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Dec L bracket - $40 ©2018 Sky-Watcher USA. Specifications and pricing subject to change without notice. 20-17021. You may have noticed that we’re using a new green on our products. It’s our new look and you’ll be seeing more of it over the next few months. Let us know what you think of it! Latitude base - $65 Counterweight kit - $30

Photographer: Carlos Guana Camera: Canon 5D IV Lens: Rokinon 14mm 2.8 Mount: Star Adventurer

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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 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. Off er expires 04/07/18 THEGREATCOURSES.COM/9ASTR 1-800-832-2412 Online Content Code: ASY1803 Enter this code at: www.astronomy.com/code MARCH 2018 to gain access to web-exclusive content VOL. 46, NO. 3 NASA/JPL-CALTECH 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 38 55 Secret Sky 16 Cassini unveils Saturn StarDome and In pursuit of exoplanets STEPHEN JAMES O’MEARA This intrepid spacecraft spent 13 Path of the Planets Two massive telescopes in the For Your Consideration 18 Lone Star State monitor 450 suns years studying the ringed planet, RICHARD TALCOTT; JEFF HESTER transforming our view of this ILLUSTRATIONS BY ROEN KELLY in the hopes of finding other captivating world. LIZ KRUESI worlds. ROBERT REEVES Binocular Universe 67 44 PHIL HARRINGTON 28 Ask Astro 60 Observing Basics 68 Saturn’s small wonders Brown dwarf jets. The photographic GLENN CHAPLE Usually known for its rings, the legacy of Lowell’s Saturn system is also home to 46 Great Refractor QUANTUM GRAVITY some of our solar system’s most A detailed look Over decades, the observatory’s Snapshot 7 intriguing moons. inside Cassini powerhouse instrument charted a FRANCIS REDDY The spacecraft’s 12 instruments new course in planetary imaging. Astro News 10 showed Saturn and its family KLAUS BRASCH 36 in unprecedented detail. IN EVERY ISSUE Sky This Month RICHARD TALCOTT 64 From the Editor 6 Mercury at its evening best. Astronomy tests Astro Letters 9 MARTIN RATCLIFFE AND 48 Celestron’s CGX mount ALISTER LING 72 minutes on Titan If you’re ready for the next level New Products 66 In 2005, the Huygens probe of telescope mounts, this may be Advertiser Index 69 pierced the moon’s shroud to the one for you. TOM TRUSOCK reveal a surprisingly Earth-like Reader Gallery 70 world. KOREY HAYNES Breakthrough 74

Astronomy (ISSN 0091-6358, USPS 531-350) is ONLINE published monthly by Kalmbach Publishing Co., 21027 Crossroads Circle, P. O. Box 1612, Waukesha, WI 53187–1612. Periodicals post- FAVORITES age paid at Waukesha, WI, and additional offices. POSTMASTER: Send address changes to Astronomy, P.O. Box 62320, Tampa, Fla. 33662-2320. Go to www.Astronomy.com My Science Ask Astro Trips and Sky This Canada Publication Mail Agreement #40010760. for info on the biggest news and Shop Archive Tours Week observing events, stunning photos, Perfect gifts for Answers to all Travel the world A daily digest of your favorite your cosmic with the staff of celestial events. informative videos, and more. science geeks. questions. Astronomy.

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WWW.ASTRONOMY.COM 5 FROM THE EDITOR BY DAVID J. EICHER Editor David J. Eicher Art Director LuAnn Williams Belter EDITORIAL Managing Editor Kathi Kube Senior Editors Michael E. Bakich, Richard Talcott Associate Editors Alison Klesman, Jake Parks Copy Editors Dave Lee, Elisa R. Neckar Editorial Assistant Amber Jorgenson Requiem ART Graphic Designer Kelly Katlaps Illustrator Roen Kelly Production Specialist Jodi Jeranek for a CONTRIBUTING EDITORS Bob Berman, Adam Block, Glenn F. Chaple, Jr., Martin George, Tony Hallas, Phil Harrington, Korey Haynes, Jeff Hester, Liz Kruesi, Ray Jayawardhana, Alister Ling, Steve Nadis, Stephen spacecraft James O’Meara, Tom Polakis, Martin Ratcliffe, Mike D. Reynolds, Sheldon Reynolds, Erika Rix, Raymond Shubinski EDITORIAL ADVISORY BOARD Buzz Aldrin, Marcia Bartusiak, Timothy Ferris, Alex Filippenko, Adam Frank, John S. Gallagher lll, Daniel W. E. Green, William K. Hartmann, Paul Hodge, Edward Kolb, Stephen P. Maran, Brian May, S. Alan Stern, James Trefil

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NASA/JPL-CALTECH ADVERTISING DEPARTMENT Phone (888) 558-1544 Advertising Sales Manager Steve Meni Advertising Sales Representative Dina Johnston, [email protected] aunched in 1997, the extraordinary touchdown studied the weird moons Ad Services Representative Cassini-Huygens on Saturn’s big moon Titan; Phoebe and Hyperion. The Christa Burbank, [email protected] spacecraft sped toward and Rich Talcott gives us list goes on and on and on. RETAIL TRADE ORDERS AND INQUIRIES Selling Astronomy magazine or products in your store: Saturn and its moons an “exploded” view of the Rarely in recent times Phone (800) 558-1544 Outside U.S. and Canada (262) 796-8776, ext. 818 for nearly seven years, spacecraft and its retinue have I seen such attachment Fax (262) 798-6592 Lentering orbit in 2004. Last of instruments. to a space probe. That hap- Email [email protected] Website www.Retailers.Kalmbach.com September, Cassini ended its The mission’s list of scien- pened with New Horizons as CUSTOMER SALES AND SERVICE journey by plummeting into tific achievements is stun- well, no doubt. But when Phone (877) 246-4835 Outside U.S. and Canada (813) 910-3616 the saturnian atmosphere, ning. The spacecraft tested Cassini took its final plunge Customer Service [email protected] burning up. Those 13 years general relativity (and last fall, Twitter lit up with Digital [email protected] Back Issues [email protected] in between gave us some of Einstein won again). It dis- science types who were CONTACT US the most incredible planetary covered seven new saturnian downright depressed, and Ad Sales [email protected] Ask Astro [email protected] science in recent memory. moons. It revealed Titan to some even to the point of Books [email protected] With Cassini in mind, be a world saturated with sending alarming, morose Letters [email protected] Products [email protected] this issue carries a special methane lakes. The mission tweets. Reader Gallery [email protected] theme of giving you every- detected and imaged water- I hope this special issue Editorial Phone (262) 796-8776

thing you need to know rich geysers and plumes will only boost your feelings Copyright © 2018 Kalmbach Publishing Co., all rights reserved. This pub- lication may not be reproduced in any form without permission. Printed about this historic mission. casting skyward from the about Cassini. This mission in the U.S.A. Allow 6 to 8 weeks for new subscriptions and address changes. Subscription rate: single copy: $5.99; U.S.: 1 year (12 issues) Four special stories surface of Enceladus. It was one of the greatest we’ve $42.95; 2 years (24 issues) $79.95; 3 years (36 issues) $114.95. Canadian: Add $12.00 postage per year. Canadian price includes GST, payable in deliver the goods: Liz Kruesi imaged and studied the seen in recent times. Now we U.S. funds. All other international subscriptions: Add $16.00 postage per year, payable in U.S. funds, drawn on a U.S. bank. BN 12271 3209 RT. Not reports on the major scien- planet’s rings and spokes in can bask in the details of the responsible for unsolicited materials. tific findings of the Cassini far greater detail than ever science while we hope fund- spacecraft at Saturn itself; before. It studied Saturn’s ing for future missions Frank Reddy describes great atmospheric storms regains a more healthy the incredible findings the and hurricanes with a dis- composure. mission made at Saturn’s tinct eyewall. The craft Follow Astronomy moons; Korey Haynes details focused on the strange hexa- Yours truly, the Huygens lander’s gon at Saturn’s north pole. It www.twitter.com/ www.facebook.com/ plus.google.com/ AstronomyMag AstronomyMagazine +astronomymagazine Follow the Dave’s Universe blog: www.Astronomy.com/davesuniverse David J. Eicher Follow Dave Eicher on Twitter: @deicherstar Editor

6 ASTRONOMY • MARCH 2018 QUANTUM QG GRAVITY EVERYTHING YOU NEED TO KNOW ABOUT THE UNIVERSE THIS MONTH . . .

BIG FUTURE COLD COMFORT LET IT SHINE HOT BYTES >> As the Sun ages, it will The James Webb The Zwicky Transient expand. ALMA has found Space Telescope has Facility at Palomar TRENDING a red giant star the size completed cryogenic Observatory in TO THE TOP our Sun is expected to testing, moving it a California saw first reach at the end of its life. step closer to launch. light November 14. NASA/JPL-CALTECH/SSI; TOP FROM LEFT: ALMA (ESO/NAOJ/NRAO)/W. VLEMMINGS; NASA; CALTECH OPTICAL OBSERVATORIES 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 The Cassini-Huygens mis- flybys of incredibly strange As we look out into the sion, which came to an end in moons like Hyperion and universe, we find that in late 2017, rewrote the book on Phoebe, and close-ups of many many places, what seems com- What Cassini Saturn and its moons. Arguably otherworldly surfaces are just a pletely strange and alien also the loveliest planet, at least from few of the countless milestones. holds connections to what we taught us an observational standpoint, But Cassini also told us a know best. Saturn is now better understood little something about our- The chemistry of the cosmos During the spacecraft’s by orders of magnitude, thanks selves. The spacecraft imaged is uniform throughout. The exploration of the Saturn to the spacecraft that visited it watery jets emanating from the presence of water on Enceladus system, we saw eerie, for 14 years beginning in 2004. moon Enceladus. The strange should remind us that we are otherworldly sights, and also There’s no doubt that Cassini moon has a subsurface, salty likely not so incredibly special, a glimpse of ourselves. took us to some weird new ocean that could contain an that life probably exists in heights. The closest ever exami- enormous amount of water. countless places in this vast nation of Saturn’s rings, storms And maybe it contains some cosmos we call home. on the planet, lakes on Titan, sort of microbial life. — David J. Eicher

WWW.ASTRONOMY.COM 7 STRANGEUNIVERSE BY BOB BERMAN Lessons learned Spectacular sky events stand out in years past — and in years to come.

he Great American Epsilon Aurigae (the nearest Eclipse is long over. little star to Capella) lost half But there’s always a its light. In those days, I was “next time” for super perilously riding my bike celestial events — through Asia. At night, I’d Tand lessons to learn. stare at Epsilon. It was amazing I recall the big events that lay to see it so dim, a three-year- Comet Halley’s February 1986 apparition was disappointing, as it lacked a long, trailing ahead when I completed school long event that occurs every 27 tail by the time the comet reached perihelion. NASA in the ’60s. I vividly remember years. Researchers at the time the sky spectacles that beck- announced they’d finally found but this time I bought lots of 14s prominences. I gasped. It deliv- oned, dreamlike, in the distant the source of the dimming: a for the group because I thought ered the most exquisite eclipse future. I ached to see them. huge orbiting cloud of particles, the high position of the Sun and views of my entire life as a dog Comet Halley’s return in 1986. each about the size of a piece of the resulting increased bright- — not a sundog, but a dog that The longest upcoming totality gravel. Weird! ness called for a darker filter. can’t help chasing these things in 1991. The expected Leonid A few eclipse chasers make But as a test, when guests like a fire engine. So now I’ll meteor storm in 1999. The their totalities a matter of stepped off their coach we asked bark even louder about the transits of Venus in 2004 and pride. They add up their cumu- each to look at the Sun using a value of good binoculars as a 2012. The mysterious eclipse of lative shadow time. They 12 and then a 14, and decide vital totality tool. Epsilon Aurigae in 2009. The scored big during the 1991 which they preferred. Both are My new top five “wonders to Great American Eclipse in 2017. totality, when they picked up equally safe. I’m glad I’d brought come” for the next two decades? I’ve not been entirely lucky. 6½ minutes in one shot. We can along a bunch of 12s — without Some are very unusual. First When I took a group to the hope that science won’t some- exception, everyone preferred the winter solstice hypercon- equator to optimally see Halley, day learn that the Moon’s the brighter image through the junction of 2020, when Saturn its tail fell off. Earth and the shadow gives you Alzheimer’s 12 to the fainter Sun through will appear as close to Jupiter as famous comet were on opposite or something. the 14. A few said that having a some of its moons! Then the sides of the Sun during its 2024 four-minute U.S. totality, February 1986 perihelion, and followed by the longest total the comet’s tail disappeared just It keeps going. Our lives, marked eclipse until the 22nd century, when it was supposed to be by dramatic events in the sky. under virtually guaranteed best. It was the worst Halley clear skies on the Nile River in apparition since the days of the 2027. Then the Friday the 13th Roman Empire. We’ll do much Farfetched? Well, in India 12 was very important to them. Apophis visit — that’s April better in 2061. they believe eclipses are So for the next two totalities 2029, when the 3rd-magnitude, The 1991 totality was won- “unhealthy,” and most still hide (both in southern South Empire State Building-sized derful, and yet, looking back, from them. I saw this firsthand America), with the Sun fairly asteroid will come just a tenth the unusual disparity between in 1980, as citizens huddled low both times, we’re definitely of the Moon’s distance and the lunar and solar disk sizes behind shuttered windows. sticking with 12s. glide across the sky, visible to made prominences harder to As for last August’s My other lesson involved the naked eye. Then in 2036, see. As for the Leonids, well, a American totality, I loved binoculars. My long-held advice Epsilon Aurigae’s next weird storm like we had in 1966 and watching as our guests were has always been to mostly eclipse occurs as it is covered by 1833 did not materialize. No swept away by the magic, and watch totality naked eye and that gravel cloud, or whatever one saw 80 meteors per second in the process I learned two use binoculars as an adjunct. its companion may be. this time around. But we still new things. But this time around, using the It keeps going. Our lives, got a heck of a nice display in First, I noticed that some best of my three image-stabi- marked by dramatic events in the wee hours of November 18, respectable news sources like lized Canons (the obscenely the sky. 2001: five brilliant green mete- The New York Times wrongly expensive 10x42 L), I have to say ors per minute with trains. urged observers to only use that the rock steadiness and Join me and Pulse of the Planet’s The first Venus transit was shade 14 welder’s goggles. I’ve clarity made the pink nuclear Jim Metzner in my new podcast, Astounding Universe, at yummy, though clouds blocked preferred shade 12 during all flames so “present,” I felt I was http://astoundinguniverse.com. the second. And in 2009, my totalities going back to 1970, closely hovering above the

BROWSE THE “STRANGE UNIVERSE” ARCHIVE AT www.Astronomy.com/Berman.

8 ASTRONOMY • MARCH 2018 ASTROLETTERS

CCD challenge accepted I often look for challenges when it comes to CCD imaging. One of the main places I go to is my monthly Astronomy magazine. In the November issue, I read “Fall into autumn galaxies” by Stephen James O’Meara, which covered my favorite sub- ject: galaxies. The 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 cam- era, and this is the result. Thank you, Mr. O’Meara. — Eugene Faulkner, Whiting, NJ

Latin pronunciations

In the October issue, there was a fas- FAULKNER EUGENE cinating article about pronunciation errors in and by Bob Sky Puppies to wonder at life and its mysteries. I have Berman. It was a very interesting read, I enjoyed Tom Trusock’s article in the trained my mind to open new doors as and I too can be very picky about correct December issue entitled “How to get kids’ they are discovered but do not close doors pronunciations. However, Mr. Berman heads in the stars.” I would add one more behind me. Intelligent design is one of made an error when discussing the Latin item to all that Trusock suggested. these doors, and I often revisit it. genitive. The Astronomical League offers a mul- If you consider yourself a scientist, you I’ve studied Latin for several years now titude of observing programs. One of will undoubtedly become a wrestler in the and intend to make it a minor in college. these programs, Sky Puppies, is designed match between science and spiritual While “ae” is the ending for the Latin specifically to help get kids’ heads in the beliefs. This event is held regularly, and genitive singular in the first declension stars. Like its other observing programs, the outcomes are always different for each (also used for the dative singular and the League offers a certificate and a pin to person. I would call your attention to two nominative plural), it is not pronounced anyone who successfully completes the men who went through this experience “EE.” Latin in the church, such as at a program. Unlike other observing pro- and left behind very clear and compelling Latin Mass, would pronounce the “ae” grams, membership in the League is NOT descriptions of their ideas. Jacob ending as “AY” like in “May” or “weigh.” required of the Sky Puppies program. Bronowski’s work, The Ascent of Man, However, I’m sure we’re all interested in There is an age limit in this program. and the essays of Allan Sandage on reli- using the classical Latin pronunciation, One must be 10 years old or younger. gion and science will give you an excel- which would be “EYE” as in “eyeball” or Details on the program can be found at lent background if and when you start “light.” So, Ursae Majoris would be the Astronomical League website, or by your wrestling match. They were, by all UR-sye mah-JOR-is, not UR-see. going to www.astroleague.org/al/ standards, great scientists who left com- However, I do still thank Mr. Berman obsclubs/skypuppy/skypuppy2.htm. pelling legacies for all of us. I would hope for reigniting my interest in this topic. — W. Maynard Pittendreigh, Orlando, FL that as you proceed with your mental (Unfortunately, I will probably continue exercises, you keep the attitude of “we do to pronounce Uranus as yor-AY-nis, but not know.” that is really just for the joke material.) Two sides of the same coin As for my personal beliefs, I view sci- — Maggie Bradley, Asheville, NC Bob Berman’s editorial in the December ence and religious beliefs as two sides of 2017 issue of Astronomy — “Intelligent the same coin. In order to spend the coin, design?” — is among the best and clear- both sides need to be present. In order to Praise for balance est I have read on the subject of scientific find truth, you must be able to see both Bob Berman’s column has always been truth and spiritual beliefs. Science has sides. — Donald Craig, Jr., Indianapolis my favorite in your magazine. I can long accepted the dual nature of man, hardly see how this will ever change after and even as we enter the age of scientific We welcome your comments at reading his December column entitled enlightenment, the more we explain, the Astronomy Letters, P. O. Box 1612, “Intelligent design?” I’ve never before more we find unexplainable. Waukesha, WI 53187; or email to letters@ seen a better, more even-handed and bal- I am a creature of this planet, and have astronomy.com. Please include your anced treatment of this subject that you been around for 74 years. I have been for- name, city, state, and country. Letters could fit onto a single page. Well said. tunate to have done many things and may be edited for space and clarity. — Richard S. Wright Jr., Lake Mary, FL experienced a great deal, and I continue

WWW.ASTRONOMY.COM 9 IT ANTI-MATTERS. Researchers at JILA in Colorado are exploring whether the electron is more egg-shaped than round, ASTRONEWS which could explain why there’s more matter than in the universe. OUR SOLAR SYSTEM RECEIVES AN INTERSTELLAR VISITOR

JUST PASSING THROUGH. 1I/2017 U1 (’Oumuamua), a visitor from another solar system, swung past our Sun in September 2017. Aside from its strange cigar-shaped appearance, the asteroid bore striking similarities to those found in our inner solar system. EUROPEAN SOUTHERN OBSERVATORY/M. KORNMESSER

n October 19, University of Hawaii metal nearly a quarter-mile (400 m) long Asteroids require careful observation and astronomer Rob Weryk noticed an and 10 times longer than it is wide. It came sensitive instruments to detect, especially if unusual 20th-magnitude streak in from the region of sky now occupied by they’re moving fast. But surveys like Pan- Oimages taken with the 1.8-meter Pan- Vega, but that star was in a different posi- STARRS and those planned with the STARRS 1 telescope. After spotting the tion 300,000 years ago when ’Oumuamua upcoming Large Synoptic Survey Telescope same object in images taken the previous would have passed through the region. should be able to detect other objects like it. night, he contacted European Space Agency While its shape is unusual, ’Oumuamua’s Such interstellar objects have been astronomer Marco Micheli. Sure enough, color and rotation rate are similar to prop- roaming the frigid depths of space for eons, his telescope in the Canary Islands had erties seen in asteroids in our inner solar with few evolutionary forces — such as caught it, too. system. “The most remarkable thing about heating or collisions — acting upon them, “Its motion could not be explained using U1 is that, except for its shape, how familiar essentially stored in deep freeze. “They are either a normal solar system asteroid or and physically unremarkable it is,” said well-preserved examples of things made in comet orbit,” Weryk said. “When both our Jayadev Rajagopal of the National Optical other star systems,” planetary scientist and datasets were fit together, it became clear Astronomy Observatory. This has led New Horizons principal investigator Alan that the only explanation was a hyperbolic astronomers to liken it to comets and aster- Stern told Astronomy. “For that reason, trajectory.” The object was an “interstellar oids believed to have been ejected from our they’re going to be valuable as the popula- interloper,” an asteroid from outside our solar system during its formation, and is tion is studied.” solar system, swinging around the Sun only likely the story behind ’Oumuamua’s fate. With these interstellar objects, astrono- once before racing away, never to return. While ’Oumuamua is the first discovered mers can learn more about other solar sys- The object is now designated 1I/2017 U1 interstellar asteroid, it’s likely not the first tems — and our own. — Robert Naeye, (’Oumuamua), a Hawaiian name loosely to have passed through our solar system. John Wenz, Alison Klesman translating to mean the first “scout” or “messenger,” signifying its discovery as the first identified traveler from one solar sys- ‘Oumuamua’s path around the Sun tem to another. It entered the solar system at 57,000 mph (25.5 kilometers per second); as it passed through, astronomers raced to observe it. “We had to act quickly,” Olivier Mercury Venus Hainaut of the European Southern Sun Observatory in Garching, Germany, said in a press release. “ ’Oumuamua had already passed its closest point to the Sun [in September 2017] and was heading back into Earth interstellar space.” Mars’ orbit ’Oumuamua was visible only for 10 days 1I/2017 U1 (’Oumuamua) and never reached a magnitude above 19.7. Observations with the European Southern QUICK TRIP. ’Oumuamua’s trajectory brought it within 23,400,000 miles (37,600,000 km) of the Sun; it passed Observatory’s Very Large Telescope revealed closest to Earth October 14, just days before its discovery. Its path will now take it out of the solar system, never an oblong, reddish, solid chunk of rock or to return. This figure shows the planets and ‘Oumuamua on October 25, 2017. ASTRONOMY: ROEN KELLY, AFTER NASA/JPL-CALTECH

10 ASTRONOMY • MARCH 2018 NIGHT SKY THREAT. The adoption of LED lighting led to a 2 percent per year increase in light pollution ASTRONEWS between 2012 and 2016, according to research published November 22 in Science Advances. Early results QUICK TAKES LIGHTNING STRIKES of NASA’s Lightning on Earth can generate gamma rays that produce antimatter particles Twins Study called positrons and kick off matter-antimatter annihilation. As part of NASA’s Twins Study, astro- naut Scott Kelly spent a year in zero BRIEFER• HISTORY gravity on the International Space The University of Cambridge Station. In the meantime, his identical made Stephen Hawking’s twin brother, former astronaut Mark Ph.D. thesis publicly available Kelly, went about his daily life on Earth. as part of Open Access Week When Scott returned, he was tempo- in October. rarily two inches taller, but his height wasn’t the only thing that changed. ANCIENT• ECLIPSE According to preliminary results Historical accounts of a solar from the study, Scott’s year in space eclipse in 1207 B.C., mentioned also drastically increased his rate of in the Bible and ancient DNA methylation, the process respon- Egyptian texts, may help sible for turning genes on and off. By better pinpoint the reign of regulating gene expression, DNA SIBLING RIVALRY. Former astronaut Mark Kelly (left) poses with his identical Ramses the Great. 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 APOLLO ASTRONAUT believed to play a major role in the the health effects of long-term spaceflight. NASA PASSES AWAY progression of many diseases, ranging Gemini XI and Apollo 12 from cancer to cardiovascular disease. Over the last year, NASA has telomeres are associated with fewer astronaut Richard “Dick” “With this study, we’ve seen thou- released a number of fascinating pre- age-related problems. Gordon passed away sands and thousands of genes liminary results from some of the 10 “This study represents one of the November 6 at the age of 88. 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 ANCIENT• SUNS pal investigator of the Twins Study. were surprised to find that Scott’s the bedrock for understanding New analyses of star “This happens as soon as an astro- telomeres — the protective caps that molecular risks for space travel as movements identified 29 naut gets into space, and some of the shield the ends of DNA strands — well as ways to potentially protect nearby suns as belonging to activity persists temporarily upon were longer than Mark’s. Previous and fix those genetic changes.” some of the first groups of return to Earth.” research has shown that longer — Jake Parks stars formed in the Milky Way. TWINKLE,• TWINKLE, LITTLE PLANET A “twinkle” every 18 months in WHERE DOES WATER FREEZE IN THE SOLAR SYSTEM? the young star EC53 suggests FREEZING TEMPERATURES. The that a newly minted planet snow line, also called the frost line or ice has formed around it. line, is the distance from a star at which the temperature drops enough that a DRONES FOR• MARS given molecule — such as water — will NASA, the SETI Institute, the freeze into a solid. The simplest models of Jupiter Mars Institute, and sensor 5.2 AU planet formation state that objects (such manufacturer Fybr are as planets) that form inside the snow line collaborating on drones for are smaller and composed of mostly rock; use in Mars’ thin atmosphere. those that form outside the snow line are more massive and include a higher per- OLD BATTLE• Ceres centage of ices. Cornell University astronomers 2.8 AU The location of the snow line depends found two massive galaxies Water on the properties of the star itself, as well merging 13 billion light-years snow line as the molecule in question, as some 3 AU away, only 500 million years molecules freeze at higher or lower tem- after the universe formed. Mars peratures than others. In our forming 1.5 AU solar system, the water snow line occurred • at about 3 astronomical units (AU; 1 AU RE-FUNDED equals 93 million miles [150 million kilome- The fifth generation of the ters]), or three times the distance between Earth and the Sun. This lies between the gained funding from the Alfred orbits of Mars and Jupiter, separating the P. Sloan Foundation, allowing it Sun Earth terrestrial and giant planets in our solar to begin continuous 1 AU Asteroid belt system. However, other young stars have observations in 2020. : ROEN KELLY ROEN : 2.2 AU 3.2 AU been observed with water snow lines tens • of AU in distance. This indicates the loca- WE ARE STARDUST tion of the snow line may evolve over time, Space dust colliding with ASTRONOMY while giant planets close to their stars organic molecules in our (inside the snow line) likely migrated there upper atmosphere could fling Although 71 percent of Earth is covered by oceans, FAST after forming farther out. — A.K. microbes off our planet — or water represents only about 0.02 percent of the planet’s mass. bring life from other planets or FACT moons to us. — J.W.

WWW.ASTRONOMY.COM 11 CALM SUN. Astronomers announced Ross 128b, a potentially habitable planet in the Ross 128 system, ASTROASTRONEWSNEWS November 15. Ross 128 is a calmer M dwarf, reducing the likelihood that it has blown away Ross 128b’s atmosphere. THE WORLD’S Positron excess may have dark matter origins 10 LARGEST REFRACTORS OBSERVATORY DIAMETER Yerkes Observatory, Williams Bay, Wisconsin 40 inches Full Moon (to scale) Roque de los Muchachos Observatory, Canary Islands 38.6 inches Lick Observatory, Mt. Hamilton, California 36 inches Geminga Paris Observatory, France 32.7 inches Leibniz Institute for Astrophysics, Potsdam, 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 inches Vienna Observatory, Austria 27 inches

PSR B0656+14 40 Yerkes Obser vator y, UNDERPOWERED. Extended gamma ray emission from the Geminga and PSR B0656+14 (yellow Williams Bay, Wisconsin 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)

The European PAMELA satellite first registered “The gamma rays that we measure [with an unexpectedly high number of positrons HAWC] are a tracer for the electrons and posi- : ROEN KELLY ROEN : in near-Earth space in 2008. Since then, two trons near the [] source. Using this, we competing theories to explain the anomaly can map out how fast the electrons and posi-

ASTRONOMY have developed. trons are moving away from the source. The simple explanation is that the extra Knowing the age and the distance of the pul- positrons — the antimatter counterparts to sars, we can figure out if [the positrons] can electrons that have a positive charge — are get here,” says the HAWC principal investiga- coming from nearby stellar remnants called tor and U.S. spokesperson, Jordan Goodman pulsars. The more exotic theory is that heavy of the University of Maryland. dark matter particles in our galaxy are mutu- The team concluded that the pulsars are ally annihilating one another whenever they not producing anywhere near enough posi- come into close proximity, self-destructing trons to explain the observed excess. 27 into a cascade of positrons and other particles. Furthermore, because these two pulsars are Vienna Observatory, Although Occam’s razor suggests the first among the closest to Earth, it seems clear Austria explanation is more likely, a recent paper that pulsars in general cannot account for published by an international team of scien- the anomaly. tists November 17 in Science all but excludes So if pulsars can’t explain the positron it. And if nearby pulsars are not responsible excess, what can? Some theorists have pro- for the excess of positrons, scientists are left posed remnants and to entertain the second option. jets as the culprits. HAWC has also detected The study uses observations taken with these objects, but “most are too far away and 4 the High-Altitude Water Cherenkov (HAWC) too young to send particles all the way to Observatory. HAWC consists of 300 large Earth,” Goodman explains. water tanks in the Mexican state of Puebla to This leaves dark matter annihilation as the detect high-energy gamma rays, the highest- most likely explanation. This theory has been Backyard telescope energy form of light. When a gamma ray on the books for many years, and it’s not con- strikes an atom in Earth’s upper atmosphere, tradicted by any astronomical observations. If it creates a shower of secondary particles that annihilating dark matter is indeed responsi- FIRST GLASS. A refractor telescope uses a lens to rain downward, triggering detections in ble for the positron excess, the particles collect and focus light. Because lenses are glass and glass HAWC’s water tanks. themselves would have whopping masses of can behave like a fluid, any lens larger than 40 inches From November 2014 to June 2016, HAWC about a thousand protons — about the mass in diameter will sag under its own weight, rendering it clearly detected high-energy gamma rays of four uranium atoms. unusable. Also, casting and figuring such behemoths coming from an extended region around two Experiments at the Large Hadron Collider are time-consuming and expensive processes. Because relatively nearby pulsars in the in Switzerland and in underground laborato- huge refractors are no longer practical, this list probably Gemini the Twins. The pulsars, known as ries around the world have yet to turn up is the final one. — Michael E. Bakich Geminga and PSR B0656+14, are roughly 800 direct evidence for dark matter particles. So and 900 light-years away, respectively. As although HAWC seems to have ruled out pul- FAST each pulsar spins, it throws off positrons and sars as the source of the excess positrons, The 40-inch Yerkes refractor FACT electrons, which, in turn, interact with nearby their origin remains a mystery, as does the collects 2.2 times as much particles to produce gamma rays. nature of dark matter. — J.P., R.N. light as the 27-inch Vienna refractor and 100 times as much as a 4-inch scope. 12 OVER THE MOON. The Indian Space Research Organisation has nearly ASTRONEWS 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 suggestive of seeping water. to the slopes of aeolian sand dry today, planetary scientists But new evidence from a dunes formed by wind found believe it could have been study conducted by the U.S. elsewhere on Mars. warmer and wetter in the past, Geological Survey (USGS), the If RSL were due to water, and that a significant volume University of Arizona, the more liquid would be neces- of water may still exist below Planetary Science Institute, and sary to form longer features. the surface. Durham University paints a dif- But the end result of dry flows In particular, dark streaks on ferent picture: RSL are more should look similar regardless the planet’s surface called likely granular flows of dry of length, which was exactly recurring slope lineae (RSL) material that don’t require the finding from the study. have been cited as evidence of water to form. “This new understanding of subsurface water flows. These Their findings, published RSL supports other evidence features appear and grow over November 20 in Nature that shows that Mars today is time during the warmest part of Geoscience, are based on a very dry,” said the lead author, the Red Planet’s summer, then study of 151 RSL and state that Colin Dundas of the USGS, in a fade away again — behavior the ends of RSL look identical press release. — A.K.

The timeframe within which astronomers expect 10 YEARS to detect gravitational waves from a merger of supermassive black holes. Comets swarm a distant star A study published October 31 in Monthly Notices of the Royal Astronomical Society out- lines a big first: the discovery of the first transit- ing exocomet. Working with Kepler data as part of the Planet Hunters project, citizen scientist Thomas MANY TAILS. A group of comets circles a distant Jacobs spotted the telltale signs of a comet’s star, leaving behind trails of debris. Citizen scientist tail in the light of the star KIC 3542116, a dim Thomas Jacobs discovered such a trail around a star M dwarf. He noticed a 0.1 percent dip in the observed with the Kepler telescope. DANIELLE FUTSELAAR star’s light occur three times. An MIT team helped confirm that the dip represents a com- discoverers to believe that the system could be et’s trail of debris or tail, which spreads over a in the middle of a “bombardment era” where much wider area than the nucleus. large planets fling smaller objects like comets The team and other collaborators spent and asteroids into the inner planetary system. months trying to figure out what the object Such events could have seeded life on Earth, or was. It resembled a disintegrating planet, but it even caused extinction-level events in the past. lacked the same periodicity expected from KIC 3542116 may be the smallest star an exo- transiting planet debris, which should be short comet has ever been discovered circling. Most based on the type Kepler usually spots. exocomets, usually discovered through spec- After they refined the data further, six total troscopy, have been around A-type stars, which comets were found in the system, leading are larger and more massive than the Sun. — J.W.

WWW.ASTRONOMY.COM 13 DENSE JETS. Magnetic fields are thought to prevent neutron stars from forming jets, but the belief is ASTRONEWS being challenged by University of Amsterdam astronomers after analyzing data from 27 telescopes. Earth stops energetic neutrinos BRIEFCASE DEEP DOWN. PROBING A PYRAMID To detect Physicists used a technique called muonography to neutrinos discover a large chamber in the Great Pyramid of Giza. zipping by, the The authors of the November 2 Nature paper revealed IceCube Neutrino the void by tracking subatomic particles called muons, Observatory which are produced when cosmic rays strike atoms in lowers “strings” the upper atmosphere, creating a cascade of secondary equipped with particles that rain down to Earth. Since muons are neutrino detectors partially absorbed by stone, the researchers could into deep holes in “image” the hidden chamber by measuring where the the Antarctic ice. flux of muons was higher than expected. Although the These detectors purpose of the void remains unknown, Egyptologists have confirmed hope the discovery will provide insights into how the that energetic 4,500-year-old pyramid was built. neutrinos are • more likely than STEADY AS SHE GOES their lower-energy Using over 60 years of observations, astronomers have counterparts discovered that microwaves from the Sun during the to interact past five solar minimums have been surprisingly with Earth. constant, whereas microwaves from the past five NSF/B. GUDBJARTSSON maximums have drastically varied. “It is very meaningful to discover a trend extending beyond a Neutrinos are chargeless, nearly massless passes closer to another particle than the single solar cycle,” said Masumi Shimojo, lead author of particles created in some of the most extreme extent of this cross section, the two will inter- the study published October 10 in The Astrophysical events imaginable: exploding stars and the act. But because a neutrino’s cross section is Journal. “This is an important step in understanding the mergers of black holes and neutron stars. small, collisions with other particles don’t occur creation and amplification of solar magnetic fields, They are small and fast, interacting with very often. which generate sunspots and other solar activity.” little in the universe — they typically zip right By measuring the rate of neutrino interac- • through most matter, including humans, tions and the types of neutrinos that do inter- MERGER MADNESS Earth, and even the instruments used specifi- act with Earth, IceCube has confirmed that Scientists confirmed yet another cally to detect them. the size of a neutrino’s cross section is deter- detection November 15, with the discovery of But while interactions are rare, they do mined by its energy, with higher-energy par- GW170608. The merging black holes that produced it occur. Now, an international team of research- ticles generating a bigger cross section. Such were about a billion light-years away and roughly seven ers associated with the IceCube Neutrino high-energy neutrinos are more likely to be and 12 times as massive as the Sun, making this the Observatory at the South Pole has brought us stopped by interactions with Earth — a pre- smallest merger observed by Advanced LIGO so far. one step closer to understanding when those diction made by our current Standard Model During the merger, energy equivalent to about 1 solar collisions are most likely to happen, confirm- of physics. mass was released in the form of gravitational waves, ing current particle physics theories and This work represents the first study of leaving behind a lone black hole nearly 18 times as shedding light on how to study neutrinos fur- high-energy neutrinos with energies 1,000 massive as the Sun. — J.P. ther. The work, published November 22 in times greater than previously measured. It is a Nature, hinges on measurements taken of the first step toward a greater understanding of number of neutrinos that do interact with not only neutrinos, but the field of particle Earth instead of passing right through. physics. “IceCube was built to both explore Each neutrino has a cross section for colli- the frontiers of physics and, in doing so, pos- sion. The cross section represents the probabil- sibly challenge existing perceptions of the ity that a particle will collide with another; if nature of universe. This new finding and oth- you picture a particle as a ball, its diameter ers yet to come are in that spirit of scientific would be analogous to its cross section. discovery,” said James Whitmore, a program Particles with larger cross sections are more director with the National Science likely to collide with others. When a neutrino Foundation’s physics division. — A.K.

MERCURY IN THE EVENING NASA ESA/HUBBLE,

15° Secrets of a cosmic snake 15 EARLY DEVELOPMENT. The Cosmic Snake is a 10 20 gravitationally lensed galaxy sitting behind the massive 10° 7 July 2 12 27 galaxy cluster MACSJ1206.2–0847. As light from this March 5 22 distant, young galaxy passes near the cluster, it is bent

Altitude 5° 6 Nov. 25 17 KELLY ROEN : by the cluster’s mass and re-formed into a squiggly, 1 snakelike image. Though warped, the image is also 11 27 22 June 17 16 Oct. 22 magnified, making the galaxy appear much brighter 0° ASTRONOMY West and its minute details easier to discern. By studying Azimuth the areas of star formation visible in this image and others like it, astronomers can piece together how stars WORLD AT DUSK. The innermost planet has a reputation for formed in very early galaxies. Studies of less-detailed being elusive because it rarely appears outside of twilight from Mercury’s best evening FAST images suggested that early galaxies host areas of star mid-northern latitudes. This chart plots Mercury’s position 45 appearance of the year FACT formation much more massive than those seen today. minutes after sunset for an observer at 35° north latitude for the comes in mid-March, But recent work studying the Cosmic Snake finds that planet’s three evening elongations of 2018. Notice that Mercury’s when it reaches a peak the galaxy’s massive star-forming regions are actually peak altitude doesn’t necessarily coincide with its greatest solar altitude of 8.3°. made up of smaller, distinct clumps of star formation, elongation (dates highlighted in yellow). — Richard Talcott more like those found in nearby galaxies. — A.K.

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WWW.ASTRONOMY.COM 15 SECRETSKY BY STEPHEN JAMES O’MEARA What’s “overhead”? Looking straight up may be harder than you think.

ecently, I went out- winter Moon. In reality, the side for a walk and Sun never passes overhead from casually noticed that these latitudes but 20° to 30° the Sun was about from it (depending on the lati- one-third of the way tude from which we observe). Rup the eastern sky. Or was it? Out of curiosity, I noted the Part pain time and later used software in the neck to determine that the Sun was Typically, when looking actually only 17° high at that straight ahead, the eye can time. How could I have been so see objects about 60° above its far off? center point. So a Sun 60° high will appear at the summit of Part psychology, the eye’s field of view. Tilting part physiology? the head back to see a 70°-high In Light & Color in the Open Sun or Moon only seems to Air, Marcel Minnaert helps us affirm that it is overhead — a to understand why it is com- location most observers will mon for us to overestimate the admit can literally be a “pain heights of objects in the sky. in the neck,” so little bother It stems from a psychological is given to the accuracy of the effect in which we naturally observation. “Overhead,” in the Determining a specific star’s altitude, or even finding the overhead point, is not as easy perceive the hemisphere of sky most casual sense, means an as you might think. A star-filled sky helps. STEPHEN JAMES O’MEARA as a flattened vault that appears object within 30° of the zenith. about twice as close overhead which speaks to Minnaert’s to within a couple of degrees of as it does to the horizon. If you Of voids and warning of having only unprej- true overhead. try to estimate the midpoint optic moths udiced observers make the The reason? I believe that between these two points, the Like Minnaert, I also found initial observations. our eyes are attracted like result will not lie at a height of that casually estimating the For instance, when I made moths to the brightest object 45° but generally somewhere midpoint of the sky during my first observation of a wan- near our invisible target desti- between 20° and 30°. the day was difficult. It was ing daytime Moon midway up nations (midpoint or zenith), “It is very important that easier on a starlit night for the the sky from the horizon, I was drawing our attention away. unprejudiced observers be simple reason that it’s hard to off by 15°. Later, I refined the When only faint stars are vis- found” who must divide “not be accurate in a blank sea of observation to within 3°, and ible, we spend more time scru- the angle but the arc” into two sky, but easier when it’s full finally to the correct value by tinizing the sky, which equal parts, Minnaert says. of stars serving as guideposts. taking the average of three sharpens our accuracy. Bright The illusion intensifies Accuracy in the perceived observations over an hour. stars may serve well as “imme- under cloud cover and lessens midpoint increases if the Sun At night, I also began by diate” signposts, but they actu- on clear and crisp starlit nights. or Moon is near it — espe- misjudging a star’s altitude but ally lead us astray. Generalities also come into cially if you take the average was able to refine it to an As always send your play. Some of us use the word of three observations (similar acceptable value. I also found thoughts and comments to overhead loosely. This is espe- to what visual Jupiter observ- that I misplaced “overhead” [email protected]. cially true for those living at ers do when making transit when a bright star was within mid-northern latitudes, when, estimates). The accuracy also 20° of the zenith. One night, Stephen James O’Meara say, we see the noonday Sun increases dramatically the however, when no bright star is a globe-trotting observer sailing “high overhead” in more you make these observa- was available near that point, I who is always looking for the next great celestial event. summer, or a “midnight” tions over the course of days, accurately selected a faint star

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16 ASTRONOMY • MARCH 2018 GRACEFUL EXIT. NASA ended the Gravity Recovery and Climate Experiment after 15 years studying gravitational anomalies around Earth. 8BUFSUPO(MBDJFS ASTRONEWS *OUFSOBUJPOBM%BSL4LZ1BSL This star exploded not once, but twice 8&/&&%:06 For the first time, astronomers have discovered a star that has 4VNNFS"TUSPOPNZ gone supernova more than once. This so-called “zombie &EVDBUJPO7PMVOUFFST star” — which reportedly exploded at least twice in the past 60 years — has baffled sci- entists by challenging many of the existing theories about how *OUFSFTUFE  massive stars end their lives. “This supernova breaks ZZZQSVJRYJODFDVWUR9,3KWP everything we thought we knew about how they work,” said Iair Arcavi, lead author of a

letter published November 9 in VIÑAS NASA/MARIA-JOSE Nature, in a press release. “It’s RINSE AND REPEAT. In this artist’s concept of a supernova, an the biggest puzzle I’ve encoun- expanding shell of dust and gas is blown outward from the star’s dense, tered in almost a decade of core. Most stars explode just once, ending their lives for good. studying stellar explosions.” However, iPTF14hls appears to have gone supernova twice in just 60 years. The undying star, named iPTF14hls, was first discovered to search through archival hot that it generates antimatter in September 2014 by research- data. They were flabbergasted in its core, causing it to become ers using the Palomar Transient to find that in 1954, another violently unstable and undergo Factory (PTF). Although the explosion was recorded in the multiple bright eruptions. supernova initially faded after exact same location. Somehow, “These explosions were only its 2014 explosion, within a few this star survived its first explo- expected to be seen in the months it began to mysteri- sion, waited 60 years, and early universe and should be ously grow brighter again. exploded again. extinct today,” said co-author Over a subsequent three-year Although researchers are Andy Howell, leader of the Las span, iPTF14hls fluctuated still uncertain about what Cumbres Observatory super- between bright and dim at caused iPTF14hls to go super- group. “This is like finding least five separate times. nova twice, one theory is that a dinosaur still alive today. If When the astronomers real- the “” is actually a you found one, you would ized iPTF14hls was not an aver- pulsation pair-instability super- question whether it truly was a age supernova, they decided nova — a star so massive and dinosaur.” — J.P.

The number of cameras the Mars 2020 23 rover will carry to the Red Planet. ESA//MPS FOR OSIRIS TEAM MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA TEAM OSIRIS FOR ESA/ROSETTA/MPS Rosetta spots a cometary dust jet NICE SHOT. On July 3, 2016, the Rosetta spacecraft caught an eruption of material from Comet 67P/Churyumov- Gerasimenko. 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 sunlight sublimating surface ice into water vapor. — J.W.

WWW.ASTRONOMY.COM 17 FORYOURCONSIDERATION BY JEFF HESTER The mind’s siren call Our craving for certainty can make us blind to true knowledge.

e all know am obliged to keep it, at least For our the feeling. for now. But if the idea can’t evolutionary ancestors like You’re sitting take that heat, out the window Australopithecus there try- it goes. afarensis, quick ing to figure Certainty pulls the rug out thinking and the feeling Wsomething out, but it just won’t from under the whole notion of of knowing come together. Frustration and justified knowledge. Logically, often made annoyance grow until suddenly once we reject the possibility the difference you get it — or at least you that we are wrong, our sup- between life and death. JOHN think you do. “Aha!” The relief posed knowledge becomes GURCHE/SMITHSONIAN comes flooding in. nothing but illusion. INSTITUTION Neurologist Robert Burton Of course, none of this talks about that feeling in his changes the fact that certainty book On Being Certain: feels really good. So like any Believing You Are Right Even addict, our natural tendency is When You’re Not. to do the worst possible thing justified knowledge, the feeling until it’s put through the Physiologically speaking, our — we try to score. We seek out of knowing can lead us down a wringer. By the way, if you brains crave certainty in the information and people that primrose path. But when rec- don’t look for the flaws in your same way a junkie craves a fix. reinforce our certainty, always ognized for what it is, that sen- pretty idea, rest assured that Satisfying those cravings acti- craving the next hit of dopa- sation can be a valuable guide. someone else will do it for you! vates the neural pathway mine while turning our backs In his book Blink: The Power of Knowledge is a slippery jux- responsible for pleasure and on anything or anyone that Thinking Without Thinking, taposition of philosophical motivation. An aha moment might call our certainty into Malcolm Gladwell discusses considerations and ages-old feels good because it releases a question. Here lies a road paved the way our brains rapidly and neurological imperatives bur- lovely hit of dopamine in the with confirmation bias, group- subconsciously combine even ied deep within our brains. brain. think, and a menagerie of other small amounts of information With that comes a practical It’s not hard to understand cognitive errors. with our previous knowledge challenge with profound real- where our addiction to cer- Once we embrace without and experience to reach tenta- world consequences for each of tainty comes from. For our question that deep, heartfelt, tive conclusions. Those us. In a complex world where evolutionary ancestors living compelling sensation of cer- thoughts, accompanied by a knowledge matters, how do we on the savanna, often the worst tainty that we so desperately feeling of knowing, enter our navigate treacherous waters possible strategy was to do crave, we become blind to real- conscious minds as intuition. filled with comfortable, spe- nothing. The feeling of know- ity. We build ourselves a house Intuition alone is never a sub- cious ideas eager to abduct our ing frees us from paralyzing of cards, believing the whole stitute for justified knowledge. all-too-willing brains? indecision. It enables us to act. time that it is made of brick. But if grounded in justified After decades in the But feeling certain has squat Knowing something (experi- knowledge and enough rel- trenches as a scientist, I can to do with being right, Burton encing the sensation of know- evant experience, intuition can share what works for me. I lis- stresses. The feeling of know- ing) and really knowing suggest a path. ten to my intuition, but I’m ing is not even a cognitive pro- something (having reasonably Science is all about justified gun-shy. When I start feeling cess. Rather, certainty is a justified belief) are two com- knowledge, but intuition is too certain about something sensation that need not be pletely different things, even if vitally important even here. that’s my cue to get out the associated with any particular we call them by the same Scientists often rely on gut feel- sledgehammer and start thought at all, he explains. name. The irony is thick ings to decide what ideas might pounding on my precious idea In The Logic of Scientific enough to cut with a knife! Our be worth pursuing, or what to see if it breaks. Discovery, Karl Popper argues brains crave certainty, but if we approach might be likely to Only then can I talk about that the foundation of knowl- want real knowledge, certainty yield good results. The abso- what I know. edge is falsifiability. “I know” is the one thing that we can’t lutely crucial caveat is that means that I have worked to allow ourselves. while intuition might be a good Jeff Hester is a keynote speaker, discover that an idea is false, Which is not to say that the place to start, it is only a start. coach, and astrophysicist. but so far have failed. If an idea feeling of knowing is always a An idea might feel right, but Follow his thoughts at jeff-hester.com. can withstand that challenge, I bad thing! When mistaken for that doesn’t matter to scientists

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18 ASTRONOMY • MARCH 2018 ODD ONE OUT. NGTS-1b, a planet whose mass is similar to Jupiter’s, was found orbiting a red dwarf star supposedly ASTRONEWS too small to have formed a planet that big, challenging planet formation theories. The case of the shrinking white dwarf Astronomers have observed 20 years. Mereghetti also found many white dwarfs over the that the white dwarf’s original years. But a study published 13-second spin period — the November 13 in Monthly Notices time it takes to complete one of the Royal Astronomical Society full rotation — was decreasing presented the first observational by about 7 nanoseconds each evidence of a contracting white year. dwarf that has been steadily Although a few nanoseconds shrinking for the past 2 million per year may not seem like years. much, for an object as massive “For decades, it has been the- and compressed as a white oretically clear that young white dwarf, this corresponds to a sig- dwarfs are contracting,” Sergei nificant shift in angular momen- Popov, an astrophysicist and tum — something that could lead author of the study, said in not be accomplished solely a press release. “Yet that very through the accretion of matter. phase of contraction has never Instead, the researchers demon- been observed in ‘real time.’ ” strated that the white dwarf’s The contracting white dwarf faster spin could be easily is part of a binary system named explained if the star were con- HD 49798/RX J0648.0–4418, tracting, much like the way a located some 2,000 light-years spinning figure skater rotates away in the constellation Puppis. faster as she pulls in her arms. Astronomer Sandro Mereghetti, “Thanks to this discovery, co-author of the study, recently astrophysicists will be able to discovered that the white study and evaluate the evolution dwarf’s rotational velocity was patterns of young white dwarfs SPEED SPINNING. In this artist’s concept, the white dwarf (left) in the binary system the fastest ever observed for — and successfully look for simi- HD 49798/RX J0648.0–4418 is spinning while surrounded by an accretion disk of matter such a remnant, and has been lar systems in the galaxy,” Popov taken from its larger companion star (right). Astronomers think the white dwarf is speeding up over the past said. — J.P. spinning faster over time because it is contracting. FRANCESCO MEREGHETTI

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WWW.ASTRONOMY.COM 19 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 the evening of September 11, The onlookers could see the beautiful Cassini captured this panoramic view 2017, Griffith Observatory rings circling Saturn, the planet’s yellowish showing the planet’s ring system in exquisite detail. The imaging team hosted an enthusiastic group cloud bands, and the orange-tinged dot of created this mosaic from 165 separate of observers. The assembled the big moon near the planet; what they images taken over a three-hour period. crowd looked through the couldn’t make out was a much smaller, NASA/JPL-CALTECH/SSI 12-inch Zeiss refracting tele- human-made target. On that late summer scope, the centerpiece of evening, the Cassini spacecraft was just the venerable public astron- 75,000 miles (120,000 kilometers) from omy venue in Los Angeles. Titan on its final path toward Saturn. The They watched as light from Saturn and its spacecraft and Titan had enjoyed their largest moon, Titan, passed through the “goodbye kiss,” as the astronomers and telescope’s optics, where lenses bent and engineers on the mission called the last focused it onto their eyes. gravitational yank that would send the

20 ASTRONOMY • MARCH 2018 2004-2017

spacecraft into the planet it had been Cape Canaveral, Florida, nearly 20 years At 4:55 a.m. PDT, they saw the last signal studying for 13 years. earlier. But their thoughts were not all on from Cassini fade away on the screen. The These observers at Griffith were no the past: Cassini was still collecting data room erupted in applause — not for the end ordinary members of the public. They and sending it back to Earth. of the mission, but for what the spacecraft were members of Cassini’s Project Science On September 15, at 3:31 a.m. PDT, and those hundreds of people had achieved. Group, watching their beloved spacecraft Cassini entered Saturn’s upper atmosphere Cassini revealed surprise after surprise on its final journey around the giant world. at a shallow angle. It would travel through at Saturn: an incredibly complex system of “It was a magical evening,” says Cassini’s the gas for nearly 1½ hours. The team moons and moonlets, rings that change Project Scientist Linda Spilker. members were gathered at NASA’s Jet structure on hourly timescales, and a beau- Over the next few days, hundreds of Propulsion Laboratory in Pasadena, tiful atmosphere wracked by huge storms. scientists and engineers on the Cassini California, where they watched and waited. The 13 years of images and measurements mission team would reminisce about the “The room got quieter and quieter as we got changed humanity’s view of the ringed spacecraft, which had launched from down to those final minutes,” says Spilker. world. But there’s still more to learn from

WWW.ASTRONOMY.COM 21 vinyl record 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 scien- tists to think the rings were 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 Above: Cassini could probe Saturn’s ring Cassini’s Ultraviolet Imaging Spectrograph, structure by sending radio signals through who has studied Saturn’s rings for more the rings. In this simulated view of the A ring and the Cassini Division (at left center), red than four decades. “Structures develop denotes particles 2 inches (5 cm) or more in within hours in the rings.” diameter; green indicates particles less than Planetary scientists have identified sev- 2 inches across; and blue signifies particles eral different types of structures. Some, less than 0.4 inch (1 cm) across. NASA/JPL-CALTECH which come and go and come back again, Right: Vertical structures at the B ring’s are called kittens — “because they seem to outer edge cast long shadows onto the rings have multiple lives,” says Esposito. Others, two weeks before Saturn’s August 2009 equinox. The structures rise some 1.6 miles (2.5 km) called propellers, migrate slightly inward above the rest of the rings, which average or outward. They are consequences of about 33 feet (10 m) thick. NASA/JPL-CALTECH/SSI gravitational interactions between a small moon embedded within the rings and the ring particles themselves. The moon tries, unsuccessfully, to clear away the particles and create a gap. the last several months of data that Cassini Solstice extension, which ultimately bled Bigger moons tend to have more notice- collected. Scientists hope those final obser- into the Grand Finale. This final stage able effects. Prometheus, for example, vations will tell them about Saturn’s interior commenced in April 2017 and featured whose diameter averages 53 miles (86 km), — in particular, how it generates a mag- 22 close-in orbits that skimmed just above “dips in almost to the edge of the F ring and netic field and how its mass is distributed. Saturn’s cloud tops. pulls out streamers,” says Spilker. Several Because Cassini’s 12 instruments were other moons also leave their gravitational An extended stay attached directly to the spacecraft, the imprints. Scientists had long known that Spacecraft already had visited Saturn three entire contraption had to rotate for an Mimas creates the Cassini Division — like times before Cassini arrived in mid-2004, instrument to point toward a specific tar- the spacecraft, named for the 17th-century so scientists had some inkling of what they get. That meant multiple instruments Italian-French astronomer Giovanni might find. But as with any new mission couldn’t observe the same spot at the same Cassini — the broadest gap in the rings. But — especially one involving a machine with time. Instead, while one looked at a moon, it took data from the Cassini probe to reveal 12 sophisticated instruments that would another might observe Saturn’s rings. that seven midsized moons combine to remain in orbit instead of flying past as And that made the last five months of the keep the outer A ring from dispersing. its predecessors had — Cassini revealed a mission — the Grand Finale — a work The rings also contain density waves complex planet full of surprises. And that’s of impressive coordination. Although that show up as variations in brightness a good thing. “If Saturn had been exactly 22 orbits might sound like a lot, they and thickness. After studying these pat- as expected, it would have been a lot more aren’t much to work with when you have to terns, scientists, including the University of boring,” says Spilker. divide the limited time during those close Idaho’s Matt Hedman, showed that these Cassini arrived at Saturn for a primary flybys among the full instrument lineup. brightness changes are tied to Saturn’s inte- mission set to last four years. But when rior. The researchers used fine-scale den- mid-2008 came, the spacecraft continued Rings, rings, and more rings sity variations in the rings as a seismometer with its Equinox Mission extension. And Saturn’s rings are the planet’s defining of sorts to learn about how the planet’s in September 2010, the mission began its characteristic. From afar, they look like a interior oscillates, in much the same way

22 ASTRONOMY • MARCH 2018 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 rings, we could be missing things about, says Luciano Iess, who is leading Cassini’s brightness variations at the Sun’s surface say, how the solar system formed.” The pro- gravity data analysis. Disentangling the correspond to its inner pulsations. cesses going on in the rings could give two will not be easy, however. The prelimi- Despite all the incredible ring structure astronomers valuable insights into how nary analysis, he says, “seems to indicate that Cassini’s cameras and spectrometers planetary systems develop. that the rings did not form with Saturn.” resolved, scientists still have questions. The Grand Finale data are getting scien- It will take more research to firm up this The biggest one concerns the ring system’s tists closer than ever to figuring out the result, and to find out when and how the mass. They don’t want to know this mass rings’ mass. During those final months, rings formed. just for knowledge’s sake. Instead, the mass Cassini flew between the inner rings and is linked to the age of the rings and how Saturn’s upper atmosphere 22 times. Cloudy weather they formed. Throughout the previous 12½ years of Beneath Saturn’s majestic rings lies the This is important because Saturn’s rings Saturn exploration, the spacecraft stayed planet’s equally magnificent cloud tops. are the closest example astronomers have of outside the rings, and thus it felt the com- Cassini unveiled churning and swirling astrophysical disks — such as the flattened bined pull from Saturn and the rings. clouds in the upper atmosphere, and places disks of gas and dust out of which solar sys- “When you are between the rings and where warm gases rise up through cooler tems form. “It’s not the same, but it’s analo- Saturn, the rings are pulling in one direc- layers and erupt into long-lasting thunder- gous,” says Hedman. And this means, “if we tion, and Saturn is pulling in the other, storms. Cassini resolved these thunder- don’t understand what’s going on in the so you can disentangle the two effects,” storm clusters into minute detail, watching

WWW.ASTRONOMY.COM 23 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 the 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 NASA/JPL-CALTECH/SSI 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 them evolve and listening to the radio Cassini’s imaging camera first saw high-frequency radio emissions created in static from lightning flashes. the storm December 5, at the same time lightning strokes. While normal photos painted pretty another instrument heard it — or at least, The jet streams in Saturn’s atmosphere pictures of the whirling atmosphere, infra- the radio bursts created by its lightning. A carried the northern hemisphere storm red images let scientists see below the cool similar phenomenon happens on Earth. If along its cloud band. By late January 2011, cloud tops to warmer regions beneath. “And you have ever been in a car listening to an it wrapped around the planet and stretched that’s our secret weapon for how to analyze AM radio station during a thunderstorm, 9,000 miles (15,000 km) north-south. As the depths of Saturn,” says Cassini scientist you probably heard what sounded like the storm progressed, scientists used the Kevin Baines. “[It’s] how Saturn was static. “That static is not actually static,” imaging instruments and RPWS to view revealed to be not this nice demure place, says William Kurth. “It’s actually radio it. In summer 2011, after some 200 days of but this roiling dynamic place.” He and his emissions from the lightning strokes and roiling, swirling, and spreading, the storm colleagues watched as clouds in the upper the thunderstorm, and they propagate at died out and the atmosphere cleared. The atmosphere blocked heat from below. They the speed of light.” Kurth is the principal region, says Baines, “has been very boring also identified vortices and a giant cyclone investigator of Cassini’s Radio and Plasma ever since.” at each of Saturn’s poles, though only the Wave Science (RPWS) instrument, which Because scientists could watch the great north pole features a hexagonal jet stream. listened in on the Great White Spot’s storm evolve with Cassini’s broad array of But one storm stood out from all the others. The Great White Spot erupted unexpectedly December 5, 2010. Earth- based observations of Saturn over the past 140 years had shown that a giant, long- lasting 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 Saturn’s north polar hexagon is a meandering A giant vortex resides at Saturn’s north pole. jet stream near 77° north latitude. Each side The storm, which appears red in this false-color scientists with an up-close look at how of the hexagon measures slightly longer than image, spans 1,250 miles (2,000 km) and has these giant storms evolve. Earth’s equatorial diameter. NASA/JPL-CALTECH/SSI winds up to 330 mph (540 km/h). NASA/JPL-CALTECH/SSI

24 ASTRONOMY • MARCH 2018 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 long- lived events. Caltech’s Andrew Ingersoll and his then-graduate student Cheng Li put forth the most likely theory. They say it’s due to a convective competition between water-rich clouds and the lighter- weight 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, radiating 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 then come the storms.” Once the storm rains out its water The Great White Spot erupted in December 2010 and quickly evolved into a massive storm. By the content, convection shuts down, and the time Cassini captured this image 12 weeks later, Saturn’s jet streams had carried the storm completely storm stops. around the planet. NASA/JPL-CALTECH/SSI

Magnetic makeup the region the field controls, called the generate, in much the same way it heard When you think of Saturn, the ornate magnetosphere. radio flashes associated with lightning. rings and cloudy atmosphere likely come Previous observations of Saturn had “We’ve been able to use the intensity of to mind first, but no object exists in isola- shown aurorae at the planet’s poles, similar these radio emissions as a proxy,” says tion. So, how does the giant planet affect its to the northern and southern lights seen in Kurth, to address questions of “how surroundings? That’s where Saturn’s mag- Earth’s polar regions. Cassini’s RPWS intense are the auroras and is there a lot of netic field factors in, and it’s why Cassini instrument monitored auroral activity activity going on.” RPWS also monitored brought along instruments to study it and by detecting the radio waves that aurorae 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 mag- netic field? To find out, scientists used Cassini’s magnetometer. This instrument measures the strength and location of the planet’s magnetic field lines, which trace how charged particles travel. Electrons, 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 gener- ates its magnetic field deep in its interior. For Earth, researchers have a pretty good Although Saturn’s north polar hexagon has lasted for at least 35 years (the Voyager spacecraft idea of how it happens. “You have heat, you 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 think an increase in solar radiation during those four years have convection taking place in the inte- caused yellowish smog to form. NASA/JPL-CALTECH/SSI/HAMPTON UNIVERSITY rior, you have rotation in the interior, and

WWW.ASTRONOMY.COM 25 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

magnetic field should be decaying — and scientists have seen no evidence of a diminishing magnetic field at Saturn. Above: Saturn’s aurora glows blue while the underlying When Cassini flew close to Saturn during atmosphere appears deep red in this infrared composite the Grand Finale, the magnetometer col- image. As on Earth, the aurora arises as Saturn’s magnetic field funnels energetic solar particles to the polar regions. lected data about the magnetic field. “We NASA/JPL-CALTECH/UNIVERSITY OF ARIZONA really expected these Grand Finale orbits to clearly measure the tilt, and all we’ve been Left: Cassini captured the ultraviolet glow from Saturn’s aurora one day before the spacecraft crashed into the planet. The able to do so far is put a limit on it,” says north pole lies at the center of this image, while the bottom Dougherty. The angle between the two faces the Sun. NASA/JPL-CALTECH/UNIVERSITY OF COLORADO/UNIVERSITY OF LIEGE-LPAP axes must be less than 0.06°. The team has had the data for only a you have flowing electrical currents,” says thick atmosphere hides the planet’s solid couple of months, however, and Dougherty Michele Dougherty, principal investigator core — assuming it has one. is confident that after she and her col- of Cassini’s magnetometer. “All of those To measure Jupiter’s day, for example, leagues complete their careful and thor- combine to give you the magnetic field that scientists track the magnetic axis and find ough analysis, they’ll know what Saturn’s you measure outside the planet.” it wobbles with respect to the planet’s rota- internal magnetic field is like. The biggest A key component in understanding tion. The magnetic field’s axis and the rota- hurdle is accurately calibrating the instru- Saturn’s magnetic field is the length of a tion axis tilt relative to each other, and that ment. The analysis requires absolute preci- saturnian day, and this was a major ques- wobble relates directly to how fast the plan- sion — the exact location and timing of the tion scientists hoped Cassini would resolve. et’s core is spinning. The problem with spacecraft’s trajectory, and knowledge of This shouldn’t be a difficult question, Saturn, though, is that the two axes are where Cassini was when the instrument right? It’s just the rotation period. But that’s nearly perfectly aligned. This makes it collected each bit of data. Researchers have a much harder problem to solve for gas awfully hard to find that wobble. predicted orbits, positions, and times, but giant planets than it is for Earth. The cloud The precise alignment also perplexes they have to know whether Cassini’s actual tops rotate at different speeds, and the researchers because it implies that the 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

26 ASTRONOMY • MARCH 2018 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, Cassini crashed into Saturn’s atmosphere September 15, 2017, at the spot marked by the oval. where day turns to night, and some high clouds This nighttime infrared view shows heat coming from the planet’s interior in red; the dark regions cast shadows on lower regions. NASA/JPL-CALTECH/SSI are silhouetted clouds. NASA/JPL-CALTECH/UNIVERSITY OF ARIZONA might there have been a half-second delay and Cassini and measured the slight chang- at different speeds than others. The because the craft felt more drag from the es in radio frequency. Those changes arose researchers still have more orbit trajectories atmosphere than expected? “It’s a really from gravitational tugs of mass pulling on to calibrate, and thus are still months away complicated process,” says Dougherty of the spacecraft — the more mass, the bigger from a major announcement. the analysis. “It’s like trying to find three the tug. So Iess and his colleagues can use Revealing that the interior doesn’t align or four needles in a haystack that’s chang- those tiny frequency changes to map the with models would be a fitting discovery ing shape and size at the time.” distribution of mass within Saturn. Because from a mission that already has found so Cassini skimmed the planet’s cloud tops many surprises at the Saturn system. Mapping gravity’s pull during its final months, it felt a stronger Cassini’s suite of instruments offered the The magnetic field analysis isn’t the only gravitational pull from those mass distribu- flexibility that allowed scientists to make one proving to be extremely complex and tions, and was able to sense finer details. those discoveries. The mission’s scientists requiring precise calibration. Scientists also Precisely understanding those Grand and engineers worked in sync for decades want to know about Saturn’s interior, and Finale orbits is crucial to the gravity analy- to perform what Spilker calls Cassini’s in particular, how the planet’s mass is dis- sis of Saturn. So far, the team has learned “intricate ballet.” tributed. To do that, they need to measure that theoretical models of Saturn’s gravity “It’s for the unknown, the unexpected,” the planet’s gravity. That’s not as simple as do not match the data. “The gravity field of she says. “That’s why you do science.” it might sound. “There is no instrument Saturn is surprising,” says Iess. “We found aboard a satellite which can reveal the grav- Saturn has features that can be explained Contributing Editor Liz Kruesi writes about ity field by itself,” says Iess. Instead, scien- only by differential rotation,” meaning distant objects from her Earthbound home tists passed radio signals between Earth some portions or layers of the planet move in Austin, Texas.

WWW.ASTRONOMY.COM 27 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 Above: NASA’s Cassini mission took images as the spacecraft approached ye candy is not in short supply at Saturn. and Calypso orbit along with Tethys — an (left) and departed (right) For visitors who tire of watching the plan- arrangement thus far unseen among any other Saturn’s moon Phoebe et’s stormy atmosphere or gazing into the moons in the solar system. during its only close flyby of the satellite. Cassini solar system’s most beautiful and complex And this is just for starters. “The Saturn sys- passed just 1,285 miles ring system, there's always the giant satel- tem is full of surprises,” says Paul Schenk, a plan- (2,068 km) above the lite Titan to explore. This colossal moon etary geologist at the Lunar and Planetary surface on June 11, 2004. is bigger than Mercury and sports a hazy Institute in Houston. There’s a satellite that likely Phoebe is thought to be a centaur that might have orange atmosphere denser than Earth’s, originated in the Kuiper Belt, the storehouse of become a Jupiter-family producing methane rains that flow across icy bodies beyond Neptune’s orbit; a piebald comet, had Saturn not Titan’s icy landscape and pool into vast lakes. moon nearly encircled by an equatorial ridge captured it. NASA/JPL-CALTECH But look again. Even Saturn’s small moons containing some of the tallest mountains in the display some unusual dynamic relationships. For solar system; a spongy-looking tumbling satellite; Opposite: Cassini took instance, Pan and Daphnis dwell in the Encke and a moon that vents its subsurface sea into this Saturn mosaic and Keeler ring gaps, respectively, where their space, providing scientists with an unexpected October 21, 2013. NASA/JPL/SSI gravity rumples the ring’s boundary and sweeps potential niche for extraterrestrial life. away particles to keep the gap clear. There’s Janus and Epimetheus, whose orbital differences are smaller than their diameters, so they should Phoebe was discovered in 1899 and is the first collide, but don’t. Instead, these “co-orbiting” satellite found photographically. NASA’s Cassini moons effectively play leapfrog, swapping orbits spacecraft made its only close flyby of the dis- over a four-year cycle. tant moon June 11, 2004, about three weeks Other small satellites orbit in the gravitational before slowing to enter orbit around the planet. safe zones — called Lagrangian points — of the At only about 130 miles (210 kilometers) across, midsized moons Dione and Tethys. Lagrangian Phoebe is about one-sixteenth the size of our points are locations 60° in front of and behind a Moon. Its heavily cratered surface is mostly larger object’s orbit where a less-massive body dark as soot with no signs of resurfacing due can move in an identical stable orbit. Dione trav- to geological activity. But bright cliffs on the els with Helene and Polydeuces, while Telesto rims of the largest craters, as well as bright rays

WWW.ASTRONOMY.COM 29 Ring king

Saturn Iapetus

Titan

Phoebe

ing e r eb Pho

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 Jupiter and Neptune. Gravitational inter- of Virginia, Charlottesville, who led the ice beneath a layer of dark material up actions with the giant planets will even- research. A 2015 study using data from to about 1,600 feet (490 meters) deep. tually destabilize their orbits. It’s thought NASA’s Wide-Field Infrared Survey Ground-based telescopes have detected centaurs originated in the Kuiper Belt Explorer found this king of rings extends the presence of frozen water, and and were perturbed into their current even farther, starting 3.7 million miles Cassini’s instruments showed the pres- orbits by Neptune. Once they start cross- (5.9 million km) from the planet and ence of frozen carbon dioxide (dry ice) ing the orbits of giant planets, centaurs reaching at least 10 million miles and organic material as well. may collide with or be captured by them, (16 million km). Continual small impacts Phoebe orbits in the opposite direction be flung out of the solar system, or be on Phoebe regularly eject dust that main- of Saturn’s spin, on a path that is both rerouted by Jupiter into orbits bringing tains the ring, while particles smaller more eccentric and more highly inclined them much closer to the Sun, where they than a few inches gradually migrate than the planet’s inner moons. On the become Jupiter-family comets. The idea inward, likely the source for dark material basis of these orbital characteristics, that Phoebe may have been a centaur found on the surfaces of other satellites, astronomers have long suspected Phoebe before being captured by Saturn is con- especially Iapetus. of being an interloper ensnared by the sistent with its surface composition, but planet’s gravity, rather than a native to it’s difficult to say more with the avail- the Saturn system. Phoebe’s chemical able observations. In 1671, Italian astronomer Giovanni makeup resembles C-type asteroids com- In 2009, infrared observations by Domenico Cassini discovered Iapetus, monly found in the farthest regions of the NASA’s Spitzer Space Telescope showed the next moon of Saturn as we move main asteroid belt, while its density sug- that Phoebe resides within a supersized, inward from Phoebe. He noticed that gests an ice-rock mixture similar to tenuous ring of ice and dust particles that when Iapetus was on one side of Saturn, Neptune’s moon Triton (itself thought to had been previously undetected. “If you it was very bright, but on the opposite be a captured object) and Pluto. could see the ring, it would span the side, it nearly disappeared from view. He Phoebe could have been a centaur. width of two Full Moons’ worth of sky, correctly proposed that Iapetus is tidally Centaurs are comet-like bodies that one on either side of Saturn,” says Anne locked — meaning it always turns the follow planet-crossing orbits between Verbiscer, an astronomer at the University same face to Saturn — just as our Moon

30 ASTRONOMY • MARCH 2018 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 subsurface ice. NASA/JPL-CALTECH/SPACE SCIENCE INSTITUTE

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 hemisphere. NASA/JPL-CALTECH/SPACE SCIENCE INSTITUTE/LUNAR AND PLANETARY INSTITUTE

WWW.ASTRONOMY.COM 31 Daphnis 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, Atlas 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 Pan 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 6 miles outer edge. NASA/JPL-CALTECH/SPACE SCIENCE INSTITUTE (10 kilometers)

is to Earth, and that the hemisphere (thanks to the initial deposits from scientists. Did it erupt or grow from the centered on the direction of motion is Phoebe), it can be sustained. As the sur- inside, when Iapetus was young and its covered with a dark material. Planetary face ice departs from Cassini Regio, interior was still warm? Or is it the scientists later named this dark feature what’s left behind is a residue native to accreted remains of a ring either kicked Cassini Regio in his honor, as well as the Iapetus mixed in with material from the up by an impact or formed when a small Cassini mission. Phoebe ring. satellite passed too close and was crushed At 927 miles (1,492 km) across, Iapetus On New Year’s Eve, 2004, the Cassini in the tides of Iapetus? is the third-largest moon of Saturn, and spacecraft approached Iapetus for the A 2014 study led by Erica Lopez the planet’s only large satellite in a highly first time, and planetary scientists were Garcia at Brown University examined the inclined orbit, which carries the moon in enthralled with the images it returned. topography of the ridge and found pre- the same direction Saturn rotates, unlike The heavily cratered body showed dominantly triangular slopes, much like Phoebe. Among the moons in the solar numerous large impact basins — the what you’d get by slowly pouring sugar system that exhibit tidally locked rota- largest in the Saturn system — but its into a pile, suggesting an external source. tion, Iapetus is by far the most distant standout feature was a conspicuous But new models exploring possible inter- from its planet. The moon’s density is ridge running almost exactly along the nal formation mechanisms continue to only slightly greater than frozen water, equator, giving the icy moon the look of a appear. “I think more people favor the indicating that rock makes up perhaps walnut. The ridge extends over 800 miles ring explanation, but the debate is still only one-fourth of its composition. (1,300 km) and cuts completely across ongoing,” says Francis Nimmo, a plan- As Iapetus circles Saturn, dust migrat- Cassini Regio. In places, it breaks into etary scientist at the University of ing in the opposite direction from the isolated mountain peaks that may reach California, Santa Cruz. Phoebe ring smacks into Cassini Regio 12 miles (19 km) high, rivaling the giant Supporting the ring interpretation are like bugs on a windshield, but the shape martian volcano Olympus Mons but equatorial bulges on other moons in the of the dark patch can’t be fully explained located on a world five times smaller. system, especially Atlas, Pan, and by this simple accumulation of material. There is nothing like it anywhere else Daphnis, which accumulate particles For one, the dark side of Iapetus is much in the solar system, and a completely sat- while orbiting in and near Saturn’s A redder than Phoebe. isfactory explanation of its origin eludes ring. And then there’s Rhea, Saturn’s In 2010, John Spencer at the Southwest Research Institute in Boulder, Colorado, and Tilmann Denk at the Free University of Berlin proposed that the dark deposits, which reflect as little sunlight as fresh asphalt, warm the leading hemisphere just enough that water ice molecules can sublimate, turning directly to a gas. These molecules migrate from the warmer lead- ing hemisphere to the colder trailing hemisphere, where they freeze onto the surface again. The slow, tidally locked Blue streaks on the moon Rhea, enhanced here to highlight color differences, reveal fresh ice spin of Iapetus produces unusually high re-exposed when material in Rhea’s ring struck the surface. The streaks form a very narrow daytime temperatures and water ice subli- 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 mation rates for a given reflectance, so Cassini imaging data returned in 2008. It remains unclear whether the moon retains a tenuous ring once this thermal migration begins today. PAUL SCHENK, LUNAR AND PLANETARY INSTITUTE

32 ASTRONOMY • MARCH 2018 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-so- distant past,” says Schenk. Inward of Iapetus lies Hyperion, 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 materials in the outer solar system. This means Hyperion truly must be spongelike, full of tiny 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 charac- teristic is its rotation. Hyperion’s irregular This color-enhanced view of Saturn’s moon Hyperion, imaged by Cassini in 2005, reveals crisp shape, eccentric orbit, and proximity to 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 Saturn’s big moon Titan create conditions low density and low gravity combine to preserve the original shapes of its craters. Impacts tend that wrench it out of any kind of stable to compress the porous surface rather than blast it out, and what little ejecta is produced is more spin and even prevent it from tidally lock- likely to leave the moon than cover up older craters. NASA/JPL-CALTECH/SPACE SCIENCE INSTITUTE ing to Saturn. Its rotational period and the direction of its spin axis can change unpredictably over days or weeks as in geological terms. “We knew there Hyperion tumbles along its orbit. Once In 1981, Voyager 2 imaged parts of was something special about Enceladus believed unique, we now know Hyperion’s Enceladus, a midsized moon, at high res- because it reflects almost all the sunlight chaotic spin is shared by at least two of olution, revealing troughs, scarps, groups striking it. It had markings indicating it Pluto’s moons, Nix and Hydra, thanks to of ridges, and craterless plains — all types had an active geologic life,” says Ed Stone, large torques generated by Pluto and its of terrain indicating internal forces had project scientist for the Voyager mis- largest moon, Charon. altered the surface comparatively recently sions. Such diversity was surprising for a satellite only 313 miles (504 km) across, leading some to suggest that Enceladus needed an unexpectedly large internal heat source to power these changes. Soon after the flyby, researchers noticed that Enceladus orbits within Saturn’s broad, diffuse E ring, which extends from about 131,000 miles to 298,000 miles (211,000 km to 480,000 km) and thickens away from the moon. Ice grains in the E ring have limited survival times because of collisions with high- energy ions trapped in Saturn’s rotating magnetic field. This process, called sput- tering, whittles away 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 supply of new grains must be ejected from Long, wispy fingers of icy particles extend from the geysers on Enceladus (black dot, center) Enceladus. In the early 1980s, several sci- and into Saturn’s E ring. NASA/JPL/SPACE SCIENCE INSTITUTE entists suggested meteorite impacts,

WWW.ASTRONOMY.COM 33 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

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 snow. Due to mutual gravitational interac- 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 stronger 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 Dramatic plumes spray water ice and vapor from locations along the tiger stripes. Alexandria south pole is a mystery.” Sulcus, the least active, is on the left; Baghdad and Damascus, the most active, are on the right. There is certainly evidence that other NASA/JPL/SPACE SCIENCE INSTITUTE parts of the surface were warmer in the past. Some impact craters look like they geysers, or volcanic eruptions as possible warm, roughly parallel trenches at the have flowed, indicating warm ice near the ejection mechanisms, but the matter moon’s south pole, nicknamed “tiger surface, and it seems fairly clear that dif- remained speculative. stripes,” erupt plumes of water vapor, ferent parts of Enceladus were heated at “Prior to Cassini, nobody would have hydrogen, and hydrocarbons that regu- different times. “There are even features anticipated that there was an ocean larly strengthen and weaken as Enceladus that look a bit like ancient tiger stripes, beneath the surface of Enceladus,” says orbits Saturn. “The eruption mechanism though this is controversial,” Nimmo Nimmo. “I think seeing actual geysers is understood, at least in outline,” Nimmo explains. “It’s hard to tell from observa- erupting was one of the biggest surprises explains. “The tiger stripes appear to be tions whether the activity was episodic, in planetary science in recent memory.” water-filled cracks, connected to an ocean but the fact that you see heavily cratered Cassini observations in 2005 revealed beneath, which open and close every orbit and very lightly cratered areas, but not Enceladus as one of the most extraordi- under the influence of tides.” The water moderately cratered areas, suggests that nary bodies in the solar system. Four exposed to the vacuum simultaneously activity was not continuous.”

34 ASTRONOMY • MARCH 2018 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 observed, some in a very pristine state sampled their composition, showing that of preservation,” says Schenk. A pair of nearly 98 percent of the gas in the plumes unusual walled depressions in the moon’s is water vapor, about 1 percent is hydro- smooth terrain might be volcanic vents; gen, and the rest is a mixture of other half of Dione’s surface is covered by what molecules, including carbon dioxide, is likely ancient volcanic ice deposits; and methane, and ammonia. Phosphorus some propose that Dione may possess a remains the only element essential to life subsurface sea. on Earth that has yet to be found in the On Tethys, a huge rift zone named Enceladus plumes, but it’s likely present. Ithaca Chasma runs nearly three-quarters With the necessary ingredients — a warm of the moon’s circumference, and subsurface sea, plentiful hydrogen that Cassini’s cameras detected a set of myste- organisms could potentially harness as a rious red arcs that appear to have formed chemical energy source, and geysers con- very recently. “So while Enceladus has veniently delivering samples into space been stealing our attention, these moons Dione’s trailing hemisphere displays a network — Enceladus is arguably the most likely have shown intermittent signs of activity, of long, deep, steep-sided fractures flanked and exciting target for the search for life. too,” Schenk adds. by bright, icy cliffs. The pattern may be related to Dione’s orbital evolution and tidal stresses On September 15, 2017, Cassini over time. Cassini imaged the moon in visible plunged into Saturn’s atmosphere, ending light in 2015. NASA/JPL-CALTECH/SPACE SCIENCE INSTITUTE While it’s hard to top Enceladus, the its mission. Scientists will be mining the neighboring midsized moons Dione and data Cassini returned on these diverse Tethys tantalize planetary scientists with worlds for decades to come. Even from Francis Reddy is the senior science writer for as-yet-unexplained features. “On Dione, a the perspective of its smaller satellites, the Astrophysics Science Division at NASA’s magnificent set of tectonic fractures was Saturn beckons. When will we return? Goddard Space Flight Center in Maryland.

WWW.ASTRONOMY.COM 35 SKYTHIS MARTIN RATCLIFFE ALISTER LING Visible to the naked eye and describe the Visible with binoculars solar system’s changing landscape as it appears in Earth’s sky. MONTH Visible with a telescope March 2018: Mercury at its evening best

after sunset and Mercury measures 6.5" across and is appears 1.8° to its lower right. 64 percent illuminated. Mercury glows at magnitude The innermost planet –1.3, about 10 times dimmer reaches its peak at greatest than its companion, but shows elongation March 15. It up nicely through binoculars then lies 18° east of the Sun and should be visible to the and stands 12° high in the naked eye in good conditions. west 30 minutes after sun- The two planets climb down. (It’s still 6° high in a slowly away from the Sun darker sky a half-hour later.) during the next two weeks. Although it has dimmed to Mercury orbits our star along magnitude –0.4, that’s brighter a faster track, however, and than any other early evening gains altitude more quickly. object except for Venus and The two come closest to each the night sky’s brightest star, other March 3, when Mercury Sirius. A telescope shows lies 1.1° to Venus’ right. Turn Mercury’s 7.3"-diameter disk, a telescope on the pair and which appears slightly less Venus sports a 10"-diameter than half-lit. disk that appears nearly full. After greatest elongation, Mercury appears 5.6" across the innermost planet sinks with an 87-percent-lit phase. lower with each passing day. The MESSENGER spacecraft revealed Mercury’s stunning geology in this Mercury passes 1.4° due It passes due north of Venus false-color image. Even under the optimal viewing conditions in March, north (to the upper right) of again March 17, this time at however, the planet appears bland through amateur scopes. NASA/JHUAPL/CIW Venus on March 5. Five days a distance of 4°. Be ready to later, Venus stands 7° above capture some twilight shots ercury and Venus But two factors make its the western horizon 30 min- on the 18th, when a crescent rule the early eve- appearance this month stand utes after sunset and Mercury Moon stands 4° to Venus’ left ning sky, a pair of out. First, the ecliptic — the appears 3° higher. A telescope and Mercury appears the bright worlds seem- apparent path of the Sun reveals essentially no changes same distance to Venus’ upper ingly tethered to across the sky that the planets to Venus’ size and shape, but right. The trio stands 10° high Meach other for much of March. follow closely — makes a steep Mercury now shows a disk that 30 minutes after sunset. Uranus joins the party late in angle to the western horizon the month as it wraps up a fine after sunset. Mercury’s elon- The innermost planet’s exceptional show evening appearance, though gation from the Sun thus you’ll need binoculars to see its translates mostly into altitude. PISCES fainter glow. Not to be outdone, But the bigger reason for the morning sky features three Mercury’s easy visibility in bright planets — Mars, Jupiter, March is its proximity to and Saturn — that grow more Venus. Earth’s closest plan- PEGASUS prominent by the week. etary neighbor shines bril- As March begins, Mercury liantly at magnitude –3.9 and Mercury appears deepest in evening stands out in the western sky Venus twilight. In the Northern after sundown. Locate it, and CETUS Hemisphere, the innermost finding nearby Mercury will planet has a deserved reputa- be a snap. The two remain in tion for being an elusive target. a single binocular field during 5° Its tight orbit around the Sun March’s first three weeks. means it never strays far from You can start searching for March 15, 30 minutes after sunset Looking west our star, so it typically appears the planets early this month. in twilight either shortly after On the 1st, Venus hangs 5° Use brilliant Venus as a guide to locating Mercury as it reaches its peak sunset or before sunrise. above the horizon a half-hour altitude for the year in mid-March. ALL ILLUSTRATIONS: ASTRONOMY: ROEN KELLY

36 ASTRONOMY • MARCH 2018 RISINGMOON A First Quarter Moon eureka moment Archimedes

Towering turrets, chains of pointy peaks, Inside the mountainous arc lies the Aristillus and a parade of rugged crater walls attractive circular form of Archimedes. highlight any First Quarter Moon. For the Acting like a canvas, the crater’s smooth best views, look along the terminator — floor displays a remarkable series of the dividing line between day and night saw-toothed shadows cast by the that cuts the Moon in half. The evening rim. Lunar cartographers named this Autolycus of March 24 provides splendid views as 52-mile-wide crater after the famed the rising Sun casts long shadows. Greek mathematician and physicist of Archimedes North of the lunar equator, Montes the third century B.C. After looking at Apenninus (Apennine Mountains) lies this striking feature, take a moment to west of Mare Serenitatis (Sea of Serenity). enjoy the smaller craters Aristillus and This mountain arc forms the southeast- Autolycus nearby. ern edge of the giant Mare Imbrium. The You can heighten your experience by massive impact that created this basin watching carefully during the evening Montes Apenninus N threw out an incredible amount of mate- hours. The long daggers of darkness rial. Later on, lava welled up from below knifing westward onto Archimedes’ E and buried much of it. But some of this floor retreat with each passing hour. By excavated material remains on view, March 25, the much shorter shadows are At First Quarter phase, jagged peaks along the rim of most prominently on the southeastern merely pointy, and they disappear in a Archimedes cast pointed shadows across its smooth side of the Apennine chain. couple more nights. floor. CONSOLIDATED LUNAR ATLAS/UA/LPL; INSET: NASA/GSFC/ASU

Mercury fades quickly as it falls toward the Sun. The METEORWATCH planet becomes a challenge in twilight by the 23rd, when it glows at 2nd magnitude. The zodiacal light’s soft glow Mercury will pass between the The Sun lights up Sun and Earth on April 1, solar system dust setting up a poor morning on Moon-free appearance in late April. March evenings. Meanwhile, Venus contin- The pyramid- shaped glow ues to climb away from the seems to point to Sun. Its ascent sets up a close the Pleiades star conjunction with Uranus cluster. DEREK MELLOTT 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 magni- tude, however, and will be hard to see in twilight. Track Dusty debris sets the evening sky aglow Venus through binoculars or a telescope as twilight darkens March is a lean month for meteor asteroid collisions and comets glow to the naked eye. To see and watch for the more distant observers. No major showers passing through the inner solar this so-called zodiacal light, planet to pop into view. occur, and the only minor one system. This dust concentrates plan to observe sometime You’ll have an easier time (the Gamma Normids) resides along the ecliptic, the plane in between March 3 and 18, when catching Uranus in early deep in the southern sky. Still, which the planets orbit. When the Moon is out of the evening March when it stands 25° high dark skies always offer a chance the ecliptic angles steeply to sky. Then, find a site far from in the west once twilight fades to see sporadic meteors. the western horizon after sun- the city, wait for twilight to to darkness. Look for it 2.3° The dust particles that give set, as it does every March, the fade away, and search for the due west of 4th-magnitude rise to meteors are debris from dust appears as a pyramidal soft glow. Omicron (ο) Piscium in the southeastern corner of Pisces OBSERVING Mercury puts on its best evening appearance of 2018 during the Fish. HIGHLIGHT — Continued on page 42 the first three weeks of March.

WWW.ASTRONOMY.COM 37 STAR N DOME

How to use this map: This map portrays the

sky as seen near 35° north latitude. Located IA

E inside the border are the cardinal directions HERCULES

and their intermediate points. To find

stars, hold the map overhead and DRACO

orient it so one of the labels matches MINOR

NE CEPHEUS the direction you’re facing. The URSA

stars above the map’s horizon

now match what’s in the sky. Polaris NCP

The all-sky map shows LOPARDALIS E how the sky looks at: BOÖTES

10 P.M. March 1

10 P.M. March 15 M81 Mizar

9 P.M. March 31 M51

Planets are shown M82

at midmonth VENATICI CANES

lla

MAJOR pe

URSA

Arcturus

BERENICES

COMA

LYNX

LEO MINOR LEO

NGP

7 M64

E Castor

GEMINI

LEO

Denebola M66 M44 Pollux

M65 VIRGO

Regulus CANCER

STAR Procyon CANIS MAGNITUDES Spica M104 MINOR SEXTANS HYDRA Sirius S 0.0 CORVUS CRATER 3.0 1.0 MONOCERO 4.0 2.0 5.0 M47 CANIS STAR COLORS MAJOR A star’s color depends on its surface temperature. PYXIS

The hottest stars shine blue h • SE ANTLIA Ad Slightly cooler stars appear white • PUPPIS • Intermediate stars (like the Sun) glow yellow • Lower-temperature stars appear orange • The coolest stars glow red NGC 2477 • Fainter stars can’t excite our eyes’ color receptors, so they appear white unless you VELA use optical aid to gather more light

38 ASTRONOMY • MARCH 2018 S Note: Moon phases in the calendar vary in size due to the distance from Earth MARCH 2018 and are shown at 0h Universal Time. SUN. MON. TUES. WED. THURS. FRI. SAT. MAP SYMBOLS

Open cluster

123

E Globular cluster CASSIOP Diffuse nebula 45678910 ANDROMEDA NW Planetary nebula Galaxy

11 12 13 14 15 16 17

869

NGC KELLY ROEN :

M33

884

E

NGC 18 19 20 21 22 23 24 ASTRONOMY CAM

25 26 27 28 29 30 31

ILLUSTRATIONS BY BY ILLUSTRATIONS

TRIANGULUM

Algol

ARIES PERSEUS )

tic Calendar of events pe p

cli Ca (e un e S 1 The Moon passes 0.9° north of 18 The Moon passes 8° south of th of Regulus, 1 A.M. EST Mercury, 2 P.M. EDT th

Pleiades Pa

Full Moon occurs at The Moon passes 4° south of AURIGA

M38 7:51 P.M. EST Venus, 3 P.M. EDT

M36 7

4 Neptune is in conjunction with 19 The Moon passes 5° south of M3

CETUS the Sun, 9 A.M. EST Uranus, noon EDT TAURUS

Hyades W M1 M35 5 Mercury passes 1.4° north of 20 Vernal equinox occurs at

Venus, 1 P.M. EST 12:15 P.M. EDT Aldebaran 7 The Moon passes 4° north of Dwarf planet Ceres is Jupiter, 2 A.M. EST stationary, 5 P.M. EDT

ORION 9 Jupiter is stationary, 5 A.M. EST 22 Mercury is stationary, 1 P.M. EDT

Last Quarter Moon The Moon passes 0.9° north of occurs at 6:20 A.M. EST Aldebaran, 7 P.M. EDT

Betelgeuse The Moon passes 4° north of 24 First Quarter Moon Mars, 8 P.M. EST occurs at 11:35 A.M. EDT M42 S ERIDANUS Rigel 10 The Moon passes 2° north of 26 The Moon is at perigee Saturn, 9 P.M. EST (229,352 miles from Earth), 1:17 P.M. EDT Sirius 11 The Moon is at apogee LEPUS (251,455 miles from Earth), 28 The Moon passes 1.0° north of 5:14 A.M. EDT Regulus, 10 A.M. EDT

M41 15 Mercury is at greatest eastern SPECIAL OBSERVING DATE elongation (18°), 11 A.M. EDT 28 Venus passes 0.07° north of Uranus in evening 17 New Moon occurs at hara SW twilight. 9:12 A.M. EDT

Mercury passes 4° north of 31 Full Moon occurs at Venus, 9 P.M. EDT 8:37 A.M. EDT

COLUMBA

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

CAS DRA Objects visible before dawn UMa

LAC HER CVN CYG LMi LYR BOÖ CrB VUL COM PEG LEO SGE oon the M EQU th of PSC SER Pa AQL OPH Sun SER VIR Celestial equator tic) clip n (e Neptune Su SEX AQR LIB the of CAP ath SCT P CET Vesta CRV HYA CRT Pluto Saturn Jupiter Mars ANT PsA SCL SGR MIC LUP CrA SCO VEL PHE CEN GRU Moon phases Dawn Midnight

19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1

31 30 29 28

The planets These illustrations show the size, phase, and orientation of each planet and the two brightest dwarf planets at 0h UT in the sky for the dates in the data table at bottom. South is at the top to match the view through a telescope.

Mercury Uranus Mars

S

WE Pluto N Saturn Venus Ceres Neptune 10" Jupiter

Planets MERCURY VENUS MARS CERES JUPITER SATURN URANUS NEPTUNE PLUTO Date March 15 March 15 March 15 March 15 March 15 March 15 March 15 March 15 March 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" 40.7" 16.2" 3.4" 2.2" 0.1" Illumination 47% 96% 88% 98% 99% 100% 100% 100% 100% Distance (AU) from Earth 0.927 1.630 1.268 1.815 4.838 10.257 20.729 30.922 33.956 Distance (AU) from Sun 0.314 0.724 1.554 2.561 5.420 10.066 19.895 29.944 33.523 Right ascension (2000.0) 0h42.4m 0h37.4m 17h52.2m 8h42.3m 15h23.4m 18h34.4m 1h38.5m 23h02.8m 19h29.2m Declination (2000.0) 6°44' 2°52' –23°23' 31°57' –17°19' –22°19' 9°40' –7°06' –21°30'

40 ASTRONOMY • MARCH 2018 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.

CAS Jupiter’s moons Objects visible in the evening LYN LAC Dots display positions PER AND Io AUR of Galilean satellites at .M. TRI 4 A EDT on the date Europa GEM PEG shown. South is at the Ceres top to match ARI S CNC Mercury appears bright in the the view evening sky in mid-March Ganymede Iris through a WE TAU telescope. N Callisto Uranus PSC 1 CMI Sun Venus 2 Io ORI MON 3 Pallas CET AQR 4 Jupiter CMA 5 LEP PsA PYX ERI FOR 6 PUP COL SCL 7 CAE PHE 8 Ganymede Early evening 9 Callisto 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. 10 11

27 26 25 24 23 22 21 20 19 18 17 16 12 13 Europa

14 Mercury Greatest eastern elongation 15 is March 15 16

17 Ceres 18 Venus Earth 19 Vernal equinox is March 20 20 Mars 21

22

Jupiter 23

24

25

26 Uranus The planets 27

in their orbits 28

Jupiter KELLY ROEN : Neptune Arrows show the inner planets’ 29 monthly motions and dots depict Saturn Solar conjunction

is March 4 ASTRONOMY the outer planets’ positions at mid- 30 month from high above their orbits. Pluto 31 ILLUSTRATIONS BY BY ILLUSTRATIONS

WWW.ASTRONOMY.COM 41 — Continued from page 37 The Galilean satellites take sides

WHEN TO VIEW THE PLANETS March 25, 5:00 A.M. EDT S Jupiter EVENING SKY MIDNIGHT MORNING SKY

Mercury (west) Jupiter (southeast) Mars (south) W Venus (west) Jupiter (south) Uranus (west) Saturn (southeast) Ganymede Io Neptune (east) 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. Once Uranus sets, the sky Jupiter’s equatorial diam- remains devoid of planets eter grows from 39" to 43" planet spins once on its axis in 11th, 24th, and 25th — but the until Jupiter rises shortly during March, plenty big less than 10 hours, so if you events on the 11th and 25th before midnight. The giant enough that any scope can don’t see it right away, you provide the best views. planet lies against the back- deliver exquisite views. The shouldn’t have to wait long. The moons all orbit Jupiter drop of Libra the Scales, but it most prominent features are Once you’ve examined in the same plane, and that is far brighter than any of that the two dark belts that strad- Jupiter’s disk, look for the plan- plane tilts significantly to our constellation’s stars. Jupiter dle a brighter zone coinciding et’s four bright moons. They line of sight this month. This shines at magnitude –2.2 in with the equator. More subtle present a constantly changing means outermost Callisto can early March and at magni- aspects abound on the periph- vista as they revolve around pass north or south of the tude –2.4 by month’s end. ery of both belts. the planet. Io shifts position planet. You can see one of If you target the planet Also keep an eye out for most quickly because it circles these rare events the night of through a telescope, wait until the Great Red Spot. This the planet in only 1.8 days, March 24/25, before Callisto an hour or two before dawn. massive storm lies near the while Europa takes 3.6 days, joins the other three moons This is when it appears high- southern edge of the South Ganymede 7.2 days, and west of Jupiter. est in the south and its light Equatorial Belt and displays a Callisto 16.7 days. These move- A few hours after Jupiter passes through less of Earth’s distinctive pinkish-red color. ments occasionally bring all rises, Mars pokes above the image-distorting atmosphere. You can see it about half the four satellites to one side of the southeastern horizon. The You’ll be rewarded with time — whenever Jupiter’s planet. For North American Red Planet has begun what sharper views of fine details rotation places it on the hemi- observers, this happens four will be its finest appearance in the jovian cloud tops. sphere facing Earth. The times in March — on the 1st, in 15 years. Although it won’t COMETSEARCH Going to California Comet PANSTARRS (C/2016 R2) N It appears as though our extend- dark-sky site during the Moon- ed drought without a bright free window from March 5 to 21. NGC 1499 comet will continue through If you happen to be out for a j the spring. But while Comet Messier marathon the weekend 21 PANSTARRS (C/2016 R2) may lack of March 16 and 17, spare a in luster, it shines for its consis- few minutes to track down 17 tency. This first-time visitor from C/2016 R2. A 4-inch or larger 54 E the distant Oort Cloud should telescope will reveal the comet, PERSEUS remain at 10th or 11th magni- but you’ll want to pump up the 13 tude for several months. power to 150x or so to see subtle Path of Comet PANSTARRS Comet PANSTARRS lies high detail. Look for a fuzzy, out-of- 9 in the west after darkness falls in round object with a brighter c March. You can find it between core and a more defined edge March 5 the star patterns representing toward the southwest. 1° Perseus the Hero and Auriga The enduring popularity of the Charioteer. Deep-sky observ- comets comes in part from their Spring’s brightest comet rides high in the evening sky during March. The ers know this region best as wide range of shapes and sizes. dirty snowball slides southeast of the California Nebula (NGC 1499). home to the California Nebula When located far from the Sun, (NGC 1499). The comet slides they often mimic elliptical galax- lies in the Milky Way, far from Both consist of gas and dust lit less than 5° southeast of the ies. And PANSTARRS qualifies — any comparison galaxies. up by starlight. And there’s no nebula in mid-March. at its closest to the Sun this May, C/2016 R2 does share some doubt that somewhere within To best see PANSTARRS, it will lie well beyond the orbit of characteristics with the nearby NGC 1499 lurks a star with an you’ll want to observe it from a Mars. But our comet currently California Nebula, however: Oort Cloud of its own.

42 ASTRONOMY • MARCH 2018 The Red Planet meets the deep sky N LOCATINGASTEROIDS Discover the nearest dwarf planet + SAGITTARIUS March offers a great opportuni- 6th-magnitude stars labeled ty to track down the king of the Sigma1 (σ1) to Sigma4 (σ4) Cancri Saturn Path of Mars asteroid belt. Ceres dims from on the chart below. Then head M20 magnitude 7.4 to 8.0 this month, outside, dark adapt, and add E which puts it within range the stars you see to the west of 19 March 31 28 25 22 through binoculars or a small this field. One of these points of M22 16 M8 telescope from the suburbs. The light will be Ceres. To find out M28 object resides in Cancer, which which one, return to this field in h stands high in the east at night- a week or so and see which dot fall and passes nearly overhead has shifted position. around 10 P.M. local daylight Astronomers once thought 1° time in mid-March. Ceres was the supposed miss- Ceres lies about two binocu- ing planet between Mars and Mars shines brightly as it passes near several bright nebulae and star clusters in northern Sagittarius during the second half of March. lar fields north of the Beehive Jupiter. It was later down- star cluster (M44). You can get graded to an asteroid and then, a head start by sketching a in 2006, elevated to dwarf peak until July, the rocky the 7th-magnitude globular framework consisting of 4th- planet status. Italian astrono- world improves noticeably in star cluster M28 on March 28. magnitude Iota (ι) Cancri and mer Giuseppe Piazzi discovered March. Observers and astro- The planet appears 1.3° north the group of four 5th- and Ceres on New Year’s Day 1801. imagers who want to get the of the cluster that morning. most out of this summer’s Mars ends the month just Ceres slides through Cancer the Crab show should start honing their 0.9° west-northwest of an skills now. even brighter globular, 5th- N Mars begins the month magnitude M22. The planet m  among the background stars then shines at magnitude 0.3, m   of Ophiuchus, some 12° east- some 60 percent brighter than m m  March 1 11 northeast of Antares in neigh- it started March. boring Scorpius. The planet Mars brightens this month Path of Ceres 21 glows at magnitude 0.8, a in part because its apparent E touch brighter than the star. diameter grows by 25 percent, 31 If you take a moment to study from 6.7" to 8.4". You should 57 the colors of the two objects, be able to tease out a few sur- you’ll understand why ancient face details during moments 61 CANCER observers named the star of steady seeing, particularly Antares, which literally means as it grows larger late in the “rival of Mars.” month. Look for the distinc- The planet moves steadily tive dark smudge of Syrtis 0.5° eastward against the starry Major — one of the planet’s f backdrop during March. It most conspicuous features The biggest object in the asteroid belt between the orbits of Mars and crosses into Sagittarius on the — on March’s final few Jupiter executes a tight loop in northern Cancer this month. 12th, setting up a series of mornings. wonderful conjunctions with If you track Mars all some of the Milky Way’s best month, you’ll notice it striking color contrast with its the 31st and likely will be lost and brightest deep-sky objects. approaching another promi- ruddy neighbor. in twilight from mid-northern It passes midway between the nent point of light. On the Enjoy viewing Saturn latitudes. (Your chances Lagoon Nebula (M8) and the 31st, the Red Planet pulls against the Milky Way back- increase the farther south you Trifid Nebula (M20) on the within 1.7° of Saturn. The drop with your naked eye or live.) It will return to view for morning of March 19, provid- ringed planet orbits much binoculars. You’ll need a tele- everyone by late April. ing a stunning binocular view farther from the Sun than scope to see its superb rings, and a prime photo opportu- Mars and thus moves east- which span 37" and tilt 26° to Martin Ratcliffe provides plane- nity. The trio climbs 20° above ward more slowly. It remains our line of sight this month. tarium development for Sky-Skan, the horizon by 5:30 a.m. local about 2° north of M22 all You also might catch a brief Inc., from his home in Wichita, daylight time, just before twi- month. Saturn appears con- glimpse of Neptune at the end Kansas. Meteorologist Alister light starts to dim the Milky spicuous at magnitude 0.5, of March. The outer planet Ling works for Environment Way star clouds. and its golden hue offers a rises an hour before the Sun on Canada in Edmonton, Alberta. As Mars continues across Sagittarius, it meets up with GET DAILY UPDATES ON YOUR NIGHT SKY AT www.Astronomy.com/skythisweek.

WWW.ASTRONOMY.COM 43 ASKASTR0 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 being studied, we do know that gas as they form, but also expel they are driven by rotating material in outflows that shoot magnetic fields in the young from their poles. Whether brown dwarf. These magnetic lower-mass objects undergo the fields sweep up material KELT-9b is the hottest gas giant discovered to date. As it orbits its huge, same process as their stellar from the gas accreting into 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 cousins was an open question the brown dwarf and drive artist’s concept. NASA/JPL-CALTECH until the first substellar object some of it toward the poles, (aka, brown dwarf) was found where it ends up being ejected Q: WHY ARE GLOBULAR than the Sun is from Earth. But undergoing this process by as a jet. Note these jets can be CLUSTERS NOT even at these distances, the Emma Whelan and collabora- quite large, often several CONSIDERED GALAXIES? magnetic fields of individual tors. Their find was published ! (One is 3.26 HOW FAR APART ARE THEIR stars would have little effect on in Nature in 2005. light-years.) STARS, AND DO THEIR each other. Mayrit 1701117 is one of the When these jets impact MAGNETIC FIELDS AFFECT If you were flying through newest examples of a young other gas along their path, they EACH OTHER? CAN A the core of a globular cluster, brown dwarf exhibiting a jet. can excite it and become CLUSTER’S STARS HAVE you likely wouldn’t need sun- In fact, it is so early in the pro- detectable on images or via PLANETS, AND WOULD YOU glasses because even at these cess of forming that it is spectroscopy, allowing us to NEED SUNGLASSES TO close separations, individual really a “proto” brown dwarf, study properties of the jet itself TRAVEL THROUGH ONE? stars would still appear dim- with plenty of accretion from as well as the nearby gas. Martin Heuer mer than our Moon. But given its surroundings left to Scott Fleming St. Petersburg, Florida that you’d have many stars in undergo. While a lot of the Archive Scientist, Space Telescope close proximity to each other, details about these jets are still Science Institute, Baltimore A: Globular clusters aren’t con- there would be nearly 1,000 sidered galaxies because they stars brighter than the planet are gravitationally bound to Venus in the nighttime sky of and orbiting galaxies like the any fictitious planet near the Milky Way, and they have rela- center of a globular cluster. tively small masses. When Their combined light would comparing the two, a typical add up to roughly the light of a globular cluster might contain Full Moon. a mass of 100,000 Suns, In the center of a globular whereas the Milky Way has cluster, conditions aren’t favor- nearly 1 trillion solar masses. able for the formation of plan- In other words, the Milky Way ets because the tidal forces of Galaxy contains 10 million passing stars could destroy any times more mass than a typical protoplanetary disks. If a planet globular cluster. did somehow form, it would The stars in a globular clus- still find itself in danger, since ter are 50 times closer to each the occasional close passage of other than the stars in our a star would likely disrupt the solar neighborhood. To put this planet’s orbit, flinging the in perspective, in a typical planet into interstellar space. The forming brown dwarf Mayrit 1701117 (bright orange-yellow) has globular star cluster, we’d Brian Murphy a 0.7 light-year-long jet, shown in green emission from ionized sulfur likely find stars separated by a Professor of Physics and Astronomy, in this image. CESAR BRICENO AND SOAR/NOAO/AURA/NSF distance 5,000 times greater Butler University, Indianapolis

44 ASTRONOMY • MARCH 2018 Q: This eclipse sequence SINCE THE SUN AND begins at upper left MOON MOVE FROM EAST TO and ends at lower right. WEST, WHY DID THE ECLIPSE During this event, MOVE FROM WEST TO EAST? the Moon moves from west to east, across the Mary Lanphier Sun’s face. BEN COOPER 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 direc- tion 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, 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, “Flying Around the Eclipse Shadow,” which illus- this; rather, it is the mass of the exoplanets, which look like learn about their atmospheres trates this geometry if you’re planet and the radiation from giant comets, with a tail of gas and environments, the more still having trouble picturing it. the star it orbits. For the atmo- streaming away from the direc- we can understand and per- You can watch it online at sphere to escape the planet, it tion of the planet’s orbit. haps work out why that did not https://svs.gsfc.nasa.gov/4579. must overcome the gravita- How the planets got to orbit happen in our own solar sys- Alison Klesman tional pull of the planet itself. so close to their stars in the tem — and whether that might Associate Editor The more massive the planet, first place is another question, be the reason I am here to the more likely it is to hold on and it is still a matter of scien- answer your question! to its atmosphere. tific inquiry. The process that Hannah Wakeford Q: MANY EXOPLANETS However, the host star can moved them so close to their Giacconi Fellow, Space Telescope ROTATE AROUND THEIR help the atmosphere escape. stars is called migration; there Science Institute, Baltimore, SUN IN DAYS INSTEAD OF Sometimes the gravity of the are a number of theories as to and Research Fellow, University YEARS, AS IN OUR SOLAR host star pulls the atmosphere what causes it. For example, of Exeter, United Kingdom SYSTEM. HOW DO YOU from the planet because it is another star could have come EXPLAIN THIS, AND HOW stronger. More commonly, the close to the system, pulling the CAN THEY MAINTAIN THEIR intense heat from the star and planet and moving it inward. Send us your ATMOSPHERES WITH THOSE energetic explosions called Or, while the planet was form- questions INCREDIBLE SPEEDS? flares give so much energy to ing, other forming planets Send your astronomy Daniel Gerritsen the particles in the planet’s could have caused an imbal- questions via email to Baarn, Netherlands atmosphere that they vibrate ance, prompting the planets to [email protected], and move enough to overcome move around until they settled or write to Ask Astro, A: You are right in thinking it the planet’s gravity and escape. and became stable. P. O. Box 1612, Waukesha, is hard to hold on to an atmo- In this way, the atmosphere The truth is, we are not sure WI 53187. Be sure to tell us sphere under those conditions, will lose its lightest constitu- yet what caused this to happen your full name and where and not all close-in exoplanets ents first, and astronomers can to more than 300 known exo- you live. Unfortunately, we have been able to do it. But it is detect hydrogen streaming planets, spiraling them in to cannot answer all questions not the speed of the planet away. We have been able to settle where we see them today. submitted. around the star that dictates measure this for some But the more we discover and

WWW.ASTRONOMY.COM 45 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 fields around the spacecraft. It helped scientists learn about Saturn’s magnetosphere as well as the interiors of the planet and its moons. IMAGING SCIENCE SUBSYSTEM (ISS) RADIO AND PLASMA This instrument WAVE SCIENCE contained a pair of digital INSTRUMENT (RPWS) cameras — a wide- As the name suggests, angle one for context this suite of antennas and a narrow-angle and sensors detected one for high resolution. radio and plasma waves. ISS photographed the The instrument “heard” Saturn system at visible, radio emissions from ultraviolet, and infrared lightning in Saturn’s wavelengths. atmosphere and from the planet’s aurorae.

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 constitu- ent 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

n October 15, 1997, the main engines of a Titan/Centaur rocket course and get close-up views of the planet’s atmosphere, rings, ignited, and the Cassini spacecraft and attached Huygens probe magnetic field, and dozens of smaller but no-less-intriguing Orose into the sky above Cape Canaveral, Florida. The rockets moons. Early on, Huygens dropped from Cassini, parachuting didn’t have enough power to get the pair out to Saturn, how- through Titan’s thick, hazy atmosphere and landing on the surface ever. So, Cassini and Huygens embarked on a circuitous trip through in January 2005. the inner solar system, stealing a bit of orbital energy from Venus Together, the two spacecraft carried 18 scientific instruments: (in April 1998 and June 1999) and Earth (in August 1999). The probes Cassini held 12 and Huygens six. These powerful tools unveiled the received an even bigger boost from massive Jupiter in December ringed world and its surroundings in unprecedented detail. And 2000, setting them on course to reach Saturn in June 2004. amazingly, only one of the orbiter’s instruments, the Cassini Plasma For 13-plus years, Cassini orbited the giant planet. Engineers Spectrometer, failed before the spacecraft burned up in Saturn’s used the gravity of Saturn’s biggest moon, Titan, to tweak its atmosphere September 15, 2017, nearly 20 years after launch.

HUYGENS PROBE RADAR This instrument sent radio signals through DESCENT IMAGER/SPECTRAL Titan’s hazy atmosphere and recorded how RADIOMETER (DISR) long they took to return, allowing scientists to build high-resolution pictures of the This instrument’s imagers built moon’s surface. up mosaics of the moon’s surface in the landing site’s vicinity. Solar sensors measured the Sun’s RADIO SCIENCE SUBSYSTEM (RSS) intensity and allowed scientists The RSS used Cassini’s high-gain antenna to to study the size and density of send radio signals to Earth through the rings airborne particles. Still another or the atmospheres of Saturn and its moons. sensor measured the atmo- Scientists then studied how the intervening sphere’s heat flow. material altered the signal to learn more about its structure. GAS CHROMATOGRAPH MASS SPECTROMETER ION AND NEUTRAL MASS HUYGENS (GC/MS)

SPECTROMETER (INMS) ATMOSPHERIC DOPPLER WIND This instrument pair UVIS NASA/JPL/UNIVERSITY PHOTO: CH/SSI; ESA COLORADO; OF HUYGENS PHOTOS: This instrument determined the chemical STRUCTURE EXPERIMENT (DWE) analyzed gases in Titan’s composition of neutral particles and low- INSTRUMENT (HASI) DWE recorded wind atmosphere at high energy ions, particularly in Titan’s upper Multiple sensors measured speeds of up to 270 mph altitudes and near the atmosphere and in Saturn’s rings and the density, pressure, (430 km/h) in Titan’s surface. The two deter- magnetosphere. temperature, and electrical atmosphere. Although mined how the abun- properties of Titan’s atmo- Cassini never received dances of nitrogen and CASSINI PLASMA SPECTROMETER sphere during the probe’s the data, Earth-based methane changed with (CAPS) descent. A microphone radio telescopes recov- altitude and discovered also recorded sounds. ered some of it. argon in the air. CAPS measured the energy, electrical charge, and direction of motion of charged particles. One of its sensors also determined the mass SURFACE of each particle. A short circuit in CAPS ended SCIENCE its life in June 2012. PACKAGE (SSP) Multiple sensors MAGNETOSPHERIC IMAGING helped determine INSTRUMENT (MIMI) the physical proper- ties of the surface MIMI’s three sensors worked in concert at Huygens’ landing to detect energetic charged particles in site, including Saturn’s vast magnetosphere, to under- its hardness and stand how it interacts with the solar wind. structure. Several of SSP’s sensors were designed to work in ON THE BACK SIDE a liquid environ- AEROSOL COLLECTOR AND PYROLYSER (ACP) COSMIC DUST ANALYZER (CDA) ment in case the The ACP pulled in aerosol particles and heated them to vapor- probe landed in a This detector established the size, speed, ize volatile substances and decompose organic compounds. sea or ocean. direction of motion, and chemical compo- It then passed these products to the GC/MS for analysis. sition of tiny dust particles near Saturn. CASSINI SPACECRAFT: NASA/JPL; VIMS PHOTO: NASA/JPL/ASI/UNIVERSITY VIMS PHOTO: NASA/JPL; CASSINI ARIZONA/UNIVERSITY OF SPACECRAFT: LEICESTER; OF NASA/JPL-CALTE PHOTO: ISS

WWW.ASTRONOMY.COM 47 72 minutes

The Huygens probe WHEN THE HUYGENS PROBE dropped into Titan’s atmosphere became — and thus far remains — the most January 14, 2005, no one knew what to expect. Would it splash distant human-made landing craft when it down into a methane ocean? Sink into a tar pit? Crash into sharp touched down on Titan’s surface in 2005. NASA 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, about Saturn’s largest moon. What the mission scientists plotted out landing sites Huygens descent probe would find was with meticulous care. Telescopes and orbit- anyone’s guess. Huygens had to be pre- ers scanned the ground, imaging danger- pared for anything. ous terrain and safe zones, and flight Alex Hayes, a Titan researcher at engineers pored over their maps and Cornell University who has been part of planned accordingly. the Cassini orbiter team since the craft’s But Titan was a mystery. Aside from a arrival at Saturn, is enthusiastically proud brief pass by Voyager 1, little was known of the probe’s success, though he didn’t

48 ASTRONOMY • MARCH 2018 In 2005, the Huygens probe pierced the moon’s shroud to reveal a surprisingly Earth-like world. onon TitanTitan by Korey Haynes

work with Huygens himself. “No matter While Huygens did touch down and Huygens might land, the mission team what data Cassini collects over the 13 years collect data on Titan’s surface, that outcome designed the craft for any condition. of its mission, there is something special was far from certain. When Voyager 1 Engineers built Huygens light enough to about reaching out and touching some- passed by the moon in 1980, it couldn’t float and with enough battery life to oper- thing,” he says. “There’s something special peer through Titan’s thick atmosphere and ate for at least a short while on the surface about landing on the surface, about getting obscuring clouds. The best it could offer — assuming it survived impact. But over- data from the surface, and Huygens pro- was a tantalizing reveal that Titan sported all, engineers designed a descent probe: vided that ground truth.” organic materials, which led to the under- Huygens would collect all of its primary standing that it was possibly covered in science during a fall through Titan’s atmo- PLANNING FOR THE UNKNOWN oceans made of methane or ethane. Later, sphere lasting two to two and a half hours. Huygens was the European Space Agency’s the massive Goldstone radio telescope Whatever it did or didn’t see afterward contribution to the greater Cassini-Huygens received radar echoes from Titan indicat- would be a fabulous bonus. mission. With very few exceptions, all of ing that at least some solid surface existed Engineers at Caltech even had a betting Huygens’ instruments and components were under the clouds. pool going for what Huygens would find on built by individual members of ESA, culmi- But without any mapping of Titan’s sur- touchdown, with options for “ice,” “tar,” nating in one magnificent spacecraft. face, and limited control over where “liquid,” “undeterminable,” “DOA,” and

WWW.ASTRONOMY.COM 49 the gases making up the air it sailed through. Another instrument 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 car- ried 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. 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 Cassini well into its jour- ney toward Saturn, an engineer took it 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 upon himself to test the communications black antennas extend from the flat top of the probe. ESA on the spacecraft. He pinged Cassini with a simulated message from the Huygens’ engi- neering 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 on Cassini handled Doppler shift- ing of signals it received. As the craft moved, any signals approaching it would shift in frequency, the same way a siren rises and falls 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 millions of miles away. Luckily, the team had four years to work on a solution. If the receiver couldn’t han- dle Doppler shifting, then the team would avoid motions that caused such shifts. But this meant changing how the orbiter and probe would maneuver through the Saturn system. Instead of releasing Huygens on its 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 first Titan pass, Cassini would now cart would deploy as expected on Titan. ESA Huygens along for a few Titan flybys, slow- ing down with each pass until it could even a facetious vote for “eaten” — as in by and how do global wind patterns flow? release the probe more gently, on a path sea monsters. What does the top of the atmosphere look that minimized the Doppler shifting of the To conduct its science, Huygens was like and how does it behave, including the probe’s transmissions. Another solution equipped with six main instruments, aimed ionosphere? And what are conditions like at called for the probe to “wake up” earlier to answer a slew of questions: What gases the bottom of the atmosphere, just above or than initially planned after separating make up Titan’s atmosphere, and what — fingers crossed — at the surface? from Cassini, since the temperature of the kinds of particles, hazes, or clouds float Huygens carried an aerosol collector to instrument also influenced the signal. there? What chemicals churn through the sample tiny particles floating through Although these changes used up pre- skies? How warm or cool is the atmosphere, Titan’s skies and a spectrometer to analyze cious fuel (for Cassini) and battery life (for

50 ASTRONOMY • MARCH 2018 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), 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. Only after Huygens was well into its descent at Titan did operators notice that only one of Cassini’s two channels was relaying infor- mation from the probe. Huygens was meant to send information over both of Cassini’s channels, Channel-A and Huygens’ final landing Channel-B. But Cassini’s programming site (left) reveals a bed of was missing a crucial command to turn on water and hydrocarbon ice, dotted with rocks the Channel-A receiver. showing smoothed edges While critical data was duplicated on and other signs of erosion. both channels, and some other transmis- This image was taken with sions were eventually recovered directly by the probe’s Descent Imager/ Spectral Radiometer and Earth-bound receivers, much other infor- colored based on spectral mation, including half of Huygens’ images, data to give a true sense of was lost forever. Even so, the information the terrain’s appearance. that Huygens sent back was enough to take A familiar image of an astronaut’s footprint from Titan from fuzzy orange ball to a fully real- the Apollo Moon landings ized world, in the span of only a few hours. (right) illustrates the scale of Huygens’ view. ESA/NASA/JPL/ GERONIMO! UNIVERSITY OF ARIZONA 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 sent Far from reassuring, the ESA team back this found the sight eerily reminiscent of the stereographic projection picture captured by the Mars Express view of orbiter the previous year of the Beagle 2 Titan’s surface probe, just before it disappeared while features from a height of dropping to the Red Planet’s surface. That 3 miles (5 km) craft would not be found for 12 years. as it descended “When Cassini took the image of toward a Huygens on its descent, we had to hope surface that appeared that wasn’t the last we saw of Huygens,” much darker recalls Ralph Lorenz, a member of the than planetary Huygens team who has also written scientists had numerous books on the subject. expected. ESA/ NASA/JPL/UNIVERSITY OF Huygens entered the atmosphere ARIZONA 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 stabi- lized, Huygens blew off its front heat shield, ready to start its science mission. Huygens immediately started analyzing and recording, snapping its first image as it drifted 89 miles (143 kilometers) above Titan’s atmospheric profile Titan’s surface. It sampled the atmosphere 140 0.003 as it passed through, measuring electrical signals and cataloging its journey in detail. Tholin haze After 15 minutes, Huygens ejected its main parachute and continued descending 120 under a smaller chute. Mission engineers had planned this switch-over to allow Huygens to explore the upper atmosphere Temperature first, then descend more quickly so it 100 0.01 would still have battery life by the time it reached the ground, if it survived. Huygens continued collecting data as it

Pressure (bar) descended more rapidly through Titan’s 80 haze and clouds, encountering some turbu- lence on the way — nothing the little probe Condensate haze 0.03 couldn’t handle. As luck would have it, Huygens did not Altitude (km)Altitude 60 land on sharp rocks or hard ice, which might have crumpled the craft. Neither did 0.1 its parachute obstruct its view — a concern held by a few members of the mission 40 team. It did not splash down in any of Methane-nitrogen clouds Titan’s numerous lakes or seas. Instead, it (CH4-N2) 0.3 thumped gently down onto a bed of some- thing with the consistency of damp sand or 20 packed snow, the ground around it strewn Plume with rocks and pebbles that wouldn’t look Ethane fog 1.0 out of place on an earthly lakeside beach. (C2H5) 0 Lake 1.5 Safely aground, Huygens continued its Possible Impacts melt mission. It assiduously recorded image River water/ammonia Organic water/ammonia (H2O/NH3) volcano sediments (H2O/NH3) after image of its final resting place for 72 80 100 120 140 160 180 minutes after touchdown. In all, it sent Temperature (K) back some 100 pictures of the same slice of terrain before Cassini and its link to Earth Titan’s atmosphere consists of a hazy upper layer and a liquid-driven weather cycle closer to the surface. Although Titan is smaller than Earth, the pressure at its surface is greater because its atmo- disappeared over Titan’s horizon. A short sphere is thicker. The green line shows Titan’s atmospheric profile: the temperature and pressure of time later, its batteries ran out, and the its atmosphere at a given height. ASTRONOMY: ROEN KELLY, AFTER ESA/NASA/JPL probe quietly shut down.

52 ASTRONOMY • MARCH 2018 South West North East South 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 drainage networks similar to those 100 miles (150 km) found all over Earth: small channels feed- ing 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, and likewise showed signs of methane rain erosion. Other riverbeds hinted at gentler streams. Scientists think these are fed not by rainfall but from “spring sapping,” where liquid methane wells up through the ground. 10 miles (15 km) 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 resem- bled a dried lake bed or floodplain. Scattered around Huygens’ base were cob- blestones, edges rounded as if shaped by flowing liquid. The stones are of a similar

1.2 miles (2 km)

0.25 miles (0.4 km)

The Huygens probe showed a landscape shaped This progression of vistas from four different altitudes, from highest (top) by the flow of liquid on Titan’s surface, such as to lowest (bottom), shows flattened (Mercator) projections of the moon this drainage system thought to channel liquid as Huygens punched through Titan’s haze to reveal its strangely Earth-like methane runoff into a larger river. NASA/JPL/ESA/ surface features. ESA/NASA/JPL/UNIVERSITY OF ARIZONA 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/ A 360° mosaic of images snapped from about 5 miles (8 km) above Titan’s surface shows a plateau (center) and Huygens’ Spectral Radiometer. eventual landing site (darker area on the right side of the image). This image and other data from the probe have been used NASA/JPL/ESA/UNIVERSITY to determine that the wind speed in Titan’s atmosphere was about 4 mph (6–7 km/h). NASA/JPL/ESA/UNIVERSITY OF ARIZONA OF ARIZONA

WWW.ASTRONOMY.COM 53 and atmosphere, and released by ice volca- noes. But whether this process is powered by Titan’s own internal heating mecha- nisms, by the heat of Saturn’s tidal pull, or even if the process truly exists at all is still under debate. While Voyager’s original methane dis- covery had raised the faint specter of alien biology as its origin, Huygens laid these hopes mostly to rest. Scientists knew that some kind of activity must refresh Titan’s methane stores, or else sunlight would destroy the gas in a matter of a few million years. But the arrangement of methane layers in Titan’s atmosphere, coupled with the carbon isotopes Huygens sampled, indicated once again that geologic pro- 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 cesses were the likely methane source. measurements of the atmosphere and imaged the world around it, continuing to beam back data However, in Titan’s haze layers, from its landing site for over an hour. ESA–C. CARREAU Huygens detected molecules similar to tholins produced in earthly laboratories. size, implying that the same currents might the temperature, pressure, and abundance Tholins are thought to be important to the have moved all of them, but scientists of gases from the atmosphere’s top all the development of life on Earth, and the com- remain unsure whether this is the case. way to the ground, creating a one-dimen- plex carbon molecules are a source of Hayes points out that the rounded cob- sional map of Titan’s skies. It revealed high active research. Their presence on Titan is bles near Huygens’ landing site appear like levels of stratification, passing from one an encouraging sign that the building stones ground smooth by a river carrying zone to another as it fell. blocks of life are not unique to Earth. them over distance. But on Earth, a river One of Huygens’ goals was to hunt for drops larger stones earlier in its path, then noble gases, such as argon. Noble gases are SMALL CRAFT, smaller stones as the flow begins to peter chemically disinclined to join with other BIG CONTRIBUTION out. “What intrigues me personally,” elements to form compounds, so their While Huygens’ science mission lasted a Hayes says, “is that in the decade since abundances hint at long histories, stretch- fraction of Cassini’s decade-plus adven- those images and data were taken, we ing back to the availability of these gases at ture, its contributions were mammoth. It started to question everything, or find the birth of the solar system. Their pres- remains the only human-made craft to that every answer you get leads to three ence helps scientists understand how touch the face of any moon other than new questions.” Titan’s atmosphere came to be — and, like- our own. And its “come what may” design Huygens quite literally scratched Titan’s wise, how other worlds like Earth might approach gives it pride of place even among surface. And by opening up an entire new have attained similar thick atmospheres. planetary exploration missions, already an world to researchers, it also jump-started a But Huygens, despite descending quite intrepid collection of engineering projects. new generation of research, inviting ques- literally through the thick of things, “All of our atmospheric knowledge is tions by the thousands. Scientists then and detected low abundances of argon com- tied to that one observation taken at the now look to Huygens as the only eyewit- pared with nitrogen, especially a particular equator,” Hayes points out. This makes ness to an entire complex world, but stud- isotope known as argon-36. Huygens found Huygens, in his words, the linchpin of ies are limited by the short time and tiny it roughly a million times less abundant Titan atmospheric science: “Any predic- area the probe could explore. than in the Sun, implying that Titan could tions or interpretations you make about So far from the Sun and under Titan’s not have gathered its atmosphere directly Titan, you have to show how it’s explained hazy skies, Huygens took its pictures in a from the early solar nebula. Instead, its by what Huygens saw, or provide a reason twilight sort of lighting. At one point, its atmosphere was likely delivered by bom- why it should be different.” vision included a dewdrop that formed on bardments of space rocks, bolstering the Over a decade later, researchers con- the probe’s exterior. While probably case for Earth’s atmosphere forming in the tinue to mine the data and publish new induced by Huygens itself and the heat same way. findings. And any future Titan mission from its landing, the single drop was none- On the other hand, detection of another will certainly start with Huygens’ success theless the first in situ sighting of liquid on isotope, argon-40, tells a different story. story. For all its brevity, the probe saw, a world other than Earth. This isotope arises from the radioactive sampled, and touched what Cassini never decay of potassium found in rocks. For could: Titan, below the veil. COLLECTING GASES Huygens to sniff out such a gas in the On its way down, Huygens sampled the atmosphere implies that Titan must have a Korey Haynes is a former associate editor gases circulating in Titan’s atmosphere and way to release it: an active geologic, or at of Astronomy who now works as a freelance confirmed they were mostly nitrogen and least cryologic, cycle where rocks or ice are science writer and outreach specialist in St. Paul, methane. More importantly, it measured churned from Titan’s depths to its surface Minnesota.

54 ASTRONOMY • MARCH 2018 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. EXOPLANETSIN PURSUIT OF

Two massive telescopes MICHAEL ENDL IS ON A MISSION: zeroing in on the answers to those questions. As a research scientist with the University in the Lone Star State of Texas at Austin, he hopes the sum of his Search techniques monitor 450 suns in astronomy career will be a chart character- Endl and his colleagues find extrasolar plan- izing exoplanets, like you might find on a ets using techniques simple in theory but the hopes of finding futuristic starship exploring faraway stars. painstaking in execution. The two most pro- other worlds. As a planet hunter, Endl is a member of a ductive are the transit and the radial velocity growing league of astronomers who seek other methods. The transit method is well suited for text and photographs worlds like our own to answer basic questions: space observatories like NASA’s Kepler space- by Robert Reeves Do certain types of stars host only certain craft. It can stare at a field of stars for weeks types of planets? What’s the frequency of while measuring any stellar brightness dip rocky planets within a star’s habitable zone? caused by a planet crossing in front of a star. Do the atmospheres of Earth-sized exoplanets The transit method is limited to detecting contain biosignatures indicating possible life? planets whose orbital planes are aligned with The exoplanet search is an exciting field. our line of sight, presumably only a small per- A generation ago, it was considered a career centage. The transit method has had success dead end. Now, Endl and his colleagues are scooping up hundreds of exoplanet candidates

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1 1. The Harlan Smith telescope’s closed tube design makes it unique. because of Kepler’s ability to stare at A row of teapot- thousands of stars at once. shaped cooling The radial velocity method is fans around the more forgiving of observational gaps base stabilizes air currents within caused by daylight or poor weather the tube, allowing and is thus better suited for Earth- the telescope based observatories. This technique to achieve 1" uses spectroscopy to measure a star’s resolution. velocity changes. These changes 2. Astronomer occur because of a planet’s gravita- Michael Endl tional pull on its star. This works ponders his next target as well for many stars, but massive he progresses ones are not affected much by the through his list pull of small Earth-like planets. of exoplanet When a planet is detected, the radial 3 candidate stars. velocity method can also be used to 3. The Harlan determine its minimum mass. Smith telescope Not all stars are candidates for planets seem to be plentiful, but we intense, and their habitable zone is peers into the night sky while searches. “Triple star systems are want to determine if Jupiter-sized closer to the star. Planets circle recording spectra. ignored because the combined stel- planets are the rule.” M-type stars in several days, and the Researchers lar gravity fields destroy planetary star’s lower mass responds more use the data formation disks,” Endl says. “Close Exoplanet science at readily to the planet’s gravitational in their search for exoplanets. binary stars are also ignored by McDonald Observatory perturbations, making them easier to radial velocity surveys because the The University of Texas exoplanet detect. A planet close to an M-type component stars have radial veloci- program is but one of many searches star can even be detected in one mul- ties in kilometers per second, mak- worldwide. It began in 1988 and tiday observing run.” Although plan- ing it impossible to discriminate the routinely observes a set of 200 ets around M-type stars may be meters-per-second velocity changes nearby stars with the 2.7-meter relatively easy to find, Endl empha- induced by a planet’s gravity.” Harlan Smith Telescope. It targets sizes that long-term observations are Endl has been spent 20 years nearby suns across all stellar classes. necessary to refine masses and using radial velocity to find exo- Another 250 stars are the targets orbital periods. planets. “Today, the existence of of the 10-meter Hobby-Eberly The Harlan Smith Telescope was exoplanets is no longer in doubt,” he Telescope at McDonald Observatory built in the 1960s with help from says proudly. “The discovery rate is in West Texas. NASA to support the Apollo pro- not as important as characterizing Small M-type stars are popular gram. The telescope can be config- the known exoplanets. The goal of search targets because they are more ured to feed light into a massive our research is to find the difference abundant than G-type stars like our spectrograph that takes up the entire between the planets around M-type, Sun. Endl explains more advantages: floor under the telescope. It is well G-type, and supergiant stars. Rocky “Being smaller, their radiation is less suited for recording spectra for radial

56 ASTRONOMY • MARCH 2018 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.

5. Like all visitors touring the telescope’s 4 spectrograph room, the author had to take a selfie using one of the instrument’s large mirrors. The dim 6 lighting inside the spectrograph room required a 10-second observed below about 25° altitude. exposure. The observer controls the tele- 6. Endl points out scope. The desired target stars are the thorium-argon listed in a software script that selects emission lamp and flat-field box the next star after each spectrum is used to calibrate recorded. An efficient autoguider the telescope’s built into the spectrograph slit guides spectrograph. 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 the expo- sure before the 20-minute limit. If the star is as bright as 4th magnitude, the 5 exposure is only a minute long. When an exposure finishes, the telescope does not automatically move velocity detection. Endl emphasized several things: to the next target. The operator must Exoplanet search time occurs “Important aspects of an observing exit the control room, walk to the tele- during the brighter Moon phases run are good coffee and good music.” scope and dome control desk, and because moonlight has little effect As we listened to his eclectic play- hold down a dead man’s switch to on spectra. Most targets are naked- list and sipped exotic coffee, I move the telescope. This keeps eyes on eye stars, but some are as dim as quickly deduced that finding exo- the telescope to prevent possible colli- 10th magnitude. The spectrograph planets is not easy. There are few sions with either the pier or objects on is sensitive enough to measure radial “Eureka!” moments when an the dome floor. The operator returns velocities down to 4 meters per sec- observer spots a planet and quickly to the control room and may record as ond, allowing detection of a Saturn- confirms it. Exoplanet searches many as 30 spectra per night. sized exoplanet 5 astronomical units require gathering extensive data that Watching the Harlan Smith from a Sun-like star. (An astronomi- are analyzed over time to prove or Telescope operate was awe-inspiring, cal unit is the average Sun-Earth disprove the existence of a planet but nothing prepared me for the distance.) around another star. stunning complexity of the spectro- The spectrograph exposures are graph. Within its room, which is as Using the Harlan Smith limited to 20 minutes, not because large as the interior of a modest-size Telescope the sensor will become saturated, house, a vast array of relay and cam- Endl’s turn with the Harlan but because Earth’s motion smears era mirrors passes the light beam Smith Telescope comes every four the spectra and makes the star’s from the spectrograph slit at the tele- months. Last summer I joined radial velocity hard to calibrate. scope’s Coudé focus through the dif- him at McDonald Observatory Because denser layers of air absorb fraction grating, then onto a CCD while he searched for exoplanets. and distort starlight, no stars are camera chip.

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7 7. A small commercial telescope intersects part The spectrograph’s CCD detector Light refracts differently as Earth’s of the light beam remains continuously below –100° atmospheric pressure and tempera- entering the Celsius (–148° Fahrenheit), cooled ture change, altering the spectrograph spectrograph and directs it with liquid nitrogen. An operator calibration. To offset this, equipment onto a light meter calibrates the detector each evening keeps the telescope and spectrograph that controls with a thorium-argon emission room at the same temperature. As the the camera’s lamp to match specific emission spectrograph creates an image, an exposure time. lines to specific pixels on the detec- iodine reference spectrum is simulta- 8. The catwalk tor. The operator can adjust the neously recorded near the star’s spec- surrounding position of the spectrum on the tra. This anchors known spectral lines 9 the telescope dome provides CCD detector vertically by tilting to known pixels in the spectrograph, gorgeous views the prism and horizontally by tilting allowing spectral shifts induced by of sunrise in the the diffraction grating. telescope flexure and atmospheric If such options are ruled out, more Davis Mountains. A stellar radial velocity of 1,500 conditions to be removed. follow-up observations search for This event signals the end of the meters per second will shift a spec- Unlike other large professional the telltale radial velocity curve observing run. tral line by one pixel on the CCD telescopes, the Harlan Smith indicating the star is slowly drifting detector. The small stellar radial Telescope has a closed tube. A venti- back and forth along our line of 9. Relay mirrors velocity shifts induced by a planet’s direct the lation system stabilizes the tempera- sight because of the minute pull of a telescope’s light gravity cannot be seen through ture in the telescope, allowing it to planet’s gravity. path onto the visual comparison of the spectra. The routinely achieve 1" resolution. spectrograph slit spectrograph data reduction software A signal from the Kepler space- A bright future and autoguider camera. The brass measures radial velocity shifts to the craft that a star has briefly dimmed Years of research indicate that eyepiece tube nearest 0.002 pixel, allowing the tele- is not sufficient proof that a planet planetary formation is a robust is used to focus scope to detect 4-meter-per-second circles it. Here is where Endl and his mechanism. Kepler data suggests the telescope. stellar velocity shifts. collaborator, Bill Cochran, dig in and that 30 percent of Sun-like G-type The spectrograph’s CCD detector do the detective work. Endl observes stars have 1 to 1.5 Earth-radii plan- is so sensitive that operators are for- the candidate star and transmits data ets within their habitable zones. bidden from turning on fluorescent from each night’s observations to However, the statistical error could lights in the room because lingering Cochran at the University of Texas. be as much as 20 percent; thus, emission will affect the instrument’s Next, the two collaborators deter- Earth-like planets could be as rare observations. Even incandescent mine if the candidate star’s periodic as existing around only 10 percent lights cannot be turned on several dimming is due to factors such as a of G-type stars or as plentiful as cir- hours before calibration or binary companion, intrinsic variabil- cling half of them. observations. ity, or large “starspots” on its surface. Hot Jupiters orbiting close to

58 ASTRONOMY • MARCH 2018 10 Data for a super-Earth 1Cnc e 30

20

10

0 dRV (m/s) –10 HJST –20 HEY KECK LICK –30 0.4 0.6 0.8 1 1.2 1.4 1.6 Phase 11 10. The spectro- graph CCD camera is cooled with their stars are rare. Researchers transiting planets with 1.5 to 2 Earth Telescope launches in spring 2019, liquid nitrogen wonder whether such planets have masses, as well as the hot Jupiters or when the WFIRST satellite comes and controlled migrated inward, destroying the rest that lie close to their stars. However, online in the next decade. through a complex electronic interface. of their star’s planetary system. They many of these exoplanets have a hazy In the meantime, researchers like would also like to know whether atmosphere whose spectra reveal lit- Endl continue their detective work 11. The 0.7-day Jupiter-sized planets are a normal tle about it. by scanning nearby stars and refin- orbital period part of planet formation. Of course, astronomers would like ing techniques. With each new of a super-Earth orbiting the The future of exoplanet research to detect biosignatures — gases pro- observation, Endl fills in the blanks star 55 Cancri is getting brighter. NASA’s 2.4m duced as a byproduct of life. To do and comes closer to answering the is displayed in Wide Field Infrared Survey this with today’s technology would persistent questions about planets this plot of data gathered by Telescope (WFIRST) satellite, sched- require a 30 m telescope or a space far beyond our solar system. Endl’s the McDonald uled for a mid-2020s launch, will mission. Perhaps the Giant Magellan dream of creating the exoplanet Observatory’s have a powerful coronagraph to Telescope under construction in chart is getting closer to reality. His 2.7m and 10m image planets close to a star. Chile will be able to detect biologi- current research will be the facts of telescopes, along with data from Understanding an exoplanet’s cally produced gases in the atmo- future textbooks that describe the Keck and Lick atmosphere is also a key area of spheres of planets orbiting the nearby amazing alien worlds undreamed of observatories. The research. The current problem is stars Proxima Centauri and several decades ago. width of the line that the spectrum of the atmosphere TRAPPIST-1. Direct imaging of a depicting the orbital plot lines of a transiting planet is a fraction of non-transiting exoplanet will be Robert Reeves is an astroimager reflects the the star’s total spectrum. Present- more efficient at detecting biosigna- and author who lives in San Antonio. uncertainty of day techniques can analyze the tures, but such searches will have to He loves to shoot the Moon in the planet’s mass. COURTESY OF MICHAEL ENDL rough atmospheric composition of wait until the James Webb Space high resolution.

WWW.ASTRONOMY.COM 59 THE PHOTOGRAPHIC LEGACY OF Lowell’s Great Refractor Over decades, the observatory’s powerhouse instrument spanning 1750 to 1850 saw major advances in optics and related instrumentation that by Klaus Brasch charted a new course in planetary imaging. extended astronomy beyond the purely visual realm. These included the invention istoric Lowell Observatory in in 1930, 14 years after Lowell’s death. of achromatic lenses and silver-coated glass Flagstaff, Arizona, is com- Amid all these undertakings, it is easy telescope mirrors. Concurrently, German monly associated with two to forget that in the first half of the 20th physicist Joseph von Fraunhofer invented notable astronomical bodies: century, the observatory and its great diffraction gratings and spectroscopy, and, HMars and Pluto. Percival Lowell, who was refractor were at the forefront of many equally important, the German equatorial determined to study the Red Planet and its other scientific discoveries. Notable among mount with its clock drive mechanism. putative canals, established the observa- these were Lowell astronomer Vesto Melvin These advances led to the production of tory in 1895. Its centerpiece was the leg- Slipher’s first spectra of spiral “nebulae” far better and more versatile refracting tele- endary 24-inch Clark refractor. (later determined to be galaxies), showing scopes, as exemplified by Fraunhofer’s leg- Lowell was convinced that the illusive that most were moving away from Earth endary 9.5-inch Great Dorpat Refractor in canals were built by intelligent beings to and thereby setting the stage for Edwin 1824. In the mid-1800s, Louis Daguerre irrigate the deserts of a dying world. In Hubble’s discovery of the expansion of the and others developed the first photographic addition to indulging such fanciful notions, universe. Slipher also discovered via spec- methods, which were quickly applied to however, Lowell was also an innovator and troscopy that the Merope Nebula in the astronomical objects. By the time Lowell competent mathematician who posited a Pleiades radiates by reflected light rather founded his observatory, the preceding trans-Neptunian planet and was deter- than emitted light, thereby confirming the technological advances made much of the mined to find it. He initiated several photo- presence of the interstellar medium. pioneering work there possible. graphic searches for “Planet X,” but sadly It’s important to remember that in sci- Notable in this regard was Lowell’s judi- he did not live to see the success of these ence, key technological advances are often cious application of photography to study efforts. Clyde Tombaugh discovered Pluto a prelude to new discoveries. The century the planets. He primarily attempted to pho- tograph his 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 work by two Lowell astronomers — Carl Otto Lampland and Slipher’s younger brother Earl C. (E.C.) Slipher — continued and per- fected planetary photography well into the 1960s. Lampland was a skilled craftsman who developed novel instrumentation, including specialized cameras for the Clark refractor to facilitate large-scale, multi- image capture on a single photographic One of Percival Lowell’s early sketches of “canals” on Mars, which he believed to be irrigation channels plate. He and E.C. Slipher also pioneered cut by intelligent beings, contrasts with one of his early photographs of the Red Planet. The photo color filter photography to highlight differ- shows no such tiny, linear features. LOWELL OBSERVATORY ARCHIVES ent levels and compositional features of the

60 ASTRONOMY • MARCH 2018 atmospheres of Venus, Mars, Jupiter, and Saturn. Arguably, E.C. Slipher’s signature contri- bution to the advancement of planetary pho- tography was his application of integration printing. By combining successive images of planets into a single print, he could minimize the inherent noise of the grainy photographic emulsions in use at that time, and also vastly improve contrast and detail in the resultant pictures. This process was, 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 Photographic Story (1962) and A 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 the venerable Clark refrac- tor again played central roles. Two NASA- funded programs were initiated at that time, both centered on the observatory and part- ner institutions. One was the International Planetary Patrol Program (IPPP), and the other was the Lunar Aeronautical Charts (LAC) project. The IPPP, led by Lowell astronomer William Baum, involved several observato- ries worldwide and was designed to constantly monitor the major planets photographically whenever they were favor- ably 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 observato- ries around the globe were modified to a stan- dard image

The 24-inch Clark refractor was built just before the turn of the 20th century. A renovation was completed in 2015. DAVID J. EICHER scale and then coupled with specially designed 35mm film cameras. These semi- automatic 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 infor- mation on the atmospheric dynamics of mar- tian clouds and dust storms, rotational currents of the jovian cloud deck, the retro- grade rotation of the venusian upper atmo- sphere, and the physical characteristics of Saturn’s rings. The Clark refractor’s last major scientific contributions did not involve direct photogra- phy, but provided visual backup for astrono- mers, geologists, and cartographers involved with the LAC project in preparation for the Apollo and early spacecraft era. This project combined the best available lunar photo- graphs from Mount Wilson, Lick, McDonald, Yerkes, and Pic du Midi observatories, with visual observations obtained with the Clark telescope. The latter resolved far finer lunar detail than the grainy photographs of the This famous planetary camera was used with the 24-inch Clark refractor for years by astronomer time could record. The LAC series charts pro- Earl C. Slipher. KLAUS BRASCH duced in the early 1960s thus marked the cul- mination of Earth-based lunar mapping efforts. Differing colors of martian storm clouds Current imaging October 16, 1973 with the scope Red Green Blue Ultraviolet 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 fortu- nate to try digital imaging through this clas- sic telescope. I was particularly interested in International Planetary Patrol Program seeing how well my results compared to the Lowell Observatory, Flagstaff, Arizona film-based images of yesteryear, and more specifically to the best lunar charts of the Mars was a frequent target of Lowell Observatory astronomers from 1960 to 1974, when they pre-Apollo era. operated the International Planetary Patrol Program at Lowell and captured Mars through various filters. LOWELL OBSERVATORY ARCHIVES 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 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 camera 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 Examples of Slipher’s finest planetary photographs made with the Clark reveal subtle more serious limitation — chromatic atmospheric and surface features on Jupiter (left) and Mars. LOWELL OBSERVATORY ARCHIVES

62 ASTRONOMY • MARCH 2018 aberration. At the relatively fast f/16 focal ratio, the classic achromat exhibits consider- able secondary color. Since the telescope is equipped with a front-end iris diaphragm, even closing it down to 18 inches left dimin- ished, 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 digi- tal image of Saturn to black and white, it The author took this image of Saturn with the Lowell refractor in 2003. It shows color unprocessed wasn’t quite on par with E.C. Slipher’s best. and processed versions (left and center), and a modern black-and-white version (right). KLAUS BRASCH 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 equip- ment, including a Canon 50 DSLR and just recently an 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 tele- scope with such a large aperture and focal length, atmospheric steadiness, or seeing, is far more critical. This is particularly impor- tant 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 experi- enced 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 This photo-geologic map of the area surrounding Copernicus Crater on the Moon was made modern images taken through the great by Gene Shoemaker and R.J. Hackman in 1960. Topographic details were based on the best Clark refractor. They include a wide-angle Earth-based photos available at the time; Lowell’s refractor contributed some of them. NASA/USGS mosaic of the Catharina, Cyrillus, and Theophilus crater trio on the 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 kilometers) in diameter. Capturing fresh images with a storied old instrument, linked to several of the great dis- coveries about the universe, is thrilling. If you haven’t visited Lowell Observatory, do so. You’ll find history and current science — in areas of solar system, galactic, and extraga- lactic — seeping from the place. It’s an amaz- ing blend of past and present.

Klaus Brasch is a retired bio-scientist and public program volunteer at Lowell Observatory. He is grateful to Lowell Observatory archivist Lauren Amundsen, public program manager Samantha A panoramic view of the Mare Nectaris region of the Moon was created by the author using the Gorney, and historian Kevin Schindler for invalu- Clark refractor. Near center is the crater trio of Theophilus, Cyrillus, and Catharina. KLAUS BRASCH able support and assistance with this project.

WWW.ASTRONOMY.COM 63 Astronomy 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, astroimagers cannot. If you’re taking pho- but often overlooked, component of any tos, you’re far better off stepping up to observing setup is the mount. At a mini- the next level of support, rather than try- mum, a mount needs to do two things: ing to make do with something that is support the payload properly and allow for barely sufficient. smooth tracking at high powers. Enter Celestron’s CGX equatorial Although these requirements seem sim- mount. This welcome piece of equipment ple, a good mount can be a difficult pur- is the company’s new entry in mid/heavy chase. Too often, amateurs deliberately class German equatorial mounts (GEM). choose mounts too small for their equip- ment, mainly due to expense. While The specs visual observers can get away with this, At a weight of 63.2 pounds (28.7 kilo- grams) — 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 observ- ing 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 before the system reaches its hard stop to prevent damage. Celestron’s Imagers will be pleased with the CGX equatorial new software to control operation and mount will drive loads imaging. Particularly attractive is the up to 55 pounds (24.9 ability to compensate for individual kilograms) with a high degree of variations in the mechanics of the accuracy. mount through multipoint mount modeling. Additionally, you’ll find an autoguider port and a USB 2.0 port (for software updates).

64 ASTRONOMY • MARCH 2018 The CGX equatorial mount and tripod comes with everything you see here.

The connector that comes with the mount Celestron also touts the fact that the mount Celestron C11 SCT can accommodate supports Programmable Periodic Error (about 30 pounds Losmandy or Vixen PRODUCT INFORMATION dovetail plates. Correction. The drive system is now belt [14 kg]). The mount driven with spring-loaded worm gears to bore the payloads well Celestron CGX equatorial mount ensure a solid connection. for visual use, and settle Weight, head: 44 pounds (20 kilograms) And, of course, the Celestron NexStar+ time was more than acceptable. Weight, tripod: 19.2 pounds (8.7 kg) control drives the entire system. If you’ve While observing through the C11, I could Load capacity: 55 pounds (24.9 kg) used the NexStar system within the past tell that Celestron has definitely improved Tracking rates: Sidereal, solar, lunar few years, you’ll feel right at home. The the damping time in this class of mount. Tripod height: 47¼ to 77½ inches 40,000-object database contains the The pointing accuracy of the NexStar+ (120 to 197 centimeters) Messier, NGC, Caldwell, and other astro- system was top-notch. For more difficult Included accessories: tripod, accessory nomical object catalogs. targets, I found the high-precision pointing tray, two counterweights, DC power mode (Precise GoTo) very helpful. For cable, NexStar+ hand controller The arrival those unfamiliar with it, you perform an Price: $2,199 The CGX comes in two boxes labeled alignment on a bright star near your target, Contact: Celestron “team lift.” If you’re getting this deliv- and then the mount determines how far off 2835 Columbia St. ered, you may want to enlist some help you are from the ideal model and compen- Torrance, CA 90503 moving it to where you plan to unpack it. sates when slewing to your next target. 310.328.9560 While the mount itself is not difficult for Astrophotographers who routinely image www.celestron.com a healthy individual to handle, it’s a little objects too faint to be seen through the awkward when boxed. optics will appreciate this. As you unpack, you’ll find a tripod, accessory tray, hand controller, GEM head, A polar scope is not included, but A positive verdict two 11-pound (5 kg) counterweights, and a Celestron notes that one will be available as In all, I was impressed with Celestron’s DC power adapter. The combined 22 pounds an option soon. The mount does support CGX equatorial mount. At its price point, (10 kg) of counterweights should be suffi- Celestron’s All-Star Polar Alignment tech- it offers a good array of standard features cient to balance most equipment while keep- nology, which allows you to use the loca- and expandability. I’d recommend anyone ing the entire setup under the 55-pound tion of any bright star to fine-tune the who carries a cellphone, tablet, or laptop (25 kg) rating. Celestron also offers an AC polar alignment. If you use this, I recom- consider purchasing the SkyPortal WiFi power adapter and 17-pound (8 kg) counter- mend running the routine a couple of module. At $99.95, this is a relatively weights as options. In the box, you’ll find times for the best accuracy. inexpensive add-on. Amateurs who want a code to download a special Celestron to bypass the manual alignment proce- edition of the Starry Night 7 software for Under the stars dure will be interested in the StarSense Windows or Mac. Field setup requires the included AutoAlign ($349.95). The dovetail that ships with the CGX is 8-millimeter Allen wrench to tighten the Celestron has developed a solid, usable, a two-in-one unit that supports both CGE head attachment bolt. Celestron engineers and expandable mount at a decent price. (otherwise known as a Losmandy D plate) have provided a place on the mount to The CGX will serve an amateur well for and CG-5 (Vixen) dovetails. Because the store the wrench, but if you frequently years of either visual or photographic use CG-5 saddle is recessed under the CGE travel to dark sites and break down the no matter which optical tube(s) you couple portion, you may find certain CG-5 dove- mount on a regular basis, I’d recommend it with. I highly recommend it. tail bars do not fit properly, although I had that you store an extra Allen (or two) in no issues. The ability to support both dove- your observing kit. Tom Trusock is an equipment guru tails is a nice touch and something that I’d I tested the CGX with a variety of pay- who does most of his testing near his like to see become standard. loads, from a lightweight refractor to a home in Ubly, Michigan.

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2017 BINOCULARUNIVERSE BY PHIL HARRINGTON Star clusters in Monoceros Little-known star groups offer gemmy views.

his month, three of 1771, he never found it. Only a the season’s brightest year later did he chance upon The sparse open star cluster M50 makes a good sight in binoculars. It is one of the stars — Betelgeuse, an errant open cluster in the highlights of Monoceros. RICHARD MCCOY Sirius, and Procyon same vicinity; it became M50. — form a large equi- Most assume that this was fall into curved lines and arcs eight faint stars that together Tlateral triangle that many know Cassini’s “nebula,” but we will that some see as heart-shaped form an angled number 3. as the Winter Triangle. probably never really know for or possibly like an arrowhead None of those stars shines Although they blaze brightly on sure. oriented east-west. If your eyes brighter than 9th magnitude, clear nights, they frame a sur- One thing we do know for are especially color-sensitive, however, so you’ll need a very prisingly barren part of the late sure is that even though M50 you may notice that M50’s clear March night to see them. winter sky. Much of this vast, lies in the middle of nowhere, brightest star shows a subtle Finally, let’s visit another starless region belongs to it’s surprisingly easy to find golden tint. All other cluster fun asterism that I first Monoceros the Unicorn. with binoculars. Begin your members appear white. bumped into more than three Although hapless Monoceros hunt at Sirius and slowly scan A trio of open star clusters decades ago while researching holds little to grab the attention northeastward toward Procyon. lies southeast of M50. The most for my book Touring the of constellation hunters, it over- Along the way, you’ll pass obvious is NGC 2353. With Universe Through Binoculars. flows with open star clusters for Theta (θ) Canis Majoris about M50 still in view, shift your Look about 5.5° west of us binocularists. Yet, while a binocular field from Sirius. attention about half a field to M50, and less than half a many of those clusters are vis- Another field-hop in the same the southeast. There, you’ll spot degree northwest of the 5th- ible in binoculars, only one direction should bring a tiny three 6th-magnitude stars in a magnitude red giant SAO found its way into Charles blur of starlight into view. gentle arc ¾° long. Look care- 133585. There, you’ll find a Messier’s catalog. That’s our That’s M50. fully, and you may see that the compact clump of seven faint first stop this month, M50. Through my 10x50 binocu- center star in the arc looks a stars forming an inverted M50’s story is an interesting lars, M50 offers a soft blur of little fuzzy and is accompanied letter V. Given the constellation one. The Italian-born French light peppered by a few faint by some fainter companions. they lie within, the group has astronomer Giovanni Cassini is points. The brightest cluster That’s the cluster. All told, some since become known as the often credited with its discov- star, at 8th magnitude, lies just 100 stars belong to NGC 2353, Unicorn’s Horn. All seven stars ery. He is said to have spotted a south of center. When I use my although most fall below the span a compact 7', so they may “nebula” between Canis Major 16x70 binoculars, more stars reach of binoculars. The other look nebulous. Since none and Canis Minor sometime emerge from the fog, while two clusters, NGC 2335 and shines brighter than 9th mag- before 1711. Although Messier under dark skies my 25x100s NGC 2343 are fainter still. nitude, 10x70 binoculars are searched for Cassini’s nebula in reveal that those stars seem to As you approach M50 from probably the smallest that will Theta Canis Major to the south, show them well. The group is you may notice a tiny zigzag clear in my 16x70s and striking β Mon clump of faint stars a little west in my 25x100s. of the line connecting those Let me know how you do two. That asterism is best in sighting the Unicorn’s Horn, α Mon known as Pakan’s 3, named as well as the other targets after amateur astronomer mentioned this month. Contact Pakan’s 3 Randy Pakan from Edmonton, me through my website, Alberta, who first noted its philharrington.net. Until next shape. Depending on your bin- month, remember that two oculars, it should be in the eyes are better than one. same field as M50, since both are separated by only 3°. Put Phil Harrington is a longtime M50 in the northeastern part of contributor to Astronomy and Pakan’s 3 is an asterism that lies close to M50. This zigzag clump was named your view, then look to the the author of many books. by amateur astronomer Randy Pakan from Edmonton, Alberta. TONY HALLAS southwest for a collection of

WWW.ASTRONOMY.COM 67 OBSERVINGBASICS EIGHT MORE DOUBLE STAR WONDERS BY GLENN CHAPLE Name R.A. Dec. Mags. Sep. P.A. 41 Aurigae 6h11.6m 48°43' 6.2, 6.9 7.4" 357° 20 Geminorum 6h32.3m 17°47' 6.3, 6.9 19.7" 211° Is this stellar Nu1 (ν1) Canis Majoris 6h36.4m –18°40' 5.8, 7.4 17.3" 264° 17 Hydrae 8h55.5m –7°58' 6.7, 6.9 4.0" 4° 35 Sextantis 10h43.3m 4°45' 6.2, 7.1 6.7" 239° pair marathon- 83 Leonis 11h26.8m 3°01' 6.6, 7.5 28.2" 149° 88 Leonis 11h31.7m 14°22' 6.3, 9.1 15.7" 332° Putting the finishing 90 Leonis 11h34.7m 16°48' 6.3, 7.3 3.5" 208° touches on the double worthy? star marathon. Key: R.A. = Right ascension (2000.0); Dec. = Declination (2000.0); Mags. = Magnitudes; Sep. = Separation; P.A. = Position angle

have to share this one with double star marathon I intro- you. At the November 9, duced in my March 2016 N 2017, meeting of my column. astronomy club, the For those of you who missed r Amateur Telescope that article and a follow-up in IMakers of Boston, member March 2017, I created the dou- PISCES John Sheff announced that ble star marathon as a counter- it was Carl Sagan’s birthday. point to the annual Messier l “How old would he have been marathon. The latter is typi- were he still alive?” I asked. cally held in mid- or late E Without batting an eye, he March when all the objects in replied, “Billions and billions!” the Messier catalog can be seen Who says astronomy enthusi- in a single evening. I picked asts are mirthless individuals? 110 stellar pairs to match the M74 I want to make a personal number typically included on d 1° plea for help — not of the men- the Messier marathon roster, tal or physical variety, though and all lie in the same areas as most folks who know me well the Messier objects. Double star Psi1 (ψ1) Piscium lies 9° northwest of spiral galaxy M74, one of the most would argue the point. No, I’m difficult Messier marathon objects. ASTRONOMY: ROEN KELLY looking for assistance in fine- Swimming with tuning and finalizing the the Fish On the weekend of this the table above, I highlight Here’s where I need your year’s Messier marathon eight that failed to make the help. California double star (Friday and Saturday nights, cut but deserve your atten- aficionado Phil Kane has sug- March 16 and 17), I’ll be tack- tion. The data come from the gested that I include Psi1 ling Psi1 before moving on to Washington Double Star Catalog, (ψ1) Piscium on the list. my main list. I encourage you which is available online at This striking pair of to back me up by doing the http://ad.usno.navy.mil/wds. I near-twin stars shine same and letting me know how have split all of them through with a pure-white you fare. If positive sightings a 3-inch reflecting telescope at hue at magnitudes outnumber the negative ones, 60x, but I suggest using a larger 5.3 and 5.5 and I’ll officially add Psi1 to the aperture to better reveal their are separated by a list. And then I’d face the colors and bumping up the comfortable 30". heartbreaking task of culling magnifying power to 100x for They lie just 9° one double from my original some of the closer pairs. northwest of the list. By the way, if you’d like to Questions, comments, or spiral galaxy M74, run the marathon this year suggestions? Email me at a notoriously dif- and don’t have the list, send [email protected]. Next ficult Messier mara- me an email, and I’ll forward month: tips for hosting an thon object because it you a copy. Astronomy Day star party. glows faintly and hangs Clear skies! low in the west on March Best of the rest evenings. Psi1 Psc would add As any diehard double star Glenn Chaple has been an a challenging element to the observer would tell you, avid observer since a friend The near-twin stars of Psi1 Psc lie in the double star marathon, but selecting 110 pairs meant showed him Saturn through a northeastern corner of Pisces the Fish. small backyard scope in 1963. JEREMY PEREZ would it be too much? omitting some real gems. In

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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, 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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70 ASTRONOMY • MARCH 2018 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 light- years. 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 IC 166 is a relatively unknown open cluster in Cassiopeia. A large telescope 3 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, 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: Astronomy Reader Gallery, P. O. Box 1612, Waukesha, WI 53187. Please include the date and location of the image and complete photo data: telescope, camera, filters, and expo- sures. Submit images by email to 4 [email protected].

5 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/Charles 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 con- stellation in which it lies. This group floats through space some 60 million light-years away. • Dan Crowson

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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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72 ASTRONOMY • MARCH 2018 TOTAL SOLAR ECLIPSE: Ancient Paths to the Present

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WWW.ASTRONOMY.COM 73 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 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 not- so-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 (STSCI)

74 ASTRONOMY • MARCH 2018

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SOUTHERN MARTIN GEORGE describes the solar system’s changing landscape SKY as it appears in Earth’s southern sky. May 2018: Jupiter at its peak

As May begins, the solar sys- disk measures 45" across the nitely notice how much brighter midevening, at least once this tem’s two brightest planets equator and is noticeably flat- it has become. The trend con- month. On a more local scale, adorn the evening sky. Venus tened — its polar diameter is 3" tinues in May — during the the spot of interest lies about disappears during twilight, less. Although two dark atmo- month’s 31 days, Mars doubles one-third of the way from Beta however, while Jupiter stays spheric belts are almost always in brightness from magnitude (β) to Eta (η) Virginis. up all night. visible, additional features pop –0.4 to –1.2. It may surprise you that this Venus shines brilliantly at into view during moments of Mars’ apparent diameter direction in the sky is called magnitude –3.9 and is easy to good seeing. Also watch for makes a similar leap. The the First Point of Libra, but that spot low in the northwest if you Jupiter’s quartet of bright ocher-colored disk grows from indeed is its name. You might have a clear and unobstructed moons. Discovered by Galileo 11" to 15" across, making it a be a bit more familiar with the horizon. On the 1st, it stands in 1610, all four stand out as great target for observing First Point of Aries, which like- nearly 10° high 45 minutes long as none of them is hiding through a telescope. To see the wise is not in the constellation after sunset. It lies 7° to the in front of or behind the planet. most detail, wait for it to climb after which it is named. It lies lower right of 1st-magnitude By midevening, glorious high in the sky after midnight. in neighboring Pisces. Aldebaran, the orange-red Saturn pokes above the eastern By late May, the planet’s south These two points are where luminary of Taurus the Bull. horizon. The ringed planet pole tips 15° toward us, deliver- the ecliptic — the apparent The planet maintains its moves slowly westward against ing impressive views of the path of the Sun across the sky brightness throughout May, the backdrop of Sagittarius the south polar cap. — crosses the celestial equator. but becomes easier to see as it Archer, north of that constella- As Saturn and Mars climb The First Point of Aries marks climbs higher. On the 31st, it tion’s conspicuous Teapot aster- high in the north before dawn, the point where the Sun crosses appears 15° high an hour after ism. Although far dimmer than Mercury rises in the east. the celestial equator from south sundown and remains visible Venus or Jupiter, magnitude 0.3 The innermost planet shines to north at the time of the past nightfall. Venus has little Saturn appears prominent brightly and remains conspicu- March equinox; the First Point to offer to observers using a among the stars of the ous for most of May. Early in of Libra denotes where the Sun telescope, however. Even at Sagittarius Milky Way. the month, the magnitude 0.3 crosses back into the southern month’s end, its 13"-diameter Saturn remains an exquisite world rises two hours before celestial hemisphere at the time disk appears nearly full. sight through any telescope the Sun and stands nearly 15° of the September equinox. On the opposite side of early once it climbs well clear of the high an hour before sunup. So, how is it that Libra has May’s evening sky, Jupiter horizon. Although its disk, And it remains a pleasing tele- a point named after it that rises shortly after sunset and which measures 18" across at scopic sight, with a nearly half- belongs to another constella- shows up easily as twilight midmonth, shows much less lit disk spanning 8". As it sinks tion? It’s all because of the phe- fades. The giant world reaches detail than Jupiter’s, the rings lower during the following nomenon of precession. The opposition May 9. Although more than make up for this weeks, Mercury grows brighter, gravitational pulls of the Sun this point typically marks a lack. The beautiful system hitting magnitude –0.3 in mid- and Moon cause Earth’s axis planet’s peak visibility, Jupiter spans 40" and tilts 26° to our May. Unfortunately, it then to rotate in the same way as a barely suffers the rest of the line of sight. You’ll also see appears smaller and shows a spinning top does. So, the axis month — it shines at magni- Saturn’s brightest moon, 8th- less interesting gibbous phase. describes a circle about 47° wide tude –2.5 throughout May. magnitude Titan; several others The planet disappears into the in both the northern and south- The planet resides in Libra all show up through 10-centimeter Sun’s glow by month’s end. ern sky that completes a circuit month, moving slowly west- and larger instruments. in about 25,800 years. Because ward against this backdrop. It By late evening, you can find The starry sky of precession, the right ascen- lies 3° east of the fine binocular Mars lurking to the lower right Approximately midway sions and declinations of the pair Alpha1 (α1) and Alpha2 (α2) of Saturn. The Red Planet between Regulus, the brightest stars change continuously, apart Librae at opposition; the gap moves eastward against the star in Leo the Lion, and Spica, from the slight changes result- closes to 1° by May 31. background stars, crossing from Virgo the Maiden’s luminary, ing from their own motions By late evening, Jupiter has Sagittarius into Capricornus in is a direction that receives through space. This causes the climbed high in the sky and will mid-May. If you’ve been follow- little attention. But I want you points where the ecliptic and be a wonderful sight through ing the planet’s appearance over to focus on this area, which the celestial equator intersect to any telescope. The gas giant’s the past few months, you’ll defi- stands high in the north in gradually progress westward. STAR S

DOME NGC 104 NGC THE ALL-SKY MAP

SHOWS HOW THE

SKY LOOKS AT: SMC

9 P.M. May 1 OCTANS

RETICULUM

8 P.M. May 15 HYDRUS

7 P.M. May 31 PAVO

Planets are shown DORADO LMC

at midmonth MENSA

SCP

CAELUM 6397 APUS 2070

GC

NGC PICTOR

CHAMAELEON AUSTRALE

TRIANGULUM

VOLANS

_

NGC 2516 NGC CARINA

Canopus MUSCA

MA

COLUMBA

OR

CIRCINUS

PUPPIS

_

4755

LEPUS _

` NGC 3372

NGC

`

b

LUPUS

a a

CRUX

c VELA

NGC 5139 NGC

¡

2477

CENTAURUS

NGC NGC

M41

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NGC 5128 NGC

MAJOR

PYXIS

CANIS

_

ANTLIA Sirius

MONOCEROS M83

W M47

LIBRA

Alphard

CORVUS

CRATER _ Spica M104 _ HYDRA ic) pt Procyon cli _ e n ( SEXTANS Su he ` f t VIRGO CANIS h o MINOR Pat CANCER

a

_ Regulus M65 M66 Denebola Ar LEO ` M44 a _ M64

` NGP Pollux

_ COMA BERENICES MAGNITUDES Castor Sirius Open cluster LEO MINOR LYNX 0.0 Globular cluster 1.0 CANES Diffuse nebula 2.0 VENATICI 3.0 Planetary nebula M51 4.0 5.0 Galaxy

N HOW TO USE THIS MAP: This map portrays the sky as seen near 30° south latitude. MAY 2018 Located inside the border are the four directions: north, south, east, and west. To find stars, hold the map Calendar of events overhead and orient it so a direction label matches the 3 Venus passes 7° north of 13 The Moon passes 5° south of direction you’re facing. Aldebaran, 17h UT Uranus, 15h UT The stars above the

map’s horizon now 4 The Moon passes 1.7° north of The Moon passes 2° south of

TELESCOPIUM Saturn, 20h UT Mercury, 17h UT GC match what’s

N in the sky. 6 The Moon is at apogee 15 New Moon occurs at 11h48m UT (404,457 kilometers from Earth), 0h35m UT 16 The Moon passes 1.2° north of

Aldebaran, 13h UT ARA

M7 M8 The Moon passes 3° north of

17 The Moon passes 5° south of

SAGITTARIUS Mars, 7h UT

M6

OR Venus, 18h UT N

SCORPIUS Eta Aquariid meteor shower M20

NGC 6231 NGC peaks The Moon is at perigee (363,776 kilometers from Earth), 8 Last Quarter Moon occurs at 21h05m UT 2h09m UT

22 First Quarter Moon occurs at _

Antares Asteroid Vesta is stationary, 3h49m UT

M4 10h UT 27 The Moon passes 4° north of 9 Jupiter is at opposition, 1h UT Jupiter, 18h UT 10 The Moon passes 2° south of 29 Full Moon occurs at 14h20m UT Neptune, 9h UT

E 12 Mercury passes 2° south of Uranus, 21h UT

Jupiter OPHIUCHUS

M5

SERPENS CAPUT

cturus Ar

STAR COLORS: Stars’ true colors depend on surface temperature. Hot stars glow blue; slight- ly cooler ones, white; intermediate stars (like BOÖTES the Sun), yellow; followed by orange and, ulti mately, red. Fainter stars can’t excite our eyes’ color receptors, and so appear white without optical aid.

Illustrations by Astronomy: Roen Kelly

BEGINNERS: WATCH A VIDEO ABOUT HOW TO READ A STAR CHART AT www.Astronomy.com/starchart. SHOP NOW FOR SCIENCE & ASTRONOMY PRODUCTS

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