the South Equatorial Belt and displays a distinctive pinkish-red color. color. You can see it about half t he time — whenever Jupiter’s rotation places it on the hemisphere facing Earth. The
Ganymede
Io
Europa
Callisto
30"
Jupiter’s four large moons line up west of the planet before dawn March 25, just a couple of hours after Callisto passes due north of the giant world.
planet spins once on its axis in less than 10 hours, so if you don’t see it right away, you shouldn’t have to wait long. Once you’ve examined Jupiter’s disk, look for the planet’s four bright moons. They present present a constantly changing vista as they t hey revolve around around the planet. Io shifts position most quickly because it circles the planet in only 1.8 days, while Europa takes 3.6 days, Ganymede 7.2 days, and Callisto 16.7 days. These movements occasionally bring all four satellites to one side of the planet. For North American observers, this happens four times in March — on the 1st,
11th, 24th, and 25th — but the events on the 11th and 25th provide the best views. The moons all orbit Jupiter in the same plane, and that plane tilts significantly to our line of sight this month. This means outermost Callisto can pass north or south of the planet. You can see one of these rare events the night of March 24/25, before Callisto joins the other three moons west of Jupiter. A few hours af ter Jupiter Jupiter rises, Mars pokes above the southeastern horizon. The Red Planet has begun what will be its finest appearance in 15 years. A lthough it won’t
COMETSEARCH
Going to California It appears as though our extended drought without a bright comet will continue through the spring. But while Comet PANSTARRS PANSTARRS (C/2016 R2) may la ck in luster, it shines for its consistency. This first-time visitor from the distant Oort Cloud should remain at 10th or 11th magnitude for several months. Comet PANSTARRS lies high in the west after darkness falls in March. You can find it between the star patterns representing Perseus the Hero and Auriga the Charioteer. Deep-sky observers know this region best as home to the California Nebula (NGC 1499). The comet slides less than 5° southeast of the nebula in mid-March. To best see PANSTARRS, you’ll want to observe it from a
42
ASTRONOMY
• MARC H
2018
Comet PANSTARRS (C/2016 R2) dark-sky site during the Moonfree window from March 5 to 21. If you happen to be out for a Messier marathon the weekend of March 16 and 17, 17, spare a few minutes to track down C/2016 R2. A 4-inch or larger telescope will reveal the comet, but you’ll want to pump up the power to 150x or so to see subtle detail. Look for a fuzzy, out-ofround object with a brighter core and a more defined edge toward the southwest. The enduring p opularity of comets comes in part from their wide range of shapes and sizes. When located far from the Sun, they often mimic elliptical galaxies. And PANSTARRS qualifies — at its closest to the Sun this May, it will lie well beyond the orbit of Mars. But our comet currently
N NGC 1499
21
54
17
E PERSEUS
13 Path of Comet PANSTARRS
9 March 5
1°
Spring’s brightest comet rides high in the evening sky during March. The dirty snowball slides southeast of the California Nebula (NGC 1499).
lies in the Milky Way, far from any comparison galaxies. C/2016 R2 does share some characteristics with the nearby California Nebula, however:
Both consist of gas and dust lit up by starlight. And there’s no doubt that somewhere within NGC 1499 lurks a star with an Oort Cloud of its own.
The Red Planet meets the deep sky
LOCATING ASTEROIDS
N
Discover the nearest dwarf planet SAGITTARIUS
Saturn
Path of Mars M20
E
31
28
25
22
19
M22
March 16
M8 M28
1°
Mars shines brightly as it passes near several bright nebulae and star clusters in northern Sagittarius during the second half of March.
peak until July, the rocky world improves noticeably in March. Observers and astroimagers who want to get the most out of this summer’s show should should start honing their skills now. now. Mars begins the month among the background stars of Ophiuchus, some 12° eastnortheast of Antares in neighboring Scorpius. The planet glows at magnitude 0.8, a touch brighter than the star. sta r. If you take a moment to study the colors of the two objects, you’ll understand why a ncient ncient observers named the star Antares, which literally means “rival of Mars.” The planet moves steadily eastward against the starry backdrop during March. It crosses into Sagittarius on the 12th, setting up a series of wonderful conjunctions with some of the Milky Way’s best and brightest deep-sky objects. It passes midway between the Lagoon Nebula (M8) and the Trifid Nebula (M20) on the morning of March 19, providing a stunning binocular view and a prime photo opportunity. The trio climbs 20° above the horizon by 5:30 �.�. local daylight time, just before twilight starts to dim the Milky Way star clouds. As Mars continues across Sagittarius, it meets up with
the 7th-magnitude globular star cluster M28 on March 28. The planet appears 1.3° north of the cluster that morning. Mars ends the month just 0.9° west-northwest of an even brighter globular, 5thmagnitude M22. The planet then shines at magnitude 0.3, some 60 percent brighter than it started March. Mars brightens this month in part because its apparent diameter grows by 25 percent, from 6.7" to 8.4". You should be able to tease out a few surface details during moments moments of steady seeing, particularly as it grows larger late in the month. Look for the distinctive dark smudge of Syrtis Major — one of the planet’s most conspicuous features — on March’s final few mornings. If you track Mars all month, you’ll notice it approaching another prominent point of light. On the 31st, 31st, the Red Planet pulls within 1.7° of Saturn. The ringed planet orbits much farther from the Sun than Mars and thus moves eastward more slowly. It remains about 2° north of M22 all month. Saturn appears conspicuous at magnitude 0.5, and its golden hue offers a
March offers a great opportunity to track down the king of the asteroid belt. Ceres dims from magnitude 7.4 to 8.0 this month, which puts it within range through binoculars or a small telescope from the suburbs. The object resides in Cancer, which stands high in the east at nightfall and passes nearly overhead around 10 P.M. local daylight time in mid-March. Ceres lies about two binocular fields north of the Beehive star cluster (M44). You can get a head start by sketching a framework consisting of 4thmagnitude Iota (ι) Cancri and the group of four 5th- and
6th-magnitude stars labeled Sigma1 (σ1) to Sigma4 (σ4) Cancri on the chart below. Then head outside, dark adapt, and add the stars you see to the west of this field. One of these points of light will be Ceres. To find out which one, return to this field in a week or so and see which dot has shifted position. Astronomers once thought Ceres was the supposed missing planet between Mars and Jupiter. It was later downgraded to an asteroid and then, in 2006, elevated to dwarf planet status. Italian astronomer Giuseppe Piazzi discovered Ceres on New Year’s Day 1801.
Ceres slides through Cancer the Crab N
March 1
11 21
Path of Ceres E
31 57 61
CANCER
0.5°
The biggest object in the asteroid belt between the orbits of Mars and Jupiter execu tes a tight loop in n orthern Can cer this mont h.
striking color contrast with its ruddy neighbor. Enjoy viewing Saturn against the Milky Wa Wayy backdrop with your naked eye or binoculars. You’ll You’ll need a telescope to see its superb rings, which span 37" and tilt 26° to our line of sight this month. You also might catch a brief glimpse of Neptune at the end of March. The outer planet rises an hour before the Sun on
the 31st 31st and likely wil l be lost in twilight twi light from mid-northern mid-northern latitudes. (Your chances increase the farther south you live.) It will return to view for everyone by late April. Martin Ratcliffe provides Ratcliffe provides planetarium development for Sky-Skan, Inc., from his home in Wichita, Kansas. Meteorologist Alister Ling works for Environment Canada in Edmonton, Alberta.
GET DAILY UPDATES ON YOUR NIGHT SKY AT www.Astronomy.com/skythisweek. WWW.ASTRONOMY.COM
43
ASKASTR0 Astronomy’ Astronomy’ s experts from around the globe answer your cosmic questions.
BROWN DWARF JETS Q: WHAT IS THE BROWN DWARF MECHANISM THAT ALLOWS AN ENERGY JET LIKE THE ONE MAYRIT 1701117 HAS? John Siller, Commerce Township, Michigan
A: Young stars not only accrete
gas as they form, but also expel material in outflows that shoot from their poles. Whether lower-mass objects undergo the same process as their stellar cousins was an open question until the first substellar object (aka, brown dwarf) was found undergoing this process by Emma Whelan and collaborators. Their find was published in Nature in 2005. Mayrit 1701117 is one of the newest examples of a young brown dwarf exhibiting a jet. In fact, it is so early in the process of forming that it is really a “proto” brown dwarf, with plenty of accretion from its surroundings left to undergo. While a lot of the details about these jets are still
being studied, we do know that they are driven by rotating magnetic fields in the young brown dwarf. These magnetic fields sweep up material from the gas accreting into the brown dwarf and drive some of it toward the poles, where it ends up being ejected as a jet. Note these jets can be quite large, often several parsecs! (One parsec is 3.26 light-years.) When these jets impact other gas along their path, they can excite it and become detectable on images or via spectroscopy, allowing us to study properties of the jet itself as well as the nearby gas. Scott Fleming
A S T R O N O M Y • MARCH
2018
Q: WHY ARE GLOBULAR CLUSTERS NOT CONSIDERED GALAXIES? HOW FAR APART ARE THEIR STARS, AND DO THEIR MAGNETIC FIELDS AFFECT EACH OTHER? CAN A CLUSTER’S STARS HAVE PLANETS, AND WOULD YOU NEED SUNGLASSES TO TRAVEL THROUGH ONE? Martin Heuer St. Petersburg, Florida
Archive Scientist, Space Space Telescope Telescope Science Institute, Baltimore
The forming brown dwarf Mayrit 1701117 1701117 (bright orange- yellow) has a 0.7 light-year-long jet, shown in green emission from ionized sulfur SOAR/NOAO/AURA/NSF in this image. CESAR BRICENO AND SOAR/NOAO/AURA/NSF
44
KELT-9b is the hottest gas giant discovered to date. As it orbits its huge, hot parent star, the planet’s atmosphere puffs up and then evaporates, likely trailing behind the planet like a giant cometary tail, as shown in this artist’s concept. NASA/JPL�CALTECH
A: Globular c lusters aren’t con-
sidered galaxies because they are gravitationally bound to and orbiting galaxies like t he Milky Way, and they have relatively small masses. When comparing the two, a typical globular cluster might contain a mass of 100,000 Suns, whereas the Milky Way has nearly 1 trillion solar masses. In other words, the Milky Way Galaxy contains 10 10 mill ion times more mass than a typical globular cluster. The stars in a globular cluster are 50 times closer to each other than the stars in our solar neighborhood. To put this in perspective, in a typical globular star cluster, we’d likely find stars separated by a distance 5,000 times greater
than the Sun is from Earth. But even at these distances, the magnetic fields of individual stars would have little effect on each other. If you were flying through the core of a globular cluster, you likely wouldn’t need sunglasses because even at these close separations, individual stars would still appear dimmer than our Moon. But given that you’d have many stars in close proximity to each other, there would be nearly 1,000 stars brighter than the planet Venus in the nighttime sky of any fictitious planet near the center of a globular cluster. Their combined light would add up to roughly the light of a Full Moon. In the center of a globular cluster, conditions aren’t favorable for the formation of planets because the tida l forces of passing stars could destroy any protoplanetary protoplanetary disk s. If a planet did somehow form, it would still find itself in danger, since the occasional close passage of a star would likely disrupt the planet’s orbit, flinging the planet into interstellar space. Brian Murphy Professor of Physics and Astronomy, Astronomy, Butler University, Indianapolis
This eclipse sequence begins at upper left and ends at lower right. During this event, the Moon moves from west to east, across the Sun’s face. BEN COOPER
Q: SINCE THE SUN AND
MOON MOVE FROM EAST TO WEST, WHY DID THE ECLIPSE MOVE FROM WEST TO EAST? Mary Lanphier Wichita Falls, Texas
A: Because Earth rotates on its
axis from west to east, the Moon and the Sun (and all other celestial objects) appear to move from east to west across the sky. Viewed from above, however, the Moon orbits Earth in the same direction as our planet rotates. So, the Moon actually moves from west to east through our sky, albeit so slowly that we almost never notice it. During a total solar eclipse, however, we can see the Moon’s true motion as it crosses the Sun’s face from west to east. As this occurs, occ urs, the Moon’s shadow follows it — moving in the same direction — and tracks a path across Earth’s surface. NASA has created a helpful video, “Flyi ng Around the Eclipse Shadow,” which illustrates this geometry if you’re still having trouble picturing it. You can watch it online at https://svs.gsfc.nasa.gov/4579. Alison Klesman Associate Editor
Q: MANY EXOPLANETS
ROTATE AROUND THEIR ROTATE SUN IN DAYS INSTEAD OF YEARS, AS IN OUR SOLAR SYSTEM. HOW DO YOU EXPLAIN THIS, AND HOW CAN THEY MAINTAIN THEIR ATMOSPHERES WITH THOSE INCREDIBLE SPEEDS? Daniel Gerritsen Baarn, Netherlands
A: You are right in thinking it
is hard to hold on to an atmosphere under those conditions, and not all close-in exoplanets have been able to do it. But it is not the speed of the planet around the star that dictates
this; rather, it is the mass of the planet and the radiation from the star it orbits. For the atmosphere to escape the planet, it must overcome the gravitational pull of the planet itself. The more massive the planet, the more likely it is to hold on to its atmosphere. However, the host star can help the atmosphere escape. Sometimes the gravity of the host star pulls the atmosphere from the planet because it is stronger. More commonly, the intense heat from the star and energetic explosions called flares give so much energy to the particles in the planet’s atmosphere that they vibrate and move enough to overcome the planet’s gravity and escape. In this way, the atmosphere will lose its lightest constituents first, and astronomers can detect hydrogen streaming away. We have been able to measure this for some
exoplanets, which look like giant comets, with a tail of gas streaming away from the direction of the planet’s orbit. How the planets got to orbit so close to their stars in the first place is another question, and it is still a matter of scientific inquiry. The process that moved them so close to their stars is called migration; there are a number of theories as to what causes it. For example, another star could have come close to the system, pulling the planet and moving it inward. Or, while the planet was forming, other forming planets could have caused an imbalance, prompting the planets to move around until they settled and became stable. The truth is, we are not sure yet what caused this to happen to more than 300 known exoplanets, spiraling them in to settle where we see them today. But the more we discover and
learn about their atmospheres and environments, the more we can understand and perhaps work out why that did not happen in our own solar system — and whether that might be the reason I am here to answer your question! Hannah Wakeford Giacconi Fellow, Space Telescope Science Institute, Baltimore, and Research Fellow, University of Exeter, United Kingdom
Send us your questions Send your astronomy questions via email to
[email protected], or write to Ask Astro, P. O. Box 1612, Waukesha, WI 53187. Be sure to tell us your full name and where you live. Unfortunately, we cannot answer all questions submitted.
WWW.ASTRONOMY.COM
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2004-2017
VISIBLE AND INFRARED MAPPING SPECTROMETER �VIMS�
VIMS comprised two cameras: One operated at visible wavelengths, the other at slightly longer infrared wavelengths. The cameras separated that light into its component colors, allowing scientists to ascertain the temperature and composition of Saturn’s atmosphere and rings as well as of the moons’ surfaces and atmospheres.
DUAL TECHNIQUE MAGNETOMETER �MAG�
Essentially a sensitive and precise compass, MAG recorded the strength and direction of the magnetic �elds around the spacecraft. It helped scientists learn about Saturn’s magnetosphere as well as the interiors of the planet and its moons. RADIO AND PLASMA WAVE SCIENCE INSTRUMENT �RPWS�
As the name suggests, this suite of antennas and sensors detected radio and plasma waves. The instrument “heard” radio emissions from lightning in Saturn’s atmosphere and from the planet’s aurorae.
IMAGING SCIENCE SUBSYSTEM �ISS�
This instrument contained a pair of digital cameras — a wideangle one for context and a narrow-angle one for high resolution. ISS photographed the Saturn system at visible, ultraviolet, and infrared wavelengths.
ULTRAVIOLET IMAGING SPECTROGRAPH �UVIS�
The four telescopes of UVIS took ultraviolet images of Saturn’s atmosphere, moons, and rings, and also split the incoming light into its constituent wavelengths to reveal the objects’ compositions.
COMPOSITE INFRARED SPECTROMETER �CIRS�
This spectrometer captured infrared radiation and split it into its component colors. Scientists used this information to deduce the temperature and composition of objects in the Saturn system.
The spacecraft’s 12 instruments showed Saturn and its family in unprecedented detail. by Richard Talcott
O
n October 15, 1997, the main engines of a Titan/Centaur rocket ignited, and the Cassini spacecraft and attached Huygens probe rose into the sky above Cape Canaveral, Florida. The rockets didn’t have enough power to get the pair out to Saturn, however. So, Cassini and Huygens embarked on a circuitous trip through the inner solar system, stealing a bit of orbital energy from Venus (in April 1998 and June 1999) and Earth (in August 1999). The probes received an even bigger boost from massive Jupiter in December 2000, setting them on course to reach Saturn in June 2004. For 13-plus years, Cassini orbited the giant planet. Engineers used the gravity of Saturn’s biggest moon, Titan, to t weak its
course and get close-up views of the planet’s atmosphere, rings, magnetic field, and dozens of smaller but no-less-intriguing moons. Early on, Huygens dropped from Cassini, parachuting through Titan’s thick, hazy atmosphere and landing on the surface in January 2005. Together, Together, the two spacecraft carried 18 scientific instruments: Cassini held 12 and Huygens six. These powerful tools unveiled the ringed world and its surroundings in unprecedented detail. And amazingly, only one of the orbiter’s instruments, the Cassini Plasma Spectrometer, failed before the spacecraft burned up in Saturn’s atmosphere September 15, 2017, nearly 20 years after launch.
HUYGENS PROBE RADAR
This instrument sent radio signals through Titan’s hazy atmosphere and recorded how long they took to return, allowing scientists to build high-resolution pictures of the moon’s surface.
DESCENT IMAGER/SPECTRAL RADIOMETER �DISR�
This instrument’s imagers built up mosaics of the moon’s surface in the landing site’s vicinity. Solar sensors measured the Sun’s intensity and allowed scientists to study the size and density of airborne particles. Still another sensor measured the atmosphere’s heat �ow.
RADIO SCIENCE SUBSYSTEM �RSS�
The RSS used Cassini’s high-gain antenna to send radio signals to Earth through the rings or the atmospheres of Saturn and its moons. Scientists then studied how the intervening material altered the signal to learn more about its structure. ION AND NEUTRAL MASS SPECTROMETER �INMS�
This instrument determined the chemical composition of neutral particles and lowenergy ions, particularly in Titan’s upper atmosphere and in Saturn’s rings and magnetosphere. CASSINI PLASMA SPECTROMETER �CAPS�
CAPS measured the energy, electrical charge, and direction of motion of charged particles. particles. One of its sensors also determined the mass of each particle. A short circuit in CAPS ended its life in June 2012. MAGNETOSPHERIC IMAGING INSTRUMENT �MIMI�
MIMI’s three sensors worked in concert to detect energetic charged particles in Saturn’s vast magnetosphere, to understand how it interacts with the solar wind.
ON THE BACK SIDE COSMIC DUST ANALYZER �CDA�
This detector established the size, speed, direction of motion, and chemical composition of tiny dust particles near Saturn.
HUYGENS ATMOSPHERIC STRUCTURE INSTRUMENT �HASI�
Multiple sensors measured the density, pressure, temperature, and electrical properties of Titan’s atmosphere during the probe’s descent. A microphone also recorded sounds.
GAS CHROMATOGRAPH MASS SPECTROMETER �GC/MS� DOPPLER WIND EXPERIMENT �DWE�
DWE recorded wind speeds of up to 270 mph (430 km/h) in Titan’s atmosphere. Although Cassini never received the data, Earth-based radio telescopes recovered some of it.
This instrument pair analyzed gases in Titan’s atmosphere at high altitudes and near the surface. The two determined how the abundances of nitrogen and methane changed with altitude and discovered argon in the air.
SURFACE SCIENCE PACKAGE �SSP�
Multiple sensors helped determine the physical properties of the surface at Huygens’ landing site, including its hardness and structure. Several of SSP’s sensors were designed to work in a liquid environment in case the probe landed in a sea or ocean.
AEROSOL COLLECTOR AND PYROLYSER �ACP�
The ACP pulled in aerosol particles and heated them to vaporize volatile substances and decompose organic compounds. It then passed these products to the GC/MS for analysis. WWW.ASTRONOMY.COM
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A S E : S O T O H P S N E G Y U H ; O D A R O L O C F O Y T I S R E V I N U / L P J / A S A N : O T O H P S I V U ; I S S / H C E T L A C � L P J / A S A N : O T O H P S S I ; R E T S E C I E L F O Y T I S R E V I N U / A N O Z I R A F O Y T I S R E V I N U / I S A / L P J / A S A N : O T O H P S M I V ; L P J / A S A N : T F A R C E C A P S I N I S S A C
772 minutes inutes The Huygens probe became — and thus far remains — the most distant human-made landing craft when it touched down on Titan’s surface in 2005. NASA
WHEN THE HUYGENS PROBE dropped
into Titan’s atmosphere January 14, 2005, no one knew what to expect. Would it splash down into a methane ocean? Sink into a tar pit? Crash into sharp rocks or tumble off a ravine? And, most importantly, what manner of world lurked beneath Titan’s thick shroud of haze and clouds?
For landings on Mars or the Moon, mission scientists plotted out landing sites with meticulous met iculous care. Telescopes and orbiters scanned the ground, imaging dangerous terrain and safe zones, and flight engineers pored over their maps a nd planned accordingly. But Titan was a mystery. Aside from a brief pass by Voyager 1, 1, little litt le was k nown 48
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MARCH 2018
about Saturn’s Saturn’s largest moon. W hat the Huygens descent probe probe would fi nd was anyone’s guess. Huygens had to be prepared for anything. Alex Hayes, a Titan researcher at Cornell University who has been part of the Cassini orbiter team since the craft’s arrival at Saturn, is enthusiastically proud of the probe’s probe’s success, though he didn’t
oonTiTitan work with Huygens himself. “No matter what data Cassini collects over the 13 years of its mission, there is something special about reaching out and touching something,” he says. “There’s “There’s something special about landing on the surface, about getting data from the surface, and Huygens pro vided that ground truth.”
PLANNING FOR THE UNKNOWN Huygens was the European Space Agency’s contribution to the g reater Cassini-Huygens mission. With very few exceptions, all of Huygens’ instruments and components components were built by individual members of ESA, culminating in one magnificent spacecraft.
While Huygens did touch down and collect data on Titan’s surface, that outcome was far from certain. When Voyager 1 passed by the moon in 1980, it couldn’t peer through Titan’s thick atmosphere and obscuring clouds. The best it could offer was a tantalizing reveal that t hat Titan sported organic materials, which led to t he understanding that it was possibly covered covered in oceans made of methane or ethane. Later, the massive Goldstone radio telescope received radar echoes from Titan indicating that at least some solid surface existed under the clouds. But without any mapping of Titan’s surface, and limited control over where
In 2005, the Huygens probe pierced the moon’s shroud to reveal a surprisingly Earth-like world. by Korey Haynes
Huygens Huygens might land, the mission team designed the craft for any condition. Engineers built Huygens light enough to float and with enough battery li fe to operoperate for at least a short while on t he surface — assuming it sur vived impact. But overall, engineers designed a descent probe: Huygens Huygens would collect al l of its primary science during a fall through Titan’s Titan’s atmosphere lasting two to two and a half hours. Whatever it did or didn’t see afterward would be a fabulous bonus. Engineers at Caltech even had a betting bett ing pool going for what Huygens would find on touchdown, with options for “ice,” “tar,” “liquid,” “undeterminable,” “DOA,” and WWW.ASTRONOMY.COM
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the gases making up the air it sa iled through. Another instru instrument ment measured the physical properties of its surroundings: temperature, pressure, the probe’s speed, and how hard it might impact the ground or the rocking motion of Titan’s waves if it hit liquid. It could even measure how the winds pushed it across Titan’s skies. It carried a whole package of sensors to analyze whatever it landed on, be it liquid sea or solid ground. And, of course, Huygens included a camera, to reveal what is still the most distant world on which humans have landed a spacecraft.
Huygens sits safely within its larger descent module, equipped with heat shields to protect the probe during its trip through the uppermost portions of Titan’s atmosphere. Its two redundant black antennas extend from the flat top of the probe. ESA
A scale model of the Huygens probe rests in the snow after surviving a drop test from a height of 24 miles (38 km) to make sure its protective shields would separate and the parachute release system would deploy as expected on Titan. ESA
even a facetious vote for “eaten” — as in by sea monsters. To conduct its science, Huygens was equipped with six main instruments, aimed to answer a slew of questions: What gases make up Titan’s atmosphere, and what kinds of particles, hazes, or clouds float there? What chemicals churn through the skies? How warm or cool is the atmosphere, 50
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and how do global wind patterns flow? What does the top of the atmosphere look like and how does it behave, including the ionosphere? And what are conditions like at the bottom of the atmosphere, just above or — fingers crossed — at the surface? Huygens Huygens carried an aerosol collector to sample tiny particles floating through Titan’s Titan’s skies and a nd a spectrometer to analyze ana lyze
CAN YOU HEAR ME? While the Huygens mission was a rousing success, it had two notable hiccups, both related to communications issues between the probe and Cassini, its relay to Earth. In 2000, with w ith Cassini well into its journey toward Saturn, an engineer took it upon himself to test t he communications communications on the spacecraft. He pinged Cassini with a simulated message from the Huygens’ engineering model on Earth, hoping to receive it back. He did receive a response, but it was gibberish. The flaw turned out to be in the way the receiver receiver on Cassini C assini handled Doppler shifting of signals it received. As the craf t moved, any signals approaching it would shift in frequen f requency, cy, the same way a siren rises and falls fal ls in pitch as it speeds toward and away from a listener. Cassini’s receiver for communication with Huygens could not adjust for these changes. Worse, the receiver’s abilities were locked in, and Cassini was already hundreds of mil lions of miles away. Luckily, the team had four years to work on a solution. If the receiver couldn’t handle Doppler shifting, then the t he team would avoid motions that caused such shifts. But this meant changing how the orbiter and probe would maneuver maneuver through the Saturn system. Instead of releasing Huygens on its first Titan pass, Cassini would now cart Huygens along for a few Titan flybys, slowing down with each pass until u ntil it could release the probe more gently, on a path that minimized the Doppler shifting of the probe’s transmissions. Another solution called for the probe to “wake up” earlier than initially planned after separating from Cassini, since the temperature of the instrument also inf luenced the signal. Although these changes used up precious fuel (for Cassini) and battery life (for
Huygens’ final resting place
Scientists wondered whether Cassini’s final view of the Huygens probe, captured December 25, 2004, would be the last they ever saw of the lander. This parting shot was taken 12 hours after Huygens detached from the larger craft on December 24, from a distance of about 11 miles (18 km). Huygens would coast ever closer to Titan for three weeks before entering the moon’s upper atmosphere January 14, 2005, successfully completing its mission. NASA/JPL
Huygens), Huygens), the spacecraft had reserves of both. And they were well worth it to avoid Cassini relaying nothing but nonsense from Huygens’ precious stream of data. By the time Cassini arrived at Saturn, the new plan was well in place. But this wasn’t the end of the mission’s communications problems. problems. Only after Huygens was well into its descent at Titan did operators notice that only one of Cassini’s two channels was relaying information from the probe. Huygens was meant to send information over both of Cassini’s channels, Channel-A and Channel-B. But Cassini’s programming was missing a crucial command to turn on the Channel-A receiver. While critical data was duplicated on both channels, and some other transmissions were eventually recovered directly by Earth-bound receivers, much other information, including half of Huygens’ images, was lost forever. Even so, the information that Huygens sent back was enough to ta ke Titan from fuzzy orange ball to a fully realized world, in the span of only a few hours.
GERONIMO! Cassini released Huygens December 24, 2004, nearly six months after first entering Saturn’s orbit. The probe then underwent a sleepy, three-week fall through space before encountering Titan’s atmosphere. The last the mission team ever saw of the probe was an image snapped by Cassini shortly after decoupling.
Huygens’ final landing site (left) reveals a bed of water and hydrocarbon ice, dotted with rocks showing smoothed edges and other signs of erosion. This image was taken with the probe’s Descent Imager/ Spectral Radiometer and colored based on spectral data to give a true sense of the terrain’s appearance. A familiar image of an astronaut’s footprint from the Apollo Moon landings (right) illustrates the scale of Huygens’ view. ESA/NASA/JPL/ UNIVERSITY OF ARIZONA
Huygens sent back this stereographic projection view of Titan’s surface features from a height of 3 miles (5 km) as it descended toward a surface that appeared much darker than planetary scientists had expected. ESA/ NASA/JPL/UNIVERSITY OF ARIZONA
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Far from reassuring, the ESA team found the sight eerily reminiscent of t he picture captured by the Mars Express Ex press orbiter orbiter the previous year of the Beagle 2 probe, just before it disappeared while dropping to the Red Planet’s surface. That craft would not be found for 12 years. “When Cassini took the image of Huygens on its descent, we had to hope that wasn’t the last we saw of Huygens,” recalls Ra lph Lorenz, a member of the Huygens Huygens team who has al so written numerous books on the subject. Huygens entered the atmosphere enclosed in a heat shield to protect it from the strain of entry. After it passed through a danger zone, it ejected the back cover and deployed its large parachute. Once stabilized, Huygens blew off its front heat shield, ready to start its science mission. Huygens Huygens immediately started analyzing ana lyzing and recording, snapping its first image as it drifted 89 miles (143 kilometers) above Titan’s surface. It sampled the atmosphere as it passed through, measuring electrical signals and cataloging its journey in detail. After 15 minutes, Huygens ejected its main parachute and continued descending under a smaller chute. Mission engineers had planned this switch-over to allow Huygens to explore the upper atmosphere first, then descend more quickly so it would still have battery life by the time it reached the ground, if it survived. Huygens Huygens continued collecting data as it descended more rapidly through Titan’s haze and clouds, encountering some turbulence on the way — nothing the little l ittle probe couldn’t handle. As luck would have it, Huygens did not land on sharp rocks or hard ice, which might have crumpled the craft. Neither did its parachute obstruct its view — a concern held by a few members of the mission team. It did not splash down in any of Titan’s numerous lakes or seas. Instead, it thumped gently down onto a bed of something with the consistency of damp sand or packed snow, the ground around it strewn with rocks and pebbles that wouldn’t look out of place on an earth ly lakeside beach. Safely aground, Huygens continued its mission. It assiduously recorded image after image of its fina l resting place for 72 minutes after touchdown. In all, it sent back some 100 pictures of the same slice of terrain before Cassini and its link to Earth disappeared over Titan’s horizon. A short time later, its batteries ran out, and the probe quietly shut down.
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A PICTURESQUE LANDSCAPE From the first images, Huygens forever changed scientists’ understanding of Titan. Its pictures showed riverbeds — channels cut clearly into Titan’s face. These rivers showed showed drainage networks similar to those found all over Earth: small channels feeding into larger rivers, which empty out into flat deltas. Bright highlands showed rough, jagged terrain. Steep river valleys and canyons indicated that Titan’s rivers could be prone to flooding, a nd likewise showed signs of methane rain erosion. Other riverbeds hinted at gentler streams. Scientists think these are fed not by rainfall rainfa ll but from “spring sapping,” where liquid methane wells up through the ground. Closer up, Huygens took stock of its landing site. The probe touched down on a dark plain. While it saw no sign of current surface liquid, the region strongly resembled a dried lake bed or f loodplain. Scattered around Huygens’ base were cobblestones, edges rounded as if shaped by flowing liquid. The stones are of a simil ar
100 miles (150 km)
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The Huygens probe showed a landscape shaped by the flow of liquid on Titan’s surface, such as this drainage system thought to channel liquid methane runoff into a larger river. NASA/JPL/ESA/
This progression of vistas from four different altitudes, from highest (top) to lowest (bottom), shows flattened (Mercator) projections of the moon as Huygens punched through Titan’s haze to reveal its strangely Earth-like surface features. ESA/NASA/JPL/UNIVERSITY OF ARIZONA
UNIVERSITY OF ARIZONA
A 360° mosaic of images snapped from about 5 miles (8 km) above Titan’s surface shows a plateau (center) and Huygens’ eventual landing site (darker area on the right side of the image). This image and other data from the probe have been used to determine that the wind speed in Titan’s atmosphere was about 4 mph (6–7 km/h). NASA/JPL/ESA/UNIVERSITY OF ARIZONA
Huygens’ final resting place is estimated to fall within the white circle on this image taken with the probe’s Descent Imager/ Spectral Radiometer. NASA/JPL/ESA/UNIVERSITY OF ARIZONA
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After entering Titan’s atmosphere, the Huygens probe deployed a parachute to slow its descent, eventually ejecting its protective outer shell and exposing its instruments. The probe took measurements of the atmosphere and imaged the world around it, continuing to beam back data from its landing site for over an hour. ESA�C. CARREAU
size, implying that the same sa me currents might have moved all of them, but scientists remain unsure whether this is the ca se. Hayes points out that the rounded cobbles near Huygens’ landing site appear like stones ground smooth by a river carrying them over distance. But on Earth, a river drops larger stones earlier in its path, then smaller stones as the flow begins to peter out. “What intrigues me personally,” Hayes says, “is that in the decade since those images and data were taken, we started to question everything, or find that every answer you get leads to three new questions.” Huygens quite literally scratched Titan’s surface. And by opening up an entire new world to researchers, it also jump-started a new generation of research, inviting questions by the thousands. Scientists then and now look to Huygens as the only eyewitness to an entire complex world, but studies are limited by the short time and tiny area the probe could explore. So far from the Sun and under Titan’s hazy skies, Huygens took its pictures in a twilight sort of lighting. At one point, its vision included included a dewdrop dewdrop that formed on on the probe’s exterior. While probably induced by Huygens itself and the heat from its landing, the single drop was nonetheless the first in situ sighting of liquid on a world other than Earth.
COLLECTING GASES On its way down, Huygens sampled the gases circulating in Titan’s atmosphere and confirmed they were mostly nitrogen and methane. More importantly, it measured 54
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the temperature, pressure, and abundance of gases from the atmosphere’s top all the way to the ground, creating a one-dimensional map of Titan’s skies. It revealed high levels of stratification, passing from one zone to another as it fell. One of Huygens’ goals was to hunt for noble gases, such as argon. Noble gases are chemically disinclined to join with ot her elements to form compounds, so their abundances hint at long histories, stretching back to the availability of these gases at the birth of the solar system. Their presence helps scientists understand how Titan’s atmosphere came to be — and, likewise, how other worlds worlds like Earth Ear th might have attained similar thick atmospheres. But Huygens, despite descending quite literally through the thick of things, detected low abundances of argon compared with nitrogen, especially a particular isotope known as argon-36. Huygens found it roughly a mil lion times less abundant than in the t he Sun, implying implying that Titan could not have gathered its atmosphere directly from the early solar nebula. Instead, its atmosphere was likely delivered by bombardments of space rocks, bolstering the case for Earth’s Eart h’s atmosphere atmosphere forming in the same way. On the other hand, detection of another isotope, argon-40, tells a different story. This isotope arises from the radioactive decay of potassium found in rocks. For Huygens to sniff out such a gas in the atmosphere implies that Titan must have a way to release it: an active geologic, or at least cryologic, cycle where rocks or ice are churned from Titan’s depths to its surface
and atmosphere, and released by ice volcanoes. But whether this process is powered by Titan’s own internal heating mechanisms, by the heat of Saturn’s tidal pull, or even if the process truly exists at all is still under debate. While Voyager’s Voyager’s original methane discovery had raised the faint specter of a lien biology as its origin, Huygens Huygens laid these t hese hopes mostly to rest. Scientists knew that some kind of activity must refresh Titan’s methane stores, or else sunlight sunl ight would destroy the gas in a matter of a few million years. But the a rrangement rrangement of methane layers in Titan’s atmosphere, coupled with the carbon isotopes Huygens Huygens sampled, indicated once again that geologic processes were the likely methane source. However, in Titan’s haze layers, Huygens Huygens detected molecules similar to tholins produced in earthly laboratories. laboratories. Tholins are thought to be important to t he development of life on Earth, and the complex carbon molecules are a source of active research. Their presence on Titan is an encouraging sign that the building blocks of life are not unique to to Earth. Eart h.
SMALL CRAFT, BIG CONTRIBUTION While Huygens’ science mission lasted a fraction of Cassini’s decade-plus adventure, its contributions were mammoth. It remains the only human-made craft to touch the face of any moon other than our own. And its “come what may” design approach gives it pride of place even among planetary exploration missions, already an intrepid collection of engineering projects. “All of our atmospheric knowledge is tied to that one observation taken at t he equator,” Hayes points out. This makes Huygens, in his words, the linchpin of Titan atmospheric science: “Any predictions or interpretations you make about Titan, you have to show how it’s explained by what Huygens saw, or provide a reason why it should be different.” Over a decade later, researchers continue to mine the data a nd publish new findings. And any future Titan mission will certainly cert ainly start star t with Huygens’ success success story. For all its brevity, the probe saw, sampled, and touched what Cassini never could: Titan, below the veil. Korey Haynes is a former associate editor of Astronomy who now works as a freelance
science writer and outreach specialist in St. Paul, Minnesota.
Almost 50 years old, the 2.7-meter Harlan Smith Telescope at McDonald Observatory in West Texas remains one of the premier instruments for recording stellar spectra.
IN PURSUIT OF
EXOPLANETS Two massive telescopes in the Lone Star State monitor 450 suns in the hopes of finding other worlds. text and photographs by Robert Reeves
MICHAEL ENDL IS ON A MISSION:
As a research scientist with the University of Texas at Austin, he hopes the sum of his astronomy career will be a chart characterizing exoplanets, like you might find on a futuristic starship exploring faraway stars. As a planet hunter, Endl is a member of a growing league of a stronomers who seek other worlds like our own to answer basic questions: Do certain types of stars host only certain types of planets? What’s the frequency of rocky planets wit hin a star’s s tar’s habitable habitable zone? Do the atmospheres of Earth-sized exoplanets contain biosignatures indicating possible life? The exoplanet search is an exciting field. A generation ago, it was considered a career dead end. Now, Endl and his colleagues are
zeroing in on the answers to those questions.
Search techniques Endl and his colleagues find extrasolar planets using techniques simple in theory but painstaking in execution. The two most productive are the transit and the radial velocity methods. The transit method is well suited for space observatories like NASA’s Kepler spacecraft. It can stare at a field of stars for weeks while measuring any stellar brightness dip caused by a planet crossing in front of a star. The transit method is limited to detecting planets whose orbital planes are aligned with our line of sight, presumably only a small percentage. The transit method has had success scooping up hundreds of exoplanet candidates WWW.ASTRONOMY.COM
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1. The Harlan Smith telescope’s closed tube design makes it unique. A row of teapotshaped cooling fans around the base stabilizes air currents within the tube, allowing the telescope to achieve 1" resolution. 2. Astronomer Michael Endl ponders his next target as he progresses through his list of exoplanet candidate stars. 3. The Harlan Smith telescope peers into the night sky while recording spectra. Researchers use the data in their search for exoplanets.
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because of Kepler’s ability to stare at thousands of stars at once. The radial velocity method is more forgiving of observational gaps caused by daylight or poor weather and is thus better suited for Earthbased observatories. This technique uses spectroscopy to measure a star’s velocity changes. changes. These changes occur because of a planet’s gravitational pull on its sta r. This works well for many stars, but massive ones are not affected much by the pull of small Earth-like planets. When a planet is detected, the radial velocity method can also be used to determine its minimum mass. Not all stars are candidates for searches. “Triple star systems are ignored because the combined stellar gravity fields destroy planetary formation formation disks,” Endl says. “Close binary stars are also ignored by radial velocity surveys because the component stars have radial velocities in kilometers per second, making it impossible to discriminate the meters-per-second velocity changes induced by a planet’s gravity.” Endl has been spent 20 years using radial velocity to find exoplanets. “Today, the existence of exoplanets is no longer in doubt,” he says proudly. “The discovery rate is not as important as characterizing the known exoplanets. The goal of our research is to find t he difference between the planets around M-type, G-type, and supergiant stars. Rocky
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planets seem to be plentiful, but we want to determine if Jupiter-sized planets are the ru le.”
Exoplanet science at McDonald Observatory The University of Texas exoplanet program is but one of many searches worldwide. It began in 1988 and routinely observes a set of 200 nearby stars with the 2.7-meter 2.7-meter Harlan Smith Telescope. It targets nearby suns across all stellar classes. Another 250 stars are the targets of the 10-meter Hobby-Eberly Telescope at McDonald Observatory in West Texas. Small M-type stars are popular search targets because they are more abundant than G-type stars like our Sun. Endl explains ex plains more advantages: “Being smaller, their radiation is less
intense, and their habitable zone is closer to the star. Planets circle M-type stars in several days, and the star’s lower mass responds more readily to the planet’s gravitational perturbations, making them easier to detect. A planet close to an M-type star can even be detected in one multiday observing run.” Although planets around M-type stars may be relatively easy to find, Endl emphasizes that long-term long-term observations are necessary to refine masses and orbital periods. The Harlan Smith Telescope was built in the 1960s with help from NASA to support the Apollo program. The telescope can be configconf igured to feed light into a massive spectrograph that takes up the t he entire floor under the telescope. It is well suited for recording spectra for radial
4. The key to the telescope’s success as a spectrographic instrument begins by routing the f/33 Coudé focus through the telescope polar axis to the massive spectrograph housed on the floor under the telescope.
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velocity detection. detection. Exoplanet search time occurs during the brighter Moon phases because moonlight has little effect on spectra. Most targets are nakedna kedeye stars, but some are as dim as 10th magnitude. The spectrograph is sensitive enough to measure radial velocities down to 4 meters meters per second, allowing detection of a Saturnsized exoplanet 5 astronomical units from a Sun-like star. (An astronomical unit is the average Sun-Earth distance.)
Using the Harlan Smith Telescope Endl’s turn with the Harlan Smith Telescope comes every four months. Last summer I joined him at McDonald Observatory while he searched for exoplanets.
Endl emphasized several things: “Important aspects of an observing run are good coffee and good music.” As we listened to his eclectic playlist and sipped exotic coffee, I quickly deduced that finding exoplanets is not easy. There are few “Eureka!” moments when an observer spots a planet and quickly confirms it. Exoplanet searches require gathering extensive data that are analyzed over time to prove or disprove the existence of a planet around another star. The spectrograph exposures are limited to 20 minutes, not because the sensor will become saturated, but because Earth ’s motion motion smears the spectra and makes t he star’s radial velocity hard to calibrate. Because denser layers of air absorb and distort starlight, no stars are
observed below about 25° altitude. The observer controls the telescope. The desired target stars are listed in a software script that selects the next star after each spectrum is recorded. An efficient autoguider built into the spectrograph slit guides the telescope during the exposure. A light meter within the optical path counts the photons and determines when a sufficient exposure has been recorded, often terminating terminating the expoex posure before the 20-minute limit. If the star is as bright as 4th magnitude, the exposure is only a minute long. When an exposure finishes, the telescope does not automatically move to the next target. The operator must exit the control room, walk to the telescope and dome control desk, and hold down a dead man’s switch to move the telescope. This keeps eyes on the telescope to prevent possible collisions with either the pier or objects on the dome floor. The operator returns to the control room and may record as many as 30 spectra per night. Watching Wa tching the Harlan Smith Telescope operate was awe-inspiring, but nothing prepared me for the stunning complexity of the spectrograph. Within its room, which is as large as the interior of a modest-size house, a vast array of relay and ca mera mirrors passes the light beam from the spectrograph slit at the telescope’s scope’s Coudé focus through the diffraction grating, then onto a CCD camera chip.
5. Like all visitors touring the telescope’s spectrograph room, the author had to take a selfie using one of the instrument’s large mirrors. The dim lighting inside the spectrograph room required a 10-second exposure. 6. Endl points out the t horium-argon horium-argon emission lamp and flat-field box used to calibrate the telescope’s spectrograph.
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7. A small commercial telescope intersects part of the light beam entering the spectrograph and directs it onto a light meter that controls the camera’s exposure time. 8. The catwalk surrounding the telescope dome provides gorgeous views of sunrise in the Davis Mountains. This event signals the end of the observing run. 9. Relay mirrors direct the telescope’s light path onto the spectrograph slit and autoguider camera. The brass eyepiece tube is used to focus the telescope.
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The spectrograph’s CCD detector remains continuously below –100° Celsius (–148° Fahrenheit), cooled with liquid nitrogen. An operator calibrates the detector each evening with a thorium-argon emission lamp to match specific emission lines to specific pixels on the t he detector. The operator can adjust the position of the spectrum on the CCD detector vertically by tilting the prism and horizontally horizontally by tilting t ilting the diffraction grating. A stellar radial velocity of 1,500 meters per second will shift a spectral line by one pixel on the CCD detector. The small stellar radial velocity velocity shifts induced induced by a planet’s planet’s gravity cannot be seen through visual compariso comparison n of the spectra. spectra. The spectrograph data reduction software measures radial velocity shifts to the nearest 0.002 pixel, allowing the telescope to detect 4-meter-per-second stellar velocity shifts. The spectrograph’s CCD detector is so sensitive that operators are forbidden from turning on fluorescent lights in the room because lingering emission will affect t he instrument’s observations. Even incandescent lights cannot be turned on several hours before calibration or observations.
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Light refracts differently as Earth’s atmospheric pressure and temperature change, altering the spectrograph calibration. To offset this, equipment keeps the telescope and spectrograph room at the same temperature. As the spectrograph creates an image, an iodine reference spectrum is simultaneously recorded near the star’s spectra. This anchors known spectral lines to known pixels in the spectrograph, allowing spectral shifts induced by telescope flexure and atmospheric conditions to be removed. Unlike other large l arge professional professional telescopes, the Harlan Smith Telescope has a closed tube. A ventilation system stabilizes tthe he temperature in the telescope, allowing it to routinely achieve 1" resolution. A signal from the Kepler spacecraft that a star has briefly dimmed is not sufficient proof that a planet circles it. Here is where Endl and his collaborator, collaborator, Bill Cochran, C ochran, dig in a nd do the detective work. Endl observes the candidate star and transmits tra nsmits data from each night’s observations to Cochran at the University of Texas. Next, the two collaborators determine if the t he candidate star’s periodic dimming is due to factors such as a binary companion, intrinsic variability, or large “starspots” on its surface.
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If such options are ruled out, more follow-up observations search for the telltale radial velocity curve indicating the star is slowly drifting back and forth along our line of sight because of the minute pull of a planet’s gravity.
A bright future Years of research indicate that planetary formation is a robust mechanism. Kepler data suggests that 30 percent of Sun-like G-type stars have 1 to 1.5 Eart h-radii planets within their habitable zones. However, the statistical error could be as much as 20 percent; thus, Earth-like planets could be as rare as existing around only 10 percent of G-type stars or as plentiful as circling half of them. Hot Jupiters orbiting close to
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their stars are rare. Researchers wonder whether such planets have migrated inward, destroying the rest of their star’s planetary system. They would also like to know whether Jupiter-sized Jupiter-sized planets are a normal part of planet formation. The future of exoplanet research is getting brighter. NASA’s 2.4m Wide Field Infrared Survey Telescope (WFIRST) satellite, scheduled for a mid-2020s launch, will have a powerful coronagraph to image planets close to a star. Understanding an exoplanet’s exoplanet’s atmosphere is also a key area of research. The cur rent problem problem is that the spectrum of the atmosphere of a transiting planet is a fraction of the star’s total spectrum. Presentday techniques can analyze the rough atmospheric composition of
transiting planets with 1.5 to 2 Earth masses, as well as the hot Jupiters that lie close to their stars. However, many of these exoplanets have a hazy atmosphere whose spectra reveal little about it. Of course, astronomers would like to detect biosignatures — gases produced as a by product of life. To do this with today’s technology would require a 30 m telescope or a space mission. Perhaps the Giant Magellan Telescope under construct ion in Chile will be able to detect biologically produced gases in the atmospheres of planets orbiting the nearby stars Proxima Centauri and TRAPPIST-1. Direct imaging of a non-transiting exoplanet will be more efficient at detecting biosignatures, but such searches will have to wait until the James Webb Space
Telescope launches in spring 2019, or when the WFIRST satellite comes online in the next decade. In the meantime, researchers like Endl continue their detective work by scanning nearby stars and refining techniques. With each new observation, Endl fills in the blanks and comes closer to answering the persistent questions about planets far beyond our solar system. Endl’s dream of creating t he exoplanet chart is getting closer to reality. His current research will be the facts of future textbooks that describe amazing alien worlds undreamed of several decades ago. Robert Reeves is an astroimager
and author who lives in San Antonio. He loves to shoot the Moon in high resolution.
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10. The spectrograph CCD camera is cooled with liquid nitrogen and controlled through a complex electronic interface. 11. The 0.7-day orbital period of a super-Earth orbiting the star 55 Cancri is displayed in this plot of data gathered by the McDonald Observatory’s 2.7m and 10m telescopes, along with data from the Keck and Lick observatories. The width of the line depicting the orbital plot lines reflects the uncertainty of the planet’s mass. COURTESY OF MICHAEL ENDL
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THE PHOTOGR PHOTOGRAPHIC APHIC LEGACY OF
Lowell’s Great Refractor Over decades, the observatory’s observatory’s powerhouse instrument charted a new course in planetary imaging. by Klaus Brasch
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istoric Lowell Observatory in Flagstaff, Arizona, is commonly associated with two notable astronomical bodies: Mars and Pluto. Percival Lowell, who was determined to study the Red Planet and its putative canals, established the observatory in 1895. Its centerpiece was the legendary 24-inch Clark refractor. Lowell was convinced that the illusive i llusive canals were built by intelligent beings to irrigate the deserts of a dying world. In addition to indulging such fanciful notions, however, Lowell was also an innovator and competent mathematician who posited a trans-Neptunian planet and was determined to find it. He initiated several photographic searches for “Planet X,” but sadly he did not live to see t he success of these efforts. Clyde Tombaugh discovered Pluto
in 1930, 14 years after Lowell’s death. Amid all these t hese undertakings, it is easy to forget that in the first half of the 20th century, the observatory and its great refractor were at the forefront of many other scientific discoveries. Notable among these were Lowell astronomer Vesto Melvin Slipher’s first spectra of spiral “nebulae” (later determined to be galaxies), showing that most were moving away from Earth and thereby setting the stage for Edwin Hubble’s discovery of the expansion of the universe. Slipher also discovered via spectroscopy that the Merope Nebula in the Pleiades radiates radiates by reflected ref lected light rather than emitted light, thereby confirming the presence of the interstellar medium. It’s important to remember that in science, key technological advances are often a prelude to new discoveries. The century
One of Percival Lowell’s early sketches of “canals” on Mars, which he believed to be irrigation channels cut by intelligent beings, contrasts with one of his early photographs of the Red Planet. The photo shows no such tiny, linear features. LOWELL OBSERVATORY ARCHIVES 60
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spanning 1750 to 1850 saw major advances in optics and related instrumentation instr umentation that extended astronomy beyond the purely visual realm. These included the inventio invention n of achromatic lenses and silver-coated glass telescope mirrors. Concurrently, German physicist Joseph von Fraunhofer invented diffraction gratings a nd spectroscopy, spectroscopy, and, equally important, the German equatorial mount with its clock dr ive mechanism. These advances led to the production of far better and more versatile refracting telescopes, as exemplified by Fraunhofer’s legendary 9.5-inch Great Dorpat Refractor in 1824. In the mid-1800s, Louis Daguerre and others developed the first f irst photographic methods, which were quickly applied to astronomical objects. By the time Lowell founded his observatory, the preceding technological advances made much of the pioneering work there possible. Notable in this regard was Lowell’s judicious application of photography to study the planets. He primarily attempted to photograph his controversial controversial martian canals, to prove beyond any doubt they were “real” and not just illusive features visible only to Lowell’s eyes. Although the results were anything but convincing, subsequent subsequent work by two Lowell astronomers — Carl Otto Lampland and Slipher’s younger brother Earl C. (E.C.) Slipher — continued and perfected planetary photography photography well into the t he 1960s. Lampland was a skilled craft sman who developed novel instrumentation, including specialized cameras for the Clark refractor to facilitate large-scale, multiimage capture on a single photographic plate. He and E.C. Slipher also pioneered color filter photography to highlight dif ferent levels and compositional features of the
atmospheres of Venus, Mars, Jupiter, and Saturn. Arguably, E.C. Slipher’s signature contribution to the advancement of planetary photography was his application of integration printing. By combining successive images of planets into a single print, he could minimiz e the inherent noise of the grainy photographic emulsions in use at that time, and also vastly improve contrast contrast and detail i n the resultant pictures. This process was, effectively, effectively, the precursor of image stacking as routinely practiced today with digital imaging. Using this approach, E.C. Slipher produced some of the finest and most detailed planetary images of the photographic era. Most of these are featured in his two classic books: Mars, the (1962) and A Photographic Photographic Story (1962) Photographic Study of the Brighter Planets (1964).
Into the modern age The arrival of the Space Age in the late 1950s heralded renewed interest and research in lunar and planetary astronomy. Lowell Observatory and t he venerable Clark refractor again played central roles. Two NASAfunded programs were initiated at that time, both centered on the observatory and par tner institutions. One was the International Planetary Patrol Program (IPPP), (IPPP), and the t he other was the Lunar Aeronautical Charts (LAC) project. The IPPP, led by Lowell astronomer William Baum, involved several observatories worldwide and was designed to constantly monitor the major planets photographically whenever they were favorably placed for observation. The goal was to garner as much information as possible on atmospheric, weather, compositional, and physical characteristics of each planet, in preparation for space probe missions to them. To these ends, telescopes in the 24- to 26-inch aperture range at eight observatories around the globe were modified to a standard image
The 24-inch Clark refractor was built just before the t urn of the 20th century. A renovation was completed in 2015. DAVID J. EICHER
scale and then coupled with specially designed 35mm film cameras. These semiautomatic devices used innovative focusing, guiding, and color filter applications to ensure as much uniformity in the resulting images as possible. By the time this program ended in the mid-1970s, it had generated over a million high-quality planetary images and provided an enormous amount of new information on the atmospheric dynamics of ma rtian clouds and dust storms, rotational currents of the jovian cloud deck, the retrograde rotation of the venusian upper atmosphere, and the physical characteristics of Saturn’s rings. The Clark refractor’s last major scientific contributions did not involve direct photography, phy, but provided visua l backup for astronomers, geologists, and cartographers involved with the t he LAC project in preparation for the Apollo and early spacecraft era. This project combined the best available lunar photographs from Mount Wilson, Lick, McDonald, Yerkes, and Pic du Midi observatories, with visual observations obtained with t he Clark telescope. The latter resolved far finer lunar detail than the grainy photographs of the time could record. The LAC series charts produced in the early 1960s thus marked the culmination of Earth-based lunar mapping efforts.
Current imaging with the scope Today, the Lowell refractor is completely dedicated to public outreach and education, and it has enchanted hundreds of thousands of visitors over the decades. As a participant in that educational effort, I have been fortunate to try digital imaging through this classic telescope. I was particularly interested in seeing how well my results compared to the film-based images of yesteryear, and more specifically to the best lunar charts of the pre-Apollo era. My first go at planetary imaging with the Clark was in October 2003 when Saturn was exceptionally well placed with nearly wide-open rings. Webcams Webcams weren’t popular yet, but at the time, I really needed a larger sensor to accommodate the image scale that a telescope of 9,770mm focal length produces. My choice was a Nikon Coolpix 995 ca mera with a (then-impressive) 3.3-megapixel CCD sensor and 4x optical zoom. I took some half-dozen exposures in quick succession and then stacked them in an early version of RegiStax. Although seeing conditions were well above average that night, I soon discovered a more serious limitation — chromatic 62
ASTRONOMY
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MARCH 2018
This famous planetary camera was used with the 24-inch Clark refractor for years by astronomer Earl C. Slipher. KLAUS BRASCH
Differing colors of martian storm clouds October 16, 1973 Red
Green
Blue
Ultraviolet
International Planetary Patrol Program Lowell Observatory, Flagstaff, Arizona
Mars was a frequent target of Lowell Observatory astronomers from 1960 to 1974, when they operated the International Planetary Patrol Program at Lowell and captured Mars through various filters. LOWELL OBSERVATORY ARCHIVES
Examples of Slipher’s finest planetary photographs made with the Clark reveal subtle atmospheric and surface features on Jupiter (left) and Mars. LOWELL OBSERVATORY ARCHIVES
The author took this image of Saturn with the Lowell refractor in 2003. It shows color unprocessed and processed versions (left and center), and a modern black-and-white version (right). KLAUS BRASCH
This photo-geologic map of the area surrounding Copernicus Crater on the Moon was made by Gene Shoemaker and R.J. Hackman in 1960. Topographic details were based on the best Earth-based photos available at the time; Lowell’s refractor contributed some of them. NASA/USGS
A panoramic view of the Mare Nectaris region of the Moon was created by the author using the Clark refractor. Near center is the crater trio of Theophilus, Cyrillus, and Catharina. KLAUS BRASCH
aberration. At the relatively fast f/16 focal ratio, the classic achromat exhibits considerconsiderable secondary color. Since the telescope is equipped with a front-end iris diaphragm, even closing it down to 18 inches left diminished, but still evident, purple color fringes. Of course, it’s important to realize that at the time of its manufacture in the late 1800s, the refractor was corrected principally for visual observations and not color photography. Even extensive image processing left residual color. After I converted my first digital image of Saturn to black and white, it wasn’t quite on par with E.C. Slipher’s best. Clearly the better way to proceed is with a monochrome webcam and tri-filter imaging, which I hope to try soon. Since then, I’ve enjoyed a number of opportunities to image the Moon with the Clark telescope and far better digital equipment, including a Canon 50 DSLR and just recently an ASI-120 ASI-120 webcam. Fortunately, chromatic aberration is not a significant issue with lunar imaging, provided one shoots in monochrome or black and white. For a telescope with such a large aperture and focal length, atmospheric steadiness, or seeing, is far more critical. This is particularly important since the Clark is always in high demand, and access to it must be scheduled well in advance with no guarantees about weather or seeing conditions. Still, I have been fortunate to have occasionally experienced seeing conditions most amateurs would rate 7 to 8 out of 10 — fair to good, but not excellent. I take pride in sharing a few examples of modern images taken through the great Clark refractor. They include a wide-angle mosaic of the Catharina, Cyrillus, and Theophilus crater trio on t he Moon, taken with the DSLR at prime focus under good seeing. The mosaic was compiled from a stack of 50 exposures combined in RegiStax 6. The smallest craterlets resolved are about 1.25 miles (2 ki lometers) lometers) in diameter. di ameter. Capturing fresh images with a storied old instrument, linked to several of the great discoveries about the universe, is thrilling. If you haven’t haven’t visited Lowell Observatory, do so. You’ll find history and current science — in areas of solar system, galactic, and extragalactic — seeping from the place. It’s an amazing blend of past and present. Klaus Brasch is a retired bio-scientist and public program volunteer at Lowell Obser vatory. He is grateful to Lowell Observatory archivist Lauren Amundsen, public program manager Samantha Gorney, and historian Kevin Schindler for invaluable support and assistance with this project. WWW.ASTRONOMY.COM
63
Asttronomy tests As tests
Celestron’s
CGX mount If you’re ready for the next level of telescope mounts, this may be the one for you. by Tom Trusock A CRITICALLY IMPORTANT, but often overlooked, component of any observing setup is the mount. At a minimum, a mount needs to do two things: support the payload properly and allow for smooth tracking at high powers. Although these requirements seem simple, a good mount can be a difficult purchase. Too often, amateurs deliberately choose mounts too small for their equipment, mainly due to expense. While visual observers can get away with thi s,
Celestron’s CGX equatorial mount will drive loads up to 55 pounds (24.9 kilograms) with a high degree of accuracy.
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ASTRONOMY
• M A R C H
2018
astroimagers cannot. If you’re you’re taking taki ng photos, you’re you’re far better off stepping up to the next level of support, rather than trying to make do with something that is barely sufficient. Enter Celestron’s Celestron’s CGX equatorial mount. This welcome piece of equipment is the company’s new entry in mid/heavy class German equatorial mounts (GEM).
The specs At a weight of 63.2 pounds (28.7 kilograms) — 44 pounds (20 kg) for the head alone — and sporting a 55-pound (25 kg) capacity, the CGX mount is the new Celestron workhorse for scopes with apertures of 6 to 11 inches. The CGX reflects an overall redesign to accommodate new control systems for both observers and imagers. It features internal cabling and two AUX ports, which support both SkyPortal WiFi and StarSense AutoAlign technology. For those observing remotely, Celestron has included sensors that return the mount to its index position in case of a power failure. Also, the sensors will shut off slewing or tracking tracking before the system reaches its hard stop to prevent damage. Imagers will be pleased with the new software softwa re to control operation and imaging. Particularly attractive is the ability to compensate for individual variations in t he mechanics of the mount through multipoint mount modeling. Additionally, you’ll find an autoguider port and a USB 2.0 port (for software updates). updates).
The CGX equatorial mount and tripod comes with everything you see here.
Celestron also touts the fact that t hat the mount supports Programmable Periodic Error Correction. The drive system is now belt driven with spring-loaded worm gears to ensure a solid connection. And, of course, the Celestron NexStar+ control drives the entire system. If you’ve used the NexStar system within the past few years, you’ll feel right at home. The 40,000-object database contains the Messier, Messier, NGC, Caldwell, a nd other astronomical object catalogs.
The arrival The CGX comes in two boxes labeled “team lift.” If you’re getting this delivered, you may want to enlist some help moving it to where you plan to unpack it. While the mount itself is not difficult for a healthy individual to handle, it’s a little awkward when boxed. As you unpack, you’ll find a tripod, accessory tray tr ay,, hand controller, GEM head, two 11-pound 11-pound (5 kg) counterweights, and a DC power adapter. The combined 22 pounds (10 (10 kg) of counterweights should be suffisuff icient to balance most equipment while keeping the entire setup under the 55-pound (25 kg) rating. Celestron also offers an AC power adapter and 17-pound (8 kg) counterweights as options. In the box, you’ll find a code to download a special Celestron edition of the Starry Night 7 software for Windows or Mac. The dovetail that ships with the CGX is a two-in-one unit that supports both CGE (otherwise known as a Losmandy D plate) and CG-5 (Vixen) dovetails. Because the CG-5 saddle is recessed under the CGE portion, you may find certain CG-5 dovetail bars do not fit properly, properly, although I had no issues. The ability to support both dovetails is a nice touch and something that I’d like to see become standard.
PRODUCT INFORMATION Celestron CGX equatorial mount Weight, head: 44 pounds (20 kilograms) Weight, tripod: 19.2 pounds (8.7 kg) Load capacity: 55 pounds (24.9 kg) Tracking rates: Sidereal, solar, lunar Tripod height: 47¼ to 77½ inches (120 (120 to 197 centimeters) Included accessories: tripod, accessory tray, two counterweights, DC power cable, NexStar+ hand controller Price: $2,199 Contact: Celestron 2835 Columbia St. Torrance, CA 90503 310.328.9560 www.celestron.com
A polar scope is not included, but Celestron notes that one will be available as an option soon. The mount does support Celestron’s Celestron’s Al l-Star Polar Alignment A lignment technology, which allows you to use the location of any bright star to fine-tune f ine-tune the polar alignment. If you use this, t his, I recommend running the routine a couple of times for the best accuracy.
Under the stars Field setup requires the included 8-millimeter Allen wrench to tighten the head attachment bolt. Celestron engineers have provided a place on the mount to store the wrench, but if you frequently travel to dark sites and break down the mount on a regular basis, I’d recommend that you store an extra Allen (or two) in your observing kit. I tested the CGX with a variety of payloads, from a lightweight refractor to a
The connector that comes with the mount can accommodate Losmandy or Vixen dovetail plates.
Celestron C11 SCT (about 30 pounds [14 kg]). The mount mou nt bore the payloads well for visual use, and settle time was more tha n acceptable. While observing t hrough the C11, C11, I could tell that t hat Celestron has definitely improved the damping time in t his class of mount. The pointing accuracy of the NexStar+ system was top-notch. For more more diff icult targets, I found the high-precision pointing mode (Precise GoTo) very helpful. For those unfamiliar with it, you perform an alignment on a bright star near your target, and then the mount determines how far off you are from the ideal model and compensates when slewing to your next target. Astrophotographers who routinely image objects too faint to be seen through t he optics will appreciate this.
A positive verdict In all, I was impressed with Celestron’s CGX equatorial mount. At its price point, it offers a good array of standard features and expandability. I’d recommend anyone who carries a cellphone, tablet, or laptop consider purchasing the SkyPortal WiFi module. At $99.95, this is a relatively inexpensive add-on. Amateurs who want to bypass the manual alignment procedure will be interested in the StarSense AutoAlign ($349.95). Celestron has developed a solid, usable, and expandable mount at a decent price. The CGX will serve an amateur well for years of either visual or photographic use no matter which optical tube(s) you couple it with. I highly recommend recommend it. is an equipment guru Tom Trusock is who does most of his testing near his home in Ubly, Michigan. WWW.ASTRONOMY.COM
65
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• Spectacular celestial photography. • Observing tips and monthly star maps. 9 7 3 0 3 P
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BINOCULARUNIVERSE BY PHIL HARRINGTON
Star clusters in Monoceros Little-known star groups offer gemmy views.
T
his month, three of the season’s brightest stars — Betelgeuse, Sirius, and Procyon — form a large equilateral triangle that many know as the Winter Triangle. Although they blaze brightly on clear nights, they frame a surprisingly barren part of the late winter sky. Much of this vast, starless region belongs to Monoceros the Unicorn. Although hapless Monoceros holds little to grab the attention of constellation hunters, it overflows with open star clusters for us binocularists. Yet, while many of those clusters are visible in binoculars, only one found its way into Charles Messier’s catalog. That’s our first stop this month, M50. M50’s story is an interesting one. The Italian-born French astronomer Giovanni Cassini is often credited with its discovery. He is said to have spotted a “nebula” between Canis Major and Canis Minor sometime before 1711. 1711. Although Messier searched for Cassini’s nebula in
1771, 1771, he never found it. On ly a year later did he chance upon an errant open cluster in the same vicinity; it became M50. Most assume that this was Cassini’s “nebula,” but we will probably never really k now for sure. One thing we do know for sure is that even though M50 lies in the middle of nowhere, it’s surprisingly easy to find with binoculars. Begin your hunt at Sirius and slowly scan northeastward toward Procyon. Along the way, you’ll pass Theta (θ (θ) Canis Majoris about a binocular field from Sirius. Another field-hop in the same direction should bring a tiny blur of starlight into view. That’s M50. Through my 10x50 binoculars, M50 offers a soft blur of light peppered by a few faint points. The brightest cluster star, at 8th magnitude, lies just south of center. When I use my 16x70 binoculars, more stars emerge from the fog, while under dark skies my 25x100s reveal that those stars seem to
β Mon α Mon Pakan’s 3
Pakan’s 3 is an asterism that lies close to M50. This zigzag clump was named by amateur astronomer Randy Pakan from Edmonton, Alberta. TONY HALLAS
The sparse open star cluster M50 makes a good sight in binoculars. It is one of the highlights of Monoceros. RICHARD MCCOY
fall into curved lines and arcs that some see as heart-shaped or possibly like an arrowhead oriented east-west. If your eyes are especially color-sensitive, you may notice that M50’s brightest star shows a subtle golden tint. All other cluster members appear white. A trio of open star clusters lies southeast of M50. The most obvious is NGC 2353. With M50 still in view, shift your attention about half a field to the southeast. There, you’ll spot three 6th-magnitude stars in a gentle arc ¾° long. Look carefully, and you may see that the center star in the arc looks a little fuzzy and is accompanied by some fainter companions. That’s the cluster. All told, some 100 stars belong to NGC 2353, although most fall below the reach of binoculars. The other two clusters, NGC 2335 and NGC 2343 are fainter still. As you approach M50 from Theta Canis Major to the south, you may notice a tiny zigzag clump of faint stars a little west of the line connecting those two. That asterism is best known as Pakan’s 3 , named after amateur astronomer Randy Pakan from Edmonton, Alberta, who first noted its shape. Depending on your binoculars, it should be in the same field as M50, since both are separated by only 3°. Put M50 in the northeastern part of your view, then look to the southwest for a collection of
eight faint stars that together form an angled number 3. None of those stars shines brighter than 9th magnitude, however, however, so you’ll need a very clear March night to see them. Finally, let’s visit another fun asterism that I first bumped into more than three decades ago while researching for my book Touring the Universe Through Binoculars. Binoculars . Look about 5.5° west of M50, and less than half a degree northwest of the 5thmagnitude red giant SAO 133585. There, you’ll find a compact clump of seven faint stars forming an inverted letter V. Given the constellation they lie within, the group has since become known as the Unicorn’s Horn. All seven stars span a compact 7', so they may look nebulous. Since none shines brighter than 9th magnitude, 10x70 binoculars are probably the smallest that will show them well. The group is clear in my 16x70s and striking in my 25x100s. Let me k now how you do in sighting sighti ng the Unicorn’s Horn, as well as the other targets mentioned this month. Contact me through my website, philharrington.net. Until next month, remember that two eyes are better than one. Phil Harrington is a longtime contributor to Astronomy and and the author of many books.
WWW.ASTRONOMY.COM
67
OBSERVINGBASICS BY GLENN CHAPLE
Is this stellar pair marathonworthy? Putting the finishing touches on the double star marathon.
I
have to share this one with you. At the November 9, 2017, 2017, meeti ng of my astronomy club, the Amateur Telescope Makers of Boston, member John Sheff announced that it was Carl Sagan’s birthday. “How old would he have been were he still alive?” I asked. Without batting an eye, he replied, “Billions and bill ions!” Who says astronomy enthusiasts are mirthless individuals? I want to make a personal plea for help — not of t he mental or physical variety, though most folks who know me well would argue the point. No, I’m looking for assistance in finetuning and finalizing the
The near-twin stars of Psi1 Psc lie in the northeastern corner of Pisces the Fish. JEREMY PEREZ
double star marathon I introduced in my March 2 016 016 column. For those of you who missed that article and a follow-up in March 2017, 2017, I created the double star marathon as a counterpoint to the annual Messier marathon. The latter is typically held in mid- or late March when all the objects in the Messier catalog can be seen in a single evening. I picked 110 stellar pairs to match the number typically included on the Messier marathon roster, and all lie in the same areas as the Messier objects.
Swimming with the Fish Here’s where I need your help. California double star aficionado Phil Kane has suggested that I include Psi 1 (ψ 1) Piscium on the list. This striking pair of near-twin stars shine with a pure-white hue at magnitudes 5.3 and 5.5 and are separated by a comfortable 30". They lie just 9° northwest of the spiral galax y M74, a notoriously difficult Messier marathon object because it glows faintly and hangs low in the west on March evenings. Psi1 Psc would add a challenging element to the double star marathon, but would it be too much?
EIGHT MORE DOUBLE STAR WONDERS Name
R.A.
Dec.
Mags.
Sep.
P.A.
41 Aurigae
6h11.6m 6h11.6m
48°43' 4 8°43'
6.2, 6.9
7.4"
357°
20 Geminorum
6h32.3m
17°47' 17°47'
6.3, 6.9
19.7"
211° 211°
Nu (ν ) Canis Majoris
6h36.4m
–18°40' –18°40'
5.8, 7.4
17.3" 17.3"
264°
17 Hydrae
8h55.5m
–7°58'
6.7, 6.9
4.0"
4°
35 Sextantis Sext antis
10h43.3m
4°45'
6.2, 7.1
6.7"
239°
83 Leonis
11h26.8m 11h26.8m
3°01'
6.6, 7.5
28.2"
149°
88 Leonis
11h31.7m 1h31.7m
14°22'
6.3, 9.1
15.7"
332°
90 Leonis
11h34.7m 11h34.7m
16°48'
6.3, 7.3
3.5"
208°
1
1
Key: R.A. = Right ascension (2000.0); Dec. = Declination (2000.0); Mags. = Magnitudes; Sep. = Separation; P.A. = Position angle
N
PISCES
E
M74
Double star Psi1 (ψ 1) Piscium lies 9° northwest of spiral galaxy M74, one of the most difficult Messier marathon objects. ASTRONOMY : ROEN KELLY
On the weekend of this year’s Messier marathon (Friday and Saturday nights, March 16 and 17), I’ll be tackling Psi1 before moving on to my main list. I encourage you to back me up by doing the same and letting me know how you fare. If positive sightings outnumber the negative ones, I’ll officially add Psi1 to the list. And then I’d face the heartbreaking task of culling one double from my original list. By the way, if you’d like to run the marathon this year and don’t have the list, send me an email, and I’ll forward you a copy.
Best of the rest As any diehard double star observer would tell you, selecting 110 pairs meant omitting some real gems. In
BROWSE THE “OBSERVING BASICS” ARCHIVE AT www.Astronomy.com/Chaple. 68
ASTRONOMY
• MARCH 2018
1°
the table above, I highlight eight that failed to make the cut but deserve your attention. The data come from the Washington Double Star Catalog , which is available online at http://ad.usno.navy.mil/wds. I have split all of them through a 3-inch reflecting telescope at 60x, but I suggest using a larger aperture to better reveal their colors and bumping up the magnifying magnif ying power to 100x for some of the closer pairs. Questions, comments, or suggestions? Email me at
[email protected]. Next month: tips for hosting an Astronomy Day star party. Clear skies! Glenn Chaple has been an avid observer since a friend showed him Saturn through a small backyard scope in 1963.
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READER GALLERY
1 1. TAKE HEART, HAVE SOUL
The Heart and Soul nebulae (IC 1805 and IC 1848) lie some 7,500 light-years away in the constellation Cassiopeia the Queen. Both are emission nebulae, which glow with a reddish hue. This false-color image made in the Hubble palette shows details that are different from the true-color version. • Kfir Simon
2. NIGHTFALL
As his telescope cooled to ambient temperature October 22, 2017, 2017, this photographer set up a camera on a tripod and captured the scope, its observatory, and the waxing crescent Moon. • Jared Bowens
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3. IN FULL BLOOM
Sharpless 2–101, also known as the Tulip Nebula, is a large emission nebula in the constellation Cygnus the Swan. The cloud spans nearly 70 light-years and lies at a distance of 8,000 lightyears. The several bright stars within the cloud emit prodigious amounts of ultraviolet radiation, exciting the gas and causing it to glow. • Georges Chassaigne
4. WORTH THE EFFORT
3
IC 166 is a relatively unknown open cluster in Cassiopeia. A large telescope will reveal a tightly packed region in a rich star field. The cluster glows softly at magnitude 11.7. • Al Kelly
5. MORNING PLANET LINEUP
On September 18, 2017, 2017, the Moon returned to the spot in the constellation Leo where, one lunar month earlier, it had eclipsed the Sun. Regulus (Alpha [α] Leonis) is to the Moon’s lower left. Venus lies above the pair, and Mars (higher, fainter) and Mercury lie near the cloud line. The photographer captured this grouping from Uludağ, Turkey. • Tunç Tezel
Send your images to:
4
5
Astronomy Reader Reader Gallery, P. O. Box 1612, 1612, Waukesha, Waukesha , WI 53187. 53187. Please include the date and location of the image and complete photo data: telescope, camera, filters, and exposures. Submit images by email to
[email protected].
6. SPARE�TIME IMAGING
From a recent email: “We work at Lowell Observatory at the Discovery Channel Telescope. A few months ago, we had a couple of hours of engineering time with nothing to do. We decided to take some pretty picture data and settled on the Ring Nebula (M57).” The galaxy in the shot is IC 1296. • Andrew Hayslip/ Jason Sanborn/Ch arles B. Ward
7. CITIES OF STARS
Spiral galaxy NGC 1365 is the largest and one of the brightest members of the Fornax Cluster, named for the constellation in which it lies. This group floats through space some 60 million light-years away. • Dan Crowson
6
NGC 1374 NGC 1382
NGC 1375
NGC 1381
NGC 1379 NGC 1399 NGC 1427
NGC 1387 PGC 013230 NGC 1404
NGC 1389
NGC 1365 NGC 1369
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BREAK THROUGH “Vermin of the skies” Astronomer Edmund Weiss coined this phrase to describe asteroids and their annoying habit of interfering with views of the distant universe. This Hubble Space Telescope image nicely illustrates his point. Seven different asteroids photobombed this field near galaxy cluster Abell 370, which lies near the plane of the solar system. These notso-magnificent seven left 16 curved and S-shaped streaks in the image. Hubble’s orbital motion around Earth caused the nearby asteroids to trace out arcs relative to the background galaxies, and several left multiple trails because the final image combined many Hubble exposures. NASA/ESA/B. SUNNQUIST AND J. MACK �STS CI�
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THE S ECR ET TO GREAT GREAT I MAGES
HIG H-RESOLUT H-RESOLUTION ION IMAGI NG WITH WITH CELESTRON What do so many of the world’s best planetary imagers have in common? They trust Celestron Schmidt-Cassegrain and EdgeHD optics. Armed with a Celestron Schmidt-Cassegrain, Christopher Go can capture more than just the cloud bands on Jupiter; his images include detailed festoons within those bands. Robert Reeves pushes his Celestron optics to reveal craters on the moon—and the smaller craterlets inside them. And Thierry Legault can capture a crisp shot of the International Space Station as it whizzes by at nearly 5 miles per second.
CHRISTOPHER GO Jupiter Celestron C14
ROBERT REEVES Ptolemaeus Alphonsus Arzachel Celestron EdgeHD EdgeH D 11 11
THIERRY LEGAULT ISS Transit Celestron EdgeHD 14
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SOUTHERN SKY
MARTIN GEORGE describes the solar system’s changing landscape
as it appears in Earth’s southern sky.
May 2018: Jupiter at its peak As May begins, the solar system’s tem’s two brightest planets adorn the evening sky. Venus disappears during twilight, however, while Jupiter stays up all night. Venus shines brilliantly at magnitude –3.9 and is easy to spot low in the northwest if you have a clear and unobstructed horizon. On the 1st, it stands nearly 10° high 45 mi nutes after sunset. It lies 7° to the lower right of 1st-magnitude Aldebaran, t he orange-red luminary of Taurus the Bull. The planet maintains its brightness th roughout May, May, but becomes easier to see as it climbs higher. On the 31st, it appears 15° high an hour after sundown and remains visible past nightfall. Venus has little to offer to observers using a telescope, however. Even at month’s end, its 13"-diameter 13"-diameter disk appears nearly full. On the opposite side of early May’s evening sky, Jupiter rises shortly after sunset and shows up easily as twilight fades. The giant world reaches opposition May 9. Although this point typically marks a planet’s peak visibil ity, Jupiter Jupiter barely suffers the rest of the month — it shines at magnitude –2.5 throughout May. The planet resides in Libra all month, moving slowly westward against this backdrop. It lies 3° east of the fine binocular pair Alpha1 (α1) and Alpha 2 (α2) Librae at opposition; the gap closes to 1° by May 31. By late evening, Jupiter has climbed high in the sky and will be a wonderful sight through any telescope. The gas giant’s
disk measures 45" across the equator and is noticeably flattened — its polar diameter is 3" less. Although two dark atmospheric belts are almost always visible, additional additional features pop into view during moments of good seeing. Also watch for Jupiter’s quartet of bright moons. Discovered by Galileo in 1610, all four stand out as long as none of them is hiding in front of or behind the planet. By midevening, glorious Saturn pokes above the eastern horizon. The ringed planet moves slowly westward against the backdrop of Sagittarius the Archer, north of that constellation’s conspicuous Teapot asterism. Although far dimmer than Venus or Jupiter, magnitude 0.3 Saturn appears prominent among the stars of the Sagittarius Milky Way. Saturn remains an exquisite sight through any telescope once it climbs well clear of the horizon. Although its disk, which measures 18" across at midmonth, shows much less detail than Jupiter’s, the rings more than make up for this lack. The beautiful system spans 40" and tilts 26° to our line of sight. You’ll also see Saturn’s brightest moon, 8thmagnitude Titan; several others show up through 10-centimeter 10-centimeter and larger instruments. By late evening, you can fi nd Mars lurking to the lower right of Saturn. The Red Planet moves eastward against the background stars, crossing from Sagittarius i nto Capricornus Capricornus in mid-May. If you’ve been following the planet’s appearance over the past few months, you’ll defi-
nitely notice how much brighter it has become. The trend continues in May — during the month’s 31 days, Mars doubles in brightness from magnitude –0.4 to –1.2. Mars’ apparent diameter makes a similar leap. The ocher-colored ocher-colored disk g rows from 11" 11" to 15" across, mak ing it a great target for observing through a telescope. To see the most detail, wait for it to climb high in the sky after midnight. By late May, the planet’s south pole tips 15° toward us, delivering impressive views of the south polar cap. As Saturn and Mars climb high in the north before dawn, Mercury rises in the east. The innermost planet shines brightly and remains conspicuous for most of May. Early in the month, the magnitude 0.3 world rises two hours before the Sun and stands nearly 15° high an hour before sunup. sunup. And it remains a pleasing telescopic sight, with a nearly halflit disk spanning 8". As it sinks lower during the following weeks, Mercury grows brighter, brighter, hitting magnitude –0.3 in midMay. Unfortunately, it then appears smaller and shows a less interesting gibbous phase. The planet disappears into the Sun’s glow by month’s end.
The starry sky Approximately midway between Regulus, the brightest star in Leo the Lion, and Spica, Virgo the Maiden’s luminary, is a direction that receives little attention. But I want you to focus on this area, which stands high in the north in
midevening, at least once this month. On a more local scale, the spot of interest lies about one-third of the way from Beta (β) to Eta (η) Virginis. It may surprise you that this direction in the sky is called the First Point of Libra, but t hat indeed is its name. You You might be a bit more familiar with the First Point of Aries, which likewise is not in the constellation after which it is named. It lies in neighboring Pisces. These two points are where the ecliptic — t he apparent path of the Sun across the sky — crosses the celestial equator. The First Point of Aries marks the point where the Sun crosses the celestial equator from south to north at the time of the March equinox; the First Point of Libra denotes where the Sun crosses back into the southern celestial hemisphere at the time of the September equinox. So, how is it that Libra has a point named after it that belongs to another constellation? It’s all because of the phenomenon of precession. The gravitational pulls of the Sun and Moon cause Earth’s axis to rotate in the same way as a spinning top does. So, the axis describes a circle about 47° wide in both the northern and southern sky that completes a circuit in about 25,800 years. Because of precession, the right ascensions and declinations of the stars change continuously, apart from the slight changes resulting from their own motions through space. This causes the points where the ecliptic and the celestial equator intersect to gradually progress westward.
STAR DOME
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THE ALL�SKY MAP SHOWS HOW THE
S M C
SKY LOOKS AT:
9 P.M. May 1 8 P.M. May 15 7 P.M. May 31
E R I T C L U U M
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Planets are shown at midmonth
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Sirius 0.0 1.0 2.0 3.0 4.0 5.0
Open cluster Globular cluster
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9 R 3 U 1 5 A C T G N N E C 8 2 1 5 C G N
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a c i p S
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Diffuse nebula Planetary nebula Galaxy
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This map portrays the sky as seen near 30° south latitude. Located inside the border are the four directions: north, south, east, and west. To find stars, hold the map overhead and orient it so a direction label matches the direction you’re facing. The stars above the map’s horizon now match what’s in the sky.
HOW TO USE THIS MAP:
M U I P O C S E L E T N
A R A
S U I P 1 3 R 2 O N 6 C C S G
S U I R A T 7 T M I 8 G M A 6 S
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MAY 2018 Calendar of events 3
Venus passes 7° north of Aldebaran, 17h UT
4
The Moon passes 1.7° north of Saturn, 20h UT
6
The Moon is at apogee (404,457 kilometers from Earth), 0h35m UT The Moon passes 3° north of Mars, 7h UT
0 2 M
N
8
Asteroid Vesta is stationary, stationar y, 10h UT
4 M
r e t i p u J
5 M
Last Quarter Moon occurs at 2h09m UT
E
9
Jupiter is at opposition, 1h UT
10
The Moon passes 2° south of Neptune, 9h UT
12
Mercury passes 2° south of Uranus, 21h UT
The Moon passes 5° south of Uranus, 15h UT The Moon passes 2° south of Mercury, 17h UT
15
New Moon occurs at 11h48m 11h48m UT
16
The Moon passes passes 1.2° 1.2° north of Aldebaran, 13h UT
17
The Moon passes 5° south of Venus, 18h UT
Eta Aquariid meteor shower peaks
s e r a t n A
S U H C U I H P O
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The Moon is at perigee (363,776 kilometers from Earth), 21h05m UT 22
First Quarter Moon occurs at 3h49m UT
27
The Moon passes passes 4° north of Jupiter, 18h UT
29
Full Moon occurs at 14h20m UT
T U P A C S N E P R E S
s u r u t c
STAR COLORS:
E S T Ö O B
Stars’ true colors depend on surface temperature. Hot stars glow blue; slightly cooler ones, white; intermediate stars (like the Sun), yellow; followed by orange and, ultimately, ulti mately, red. Fainter stars can’t excite our eyes’ color receptors, and so appear white without optical aid. Astronomy y : Roen Kelly Illustrations by Astronom
BEGINNERS: WATCH A VIDEO ABOUT HOW TO READ A STAR CHART AT www.Astronomy.com/starchart.
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