STARS, MOONS, AND PLANETS: Night sky guide

OCTOBER 2017 VOYAGER

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The world’s best-selling astronomy magazine REMEMBERING VOYAGE R YEARS 40 LATER The “Grand Tour” of the solar system opened our eyes like never before

Saturn’s stunning rings revealed p. 28

www.Astronomy.com Unveiling Jupiter’s $5.99 storm-wracked globe p. 20 10 Vol. 45

• Issue 10 Issue Unsolved mysteries of the ice giants p. 46

0 71486 01096 8 Voyager’s great cruise illustrated p. 50 “When I went back to viewing, I wanted the best... 24” f/3.85 Slipstream telescope and Tele Vue eyepieces.” —Tony Hallas

M24 region imaged by Tony Hallas using a Tele Vue-NP101is refractor. Tony Hallas, Renowned Astrophotographer, Returns to the Eyepiece (from an unsolicited e-mail to David Nagler) Hi David and Al, Although I am still active in imaging, I have decided to go back to viewing and have taken possession of a new 24” f/3.85 Slipstream telescope from Tom Osypowski. You will be happy to know that I have acquired a treasure trove of TeleVue eyepieces to complement this telescope, specifically: 26 and 20mm Nagler Type 5, 17.3, 14, 10, 6, 4.5mm Delos, Paracorr Type 2, and 24mm Panoptics for binocular viewing. After using a Delos, “that was all she wrote;” you have created the perfect eyepiece. The Delos eyepieces are a joy to use and sharp, sharp, sharp! I wanted to thank you for continuing your quest to make the best eyepieces for the amateur community. I am very glad that you don’t compromise ... in this world there are many who appreciate this and appreciate what you and Al have done for our avocation. Hard Tony with his Tele Vue eyepiece collection awaits to imagine what viewing would be like without your creations. a night of great observing at his dark-sky site. Best, Tony Hallas

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WWW.ASTRONOMY.COM 3 OCTOBER 2017 VOL. 45, NO. 10 NASA/JPL/USGS ON THE COVER 20 Forty years ago, Voyagers 1 and 2 made their “Grand Tour” of the solar system, changing our view CONTENTS of the universe forever. FEATURES 20 COVER STORY 38 56 COLUMNS Unveiling a giant StarDome and How an interplanetary A storm-wracked world and Path of the Planets mission changed the Strange Universe 10 BOB BERMAN moons with erupting volcanoes RICHARD TALCOTT; world and underground oceans were ILLUSTRATIONS BY ROEN KELLY Voyager 1 and 2 revealed much For Your Consideration 16 just some of the surprises the about the cosmos — and let a JEFF HESTER Voyager spacecrat revealed. 44 giant mark on pop culture. 18 FRANCIS REDDY Observing Basics Ask Astro SHANNON STIRONE GLENN CHAPLE he hidden Earth. 28 58 Secret Sky 66 Close encounters with 46 Talk astronomy with your STEPHEN JAMES O’MEARA the ringed planet The unsolved mysteries un-cosmic friends Binocular Universe 68 he probes not only returned of the ice giants Here’s a concise guide to PHIL HARRINGTON stunning images of Saturn and Geysers on Triton, furious storms avoid going over the heads of its rings — they also set the stage on Neptune, and a seemingly se- your science-avoiding peeps. for the highly successful Cassini QUANTUM GRAVITY rene Uranus that turned out to be MICHAEL E. BAKICH mission. LIZ KRUESI a cosmic oddity. KOREY HAYNES Snapshot 8 62 Astro News 12 34 50 Memories from a Voyager revealed! The Grand Tour backyard observer Incredibly, a few instruments IN EVERY ISSUE he twin spacecrat took long Decades-old observing jour- aboard both spacecrat continue journeys to explore the outer nals recall a time when Voyager From the Editor 6 to function. JOHN WENZ solar system. RICHARD TALCOTT utterly transformed what we Astro Letters 11 36 know about the outer planets. New Products 69 52 RAYMOND SHUBINSKI Sky This Month Image the giant planets Reader Gallery 70 Uranus glows brightly. With the right camera and Advertiser Index 73 MARTIN RATCLIFFE AND telescope, you can take the same Breakthrough 74 ALISTER LING “Grand Tour” the Voyagers did. DAMIAN PEACH

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JULY 2017 JULY p. 44 p. 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 Editor Alison Klesman Copy Editors Dave Lee, Elisa R. Neckar Voyager’s Editorial Assistant Nicole Kiefert ART Graphic Designer Kelly Katlaps Illustrator Roen Kelly great legacy Production Specialist Jodi Jeranek 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 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, Anne L. Kinney, Edward Kolb, he end of the 1970s 1979, the science world lit Stephen P. Maran, Brian May, S. Alan Stern, James Trefil was a magical time up. Some 217,000 miles for astronomy (349,000 kilometers) from Kalmbach Publishing Co. enthusiasts. We were Jupiter’s core, the spacecraft Senior Vice President, Sales & Marketing Daniel R. Lance T Vice President, Content Stephen C. George still coming off the imaged volcanic activity on Vice President, Consumer Marketing Nicole McGuire afterglow of the Apollo Jupiter’s moon Io, a startling Art and Production Manager Michael Soliday Corporate Advertising Director Ann E. Smith program. Comet West had discovery. Each aspect of Circulation Director Liz Runyon New Business Manager Cathy Daniels recently dazzled skygazers. Jupiter and every one of its Retention Manager Kathy Steele Astronomers had discovered moons offered scientists a Single Copy Specialist Kim Redmond rings around Uranus and a new world of surprises. 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The of dark space. permission. Printed 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 Voyager 1. The twin space- spacecraft discovered many In the end, the Voyager (12 issues) $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, craft carried a battery of moons and studied the plan- Grand Tour gave us our payable in 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 instruments, including imag- et’s unusual axial tilt that most detailed understanding 3209 RT. Not responsible for unsolicited materials. ing systems, radio science resulted from a major colli- of the solar system up to that systems, spectrometers, mag- sion with another body early time, and also put Earth in netometers, cosmic ray detec- in the solar system’s history. cosmic perspective. tors, and photopolarimeters. Three years later, the space- Follow Astronomy When Voyager 1 whizzed craft encountered Neptune, Yours truly, past Jupiter on March 5, recording amazing images of 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

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P31155 Learn more Astronomy.com/magazine/trips-tours/2018-costa-rica QUANTUM GRAVITY QGEVERYTHING YOU NEED TO KNOW ABOUT THE UNIVERSE THIS MONTH . . .

IN REVIEW TEAMWORK BLACK HOLE HOT BYTES >> NASA has established an The European Space ANALOG independent committee to Agency announced its Researchers at the TRENDING review costs and scheduling intention to develop LISA, University of Nottingham TO THE TOP associated with the flagship the Laser Interferometer used a water bath to Wide Field Infrared Survey Space Antenna, in col- simulate the space Telescope (WFIRST). laboration with NASA. around a black hole.

SNAPSHOT Voyager 2 captured this image of Saturn (left) on its approach on August 4, 1981, Reflections from a distance of 13 million miles (21 million on Voyager kilometers). Aside from the planet’s We’ve come a long way since, but the colorful cloud tops and rings, the image Voyagers’ Grand Tour set the stage. shows (in order of distance from the planet) the moons Tethys (closest), In 1977, when the twin Voyager space- Dione, and Rhea, as craft launched, our knowledge of the well as the shadow solar system was pretty primitive. Twelve of Mimas. Some years later, after Voyager 2’s exploration of 23 years later, the Cassini spacecraft Neptune, the first stage in understanding captured Saturn our family of planets — the “Grand Tour” (bottom) in one — was over. Now the Voyagers are more of its greatest composite shots. than 100 times the Earth-Sun distance away, silently moving still farther out, and still providing data about the outer part of the solar system. The Voyagers’ Grand Tour gave us the NASA; SA; first detailed exploration of Jupiter and Saturn, although those missions were fly- bys, and the initial close-up looks at Uranus and Neptune. We only explored Pluto for the first time, of course, many years later with the New Horizons spacecraft. For those who weren’t around in the 1970s and ’80s, it’s hard to understand the Voyagers’ thunderous impact. Previous explorations of Jupiter and Saturn by the Pioneers were primitive by comparison. When the Voyager flybys took place, the NASA/JPL AND NASA/JPL/SPACE SCIENCE INSTITUTE; TOP FROM LEFT: NA SCIENCE INSTITUTE; LEFT: FROM TOP NASA/JPL/SPACE AND NASA/JPL floodgates of data and what were then NOTTINGHAM OF UNIVERSITY deemed to be mind-blowing images opened. The whole astronomy community endless. It’s now tempting to compare the early ’80s. There’s no doubt that we have a was stunned. Voyager images with more recent ones, such long way to go to fully understand our Sun The discoveries of moons, of planetary as looking at Voyager Saturns next to Cassini and family of planets. But the Voyagers will features, of strange properties of the worlds Saturns, and contemplate the quantum leaps always mark a time when humans took the we had not yet seen up close, seemed in understanding that have taken place since first big steps to do so. — David J. Eicher

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WWW.ASTRONOMY.COM 9 STRANGEUNIVERSE BY BOB BERMAN Say Betelgeuse How do you pronounce that star, that constellation, or that astronomer’s name?

etelgeuse is return- Sumerians saw the constellation ing to the morning of Orion not as a hunter, but as sky, but few can a sheep — so “the armpit of the pronounce it. That’s sheep” is the correct meaning. because almost none His research first appeared Bof our friends know the stars in 1944 in Popular Astronomy and constellations, so we rarely and was reprinted in 1971 by hear them spoken. But even Sky Publishing Co. I bought it during star parties, mispronun- in 1972 and have treasured that ciations abound. yellowing 24-page booklet all A red supergiant star, Betelgeuse (or Bet’l’jooz) stands out as the shoulder of Orion the Gibberish is nobody’s fault. these 45 years. Nowadays, you Hunter. Its name, however, is derived from a time when the Sumerians envisioned its In the case of Orion’s alpha star, can find it online at http:// constellation as a sheep. © PERESANZ | DREAMSTIME.COM the movie Beetlejuice, starring articles.adsabs.harvard.edu/ Michael Keaton, permanently full/1944PA.....52....8D. then add an “is” sound? No! When observing the deep red implanted that pronunciation Bottom line? Bet’l’ jooz is You must change every syllable! star Mu (μ) Cephei, do you say in everyone’s mind. Looking it indeed the most mispro- Betelgeuse is pronounced “Moo” or “Myoo”? I prefer up in the dictionary is of little nounced bright star, but run- (Alpha) oh-ree-OH-nis. ZAY-tuh and THAY-tuh, but help — even its meaning varies ners-up include Vega and Spica Now go ahead and learn the some pronounce them ZEE-tuh with each reference book. Grab (say SPY-kuh, not SPEE-kah). genitives of all 88 constellations! and THEE-tuh. Happily, as the nearest dictionary, and Constellations can be trou- Those with two words, like Ursa with Jupiter’s moon Io (say you’ll find that the word blesome, too. Seven years ago, Major, have both changed, mak- either EYE-oh or EE-oh), Betelgeuse means “the shoulder our own Michael Bakich cre- ing it UR-see mah-JOR-is. increasing numbers of celestial of the giant,” “the armpit of the ated an extremely helpful online Fortunately, no one can force objects are now greenlighted for sheep,” “the House of the pronunciation guide, though I you to memorize all this. I multiple “correct” pronuncia- Twins,” or one of several other regard cah-RYE-nuh as the pre- obsessively learned them in the tions. A few modern dictionar- contradictory things. ferred way to pronounce the ’70s when I started lecturing, to ies even obscenely allow The final judge? My favorite yor-AY-nis for the seventh planet. authority was the late George I rarely correct people Davis of Buffalo, New York, an Gibberish is nobody’s fault. because it’s obnoxious, and I attorney, avid amateur astrono- want to be liked. Also, many mer, and noted Arabic scholar. mispronunciations ultimately Starting in the 1930s, he spent Southern constellation some- avoid the embarrassment of say- prove transient: If you wait long seven years researching star times listed as cah-REE-nuh. ing things wrong. But there are enough, your articulation may names, traveling to the East to But it’s the Latin genitive issues even today, because the eventually be deemed OK. Even seek original sources. Most star form that really throws people. British and Americans often transient, which everyone names come from Arabic, but Stars within constellations are utter genitives (and other things) started pronouncing with three that language, like all others, labeled using the Latin genitive differently, like with antennae syllables in the 1970s, and has changed over the centuries. form of the constellation, pronounced an-TEN-ee versus which still bugs me because it That’s one reason why so many denoting possession or associa- an-TEN-nay versus an-TEN-nye. classically has two (TRAN- myths and false ideas appear in tion. Vega is also known as So if you’d like to discuss the shent), is now sometimes listed print. To get at the truth, Davis Alpha (α) Lyrae; add an “e” to amazing irregular supernova as acceptable with three. started with 2,000-year-old the constellation of Lyra to form and nebula named Eta Carinae, Maybe all this is an argu- Arabic and then traced those the genitive. But how to say it? you could say either cah-REE- ment for solitary observing. star names to their roots from This one’s simple: You change nay or cah-RYE-nee, though I Then any heated dispute would the even earlier Sumerian. LY-ruh to LY-ree. prefer the latter. occur only in your mind — Davis managed to document The genitive often seems Even the Greek letters are which is never a good sign. that, yes, Vega is pronounced easy, like with Betelgeuse’s des- controversial. Probably 20 per- VEE-gah, which meant “the ignation as Alpha (α) Orionis. cent of the letters are ambigu- Contact me about falling eagle.” As for Bet’l’ jooz, But should you simply say ous. Returning to Eta Carinae, my strange universe by visiting http://skymanbob.com. the name derives from when the “Orion” the normal way and you can say EE-tuh or AY-tuh.

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

10 ASTRONOMY • OCTOBER 2017 ASTROLETTERS The best $299 eyepiece you’ll ever buy. No computer required. Battery- powered 7” color monitor included. E. KARKOSCHKA (UNIV. OF ARIZONA LUNAR & PLANETARY LAB) AND NASA/ESA AND LAB) PLANETARY & LUNAR ARIZONA OF (UNIV. KARKOSCHKA E. M16 Eagle Nebula Fourth time’s the charm think there are a trillion galaxies, leav- 8” Celestron Evolution There’s one astronomy event I latched ing the reader to think that we don’t need Metropolitan Skies onto when I got my 58x telescope in the dark matter because there is much more spring of 1980. About every 15 years, regular matter out there than we thought. REVOLUTION IMAGER Earth’s viewpoint crosses Saturn’s Conselice is careful to point out that RevolutionImager.com rings, and they appear edge-on, or even the number of galaxies is greater than disappear. we thought, but that the early ones were I am 65 now and have seen this three small and numerous and the total mass Oceanside Photo & Telescope Woodland Hills Telescope times in a row. The second time was remains the same. His explanation of why High Point Scientiic January 25, 1996, using the same tele- the sky looks dark to us between galaxies Agena AstroProducts scope. That time, the rings appeared as a is concise and easily understood. Skies Unlimited thin, misty line. Anyway, thanks for this article. I found Camera Concepts & Telescope The third time was April 4, 2009, at a it both clear and fun as the author also public gathering on the grounds of the injects a bit of humor here and there. That U.S. Naval Observatory. On that occasion, is why I also enjoy Bob Berman’s writing the rings looked like a thick, bright line. so much. — Robert Walty, Stephens City, VA I recently emailed Geoff Chester at the observatory and he said the next Earth crossing will be March 23, 2025, when I A perfect ellipse will be 73. Four times in a row — it beats Just a quick response to Bob Berman’s waiting for Halley’s Comet. “A perfect circle” article in the May 2017 — John Lockwood, Washington, DC issue. The universe prefers ellipses. Most planets are “squished” due to their rota- tion. Also, most orbits of planets and The concise explanation moons are ellipses. Sorry, but the universe I wanted you to know how much I is happy with imperfect circles, aka ellips- enjoyed the article “Our trillion-galaxy es! — Douglas Kaupa, Council Bluffs, IA universe” in the June issue. Christopher Conselice is a very good writer who puts Bob Berman responds important ideas across to us armchair Hey Douglas, maybe you were excessively astronomers clearly. Sometimes articles speed-reading my column because your in Astronomy have cloudy spots in them letter oddly says, “Sorry, but” followed by a where the writer seems to know the “correction” that essentially says the same issue but does not give us, the reader, things I wrote! quite enough information to understand My article said: “Reality ultimately fully. Some writers might say we now intruded. As it turned out, only Venus’ orbit would look circular to an alien study- We welcome your comments at ing it from a distance, with other planets Astronomy Letters, P. O. Box 1612, displaying varying degrees of elliptical Waukesha, WI 53187; or email to letters@ shapes. And fair enough: Ellipses rule . . . ” astronomy.com. Please include your Indeed, your letter says that “most plan- name, city, state, and country. Letters ets and moons” have elliptical orbits. may be edited for space and clarity. In fact, all of them do. — Bob Berman, contributing editor

WWW.ASTRONOMY.COM 11 RULED OUT. Physicists looking to explain the cosmic inflation that happened shortly after the Big Bang have ruled out ASTRONEWS the presence of the light inflaton, a particle with properties similar to the Higgs boson but a lower mass. HUBBLE WEIGHS A WHITE DWARF BRIEFCASE COSMIC MAGNIFYING GLASS limit (1.4 solar masses), it will explode as a A team led by James Lowenthal of Smith College has supernova, which astronomers can use to discovered that gravitational lensing may be respon- accurately measure distances. If our under- sible for the brightest galaxies in the sky. Of a sample of 31 of the brightest galaxies at infrared wave- standing of the relationship is incorrect, it lengths, 11 have been observed with the Hubble would affect the distances calculated. Space Telescope. All of them are gravitationally Only three other white dwarfs have been lensed star-forming galaxies seen 8 billion to 11.5 bil- weighed, but their masses were measured lion years ago birthing stars at a rate 1,000 or more times that of the Milky Way. Despite the distorted via their gravitational effects on a binary images created by the lenses, the team can use these companion. The masses derived for white new, clearer images to reconstruct the galaxies and dwarfs in binary systems could be affected better understand their high• rates of star formation. by a process called mass transfer between OUR LOCAL VOID NOT QUITE NEIGHBORS. The white dwarf the stars, which would contaminate the Stein 2051B (bright central object) can be seen Researchers at the University of Wisconsin–Madison superimposed on the sky with a background star mass-radius relationship. Although Stein have discovered evidence that the Milky Way and its (lower left), whose light was bent by the white dwarf 2051B has a binary companion, it’s so far Local Group may exist inside a large void. A local void could explain the “tension” currently observed in the passing in front of it. NASA, ESA, AND K. SAHU (STSCI) away — at least 5 billion miles (8 billion Hubble constant, which measures how fast the uni- km) — that the two cannot exchange mass. verse is expanding. The effects of this void, and the ccording to the theory of general rela- Once the mass of 2051B was measured, pull of the mass beyond it, might contribute to dif- tivity, mass bends space-time like a Sahu’s team plotted it on the white dwarf ferent values observed when measuring the Hubble constant using nearby versus faraway indicators. Abowling ball depressing a mattress, mass-radius relation. Their result fit per- • causing light to follow a curved path. fectly with the model. “When I saw that it MOLECULAR BLACK HOLE In 1919, Sir Arthur Eddington measured was right exactly on here, I almost fell off Scientists unexpectedly generated a “black hole” in a the deflection of background stars caused my chair,” Sahu explained. lab using X-ray pulses. Using the Linac Coherent by our Sun during a total solar eclipse. Stein 2051B’s excellent fit confirms our Light Source (LCLS) X-ray free-electron laser at the Nearly a century later, astronomers have SLAC National Accelerator Laboratory, researchers current evolutionary theory of white dwarfs stripped iodine atoms of their electrons with a burst used the Hubble Space Telescope (HST) to and agrees with our understanding of the of concentrated energy (100 quadrillion kilowatts per observe this effect caused by a star outside matter that makes up these objects. “This is cubic centimeter). As a result, the iodine atoms our solar system for the first time. began pulling in electrons from surrounding atoms. really a confirmation of the theory that we This is similar to the forces at work in a black hole, Kailash Sahu of the Space Telescope have been using so far,” Sahu said. though these objects exhibit a gravitational pull Science Institute announced the result at — Alison Klesman instead of electromagnetic attraction. — J. W., A. K. the 230th Meeting of the American Astronomical Society. Sahu’s team used the HST to capture the deflection of light from The number of Jupiter masses an object a background star as a white dwarf, the 70 needs to ignite hydrogen fusion as a star. remnant of a star once like our Sun, passed in front of it. Although this deflection was about 1,000 FIRE IN THE SKY The largest fireball occurred above FAST times smaller than the one Eddington mea- Chelyabinsk, Russia, on February 15, 2013, FACT and released about 440 kilotons of energy. sured, Hubble’s precision allowed astrono- mers to see it clearly. From the deflection, they were able to measure the mass of the white dwarf, Stein 2051B. The work appeared in the June 7 issue of Science. Stein 2051B is 17 light-years away. In March 2015, it passed in front of a star 5,000 light-years distant, causing its light to bend by 2 milliarcseconds. Seeing that bend was like trying to watch an insect crawl across the face of a quarter from a distance of about 1,500 miles (2,400 kilometers). : ROEN KELLY, AFTER ALAN B. CHAMBERLIN (JPL/CALTECH) CHAMBERLIN B. ALAN AFTER KELLY, ROEN : This is the first time a deflection due to general relativity has been measured using a star other than our Sun. And measuring ASTRONOMY SKYLIGHT. Astronomers define a fireball as any Impact energy (kilotons) the mass of Stein 2051B is the first “clean meteor brighter than magnitude –3. For the past test for [the] mass-radius relationship,” three decades, U.S. government sensors have been Sahu explained. recording such events across the globe. Although 0.1 1 10 100 The mass-radius relationship for white the network doesn’t record every fireball, it gives a good picture of the pummeling Earth receives from solar system debris. This chart depicts the brightest dwarfs leads to the Chandrasekhar limit. If events from April 15, 1988, to June 20, 2017; the relative size and color of each circle represents the fireball’s a white dwarf accumulates mass over this estimated total impact energy. — Richard Talcott

12 ASTRONOMY • OCTOBER 2017 BROWN DWARF DANCE. Astronomers using the Hubble Space Telescope to watch a pair of brown dwarfs ASTRONEWS 6 light-years away have determined there is no third object in the system, as was originally suspected. A new look at the Orion Nebula QUICK TAKES SPECIAL DELIVERY Observations of Comet 67P/ Churyumov–Gerasimenko suggest comets may have delivered up to 22 percent of Earth’s atmospheric xenon.

HEAT• IT UP The network of valleys surrounding Mars’ Lyot Crater was likely carved by water from an ice layer melted by hot ejecta from the impact.

INFINITE •DIVERSITY Samples collected by the Curiosity rover near Mars’ Mount Sharp show diverse mineral content suggestive of changing conditions in the area.

LOCKED •IN PLACE A Lowell Observatory-led study shows that massive galaxy cluster centers have been aligned with their surroundings

GBO/AUI/NSF for 10 billion years. STAR-FORMING STRING. The Orion Nebula is visible to the naked eye, located just beneath the Hunter’s three-star belt. But gas • and dust hide much of what’s going on in this nearby stellar nursery. Using radio observations taken of the nebula with the Robert C. BIG BANGS Byrd Green Bank Telescope in West Virginia, astronomers have identified a filament of ammonia molecules spanning 50 light-years. Researchers from the The ammonia is visible as orange in this composite image, which also shows other gas, imaged with NASA’s Wide-field Infrared University of Colorado Boulder Survey Explorer telescope, in blue. Ammonia in particular traces the dense gas that ultimately collapses to form stars. Studying the found that hot intracluster gas concentration and location of ammonia in the Orion Nebula will help astronomers determine whether its densest regions of gas are may result from turbulence as stable or likely to form new stars in the near future. — A. K. massive galaxy clusters collide.

JETTING• OFF Astronomers are using data from the Chandra X-ray LIGO detects third gravitational wave Observatory to trace previous LIGO has detected gravitational outbursts in the R Aquarii waves for a third time, helping GW150914 binary system, as well as predict astronomers further understand 70 future events by watching black holes. blobs of hot gas in the jets 60 All objects distort the fabric of GW170104 associated with the stars. space-time by some amount. The 50 • interaction of more massive objects LIGO detections METEORITE MYSTERY can cause detectable “ripples” in it, 40 Researchers have used high- sort of like a ripple in a pond, except pressure experiments to 30 determine how meteorites can these ripples move at the speed of X-ray studies GW151226 contain multiple types of light. But these ripples are also small 20 silica, despite the differing because gravity is the weakest of the LVT151012 conditions required for them 10 fundamental forces. (tentative to form. The results shed light detection) Finding ripples of gravity is there- Black hole mass (solar masses) 0 on impact processes fore far more difficult than, say, throughout the solar system.

detecting a massive electromagnetic SIMONNET) (AURORE STATE LIGO/CALTECH/SONOMA event. That’s why LIGO, short for the FILLING THE GAPS. LIGO detectors have discovered a pair of merging black holes EXTREME• HEAT Laser Interferometer Gravitational- with masses higher than those detected in X-rays. On the left are previously known The gas giant planet KELT-9b wave Observatory, uses two (soon to black holes found via X-rays; on the right are three strong and one tentative gravitational has a dayside temperature of be three) locations to tune in to espe- wave detections made by the observatory, sorted by mass on the vertical axis. 7,800° F (4,300° C), making it cially violent events, like the merger hotter than many stars. of two black holes. When black holes black hole 49 times the mass of the Physical Review Letters, may indicate a merge, the magnitude of their gravi- Sun. A black hole of this mass helps shift toward one of the LIGO team’s GALACTIC• tational waves changes from the “fill in” the spectrum of black holes, hopes: that detecting gravitational WEIGHT LOSS effects of a “pebble” in the pond of as it falls between the masses of the waves may become commonplace As galaxies join groups of space to the equivalent of tossing in a two black hole mergers previously events. The addition of the European 20–30 members, they lose up boulder, sending out much larger detected by LIGO (62 and 21 solar Gravitational Observatory’s Advanced to 40 percent of their initial gravitational waves in all directions. masses). Before LIGO, astronomers Virgo detector, as well as a third mass, astronomers have LIGO’s third detection is a merger had measured black holes a few LIGO observatory recently approved found. This is more than the that took place 3 billion light-years times the mass of the Sun and those for construction in India, will go a mass loss experienced by away and reached Earth on January 4. several million times the mass of the long way toward increasing the sensi- single galaxies falling into The event occurred when two black Sun, with nothing in between. tivity of the project in the future. clusters of hundreds or holes combined to create a single The results, published June 1 in — John Wenz thousands of galaxies. — A. K.

WWW.ASTRONOMY.COM 13 BIG RINGS. A huge gas giant planet with a ring system is blotting out light from the star PDS 110 as it orbits once every ASTRONEWS 2.5 years, according to a team led by astronomers at the University of Warwick.

I SPY . . . Curiosity landed on Mars WHAT IS EARTH MADE OF? to investigate BREAK IT DOWN. the possibility of Our planet contains all of the 92 microbial life on naturally occurring elements. But which ones are most the planet and to abundant? This chart shows the top eight elements study the 3-mile- found within Earth — all of it, not just the crust. These high (5km) Mount elements each account for more than 1 percent of our Sharp, which sits world’s mass. — Michael E. Bakich inside the 96-mile- Calcium wide (154km) Nickel 1.5% Gale Crater. HiRISE captures pictures 1.8% Aluminum of Curiosity about Sulfur 1.4% every three months 2.9% Iron while monitoring 32.1% the area for erosion Magnesium or migration of the 13.9% surrounding dunes. NASA/JPL-CALTECH/UNIVERSITY OF ARIZONA Curiosity climbs Mount Sharp NASA’s Mars Reconnaissance Orbiter has the informally named “Vera Rubin Ridge” when most powerful telescope ever sent to the the High Resolution Imaging Science Red Planet. While monitoring Mars on June 5, Experiment (HiRISE) camera imaged it. 2017, it captured a picture that included the The camera’s filters, which are meant to Silicon

Curiosity rover climbing Mount Sharp, the enhance colors to help scientists better pick 15.1% KELLY ROEN : mountain inside a crater that Curiosity has out subtle differences in the materials that Oxygen been exploring for about five years. make up Mars’ surface, make the rover appear The orbiter was about 169 miles (271 kilo- bluer than it really is. 30.1% ASTRONOMY meters) away when it took the picture. The Once Curiosity reaches its next destination, rover, which measures 10 by 9 feet (about 3 it will take a closer look at the active dunes Earth weighs 6.58 sextillion tons. That’s FAST by 2.8 meters), was headed from the sand where the mineral hematite has been spotted 6,580,000,000,000,000,000,000 tons. FACT dunes on lower Mount Sharp toward the from orbit. — Nicole Kiefert ALMA uncovers more ingredients for life IRAS 16293–2422 became famous The ALMA observations, which in 2012 when astronomers using covered several wavelengths in the Atacama Large Millimeter/ the radio spectrum, indicate that submillimeter Array (ALMA) methyl isocyanate is found in the detected glycolaldehyde, a simple cocoons of gas surrounding the sugar, in the system. Now, the young, still-forming stars. After system is making news again isolating the molecule’s signature, with the discovery that it contains the teams used a combination of methyl isocyanate, a complex computer modeling and labora- prebiotic molecule. tory experimentation to better IRAS 16293–2422 contains understand how the molecule three young protostars with had formed in the system. masses similar to the Sun and lies This discovery allows astrono- about 400 light-years away. mers an indirect peek into the past Two groups of astronomers of our own solar system, which discovered and studied the likely formed under similar chemi- molecule via observations cal conditions. It also has far-reach- taken as part of the Protostellar ing implications for the possibility Interferometric Line Survey with of Earth-like life on other planets, ALMA. Their results were pub- because a star’s planets form from lished in two papers in the the material that surrounds it. April 17 issue of the Monthly “Our laboratory experiments Notices of the Royal Astronomical show that methyl isocyanate can Society. Co-authors Niels Ligterink indeed be produced on icy parti- and Audrey Coutens explained cles under very cold conditions the profundity of the discovery, that are similar to those in inter- saying, “This family of organic stellar space. This implies that this molecules is involved in the molecule — and thus the basis for THE RIGHT ENVIRONMENT. The Rho Ophiuchi star formation region contains synthesis of peptides and amino peptide bonds — is indeed likely the triple-protostar system IRAS 16293–2422, where astronomers discovered clouds acids, which, in the form of to be present near most new of methyl isocyanate gas surrounding the forming stars. ESO/DIGITIZED SKY SURVEY 2; proteins, are the biological basis young solar-type stars,” said ACKNOWLEDGEMENT: DAVIDE DE MARTIN for life as we know it.” Ligterink. — A. K.

14 ASTRONOMY • OCTOBER 2017 FLARE-UPS. A new catalog based on data from the Galaxy Evolution Explorer spacecraft will allow astronomers ASTRONEWS to determine how stellar flares from cool dwarf stars could affect the habitability of their planets. TRAPPIST-1’s planets may trade life The TRAPPIST-1 planetary system could be one of the most promising places to find life — if its star isn’t too volatile. A paper by Manasvi Lingam and Abraham Loeb, published June 14 in the Proceedings of the National Academy of Sciences, suggests that if the sys- tem is habitable, its planets might more or less share the same life. The TRAPPIST-1 system was initially announced in May 2016 as a three-planet system around a star slightly larger than Jupiter. At the time, astronomers believed two of its planets were habitable. Further study revealed that the third, erratic planet was actu- ally several planets, bringing the total number of plan- ets (so far) to seven. The planets are tightly packed, with distances between them similar to the separation of Earth and the Moon. Each orbits in resonance with the others. But these close distances could have a strange effect: It wouldn’t be hard for meteorites to hop from one planet to another, bringing life along with them. The likely “passengers” would be microbial. The paper suggests this hypothesis could be proven if the same biosignatures are found on multiple planets, something that might be possible by comparing the planets’ atmospheres using the upcoming James Webb Space Telescope. Like many small M-dwarf stars, TRAPPIST-1 is fairly active, shooting out flares that could strip its planets of their atmospheres. A recently discovered planet CLOSE NEIGHBORS. The TRAPPIST-1 system, seen here in an artist’s depiction, is packed so tightly around an M dwarf appears to have retained its atmo- that the planets orbit in resonance with less than a million miles between neighbors. If the star hasn’t sphere, however, meaning it is possible that these fried away the planets’ atmospheres, the system could be a promising place to find alien life. NASA/ planets could cling to their gas layers. — J. W. JPL-CALTECH Amateur sleuths find a new nearby brown dwarf Citizen scientists using Zooniverse’s Backyard Worlds: Planet 9 tool have dis- covered a brown dwarf just over 100 light- years from Earth, increasing the number of objects in our Sun’s neighborhood. The project, which launched in February, almost immediately began yield- ing results. For instance, the dim brown dwarf pictured was found just six days into the effort. Australian schoolteacher Bob NEW FIND. Citizen scientists discovered this NASA/CXC/RUTGERS/J. WARREN AND J. HUGHES ET AL. HUGHES J. AND WARREN NASA/CXC/RUTGERS/J. Fletcher is credited with the initial discov- dim brown dwarf through the Backyard Worlds: ery, and three other teams identified it Planet 9 project. It had eluded detection by around the same time. surveys looking for brighter objects. NASA Tycho in 3-D Brown dwarfs, often called “failed stars,” are more massive than planets but Worlds: Planet 9 was designed with a hard- WHAT’S IN A SHAPE? Researchers built this 3-D image less massive than stars, so they are unable to-find target in mind: Planet Nine, a possi- of the Tycho supernova remnant to better understand the expansion of the violent stellar event. The NASA-led team to fuse hydrogen into helium. While some ble cold ice giant about the size of sought to understand asymmetries in supernova remnants brown dwarfs can be quite warm because Neptune at the margins of our solar sys- to determine if their shape develops during or after the they fuse deuterium (a relatively scarce tem. Because of the search parameters supernova. The model used 12 years’ worth of data from isotope of hydrogen), WISEA required to find such an object, the the Chandra X-ray Observatory to determine that various J110125.95+540052.8 — some just call it Backyard Worlds project is also primed to “knots” are moving at different velocities. They concluded “Bob’s dwarf” — is comparatively cold, just discover other nearby cold objects, includ- the explosion was likely symmetric; the asymmetries a few degrees warmer than Jupiter. ing dwarf planets and brown dwarfs. were caused by interactions with matter in the interstellar The object was too faint to be detected The research was published May 24 in medium as the shock wave expanded outward. — J. W. by previous sky surveys, but Backyard The Astrophysical Journal Letters. — J. W.

WWW.ASTRONOMY.COM 15 FORYOURCONSIDERATION BY JEFF HESTER Entropy’s rainbow The statistically likely path to complexity.

he early universe disorder increases with time.” logarithm of the number of Nope. The molecules are was remarkably To this very day, that statement microstates corresponding to indeed more localized in both uniform. Today’s remains de rigueur in class- the same macrostate.) space and momentum, so their universe is anything rooms (and blogs) any time the Rather than “disorder entropy is clearly lower than it but. What gives? subject crops up. But it is an increases,” what the Second Law started out. But what about the TThe details of how we got from oversimplification that does of Thermodynamics actually photons that were radiated there to here form one of the more harm than good. says is that random changes away? There are all sorts of grand themes of science. But Identifying entropy with tend to move things toward sta- possible ways for those photons viewed from 30,000 feet, the “disorder” is a lot like equating tistically more likely configura- to be arranged, meaning that transformation can be summed energy with “bombast.” It may tions. What happens along the there are lots and lots of pos- up in one word — entropy. well be that adding energy often way can be really amazing. sible configurations for the “Hold on a minute,” you say. makes things more bombastic. Imagine watching a giant system as a whole. The “Entropy doesn’t build stuff. Talking about bombastic events cloud of interstellar gas as it starts increased entropy of the pho- Entropy tears stuff down!” might even help someone visu- to collapse under its own weight. tons is more than enough to That is certainly the common alize what can happen when As the cloud gets smaller, it sure make up for the decreased impression people have lots of energy is dissipated at looks like things are getting more entropy of the cloud itself. about the Second Law of one time. But there is no such “ordered,” so perhaps you com- As energy (and entropy) is Thermodynamics. It’s also thing as “conservation of bom- plain about the seeming violation radiated away, the cloud loses dead wrong. bast.” Trying to apply such a of the Second Law. pressure support and collapses Rewind the clock to the mid- further. Eventually that col- dle of the 19th century. In one lapsing cloud settles into a of the all-time greatest advances disk. Stuff in the disk clumps in the history of science, up. The gas at the center gets so Ludwig Boltzmann has just hot and dense that a fusion used the statistics of how mol- reaction kicks in. Where once ecules move to explain why heat there was a big, spread out, always flows from hot things to “disordered” clump of inter- cold things. It is a beautiful, stellar gas, now there is a elegant piece of work. It is also young star surrounded by counterintuitive to a generation The tombstone of Ludwig Boltzmann (1844–1906), who defined entropy, carries planets. So much for tearing of scientists who are only begin- the equation for the Boltzmann constant. MICHIGAN STATE UNIVERSITY things down! ning to get used to the atomic As it goes with the forma- theory of matter. notion to the physical world What you don’t see is that the tion of a planetary system from Boltzmann has trouble would lead to all sorts of collapsing cloud is also getting an interstellar cloud, so it goes explaining what he is on about, absurdity. hotter, meaning that the mol- for everything beginning with until he starts talking about In like fashion, it is wrong, ecules in the gas have the energy that early uniform universe, entropy as “disorder.” A low even nonsensical, to identify to move faster. The wider range right up to you sitting there entropy state, says Boltzmann, entropy with disorder. of velocities (more correctly reading this article. The glori- is more ordered. A high entropy “Increasing disorder” is not a momentum states) that mol- ous, complex, awe-inspiring state is more disordered. It’s physical law. It can’t be. ecules can take on more than structure that surrounds us is a easier to jumble things up than Disorder isn’t even a physical makes up for the decrease in byproduct of increasing to go the other direction, which quantity. Rather than a measure places the molecules can be. entropy, the blind, unguided, is why entropy always increases. of disorder, entropy is actually a Entropy increases as it must. inexorable march of the uni- It is a simple, visually compel- quantitative measure of how But that hot molecular gas verse toward ever-more statisti- ling explanation that people can many different ways you can gives off radiation, which takes cally likely states. sink their teeth into. The idea of rearrange the components of energy away from the mole- entropy as disorder sticks. And something without changing its cules, reducing the available Jeff Hester is a keynote speaker, sticks. And sticks . . . bulk properties. (If you want momentum states. OK, surely coach, and astrophysicist. “The Second Law of that in more formal terms, this is a violation of the Second Follow his thoughts at jeff-hester.com. Thermodynamics says that entropy is a constant times the Law!

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16 ASTRONOMY • OCTOBER 2017 RADIATION WARNING. NASA’s Human Research Program is studying simulated space radiation to determine ASTRONEWS the long-term effects of space flight on astronauts’ health in preparation for manned missions to Mars. Juno’s early results reveal a mysterious Jupiter Since arriving at Jupiter on July 4, atmosphere, while bands at other 2016, the Juno spacecraft has latitudes spawn additional atmo- been showing planetary scientists spheric structures. a brand-new view of the planet, Juno revealed a magnetic field including a deeply turbulent 10 times stronger than any on atmosphere and a “lumpy” mag- Earth, varying in strength across netic field. the planet. “This uneven distribu- “It was a long trip to get to tion suggests that the field might Jupiter, but these first results be generated by dynamo action already demonstrate it was closer to the surface, above the well worth the journey,” said layer of metallic hydrogen,” said Diane Brown, a Juno program Jack Connerney, the mission’s executive at NASA’s Washington deputy principal investigator and Headquarters, in a May press leader of the probe’s magnetic release. The results from Juno’s field investigation at NASA’s first data-collection pass of the Goddard Space Flight Center. planet — at an altitude of 2,600 “We knew, going in, that Jupiter miles (4,200 kilometers) on would throw us some curves,” said August 27, 2016 — were recently Scott Bolton, Juno principal inves- published in Science and tigator from the Southwest Geophysical Research Letters. Research Institute in San Antonio. RI/MSSS/GERALD EICHSTÄDT/SEÁN DORAN EICHSTÄDT/SEÁN RI/MSSS/GERALD Those results include images “But now that we are here, we are w from JunoCam, which showed finding that Jupiter can throw the Earth-sized storms clustered at heat, as well as knuckleballs and the planet’s north and south sliders. There is so much going on

poles. Data from the Microwave here that we didn’t expect that we NASA/JPL-CALTECH/S Radiometer demonstrated that have had to take a step back and LIGHT AND DARK. Jupiter’s atmosphere contains numerous bands and belts; Jupiter’s equatorial belt pene- begin to rethink of this as a whole each band in this JunoCam image is wider than Earth. Three white, oval-shaped trates deep into the planet’s new Jupiter.” — A. K. storms are visible, part of the planet’s “String of Pearls.”

The number of potential exoplanets currently identified 4,034 using data from the Kepler spacecraft.

EXPLOSIVE END. Though there are many subclasses, supernovae fall into two general types: massive stars reaching the end of their lives (type II) and white dwarfs accreting more matter than they can physically support (type Ia). Type II supernovae are responsible for distributing heavy elements throughout the cosmos; type Ia supernovae are “standard candles” used to accurately measure distances to very distant galaxies, thereby determining the expansion rate (and acceleration) of the universe. — A. K. White dwarfs are themselves remnants of Sun-like stars (<8 MSun) that did not explode upon death. If a white dwarf has a binary companion, it can pull material off this nearby star and grow until a critical mass (called

the Chandrasekhar limit, about 1.4 MSun) is reached. This triggers a runaway nuclear reaction that tears the white dwarf apart, causing a type Ia supernova.

Because type Ia supernovae Alternatively, type Ia supernovae may always occur at roughly the same mass (1.4 MSun), they During a massive star’s (≥8 MSun) lifetime, pressure generated by fusion processes occur when two white dwarfs in a binary hold the core up. As fusion slows, the pressure diminishes and the core shrinks. system spiral inward (left) and eventually have the same intrinsic

Eventually, the material inside the core reaches a critical point, triggering a recoil merge (right). If the resulting object is brightness and the observed KELLY ROEN : effect. The resulting explosive shock wave tears through the star’s outer layers. greater than the Chandrasekhar limit, it brightness can be used to calculate distance. Stars of any mass greater than 8 MSun can cause a type II supernova, so these will also explode in a type Ia supernova. events vary greatly in brightness. ASTRONOMY Supernova 1987A is one of the most famous modern supernovae. This FAST type II supernova had a progenitor about 18 times the mass of our Sun. FACT WWW.ASTRONOMY.COM 17 OBSERVINGBASICS Oct 1 N λ BY GLENN CHAPLE 16 16 α

κ Track an Sept 1 η E 31 β asteroid pair Path of Iris θ γ Pallas and Iris come within reach of small telescopes these October nights. ARIES Nov 15

ι ver the years, I’ve star-hopping with my telescope taken attendees to the appropriate location, 30 at public star and looking for a “star” in the 1° parties on tele- position where the asteroid is scopic voyages supposed to be. I make a sketch The brightest asteroid this autumn is 7th-magnitude Iris, which conveniently lies near Oto all corners of the universe. that plots my suspect and the the brightest stars in Aries. ASTRONOMY: ROEN KELLY We’ve viewed the Sun during surrounding stars, being sure daytime (with an over-the- to include those near where the evening of August 13, 1847. 79 million miles away, and thus front filter or by solar projec- asteroid’s path will take it on This was Hind’s first asteroid it peaks at magnitude 6.9. tion, of course) and explored subsequent evenings. find; over the next seven years, If you’ve never seen an virtually everything the night If a follow-up observation he would add nine more. Hind asteroid, this is one you’ll sky has to offer, except for one confirms that my suspect is no is perhaps better known for want to check out. Not only is class of objects: asteroids. longer there and a new “star” discovering the striking red- it bright enough to be picked Most observers can recog- appears where I saw none ear- hued carbon star R Leporis, up through binoculars, but it nize the Sun and the Moon, lier, I carve another notch in also known as Hind’s Crimson also spends autumn close to planets and comets, and deep- my telescope tube. Well, actu- Star, and Hind’s Variable the three bright stars that form sky objects from double stars ally, I record the asteroid Nebula (NGC 1555), which the the head of Aries the Ram. To to nebulae, clusters, and galax- name and date of sighting in star T Tauri illuminates. capture Iris and put a notch in ies at first glance. But an aster- a logbook. Through my little Like most asteroids, Iris your telescope tube (or, prefer- oid? As the name implies 3-inch Edmund Scientific orbits the Sun in the gap ably, a note in your logbook), — aster is the Latin word for reflector, I’ve seen more than between Mars and Jupiter. use the process I described “star” — it looks like a star, so earlier. If you’re successful, it doesn’t stand out when real go ahead and try your luck stars clutter the same field. Through my little 3-inch reflector, with October’s other bright Things would get more inter- asteroid, Pallas. It’s a bit more esting if the viewer were to I’ve seen more than 100 asteroids. than a magnitude fainter than return the next night and Iris and drifts across a rather notice that one of the “stars” barren region near the border had moved. But star parties 100 asteroids. If I’d notched the Studies of its brightness indi- between Eridanus and Fornax, typically are one-night deals, tube each time I captured one, cate that this 130-mile-wide but you can do it. so asteroids aren’t on the menu. I’d have whittled the poor asteroid rotates once every 7.1 Although asteroids might not Before I continue, let me thing down to nothing! hours. Astronomers classify it be high on the list of showpiece explain why I use asteroid as an S-type, or silicaceous. celestial sights, asteroid hunting rather than the broader astro- All eyes on Iris Most of these stony asteroids is a fun and relaxing way to test nomical term minor planet. This month, two of the first inhabit the inner part of the your observing skills. Simply put, I like “asteroid” 10 asteroids discovered — asteroid belt. Questions, comments, or better. Maybe it’s the negative 2 Pallas and 7 Iris — will be Iris reaches opposition and suggestions? Email me at connotation of “minor planet.” within reach of small tele- peak visibility October 29, [email protected]. Next Asteroids aren’t minor at all. scopes and even binoculars. when it lies about 2° south of month: touring Andromeda Just ask the dinosaurs. Because I gear this column to the 2nd-magnitude star Hamal the Princess with a small tele- Asteroids may not be jaw- the novice, let’s set our sights (Alpha [α] Arietis). At a typical scope. Clear skies! dropping telescopic sights, on Iris, which is brighter and opposition, Iris glows at 8th or but it’s fun to track their conveniently located near some 9th magnitude, but this time Glenn Chaple has been an motion over several evenings. bright guide stars. around, opposition occurs avid observer since a friend I enjoy taking out a finder English astronomer John when the asteroid is about as showed him Saturn through a small backyard scope in 1963. chart for a particular asteroid, Hind discovered Iris on the close to Earth as it can get, just

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18 ASTRONOMY • OCTOBER 2017 DEATH BLOW. Using data from LIGO, Rochester Institute of Technology researchers have determined that an exploding ASTRONEWS star may give the resulting black hole a “kick” that affects its alignment in space. Enceladus’ odd tilt from an ancient collision?

Equator

Tiger stripes

Tiger stripes

Original south pole Current south pole NASA/JPL-CALTECH/SPACE SCIENCE INSTITUTE/CORNELL UNIVERSITY INSTITUTE/CORNELL SCIENCE NASA/JPL-CALTECH/SPACE POLAR WANDERER. Enceladus’ “tiger stripes” are geological scars normally associated with equatorial regions. The left-hand image depicts what may have been the moon’s original orientation, while the right- hand image shows how the moon’s tilt has dramatically changed likely due to a large asteroid impact.

Saturn’s oddball moon Enceladus may be a equator, shifting that region toward the cur- jumbled mess, according to research pub- rent south pole. Incidentally, that’s where the lished April 30 in Icarus. moon’s geysers are most active. Enceladus Researchers at Cornell University, NASA, has an ocean underneath its ice crust that fre- and the University of Texas say the poles of quently spurts water into space. the moon are about 55 degrees off from The initial aftermath of the collision would where they should be. The telltale signs of have left Enceladus without a stable axis, this misalignment are a group of basins form- instead chaotically wobbling around for a time. ing a rough line across the moon’s surface The moon would have resettled after about a that would be expected at its equator — million years with an entirely new orientation. except this chain is currently far off from the Such a collision may also explain why the current equator as defined by its spin. moon’s south pole is rife with geological This means something changed Enceladus’ activity, while its north pole is relatively inert. tilt, likely an asteroid striking near the moon’s — J. W. All stars may be born in pairs The Perseus molecular cloud may be reveal-

ESO ing a strange truth about the universe: Most stars, if not all, are born in pairs. This means Three for the that the Sun could have lost a companion that’s somewhere out there — and it may be one of several known stars. price of one All stars form in molecular clouds, such as SEEING TRIPLE. The European Southern the one in Perseus. Radio observations of the Observatory snapped this 3-gigapixel picture of area showed many of its stars are forming in (bottom to top) the Omega Nebula, the Eagle Nebula, multiple-star systems. Models based on the and a gas cloud called Sharpless 2–54. The objects are data further showed that the conditions all about 7,000 light-years away and may have formed inside the cloud could be reproduced only if from the same primordial gas clouds. Each has since all the stars were originally born in pairs. become an area of star formation. The Eagle Nebula Protostars have been observed to form in is home to the so-called “Pillars of Creation,” a star egg-shaped cores within larger molecular forming region that is one of the most famous images

clouds. The models based on the Perseus NRAO/AUI/NSF (ESO/NAOJ/NRAO), ALMA SAXTON, BILL taken with the Hubble Space Telescope. — J. W. molecular cloud show that these cores tend MISSING SIBLING. ALMA captured this radio to form two concentrations of mass as they image of a multiple-star system forming inside collapse, and thus two stars rather than one. the Perseus molecular cloud. Modeling based on So why doesn’t the Sun have a binary com- observations of the cloud indicates that all stars panion (well, depending on whom you ask)? It may be born in systems of two or more stars. 20 years seems that 60 percent of stars lose their binary sister over time, gaining a wider dis- The authors of the paper, published in the How long the United States tance from their partner until they are gravita- Monthly Notices of the Royal Astronomical tionally severed. The pairs that form also may Society, say more work is needed to confirm has been continually operating not include two objects with similar masses, their hypothesis. But if it’s true, the hunt may meaning that some former companions could be on for the companion the Sun once had. spacecraft on or around Mars. be brown dwarfs cast out by larger stars. — J. W.

WWW.ASTRONOMY.COM 19 The huge, umbrella- shaped plume of the volcano Pele rises some 185 miles (300km) above Io’s surface. Ejecta from the massive eruption covers an area the size of Alaska. NASA/JPL/USGS

UNVEILING A A storm-wracked world and moons with erupting volcanoes and underground oceans GIANT were just some of the surprises the Voyager spacecraft revealed. by Francis Reddy ntil the mid-1990s, when planets were at last being discovered orbiting distant stars, Jupiter was the larg- est world known. Eleven times Earth’s diameter and 318 times our planet’s mass, Jupiter possesses Umore than twice the com- bined matter of all the other planets, moons, asteroids, and com- ets in the solar system. Made mostly of hydrogen (89.8 percent by volume) and helium (10.2 percent), Jupiter is essentially a bottomless atmosphere that, beneath the clouds, transitions into an exotic fluid. Its largest moons, so big they rival Mercury in size, appear to hold substantial amounts of liquid water and beckon scientists as poten- tial abodes for life beyond Earth. For centuries, humanity could view this giant world only through ground-based telescopes. But in 1973 and 1974, respec- Voyager’s views of Jupiter’s colorful cloud tops reminded many people of an impressionist painter’s tively, the Pioneer 10 and 11 spacecraft canvas. Fine details in and around the Great Red Spot changed significantly in the four months raced past the planet, providing the first between the Voyager 1 flyby and Voyager 2 (pictured). NASA/JPL close-up images of its stormy atmosphere, probing its internal structure, and charting its intense radiation belts and magnetic cruise away from our planetary system and By early February, the resolution and field. The Pioneer probes blazed a trail for journey into the unending night of inter- image quality were comparable to the best further exploration of the outer solar sys- stellar space. Voyagers 1 and 2 are now so pictures returned by the Pioneers. From tem. Even as scientists reveled in the data far away that their radio signals take more that point on, Jupiter would be seen as the probes returned, NASA already was than 15 hours to reach us. The more dis- never before. working on a far more ambitious encore. tant probe, Voyager 1, has passed beyond When viewed from Earth through a The mission, originally called Mariner the heliopause, a boundary separating the small telescope, the planet’s atmosphere Jupiter-Saturn 1977, received a name interstellar environment from charged par- shows alternating bright white zones and change before launch to the one we know ticles and magnetic fields flowing from the darker brown belts. These are the visible today: Voyager, the longest-running space Sun, known as the solar wind. Voyager 2 is manifestations of east-west jet streams that mission in history. The twin Voyagers car- cruising through the heliosheath, a zone alternate direction from the equator to the ried better cameras, more advanced instru- where pressure from interstellar gas begins poles and carry oval-shaped weather sys- ments, and more computing power than slowing the solar wind. tems of all sizes. the Pioneers. Both spacecraft launched in “It’s been a remarkable journey,” says The Voyager imaging team tracked a 1977 and are still returning data as they Voyager project scientist Ed Stone at the pair of similarly sized brown ovals, watch- California Institute of Technology in ing them merge, tumble, and eject a dark Pasadena. “We keep discovering things streamer. Atmospheric models had not nobody knew we were going to discover.” predicted such strange behavior. Eddies, He says both spacecraft may be able to waves, and turbulent clouds churned return measurements from a single science everywhere. At a February 28 press brief- instrument until 2030 if their power ing, imaging team leader Bradford Smith sources — called radioisotope thermoelec- described his team as “happily bewildered” tric generators — hold out as expected. by what they were seeing. “Jupiter is far more complex in its atmospheric motions Encounter with Jove than we had ever imagined,” he said. On January 6, 1979, Voyager 1 was 36 mil- The planet’s largest feature, a vast lion miles (58 million kilometers) from southern storm called the Great Red Spot, Jupiter and two months from its closest had been observed continuously from approach. Views of the planet’s cloudy, Earth for 150 years. But now, for the first banded disk already exceeded the best time, scientists could study its rotation and Voyager 1 captured this view of Jupiter on images from Earth. Among other assign- watch it interact with neighboring features. January 9, 1979. Even though the spacecraft was 34 million miles (54 million km) away, ments, the probe began accumulating a Large enough to hold a pair of Earth-sized its camera captured details impossible time-lapse movie by taking images every planets, the Great Red Spot rolls between to see from Earth. NASA/JPL 10 hours, one for each Jupiter rotation. two jet streams and completes a rotation in

WWW.ASTRONOMY.COM 21 clouds formed by ammonia ice crystals occupies this level. Plunge deeper, to twice Earth’s surface pressure, and the temperature rises to –76° F (–60° C). Here we encounter a tawny, yellow-orange cloud deck built from droplets or crystals of ammonium hydro- sulfide. Usually we cannot see past these clouds, but spacecraft measurements and models indicate that the next cloud layer — at pressures of about 3 to 7 bars and temperatures from freezing to about 60° F (16° C) — contains water ice crystals at the colder levels and water droplets lower Io (in front of the Great Red Spot) and Europa appear against Jupiter’s cloud tops in this Voyager 1 down, just like clouds on Earth. Any famil- image from February 13, 1979. Io’s odd coloration already stands out from a distance of 12 million iarity pretty much ends there, though. miles (20 million km). NASA/JPL Minute frequency changes in spacecraft when ultraviolet (UV) sunlight breaks radio signals allow scientists to map the down ammonia and acetylene, gases com- structure of Jupiter’s gravitational field; this mon in Jupiter’s upper atmosphere. The enables them to develop models of what high clouds within the Great Red Spot and lies beneath the clouds. Pressures and tem- similar storms receive more solar UV, and peratures increase steadily, but the hydro- the storms’ rotation helps retain colored gen atmosphere simply grows denser and particles that result in stronger shades. hotter with depth until, hundreds of miles Scientists suspect these changes in color beneath the clouds, molecular hydrogen and size are related to storm strength, but starts to resemble a hot liquid. At depths they remain poorly understood. 10 times greater, only 20 percent of the way In 1998, two of three 60-year-old white to Jupiter’s center, pressures approach a oval storms in a cloud band south of the million bars, and temperatures soar to The Voyagers revealed several brown ovals in Great Red Spot merged, and in early 2000, 10,000° F (5,700° C) — nearly as hot as Jupiter’s atmosphere. These appear to be open- the third oval joined them. The resulting the Sun’s surface. Here the interior trans- ings in the high-level clouds that give a peek to darker regions below. Note the high white cloud weather system, named Oval BA, is about forms into a more exotic substance called protruding over this oval’s northern edge. NASA/JPL half the size of the Great Red Spot and per- liquid metallic hydrogen, an electrically sists today. In August 2005, amateur astron- conductive soup of protons and electrons omers noticed it was acquiring a reddish that makes up most of Jupiter’s mass. about six days. It spins counterclockwise, color. The hue grad- the opposite direction as hurricanes in ually deepened; by Earth’s Southern Hemisphere, classifying it 2006, the storm was DESPITE THE VOYAGER PROBES AND LATER as a high-pressure system. Its cloud tops nicknamed “the MISSIONS, VITAL QUESTIONS ABOUT extend nearly 5 miles (8km) above neigh- Little Red Spot” and boring layers. Although winds whip around “Red Spot Jr.” JUPITER’S ATMOSPHERE REMAIN. its periphery at 425 mph (680 km/h), the Yet despite the interior is calm. Its size and position vary Voyager probes and slightly, and long-term ground-based mon- later missions, vital questions about Jupiter’s Some 28,000 miles (45,000km) farther itoring shows that the longest-lived storm atmosphere remain. Why are the jet streams down, about 80 percent of the way to the known to science is shrinking steadily. and large storms stable for so long? What’s planet’s center, the composition may Amy Simon at NASA’s Goddard Space the energy source for the jets? And do the change to a mix of water, methane, and Flight Center in Greenbelt, Maryland, leads winds continue into the planet’s interior? ammonia at enormous temperatures and a team studying Jupiter with the Hubble pressures. Another 4,400 miles (7,000km) Space Telescope. The observations show the Diving into Jupiter down, and we’re 10 percent from the center; storm’s long axis is half what was reported The top of Jupiter’s atmosphere consists the pressure rises to around 40 million bars in the 1880s and about 30 percent smaller of haze layers formed by complex hydro- and the temperature to some 40,000° F than during the Voyager flybys. And since carbons like ethane, ethylene, and acety- (22,000° C). At this point, Jupiter’s compo- 2014, the Great Red Spot has turned an lene. These chemicals assemble from the sition may gradually morph into a dense unusually intense shade of orange. fragments of methane molecules broken core, perhaps containing up to 20 Earth Kevin Baines and colleagues at NASA’s apart by solar UV, a process similar to how masses in a mix of rock and iron that may Jet Propulsion Laboratory (JPL) in Pasadena smog forms in Earth’s atmosphere. About also include water, methane, and ammonia. have conducted laboratory experiments 25 miles (40km) deeper, the pressure At these pressures, dense materials may that suggest the storm’s color, which has approaches 60 percent of that at Earth’s become soluble in liquid hydrogen, some ranged from pale orange to brick red, is a surface (1 bar), but the temperature is only scientists suggest. This means Jupiter’s orig- result of reddish coloring agents produced –193° F (–125° C). A deck of bright white inal core may have dissolved partially or

22 ASTRONOMY • OCTOBER 2017 UNDER THE VEIL

NASA’s Juno mission entered an elon- “On Jupiter, a gas planet, there is no gated 53-day orbit around Jupiter on iron-bearing crust between us and the July 4, 2016. The spacecraft is mapping dynamo, and the dynamo is much, much the planet’s gravitational and magnetic closer to the surface of Jupiter than is the fields in unprecedented detail, allowing case in the Earth,” says John Connerney scientists to understand how material at NASA’s Goddard Space Flight Center, moves deep within the planet. Juno also lead scientist for Juno’s magnetometer is measuring atmospheric dynamics to instrument. Initially, scientists thought pressures beyond 100 bars, far below Jupiter’s dynamo might be between Jupiter’s visible clouds, and providing 15 and 25 percent of the way down, or scientists with an improved upper limit 0.85 to 0.75 Jupiter radii from the plan- on the mass of a possible solid core. et’s center. “More contemporary thinking Meanwhile, Juno’s magnetometer is is that maybe the dynamo is actually creating the first 3-D map of the planet’s closer, up to 0.93 jovian radii, in which magnetic field, providing additional case it’s really just beneath the visible insights into Jupiter’s internal structure. surface,” adds Connerney. The movement of electrically charged To perform its mission, Juno flies over material, called a dynamo, generates the Jupiter’s poles, passing about 2,600 miles field somewhere within the planet. (4,100 kilometers) from its cloud tops. Juno’s observations could provide sci- The elongated polar orbit helps limit the entists with their most detailed look at a spacecraft’s exposure to Jupiter’s punish- planetary dynamo. Earth’s dynamo lies ing radiation belts, but more protection about halfway to its center, but the still was needed. So Juno’s critical elec- The Juno spacecraft’s camera preferentially targets abundant iron-bearing materials in the tronics are placed in a radiation-shielded Jupiter’s poles. This view of the south pole reveals crust makes mapping its location from “vault,” a titanium cube with walls a third several oval cyclones up to 600 miles (1,000km) in diameter. orbit complicated. of an inch (1 centimeter) thick. — F. R. NASA/JPL-CALTECH/SWRI/MSSS/BETSY ASHER HALL/GERVASIO ROBLES completely away, its high-density materials charged particles inside it, forming a vast, look up and identify Jupiter in the sky, its dispersed throughout a larger portion of comet-shaped bubble — called a magneto- magnetosphere would be about the size of the planet. One of the main goals of NASA’s sphere — that shields the planet from direct the Moon,” says Goddard’s John Connerney, Juno mission, which has been orbiting the exposure to the solar wind. Pressure from deputy principal investigator for the Juno planet since July 2016, is to answer the the solar wind pushes the Sun-facing side mission. Its bow shock typically lies about many remaining questions about how the into a rounded bow shock that slows and 40 Jupiter diameters sunward of the planet solar system’s largest world is put together. deflects most of the incoming charged par- and its magnetotail stretches some 3,500 (See “Under the veil,” above.) ticles in much the same way that water diameters behind it, nearly reaching Saturn’s flows around the bow of a moving ship. orbit. We know this because as Voyager 2 Magnetic tango The opposite side tapers into an immense approached Saturn in 1981, two years and Like Earth, Jupiter generates a magnetic magnetotail whose farthest portions wave 400 million miles (650 million km) after its field, which at the cloud tops is about 15 and flap like the tattered end of a windsock. Jupiter encounter, the spacecraft slipped in times stronger than our planet’s. The field Jupiter’s magnetosphere is the solar sys- and out of Jupiter’s frayed magnetotail traps, stores, and controls the flow of tem’s largest planetary structure. “If you multiple times. (By comparison, Earth’s bow shock lies three to five Earth diam- eters toward the Sun, and its magnetotail may extend hundreds of diameters.) Between February 28 and March 2, 1979, the magnetosphere seemed to be playing hard to get with the approaching Voyager 1. The solar wind was gusty, producing unusually strong and variable pressures that pushed the bow shock closer to the planet. When the wind eased, the bow shock re- expanded. Pioneers 10 and 11 made their crossings 50 diameters from Jupiter, but Voyager 1 crossed it five times, the last at scarcely half that range (28 diameters). On March 5, the spacecraft made its clos- Scientists discovered the first volcanic eruptions Sinuous rivers of lava flow down the flanks est approach, passing within 128,400 miles beyond Earth on this navigation image taken of Io’s volcano Ra Patera. The volcano’s vent (206,700km) of Jupiter’s cloud tops, barely March 8, 1979. The photo shows a crescent Io measures about 19 miles (30km) across, while one-third the distance at which Voyager 2 with a volcanic plume rising more than 150 miles the lava flows extend up to 155 miles (250km) (260km) above the limb. A second plume catches from there. The summit rises only about 0.6 mile would pass July 9. Scientists selected this the rays of the rising Sun. NASA/JPL (1km) above the surroundings. NASA/JPL path so they could measure a hypothesized

WWW.ASTRONOMY.COM 23 The largest structure in the jovian system isn’t the planet or its Jupiter’s magnetic personality rings, but a mostly invisible region built from ionized gas and the gas giant’s strong magnetic field. The field sweeps charged gas particles from Io’s atmosphere into a doughnut-shaped ring (red). Other interactions power an electrical current (green) that arcs from Jupiter to Io; similar interactions create weaker electrical links with Europa and Ganymede. JOHN SPENCER

Magnetic field lines

Electrical current Io torus Jupiter Neutral sodium cloud Europa

Io

Ganymede

Callisto

electrical circuit connecting Jupiter and its satellites do not disappoint. Discovered by explored a different solar system.” moon Io, and it took Voyager 1 deep into Galileo Galilei in 1610, they are Io, Europa, “They’re quite distinct,” says Stone, the most hazardous radiation belts in the Ganymede, and Callisto (in order of dis- “and I think the one thing we have learned solar system. Based on measurements from tance from the planet). Ganymede, the is that nature is remarkably diverse, and the Pioneers, the Voyager design included largest and most massive moon in the solar you don’t see replicas. Each body seems to shielding that hardened sensitive electron- system, is slightly bigger than Mercury, have its own life history written on the sur- ics to the bombardment of high-energy while Callisto is nearly the planet’s diame- face and in its interior.” electrons, protons, and ions stored in ter. Both Io and Europa are roughly the There had long been hints that the big Jupiter’s equivalent of Earth’s Van Allen size of Earth’s Moon. moons might be doing something interest- The moons’ ing. Galileo’s 17th-century plots showed complexity and that Europa and Io always meet up on the AN EXTRAORDINARY PLANET DESERVES individuality, which side of Jupiter exactly opposite from where scientists hardly Europa and Ganymede do. In the 7.15 days EXTRAORDINARY MOONS, AND JUPITER’S suspected despite it takes Ganymede to go around Jupiter FOUR BIG SATELLITES DO NOT DISAPPOINT. centuries of tele- once, Europa orbits twice and Io four scopic observations, times. This 1:2:4 resonance forces the proved a major sur- moons’ orbits to maintain a slight eccen- Belts. But an unprotected human passenger prise of the Voyager missions. Reporting tricity, which in turn causes their bodies to riding aboard Voyager 1 during closest their results in the journal Science three flex slightly due to tides raised by Jupiter’s approach would have received a thousand months after the Voyager 1 encounter, the gravity. Just as repeatedly bending a paper times the lethal radiation dose. imaging team noted that the large moons clip warms up the metal, this forced flexing do not closely resemble either the planets warms the interiors of Jupiter’s big moons. Four new worlds in the inner solar system or one another. Stanton Peale at the University of An extraordinary planet deserves extraor- “The sense of novelty,” they wrote, “would California, Santa Barbara, and his col- dinary moons, and Jupiter’s four big probably not have been greater had we leagues first described the mechanism in a

24 ASTRONOMY • OCTOBER 2017 The Galilean moons

Io Europa Ganymede Callisto The solar system’s most Likely supports an The solar system’s largest The solar system’s third- volcanic body; 5 percent ocean beneath an icy satellite, 8 percent larger largest moon, 1 percent larger than Earth’s Moon. crust; 90 percent the than the planet Mercury; smaller than Mercury; it size of Earth’s Moon. it is the only one known to has the oldest surface and 0 1,000 2,000 miles generate a magnetic field. is nearly saturated with impact craters. 3,000km 2,160 miles Diameter of Earth’s Moon The Voyager flybys brought Jupiter’s diverse family of moons into sharp focus. Observations of the four Galilean satellites proved to be as much 3,476km 3,270 miles Diameter of Ganymede of a revelation as those of Jupiter itself. NASA/JPL

0 5,562km

paper submitted to Science in January 1979. They presciently concluded: “Voyager images of Io may reveal evidence for a plan- etary structure and history dramatically different from any previously observed.” The paper was published March 2, three days before Voyager 1’s closest approach. By then, images of the weirdly colored moon, reminiscent of a cheese pizza, already were making headlines. Scientists had expected Io to resemble our Moon, but it lacked anything that looked like an impact crater and was coated with richly colored deposits that wouldn’t look out of place around active fumaroles on Earth. On March 8, as Voyager 1 raced out of Giant Ganymede — larger than Mercury — the Jupiter system, its camera captured a sports two different types of terrain. The dark material is oldest, while the lighter bands cut routine navigation image of a crescent Io as by grooves and ridges are significantly younger. part of a program to refine knowledge of A relatively recent impact threw out the bright The Voyagers revealed dark streaks crisscrossing the spacecraft’s trajectory. The next day, material at bottom. NASA/JPL the surface of Europa, the smallest of Jupiter’s JPL engineer Linda Morabito enhanced Galilean moons. In some cases, the streaks this image to locate background stars and stretch more than 1,000 miles (1,600km). Scientists later deduced that the streaks are uncovered another crescent shape on Io’s ejecta associated with a heart-shaped vol- cracks in the moon’s icy crust filled with material sunlit limb. It looked like the edge of cano now known as Pele. Remarkably, the from its interior. NASA/JPL/USGS another satellite peeking out from behind same picture also held a second plume, one Io, but scientists determined that an erupting from a feature now called Loki unknown satellite so large would have been and catching sunlight as it rose high over paint Io’s terrain in garish hues, and the detectable from Earth. “No one understood Io’s night side. rest forms a thin, distended atmosphere what they were seeing, reinforcing the Scientists now know of at least 150 around the moon. Particle interactions ion- degree of difficulty associated with inter- active volcanoes on Io. Some of them blast ize some of these atoms, and they then preting this image,” she later wrote. umbrella-shaped plumes containing become swept up in Jupiter’s fast-moving By March 12, the imaging team had sodium, potassium, sulfur, sulfur dioxide, magnetic field, which rotates with the confirmed the presence of active volcanoes. and more to altitudes as high as 310 miles planet’s 10-hour rotation. “About a ton per What Morabito had noticed was a plume of (500km). Some of the ejecta falls back to second of that material is picked up by the

WWW.ASTRONOMY.COM 25 Jupiter’s ghostly rings lie well inside the orbits of the Galilean moons, though four much A giant’s ghostly rings smaller moons orbit among the tiny dust particles. The Voyager spacecraft discovered these rings along with the moons Metis, Adrastea, and Thebe. ASTRONOMY: ROEN KELLY; IMAGES: NASA/JPL

Main ring

NASA’S New Horizons probe found Halo a gap in the main ring during its 2007 flyby. This may be caused by Gap an unknown moon. NASA/JHUAPL/SWRI

Main ring Thebe ring Thebe extension Halo ring Amalthea ring

Adrastea Amalthea Thebe

Metis

0 5,000 10,000 miles

0 300 miles 0 10,000 20,000km Adrastea Thebe 0 500km Rings are labeled in yellow. Moons are labeled in white. Amalthea Metis Sulfur debris from Io’s eruptions may cause the moons’ red color.

jovian magnetic field, and that mass of stuff a spot of infrared emission in Jupiter’s and its images revealed frozen plains criss- inflates the Jupiter magnetosphere to about polar atmosphere. The glow tracked with crossed by dark streaks, giving it the look twice the size it should be,” Stone says. Io in its orbit and arose from energy cours- of a cracked egg. Europa’s surface is the The ionized gas spreads along Io’s orbit ing down the flux tube. But Io isn’t alone in smoothest in the solar system. Features to form a doughnut-shaped cloud called this regard. In 2002, Hubble imaged Io’s display so little topographical relief that the Io plasma torus. Some of the heavy ions spot in the UV and found two more glows imaging team member Larry Soderblom in the torus migrate outward, and their from Europa and Ganymede, showing they compared the moon to a billiard ball. Later pressure supersizes the magnetosphere. As generate their own flux tubes. “The system that year, the scientists who explained the Io moves through the torus, it continuously is highly coupled and connected, where the heating of Io suggested that tidal flexing of magnetic fields and Europa could provide enough heat to sus- THE JUPITER FLYBYS MARK THE FIRST the particles are all tain an ocean beneath its icy shell, which is interacting with the widely thought to be 10 to 15 miles (16 to CHAPTER IN THE VOYAGER’S EXPLORATION moons,” Stone says. 24km) thick. The ocean itself may be at And the phenom- least 30 miles (48km) deep, or more than OF THE OUTER SOLAR SYSTEM. enon isn’t unique to 10 times the average depth of Earth’s seas. Jupiter. In 2011, sci- In fact, several lines of evidence now entists identified a support the presence of briny global oceans generates an electrical current that flows UV spot associated with the active moon within Europa, Ganymede, and Callisto, along a conduit, called the Io flux tube, Enceladus in images from the Cassini mis- each containing more water than Earth’s linked to Jupiter’s upper atmosphere. Two sion orbiting Saturn. seas. NASA’s Galileo spacecraft, which in billion kilowatts flow through the flux tube, 1995 became the first to orbit Jupiter, flew comparable to the average global power The outer trio close to these moons and found that consumption on Earth. The Voyager 1 Europa, the next moon out from Jupiter, Jupiter’s rotating magnetic field induces team deliberately tried to pass through the couldn’t be more different from its siblings. currents in electrically conducting layers tube, but the material around Io shifted its Low-resolution images from Voyager 1 within them. These currents, in turn, gen- position from what was expected, and the showed a bright surface of frozen water erate secondary magnetic fields Galileo spacecraft instead flew alongside it. with no discernible craters, along with could detect. Europa’s induced response While imaging from Earth in 1993, hints of dark linear features. Voyager 2 matches what researchers would expect for Connerney and his colleagues discovered passed much closer to Europa on July 9, a salty subsurface ocean many miles thick.

26 ASTRONOMY • OCTOBER 2017 The multiple-ring impact basin Valhalla dominates this Voyager 1 view Voyager 1 discovered Jupiter’s faint, dusty rings during its flyby, and they of Jupiter’s outermost major moon, Callisto. The lighter, central area posed for this Voyager 2 portrait once the probe passed into the planet’s spans about 375 miles (600km) while the outer rings extend about shadow. The curved rings appear orange-red; Jupiter’s multicolored limb 1,100 miles (1,800km) from the center. NASA/JPL comes from the long exposure through two filters. NASA/JPL

And different teams using Hubble in 2012 suggested that tidal heating had powered a frozen remnant of a giant impact. Galileo and 2016 discovered tantalizing evidence brief period of intense tectonic activity in spacecraft observations indicate the pres- that Europa occasionally erupts plumes of Ganymede’s icy crust early in its history. ence of a salty, subsurface global ocean water vapor reaching heights of 125 miles The Galileo mission revealed in 1996 despite little tidal heating at Callisto now. (200km), suggesting the icy shell may be that Ganymede generates its own perma- Perhaps ammonia and other contaminants quite thin in some locations. nent magnetic field, the only moon in the lower the freezing point enough for a liquid The first low-resolution glimpses of solar system known to do so, and therefore layer to survive. Ganymede revealed two starkly different makes its own miniature magnetosphere. terrain types. A dark material covers about This complicates the interpretation of its Opening act 35 percent of the surface. Peppered with induced field, but recent models, as well The Jupiter flybys mark the first chapter impact craters and their bright halos of icy as Hubble observations of Ganymede’s in the Voyagers’ exploration of the outer ejecta, this is the moon’s geologically oldest aurorae, suggest the interior contains solar system. They provided new views surface. The rest of the moon features light shells of different phases of water ice sepa- of an enormous, complex, and dynamic materials crosscut by an intricate patchwork rated by salty seas. atmosphere that is still far from under- of grooves and ridges. Voyager 1 images Callisto, the farthest of Jupiter’s big stood. They explored a vast magnetosphere showed that fault lines cut some of the light moons, hosts the solar system’s most heav- loaded with particles from its moons, bands, which were then offset by surface ily cratered and geologically ancient sur- especially Io, and intimately connected movement. “There is transverse motion face. Its terrain is nearly saturated with to them. Close-ups of unique new worlds along these faults,” Soderblom said at a bright impact craters. The largest visible uncovered incredible properties, including March 6 briefing. “Things get offset, appar- feature, named Valhalla, resembles a bull’s- the first example of active extraterrestrial ently, for hundreds of kilometers.” This eye about 2,200 miles (3,600km) across, the volcanism and the first clues that frozen moons could sport internal seas. Further discoveries included a faint ring of dust SPACECRAFT VISITS TO JUPITER extending 80,000 miles (129,000km) from Probe Status Agency the planet’s center, and two new moons, Metis and Adrastea, orbiting just beyond Pioneer 10 Flyby and gravity assist, 1973 NASA it. The probes also found a third satellite, Pioneer 11 Flyby and gravity assist, 1974 NASA Thebe, in a more distant orbit, though still Voyager 1 Flyby and gravity assist, 1979 NASA well inside Io’s. Voyager 2 Flyby and gravity assist, 1979 NASA With Jupiter now in the rearview mir- Ulysses Flyby for gravity assist, 1992 and 2004 NASA/ESA ror, Voyager scientists could begin digging Galileo Orbiter, 1995 to 2003 NASA deeper into the data — and wondering Cassini-Huygens Flyby for gravity assist, 2000 NASA/ESA what awaited them at their next destina- tion, Saturn. New Horizons Flyby for gravity assist, 2007 NASA Juno Orbiter, reached Jupiter in 2016 NASA Francis Reddy is the senior science writer for Jupiter Icy Moons Orbiter Orbiter, in development for 2022 launch ESA the Astrophysics Science Division at NASA’s Europa Clipper Orbiter, in development for 2020s launch NASA Goddard Space Flight Center in Greenbelt, Maryland.

WWW.ASTRONOMY.COM 27 Close encounters with the

The probes not only returned stunning 2 images of Saturn and its rings — they also set the stage for the highly successful Cassini mission. by Liz Kruesi

1

ome call Saturn the jewel of the the beautiful and intriguing ringed world solar system, with its distinc- and its equally fascinating system of tive rings, variety of moons, moons. The planet hosts a wide array of Sand swift atmospheric winds. astronomical processes and structures, and The first glances of the planet the Voyager probes were the first to show through a telescope certainly form a mem- scientists how incredible the Saturn system ory that sticks in peoples’ minds. Whether truly is. The system they uncovered was you’re a backyard observer or a planetary too intriguing not to revisit, laying the scientist, Saturn is a fascinating world. foundation for the groundbreaking Cassini For centuries, astronomers using tele- mission decades later. The pair of probes revealed many unex- scopes spied a ring of material circling the pected details about the Saturn system, but planet’s equator. Pinpricks of light, the Toward the ringed world Voyager 1 was not the first mission to snap planet’s moons, floated nearby. Technical The twin Voyager spacecraft launched 16 an up-close view of the planet. That title advances led to better views, until scientists days apart in 1977, beginning their Grand belongs to Pioneer 11, which flew by the could see gaps or divisions in the material Tour of the outer solar system. They took ringed world in 1979. Its photographs, ringing Saturn. They also saw oddities on separate journeys to the ringed planet and combined with ground-based detections, its moons: One seemed to hold a methane its diverse moons. Voyager 1 arrived in helped planetary scientists better plan the atmosphere laced with clouds, while November 1980, and afterward used the Voyagers’ Saturn flyby routes, as well as another was two-faced, bright on one half planet’s gravity to slingshot itself out of choose which targets to focus on. but dark as asphalt on the other. the solar system’s plane. Voyager 2 swung Each closest approach was a quick But it would take Voyager 1 and 2, a through in August 1981, continuing on to encounter — after all, the probes were pair of visiting spacecraft, to fully reveal Uranus and Neptune. traveling faster than 9 miles per second

28 ASTRONOMY • OCTOBER 2017 he RINGED PLANET

(15 km/s). But the mission team began col- rings: the Cassini Division and the Encke 1 and 2. Although Pioneer 11 holds lecting detailed observations of each target Gap. But based on observations from both the distinction of taking the first close-up image of Saturn (left, with its weeks in advance. And for the two weeks Pioneer and ground-based telescopes, “we largest moon Titan also in the frame), surrounding each nearest encounter of thought we would find bland, featureless the superior resolution afforded by Saturn, all the science teams would con- sheets of material separated by gaps,” says the Voyager probes (right, seen with verge at the Jet Propulsion Laboratory in Linda Spilker, who studied the rings as moons Tethys, Dione, and Rhea to the lower right, while Mimas appears as a Pasadena, California, for an intense part of the Voyager team and is now the dim shadow just beneath the rings on observing session. Cassini project scientist. Instead, the twin the left) uncovered details previously spacecraft revealed the rings are anything unimagined by planetary scientists. Remarkable features but bland. 1: NASA AMES; 2: NASA/JPL/USGS of the rings On October 6, 1980, when Voyager 1 Before Voyager arrived at Saturn, scientists was still 30 million miles (50 million kilo- knew of two empty paths splitting Saturn’s meters) from Saturn, researchers first made

WWW.ASTRONOMY.COM 29 3 4 5

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3. Voyager was the first to image out dark streaks in the rings. The streaks from Delta Scorpii as the rings passed in Saturn’s rings in enough detail to make didn’t circle the planet, but instead looked between that background star and the out features like the “spokes” seen here in the B ring on August 22, 1981, like they radiated away from it. These spacecraft. Called an occultation, this fil- from a distance of 2.5 million miles “spokes” were the first of many unexpected tered view allowed researchers to see even (4 million km). These dusty features, details in the ring system. finer details in the rings. In fact, they saw which can stretch up to 12,400 miles As Voyager 1 neared closest approach the particles in the rings with a resolution (20,000km) in length, vary with time and have no agreed-upon origin. NASA/JPL-CALTECH and the resolution improved, the team 10 to 20 times better than by just photo- could make out more details. “It looked graphing the rings directly. 4. Saturn’s skinny F ring, imaged like grooves on a phonograph record,” On the evening of nearest approach, November 12, 1980, from 466,000 miles (750,000km), is a complex structure says Spilker of the rings. Hundreds of when Voyager 2 was just over 100,000 miles composed of two bright rings encircling concentric rings circled Saturn. Scientists (161,000km) from Saturn, the photopola- a third, fainter ring. Clumps, kinks, and saw waves along the edges of gaps between rimeter (an instrument that records how other structures in these rings give them those rings, as well as braided features light scatters off particles) observed the a braided appearance. NASA/JPL-CALTECH and spiral structures within the rings — rings for two and a half hours. Those data 5. The Voyagers unveiled not only a all due to the gravitational influence were printed on a roll of paper using a pen- dynamic planet, but an intriguing of small moons embedded in and sitting plotter. “I remember unrolling it out in the and varied system of moons as well. Voyager imaged Mimas on November just outside of the rings. They even saw hallway, putting a book on one end and a 12, 1980, from a distance of 264,000 patterns that looked like propeller wings book on the other end and walking miles (425,000km). The moon’s terrain spiraling out from moonlets, showing through and just looking at the data,” is marred by the huge Herschel crater, how large boulders clear material along recalls Spilker. “It was like walking which is 80 miles (130km) across and covers more than a quarter of Mimas’ gaps in their orbit. Studying the behavior through the rings.” surface. NASA/JPL of moonlets in a “debris disk” such as With those data, scientists could esti- Saturn’s rings has allowed scientists to mate the thickness at the edge of each ring: 6. Tethys, imaged by Voyager 2, showed indirectly study how planets form around between 33 and 656 feet (10–200m). They areas of both old (brighter, more heavily cratered areas) and new (darker) crust, stars in protostellar disks. “The rings were saw smaller structures in the rings: clumps, as well as a vast system of canyons that just so much more than I had imagined,” twists, and waves — all due to the gravity nearly encircles the small moon from top adds Spilker. of Saturn’s satellites. That occultation using to bottom. NASA/JPL-CALTECH But it wasn’t just the beautiful images of Delta Scorpii was one of the most crucial the photogenic ring system that surprised observations Voyager made at Saturn. And scientists. When Voyager 2 approached with only one such event, the data was Saturn in August, it observed starlight extremely precious, says Spilker.

30 ASTRONOMY • OCTOBER 2017 WHAT IF? 9

When Voyager 2 flew by the Saturn sys- tem in August 1981, one of its goals was to further study Enceladus. Unfortunately, the spacecraft’s swiveling platform mal- functioned for several hours, and its five instruments were out of commission dur- ing closest approach to Saturn and its inner moons. That meant the science team didn’t get all the images and measurements they were hoping for, including close-up mosaics of Enceladus. During the flyby, that moon and the Sun aligned in such a way to illuminate Enceladus’ south polar region, where scientists now know geysers of water and organic materials spew. Would sunlight have glinted off the icy particles in those jets, revealing their presence in 1981? “I always wonder: Would we have found Enceladus’ plumes with Voyager had the Voyager 2 scan platform not gotten stuck?” asks Linda Spilker. Instead, the Cassini spacecraft discovered the geysers more than two decades later, during a 2005 flyby. — L. K.

What a difference a generation of tech- 10 7. Enceladus is one of Saturn’s most nology design makes: Today, scientists have compelling moons. Voyager 2 passed much closer to the moon than Voyager hundreds of occultation observations from 1, and it took this image August 25, the Cassini spacecraft, which studied the 1981, from a distance of 74,000 miles Saturn system from 2004 until September (119,000km). Looking at data from the of this year. Cassini’s 13 years of observa- probes, planetary scientists became intrigued by Enceladus’ widely varying tions provided answers about how the terrain. Swaths of the moon appeared moons and Saturn itself shape the rings. devoid of craters, indicating ages in those areas of less than a few hundred Saturn’s satellites million years. NASA/JPL-CALTECH Before either Voyager arrived at Saturn, 8. Voyager 1 sent back images of Dione most of the planet’s moons were no more on November 12, 1980, from 149,000 than pinpricks of light. Ground-based tele- miles (240,000km), that showed numerous impact craters marred by scopes couldn’t resolve their surfaces, so fractures. These fractures appeared as scientists had little information about the bright, wispy streaks, and range from immense variation these worlds hold. Voyager scientists stationed at JPL, recalls tens to hundreds of thousands of miles First up was Titan, the planet’s largest planetary scientist Carolyn Porco, that liq- long. NASA/JPL moon. Voyager 1 made its closest approach uid nitrogen might exist on the surface of 9. More than 30 years after Voyager’s to the orange sphere in the late hours of Titan. “It turned out the initial analysis was last glimpse of Titan, Cassini finally November 11, 1980, when it flew less than incorrect,” she says. “But I’ll never forget revealed the moon’s surface via infrared imaging November 13, 2015. The surface 310,000 miles (500,000km) from the moon. the indescribable thrill of hearing that is surprisingly Earth-like, with features Scientists hoped to see through the thick rumor. It felt, for a moment, like all of us . . . that include sand dunes, mountain atmosphere to learn about the surface, but were crewmates on the starship Enterprise, ranges, and liquid hydrocarbon seas. Titan’s mysteries weren’t so easy to solve. and we had just come upon the most alien NASA/JPL/UNIVERSITY OF ARIZONA/UNIVERSITY OF IDAHO The visible and infrared cameras could not of worlds yet seen. We were indeed plan- 10. Haze layers sit just above Titan’s penetrate the clouds. Fortunately, research- etary explorers.” Porco later explored the thick atmosphere, at heights of ers could get a radio signal to the surface Saturnian system as a member of the about 300 miles (500km), above the moon’s surface in this Voyager image. and back, and used it to calculate the atmo- Cassini team. NASA/JPL-CALTECH sphere’s density: 1.6 times that of Earth. What Voyager did reveal of Titan, During the analysis of the radio data, a though — knowledge of its atmosphere’s hushed rumor spread among the dozens of density and composition, the possibility of

WWW.ASTRONOMY.COM 31 11 Measuring a day

Jupiter 3.1° Rotation axis Magnetic axis 10° NORTH Saturn 26.7° 47° 28.3° Uranus Neptune

Ecliptic plane –97.8°

60°

The outer planets have no solid surface to track as they rotate in order to measure the length 12 of their day. Instead, astronomers track the rotation of the planets’ magnetic poles, which typically are offset from the axis of rotation. Jupiter’s magnetic pole is offset from its rotational axis by about 10°, while Uranus’ axes are offset by 60° and Neptune’s by 47°. Saturn’s magnetic and rotational axes are almost perfectly aligned, with an offset of less than 1°, making measurements of the length of its day challenging. Both Voyager spacecraft measured the length of Saturn’s day to be about 10.7 hours, but the Cassini spacecraft has measured the length to be slightly shorter (10.6 hours) or longer (10.8 hours), depending on the hemisphere in which measurements are taken. ASTRONOMY: ROEN KELLY

13 moon somehow feeds the ring. Could there be ice volcanoes on Enceladus, providing the sloshy material that would fill in impact craters? If so, some of that gushing material perhaps could escape the surface and orbit Saturn as part of the E ring. Those first detailed observations from Voyager trig- gered an ongoing fascination with this small, reflective moon, adds Ingersoll. Scientists now know from the Cassini mission that an underground water ocean 11. Enhanced color image processing hydrocarbons perhaps in liquid form at the feeds geysers at Enceladus’ south pole. increases the ability to see clouds and surface — made Titan even more intrigu- They’ve also discovered likely storms in Saturn’s atmosphere, such as the convective clouds in this image ing for further study. In fact, says Spilker, hydrothermal activity at the ocean floor. taken November 5, 1980, from a range of “it was really the Voyager flyby of Titan, On Earth, biological ecosystems thrive in 5 million miles (8 million km). NASA/JPL-CALTECH and what we learned and what we didn’t such environments. Could they do the learn, that led to this strong desire to go same on Enceladus? That’s a question a 12. The spacecraft revealed oval-shaped storms in Saturn’s atmosphere, similar back.” And ultimately, it was that flyby that future dedicated mission to the small to those seen on Jupiter. Imaged sparked the Cassini mission. moon might answer. November 7, 1980, from a distance The Titan discoveries were just the of 4.6 million miles (7.5 million km), these brown ovals in Saturn’s northern beginning of revealing the saturnian satel- The weather out there hemisphere span about 6,000 miles lites. From afar, two of the planet’s closer- Scientists didn’t send Voyager to Saturn to (10,000km) in diameter. NASA/JPL in moons, Mimas and Enceladus, had been study only its rings and moons. The planet considered twins due to their similar sizes. and its atmosphere were also a science 13. Winds in Saturn’s upper atmosphere are estimated to reach speeds of But on November 12, 1980, Voyager found focus. Like that of its sister giant planet, 1,118 mph (1,800km/h), faster than the that their surfaces look vastly different. Jupiter, Saturn’s atmosphere hosts incred- winds on Jupiter. This ribbonlike feature While Mimas was pockmarked with the ible storms and enormous jet streams, and is the result of a westward jet stream imaged on November 10, 1980, from a evidence of billions of years of collisions the Voyager twins were the first spacecraft distance of 2.2 million miles (3.5 million from space debris, Enceladus, oddly, was to photograph the details in those cloud km). It was still visible to Cassini in 2012, not. Scientists had expected a heavily cra- tops up close. though its shape had changed in the tered surface, “and yet there were parts of While reanalyzing Voyager images of intervening years. NASA/JPL Enceladus that were smooth,” says Saturn’s poles, astronomer David Godfrey 14. Saturn’s polar hexagon spans a Caltech’s Andrew Ingersoll, who was a discovered a hexagonal shape surrounding diameter of 20,000 miles (32,000km) and member of the Voyager imaging team. The the northern pole. That surprising shape reaches depths of 60 miles (100km) in the world was also very reflective, similar to comes from jet streams moving at differ- planet’s atmosphere. The bizarre feature was first identified in Voyager images and the brightness of freshly fallen snow. ent speeds. “It’s really just a meandering later confirmed by the Cassini spacecraft, Observations from Earth of a ring current flowing east a little faster than the which snapped this image June 14, 2013. around Saturn at the distance of Enceladus’ rest of its neighbors,” explains Ingersoll, NASA/JPL-CALTECH/SPACE SCIENCE INSTITUTE orbit already hinted that perhaps that small who was also the head of the Voyager

32 ASTRONOMY • OCTOBER 2017 14 15

atmosphere working group. This hexagon 16 was still there decades later, when Cassini reached Saturn. And then there are Saturn’s winds them- selves. “I remember being amazed at how fast the winds were blowing,” recalls Ingersoll — although how fast isn’t actually known yet. That’s because scientists don’t have a reference against which to measure the wind speeds, explains Ingersoll, who has studied planetary atmospheres for decades. “On Earth, we measure the wind relative to the continents,” explains Ingersoll, and Saturn, of course, doesn’t have any conti- nents. But if scientists could measure how fast the planet’s solid core rotates, that speed would serve as the reference. To get at that rotation rate for a giant planet, researchers track the planet’s mag- netic field, which is produced in the solid core. On Jupiter, the magnetic field’s axis is 15. False-color images, such as this one tilted in relation to the rotation axis, which questions about the ringed world and its taken August 23, 1981, from 1.7 million means as the core rotates, the magnetic system, as well as introduced new mysteries miles (2.7 million km), highlight the field wobbles. “You see the magnetic field for future spacecraft to resolve. varying chemical composition of the wobble back and forth like a . . . top, and so This world holds beautiful rings that rings, seen as different colors. The C ring appears blue in this composite, while the that tells you how fast the interior of the mimic some characteristics of disks around B ring appears yellow-orange. NASA SPACE planet is rotating,” says Ingersoll. young stars, intense atmospheric storms, SCIENCE DATA COORDINATED ARCHIVE Unfortunately, Saturn’s magnetic field axis and a variety of moons — including one 16. Voyager 1 took this image of Saturn and rotation axis are too similar to produce with an Earth-like weather system and as it departed, looking back from its a measurable wobble on the Voyagers’ another with the ingredients of a habitable vantage point of 3.3 million miles instruments. But in its final mission phase, environment. “The study of Saturn has (5.3 million km) four days after closest skimming just above the cloud tops, provided scientists the means to study pro- approach. Spokes can be seen as bright patches in the rings from this distance. Cassini may finally get close enough to the cesses that are at work all across our solar NASA SPACE SCIENCE DATA COORDINATED ARCHIVE planet to better track the wobble. system and scale-invariant across the cos- On September 15, Cassini will end its mos,” says Porco. “No other planet can study of Saturn, closing the door on up- claim as much.” Liz Kruesi is an Astronomy contributing close observations of the ringed planet, just Because of Voyager 1 and 2, we know editor who writes about distant objects like Voyager’s departure in 1981. Both mis- why the Saturn system continues to tempt from her Earthbound home in Austin, sions have answered long-standing planetary explorers. Texas.

WWW.ASTRONOMY.COM 33 on. by John Wenz Incredibly, a few instruments aboard both spacecraft continue to functi

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ed Above: Voyager 1’s Plasma Wave Science instrument measured the bursts of energy seen in this image, Above: Voyager had a Cosmic Ray Subsystem to monitor this radiation as it passination all but confirming the spacecraft had finally entered through the crafts. An increase showed the escape of both craft from the term ager 1 interstellar space, where the Sun’s magnetic fields end. shock, where solar wind particles travel at speeds less than that of sound. Voys the entire Voyager 1 still has a few thousand years to go before crossed this threshold in 2003. Voyager 2 crossed in 2007. The heliosphere ASTRONOMY: ROEN i KELLY NASA/JPL exiting the solar system entirely. bubble-shaped region where the Sun has its greatest influence. Right: Two engineers install the Golden Record, an encapsulation of humanity as it was in the mid-1970s. Voyager 1’s final image was the famous This LP-sized object represents a message from Earth to “Pale Blue Dot” photo. The craft looked whomever may find the probe. The record’s cover shows back at the solar system, with Earth where the spacecraft came from. The Voyagers will not appearing as a small dot. Carl Sagan encounter other stars for tens of thousands of years. conceived of the image; Candy Hansen This may turn out to be a last message fromNASA humanity and Carolyn Porco took it. NASA/JPL-CALTECH should our civilization come to an end.

KEY FINDING: The PPS instrument 8. KEY FINDING: This instrument gath- discovered a more detailed structure 1. ered the bulk of the data regarding - to Uranus’ and Neptune’s ring systems, SHUTDOWN DATE: Essentially never SHUTDOWN DATE: Still active planetary atmospheres, helping estab including discovering additional rings PURPOSE: Protect the instruments PURPOSE: Communications lish what the gas giants were made of in each. at the rear of the craft from dust and KEY FINDING: This is the craft’s main — and that Uranus and Neptune were other particles that could interfere contact point with Earth. It once sent different from Jupiter and Saturn in with their operations. composition. 6. KEY FINDING: Unknown, but it may back the robust data from the craft; to- - day, it sends out basic information from have spared the craft plenty of head the low-power instruments still online. Still active SHUTDOWN DATE: - aches from tiny particles. 4. PURPOSE: Measure ions and their dis tribution in the outer solar system. 2. KEY FINDING: A violent burst of 9. SHUTDOWN DATE: 1998 solar activity in 1993, coupled with PURPOSE: Monitor temperature and the Low-Energy Charged Particle SHUTDOWN DATE: Still active SHUTDOWN DATE: Still active other information inside the planets. Instrument, helped researchers locate PURPOSE: Help calibrate the craft’s PURPOSE: Measure the magnetic fields KEY FINDING: The IRIS instrument the heliopause. charged particle instrument, infrared discovered a heat source deep within radiometer, and cameras. of the Sun and the outer planets. - Voyager’s instruments KEY FINDING: In 2015, the craft discov Neptune that leaves the planet roughly 7. KEY FINDING: - ered that even past the heliopause, solar the same temperature as Uranus. This SHUTDOWN DATE: Still active pointed to the target plate for calibra winds can redirect the magnetic field of is likely due to slightly higher methane- PURPOSE: Propel the craft and keep tion countless times in its travels. charged particles they encounter. content trapping more heat and mak it warm. ing the planet a few degrees warmer. KEY FINDING: While Voyager 1 was in 10. 3. transit to the outer solar system, the 5. Pioneer 11 craft revealed a substantial atmosphere on Saturn’s moon Titan. SHUTDOWN DATE: 1998 SHUTDOWN DATE: 2008 1991 Rather than steer toward a possible PURPOSE: Monitor the composition of SHUTDOWN DATE: encounter with Pluto, NASA used the PURPOSE: Measure radio emissions PURPOSE: planetary atmospheres in ultraviolet Gather information on craft’s thrusters to send it toward Titan, from the gas giants. wavelengths, which also show solar the gas giants while monitoring their eventually flinging Voyager 1 above interaction. atmospheres. the plane of the solar system.

34 ASTRONOMY • OCTOBER 2017 3

4

5 6

8 7

9

1

KEY FINDING: It measured radio waves from Jupiter and Saturn, resulting in a creepy album called which Voyager: Sound of the Cosmos makes Jupiter sound like even more 10 of a monster. 11.

SHUTDOWN DATE: Still active, but 2 waning. PURPOSE: Power the craft and keep it warm with Plutonium-238. 11 KEY FINDING: It has allowed for the 28-year extended mission of both craft, though the radiation given off may drop too low to continuously power the craft past the early 2020s.

John Wenz is a former associate editor of Astronomy.

AFTER NASA/JPL

ASTRONOMY,

JAY SMITH FOR SKYTHIS MARTIN RATCLIFFE ALISTER LING Visible to the naked eye and describe the Visible with binoculars MONTH solar system’s changing landscape as it appears in Earth’s sky. Visible with a telescope October 2017: Uranus glows brightly

in evening twilight at the end sight — the maximum angle of the month. The planet then possible. The rings haven’t shines at magnitude –0.4 and appeared this open since hangs a couple of degrees high 2003, and they won’t approach in the west-southwest 30 min- this tilt again until 2032. utes after sunset. The steep angle means After straining to see our observers will get superb first two targets, skygazers get views of ring structure. The some welcome relief with the Cassini Division — the dark appearance of Saturn. The gap that separates the outer ringed planet lies 20° above A ring from the brighter the southwestern horizon at B ring — will be obvious the end of twilight in early through any scope. And this October. Although it dips will be a great chance to spot lower as the month progresses, the Encke Division, a narrow, it remains conspicuous. Saturn challenging feature near the shines at magnitude 0.5 A ring’s outer edge. against the backdrop of south- If you can tear yourself ern Ophiuchus. away from the rings, Saturn The planet drifts slowly also boasts a family of bright eastward relative to these moons. The biggest and background stars, beginning brightest, 8th-magnitude October 3° north-northeast Titan, completes a circuit of Earth-based telescopes easily show Uranus’ distinctive blue-green color. θ This 2006 Hubble Space Telescope image also reveals banding in the of 3rd-magnitude Theta ( ) the planet once every 16 days. atmosphere as well as a rare transit by the planet’s moon Ariel (the white Ophiuchi and ending the Look for it due south of dot) and its shadow (to the right). NASA/ESA/L. SROMOVSKY (UNIVERSITY OF WISCONSIN–MADISON) month 4° northeast of this star. Saturn on October 5 and 21 Now swing your telescope and due north on the 13th and he giant worlds of the chance to see Jupiter in the to Saturn. The planet’s disk 29th. It appears 1.2' from the outer solar system take evening sky this year. Those appears 16" across, and the ringed world on those dates. center stage on October with a clear, unobstructed rings span 36" in mid-October. Outermost Iapetus glows evenings. While Jupiter western horizon can find the But more significantly, the ring at 10th magnitude when it hangs low in twilight, giant planet 3° high a half- system tilts 27.0° to our line of reaches greatest western TSaturn stands higher in a hour after sunset. Fortunately, dark sky. The more distant it’s bright — shining at mag- Uranus swims with the Fish world should look spectacular nitude –1.7 — so you can N because its rings open wider track it nearly to the horizon than they’ve been in 14 years. despite the bright twilight. PISCES But the stars of the evening Jupiter dips lower each show lie even farther afield. evening and finally suc- π ARIES Neptune grabs the spotlight cumbs to the Sun’s glare October 5/6 when its largest after October’s first week. It moon, Triton, dramatically passes behind the Sun on the ο Uranus cuts off the light of a distant 26th and will reappear before E sun. And Uranus reaches oppo- dawn in early November. sition and peak visibility this Mercury plies nearly the ζ month, making it a tempting same parcel of real estate as target all night. As this planet Jupiter. The innermost planet sinks low in the west before slides behind the Sun at supe- μ dawn, Venus and Mars make rior conjunction October 8. 1° a striking pair in the east. Viewers in the southern ν Early October provides United States and points Uranus reaches magnitude 5.7 at opposition October 19. It then lies among observers with their final south might glimpse Mercury the background stars of Pisces the Fish. ALL ILLUSTRATIONS: ASTRONOMY: ROEN KELLY

36 ASTRONOMY • OCTOBER 2017 RISINGMOON See the Moon in a new light Aristoteles and Protagoras

Most skywatchers observe between the snaking Serpentine during the evening and shun Ridge and Mare Serenitatis. the morning hours except From there, jump halfway Protagoras for special events. That’s why to the pole and find the large lunar observers tend to focus crater Aristoteles. The low Sun on the Moon’s waxing phases angle transforms its apron of visible after sunset and ignore impact splatter into a finely Mare Frigoris Aristoteles the waning phases best seen textured expanse. Check out before dawn. But this month the shadow cutting across provides a nice opportunity to Aristoteles’ middle and trace the N view the waning gibbous Moon huge “divot” of light back to the before midnight. You might be small crater that breaches the E surprised at what the different western flank and allows sun- lighting reveals. light to reach the floor. Two distinctive craters show up on the frozen lava of Mare Frigoris Target Luna around 11 P.M. Farther to the northwest lies on October 9’s gibbous Moon. CONSOLIDATED LUNAR ATLAS/UA/LPL; INSET: NASA/GSFC/ASU local daylight time October 9. As the crater Protagoras, which you scan along the terminator resembles a golf hole in the flow came right up to the west- suspect this unusual feature was — the dividing line between putting green formed by the ern lip of Protagoras, so there’s hiding in plain sight. day and night — features may solidified lava of Mare Frigoris. no rim to catch the Sun’s rays. The shadows intensify on look odd because you’re view- Normal craters are surrounded Under the more familiar lighting the 10th as the Sun drops lower ing lunar sunset and not sunrise. by a raised rim whose western conditions of a waxing crescent in the lunar sky, though the The reverse lighting makes it flank glows brightly in the set- Moon on the evening of Moon doesn’t climb to a decent easy to see the relationship ting Sun. But an ancient lava October 25, you might never altitude until midnight. elongation October 13. It then METEOR lies 8' from Saturn and shows WATCH up nicely through 4-inch and larger scopes. The same Dark skies for the Orionid meteor shower instrument reveals a trio of Aldebaran similarly bright moons — Hunter’s show TAURUS Tethys, Dione, and Rhea — that circle the planet inside Halley’s Comet last appeared in GEMINI Radiant our sky more than 30 years ago, Titan’s orbit. Castor ORION but it still makes its presence As Saturn prepares to Betelgeuse known. Every October, Earth Pollux set, turn your attention to Rigel Neptune. This distant ice plows into debris left behind by the periodic comet during its trips giant reached opposition LEPUS through the inner solar system. and peak visibility in early As this dusty debris burns up in September, but the view in Procyon our atmosphere, we see flashes 10° October barely suffers in of light known as meteors. The Sirius comparison. It appears in the flashes appear to radiate from the CANIS MAJOR southeast as darkness falls constellation Orion the Hunter, in early October and climbs which lends its name to the October 21, 2 A.M. Looking southeast highest in the south around Orionid meteor shower. Orionid meteors 11 p.m. local daylight time. (It Conditions this year should be With the Moon absent for this reaches the same position two ideal. The shower peaks before month’s premier shower, Active dates: Oct. 2–Nov. 7 hours earlier by month’s end.) dawn October 21, just two days observers can expect to see Peak: October 21 up to 20 meteors per hour. Moon at peak: New Moon Neptune glows at magni- after New Moon. Observers under Maximum rate at peak: tude 7.8, so you’ll need bin- a dark sky can expect to see 20 meteors per hour in the 20 meteors/hour oculars or a telescope to see close to the maximum rate of hours before dawn. it. The planet resides in Aquarius, less than 1° from λ OBSERVING The zodiacal light glows in the predawn sky for Northern Hemisphere magnitude 3.8 Lambda ( ) HIGHLIGHT — Continued on page 42 observers at dark sites from October 17 to November 2.

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ε α i χ

p η α

t β ic η ) P E G A S U S β

S E C S I P ζ

κ γ α α μ M27

Uranus

γ α

ζ α SAGI Mira ο β β Enif M15 γ ζ Altair α C E T U S ε β STAR DELPHINUS δ θ α η ζ MAGNITUDES γ E Q U U L E U S α η AQUARIUS θ Sirius λ α η β ν 0.0 3.0 1.0 τ η 4.0 α 2.0 5.0 δ γ β δ β π ξ μ STAR COLORS NGC 253 SGP λ A star’s color depends σ on its surface temperature. τ ϕ α α The hottest stars shine blue SE S C U L P TO R δ • Fomalhaut CAPRICORNUS ζ γ • Slightly cooler stars appear white Intermediate stars (like the Sun) glow yellow P I S C I S • ε AU S T R I N U S γ η • Lower-temperature stars appear orange α The coolest stars glow red • MICROSCOPIUM α β • Fainter stars can’t excite our eyes’ color receptors, so they appear white unless you use optical aid to gather more light β G RU S α

38 ASTRONOMY • OCTOBER 2017 S

Note: Moon phases in the calendar vary ο in size due to the distance from Earth

OCTOBER 2017 and are shown at 0h Universal Time. α

SUN. MON. TUES. WED. THURS. FRI. SAT.

δ MAP SYMBOLS

ε M A S R U Open cluster

1 2 3 4 5 6 7

Mizar Globular cluster

M51

ζ

η Diffuse nebula

α

η

β 8 9 10 11 12 13 14 α

ε Planetary nebula

α

ζ NW

β Galaxy

γ 15 16 17 18 19 20 21 γ

: ROEN KELLY ROEN :

α ι

δ DRACO

β

ε 22 23 24 25 26 27 28 ASTRONOMY

α

η

BOÖTES

γ

γ

ζ

ε δ ζ

29 30 31 β

δ BY ILLUSTRATIONS δ

ρ

β

γ η ν

β

ι α

β

BOREALIS

CORONA

α

α

γ Calendar of events

η

γ δ

η M13

μ

β

ζ 3 ζ The Moon passes 0.7° south SPECIAL OBSERVING DATE

β

HERCULES A.M.

β of Neptune, 8 EDT 21 The annual Orionid π

α meteor shower peaks

β Vega 5

δ Venus passes 0.2° north before dawn under a ε α

β of Mars, 9 A.M. EDT

α Moon-free sky. α

α δ

ο

μ

γ

δ Full Moon occurs at CAPUT

SERPENS β

LYRA 24

γ P.M.

γ The Moon passes 3° north

α η 2:40 EDT A.M.

M57 W of Saturn, 8 EDT ε α η χ α 6 The Moon passes 4° south η β β of Uranus, noon EDT The Moon is at apogee ζ κ γ (251,751 miles from Earth), α α μ 7 Mars is at aphelion 10:26 P.M. EDT (154.9 million miles from γ α VULPECULA the Sun), 6 P.M. EDT 26 Jupiter is in conjunction with ζ α the Sun, 2 P.M. EDT ο SAGITTA β β 8 Mercury is in superior γ P.M. 27 ζ OPHIUCHUS conjunction, 5 EDT First Quarter Moon α SERPENS AQUILA P.M. ε CAUDA occurs at 6:22 EDT β 9 θ α δ η ζ The Moon is at perigee (227,953 γ miles from Earth), 1:55 A.M. EDT 28 Asteroid Pallas is at opposition, α η 8 P.M. EDT θ λ α The Moon passes 0.6° north η β ν of Aldebaran, 3 P.M. EDT 29 Asteroid Iris is at opposition, M11 8 P.M. EDT M16 η τ 12 Last Quarter Moon α SCUTUM occurs at 8:25 A.M. EDT 30 The Moon passes 0.9° south P.M. δ γ M17 of Neptune, 5 EDT β δ β 15 Saturn The Moon passes 0.2° north π ξ μ M20 A.M. M22 of Regulus, 7 EDT λ 17 The Moon passes 1.8° north σ M8 τ of Mars, 6 A.M. EDT ϕ α α δ SW The Moon passes 2° north ζ γ of Venus, 8 P.M. EDT

ε 19 γ η Uranus is at opposition, SAGITTARIUSα 2 P.M. EDT α β CORONA New Moon occurs AUSTRALIS at 3:12 P.M. EDT

β α BEGINNERS: WATCH A VIDEO ABOUT HOW TO READ A STAR CHART AT www.Astronomy.com/starchart.

WWW.ASTRONOMY.COM 39 PATHOF THE PLANETS The planets in October 2017

LYN PER AND CV Objects visible before dawn LM AUR GEM AsteroidTRI Iris reaches COM CNC opposition October 29 Ceres Massalia ARI PEG P Venus passes 0.2° north ath o LEO Path o f the BOÖ of Mars on October 5 f the Sun Flora Moo (ec ORI TAU n lipt ic) PSC CM Nysa Mars

VIR Venus SEX MON Uranus reaches its peak in mid-October Sun HYA CET CRT ERI CRV CM LEP SCL PYX ANT Asteroid Pallas reaches opposition October 28 FOR COL PUP CAE VEL PHE CEN

Moon phases Dawn Midnight

20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 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. 31

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 Jupiter Neptune 10"

Planets MERCURY VENUS MARS CERES JUPITER SATURN URANUS NEPTUNE PLUTO Date Oct. 31 Oct. 15 Oct. 15 Oct. 15 Oct. 1 Oct. 15 Oct. 15 Oct. 15 Oct. 15 Magnitude –0.4 –3.9 1.8 8.7 –1.7 0.5 5.7 7.8 14.2 Angular size 4.9" 10.8" 3.8" 0.5" 30.9" 15.8" 3.7" 2.3" 0.1" Illumination 93% 93% 98% 96% 100% 100% 100% 100% 100% Distance (AU) from Earth 1.367 1.552 2.495 2.694 6.374 10.503 18.917 29.175 33.496 Distance (AU) from Sun 0.466 0.719 1.666 2.619 5.444 10.063 19.911 29.946 33.426 Right ascension (2000.0) 15h13.5m 12h04.1m 11h42.9m 8h41.0m 13h44.5m 17h29.4m 1h39.1m 22h53.6m 19h12.0m Declination (2000.0) –19°19' 1°12' 3°07' 22°49' –9°42' –22°13' 9°39' –8°04' –21°49'

40 ASTRONOMY • OCTOBER 2017 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.

LAC CV Jupiter’s moons CYG Objects visible in the evening Dots display positions LYR HER BOÖ Io CB of Galilean satellites at COM 10 P.M. EDT on the date Europa VUL shown. South is at the DEL top to match S SGE Julia the view Ganymede through a WE EQU SER N AQL VIR telescope. Callisto Celestial equator OPH AQR 1 Neptune LIB Jupiter 2 SCT SER Mercury Sun 3 CRV CAP Pluto Saturn 4 Callisto

PA SGR 5 MIC 6 CA SCO TEL 7 LUP CEN GRU 8

Early evening 9

10 3 2 1 11 Jupiter

12 30 29 28 27 26 25 24 23 22 21 20 19 18 Io

13

14

15

The planets 16 Mars Ceres Aphelion is in their orbits 17 October 7 Arrows show the inner planets’ 18 Ganymede monthly motions and dots Venus Earth depict the outer planets’ 19 positions at midmonth from high above their orbits. 20 21 Mercury 22 Europa Jupiter Superior conjunction is October 8 23

Uranus 24 Opposition is 25 October 19 26

27 The planets Jupiter Solar conjunction in their orbits 28 is October 26 KELLY ROEN : Arrows show the inner planets’ Neptune 29 monthly motions and dots depict Saturn the outer planets’ positions at mid- 30 ASTRONOMY month from high above their orbits. 31 Pluto ILLUSTRATIONS BY BY ILLUSTRATIONS

WWW.ASTRONOMY.COM 41 — Continued from page 37 Watch a star disappear WHEN TO VIEW THE PLANETS N 78 EVENING SKY MIDNIGHT MORNING SKY Mercury (southwest) Uranus (southeast) Venus (east) Jupiter (west) Neptune (southwest) Mars (east) Saturn (southwest) Uranus (west) Uranus (east) λ Neptune (southeast) E 4U 410-143659 Aquarii. Neptune appears 0.6° passes directly in front of a east-southeast of Lambda on star. Viewers in the northeast- Neptune October 1 and the same dis- ern United States and eastern tance due south of the star Canada can view this rare on the 31st. occultation on the evening of AQUARIUS You should be able to iden- the 5th; those in the United tify Neptune from its position Kingdom, western Europe, 0.2° because no star in the immedi- and northwestern Africa can Neptune’s moon Triton slides in front of 12th-magnitude 4U 410-143659 ate area shines as brightly. But witness it on the morning of the night of October 5/6. The star will dim visibly for up to 161 seconds. you can remove any doubt by the 6th. A 6-inch scope will pointing a telescope in its show the event. and 24h00m UT), depending bright as possible. But the direction. Only the planet The magnitude 12.4 star, on their location. The finder appearance of an outer planet shows a discernible disk, which cataloged as 4U 410-143659, chart above pinpoints the star. changes slowly, and Uranus has a diameter of 2.3" and a dims by 1.4 magnitudes as the Uranus reaches opposition maintains its magnitude 5.7 distinctive blue-gray color. magnitude 13.5 moon occults October 19. It then lies directly peak throughout October. Keep your scope handy for it. Viewers will see the star opposite the Sun in our sky, so The ice giant lies among Neptune’s event of the month: disappear for up to 161 sec- it remains visible all night. It the background stars of On the night of October 5/6, onds sometime between 7:44 also lies closest to Earth at Pisces. It starts the month 1.3° the planet’s big moon, Triton, and 8:00 p.m. EDT (23h44m opposition, rendering it as northwest of magnitude 4.3 COMETSEARCH

The Pleiades adds a sister Comet PANSTARRS (C/2015 ER61) N For the past two months, Comet should be ideal. PANSTARRS PANSTARRS (C/2015 ER61) has then appears some 30° above Pleiades been floating near the Pleiades the eastern horizon at 11 P.M. star cluster (M45), or Seven local daylight time. Sisters, in northwestern Taurus. Apart from sharing the same They remain neighbors during area of sky, the comet and E Path of October, though PANSTARRS’ Pleiades showcase a common Oct 1 Comet PANSTARRS ARIES slow westward motion carries it phenomenon of physics: light 6 into eastern Aries by the month’s scattering (or reflecting) off 11 final week. The solar system dust. Comet dust arises when 16 τ object should glow around 11th sunlight turns the ices on the 21 TAURUS magnitude as it heads back body’s nucleus directly into a 26 toward the distant Oort Cloud gas, and carries dust away in the 31 from which it came. That bright- process. These particles form a 1° ness pushes the limits of a shroud around the nucleus that 4-inch telescope under a dark appears yellow because it This visitor from the distant Oort Cloud should glow at 11th magnitude sky, but should pose little prob- reflects sunlight. Far in the back- in October as it gradually moves away from the Pleiades star cluster. lem through an 8-inch or larger ground, the Pleiades’ stars are instrument. plowing through a cloud of cos- dust production is dropping the center. The comet’s small You’ll want to avoid viewing mic dust. These particles scatter with each passing night. size and low surface brightness during October’s first 10 days, the blue light radiated by the Through most telescopes, it mean you might not see this when a bright Moon shares the cluster’s many hot, young stars. likely will look like an unimpres- structure at first under low evening sky. The two-week win- As Comet PANSTARRS moves sive elliptical galaxy — mostly power, but at 120x or more, it dow of dark skies that follows steadily away from the Sun, its round and a bit brighter toward should show quite nicely.

42 ASTRONOMY • OCTOBER 2017 Morning planets greet the Moon LOCATINGASTEROIDS Keep an eye on Iris

The space between the orbits Iris lies within one binocular of Mars and Jupiter is packed field of Aries the Ram’s bright- with hundreds of thousands of est star — magnitude 2.0 small bodies known as aster- Hamal (Alpha [α] Arietis) — all oids. Only a handful of these month. This area stands high in objects are big enough and the east by midevening. Simply come close enough to Earth to walk outside, dark adapt for show up easily through binocu- five minutes, and then focus in lars. Fortunately, October’s sky on Hamal. features a fine specimen of this The easiest time to find Iris rare breed. comes during October’s final Asteroid 7 Iris ranks second week, when it shines brightest only to 4 Vesta as the brightest and slides 1.7° due south of asteroid of 2017. Iris glows at Hamal and 1.0° due south of Venus appears 0.2° from Mars on October 5. On the 17th, a crescent Moon magnitude 6.9 when it reaches magnitude 5.0 Kappa (κ) Ari. To joins the two, mirroring this scene from January 31, 2017. MATTHEW DIETERICH opposition and peak visibility confirm a sighting, sketch the late this month. Eagle-eyed field with Alpha, Kappa, and Omicron (ο) Piscium and ends 4th-magnitude star Sigma (σ) observers under pitch-black several of the stars near the the month 2.2° due west of this Leonis 0.3° north of Venus. skies might be able to glimpse asteroid’s position plotted star. Once you locate Uranus Unfortunately, a telescope it with their naked eye, but below. Return a night or two through binoculars, set them doesn’t add much. Venus even newcomers should have later and identify the point of aside and try to spot the planet spans 11" while Mars mea- little trouble finding it through light that changed position. with your naked eye. You sures 4" across, and both binoculars from the suburbs. That “star” is Iris. should be able to from under appear nearly full. a dark sky. A telescope reveals The two planets move Iris butts the Ram’s head Uranus’ 3.7"-diameter disk and eastward relative to the back- impressive blue-green color. ground stars during October, N The telescopic view should with Venus in its inner orbit be especially good this month moving faster. It crosses the because Uranus appears so border into Virgo on the 9th, Oct 1 α high. From mid-northern lati- setting up a string of close stel- 6 11 tudes on the night of opposi- lar conjunctions. The bright Path of Iris 16 κ tion, the planet lies 60° above planet passes 0.8° northeast E the southern horizon at its of 4th-magnitude Beta (β) 21 peak around 1 a.m. local day- Virginis on October 12, light time. This is the highest 0.2° north of 4th-magnitude 26 it has appeared at opposition Eta (η) Vir on the 18th, 1.3° since February 1963. southwest of 3rd-magnitude ARIES η 31 By the time twilight starts Gamma (γ) Vir on the 0.5° to paint the morning sky, 22nd, and 0.3° south of 4th- Uranus dips low in the west. magnitude Theta Vir on the The action then shifts to the 29th. By the 31st, Venus stands This 7th-magnitude asteroid should be easy to find as it slides south of eastern sky, where Venus and 16° east of Mars. 2nd-magnitude Alpha (α) Arietis, the brightest star in Aries the Ram. Mars put on a marvelous show. The Red Planet runs a sim- On October 1, they appear 2.5° ilar gauntlet, but at a slower apart against the background pace. It crosses into Virgo on But the most spectacular appears only 2 percent illu- stars of Leo. Magnitude –3.9 October 12 before sliding 0.5° conjunction takes place when minated and 6° to Venus’ Venus rises 13 minutes before north of Beta Vir on the 18th. the waning crescent Moon lower left. magnitude 1.8 Mars. On the 30th, it lies 0.3° south- passes the two planets just The gap between the two west of Eta Vir and nearly on after midmonth. On October Martin Ratcliffe provides plane- closes until October 5, when top of 6th-magnitude 13 Vir. 17, the 5-percent-lit crescent tarium development for Sky-Skan, just 0.2° — about half the Full As twilight begins on the East Moon stands 2° to Mars’ left Inc., from his home in Wichita, Moon’s diameter — separates Coast, they lie just 44" apart; and 6° above Venus. Our satel- Kansas. Meteorologist Alister them. The two haven’t been this from the West Coast, 4.2' sep- lite is much harder to see the Ling works for Environment close since November 1995. If arate the two. following morning, when it Canada in Edmonton, Alberta. you view the pair through bin- oculars, you’ll also notice the 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. THE HIDDEN EARTH Q: IF ANOTHER PLANET EXISTED THAT WAS PRECISELY OUR SIZE AND IN OUR EXACT ORBIT, BUT PLACED BEHIND THE SUN, WOULD WE HAVE THE MEANS TO DETECT

ITS EXISTENCE? Gary Pyke, McEwen, Tennessee

A: The first person we know to These disparities would have proposed such a “counter show themselves in the Earth” was Philolaus, a Greek motions of comets, and in philosopher who lived in the spectacular ways. Comets fifth century b.c. In the days appear large, but the actual before English mathematician bodies are no more than a few Isaac Newton introduced his miles in diameter. The gravita- theory of gravitation, a few tional influence of another other notable philosophers body as massive as Earth Earth is the only planet in its orbit. There can be no other body hidden brought forth similar ideas. would throw their orbits — from our view on the opposite side of the Sun — the law of gravitation Today, however, we know that that astronomers calculate with would quickly make such an object known to us. NASA/GSFC/NOAA/USGS such a world could not remain such precision — into chaos. undetected for many reasons. So, a counter-Earth would where we could view it directly. with light pillars, they also can Let’s examine just two of them. affect the motions of all nearby And all the while Mercury, originate with the Moon or The gravitational influence objects, but the reverse also is Mars, and even Jupiter would even bright terrestrial lights. of another Earth in that part of true. Let’s take Venus as an be adding their not insignifi- Tiny six-sided ice crystals the solar system would have a example. It approaches Earth cant influences. slowly falling through the wide-reaching effect on all as close as 25 million miles Michael E. Bakich atmosphere create the effect. nearby objects: Mercury, every 584 days. Logically, Senior Editor As they fall, the crystals align Venus, us, the Moon, Mars, Venus would also approach the themselves parallel to the hori- and beyond. Over time, astron- counter-Earth that closely. zon, in effect simulating a omers would be able to detect Venus has about 80 percent of Q: ONE WINTER MORNING colossal mirror made of bil- the tiny variations in position Earth’s mass, so after even a AS I WAS DRIVING TO lions of individual pieces. and, using the law of gravita- few dozen passes, Venus’ gravi- WORK, I LOOKED TO THE Wind moving the crystals tion, quickly calculate the tational influence would move EAST, AND THE SUN WAS causes the reflected light to source of the problem. the hidden world to a position ABOUT 20° ABOVE THE stretch into a column. HORIZON. A SOLID BRIGHT You saw the pillar below the COLUMN OF LIGHT CAME Sun. But pillars also occur that STRAIGHT DOWN FROM IT are above it or the Moon. TO THE HORIZON. WHAT Indeed, either of these celestial CAUSES THIS, WHAT IS IT objects can be below the hori- CALLED, AND IS THIS zon while still creating pillars. RELATED TO ANY ARCHAEO- And pillars formed by earth- LOGICAL FINDS SUCH AS bound lights (which sit on the THE OBELISKS OF EGYPT? horizon) are always seen above John Dally the lights. Colorado Springs, Colorado Indeed, some Egyptologists believe that Sun pillars were A: What you saw was an opti- the inspiration for the great cal effect called a Sun pillar, obelisks created by that culture also sometimes called a light in the same way that crepuscu- This Sun pillar was captured about 20 miles (35 kilometers) from Lisbon, Portugal, on April 26, 2010. The photographer was struck by the pillar’s pillar. And although the Sun is lar rays inspired them to con- height and its white color. MIGUEL CLARO usually the object associated struct the pyramids. For more

44 ASTRONOMY • OCTOBER 2017 about this, see “Stargazing in ancient Egypt,” by Patricia Blackwell Gary and Richard Talcott, in the June 2006 issue of Astronomy. Michael E. Bakich Senior Editor

Q: I OFTEN READ ABOUT A SUNSPOT ON THE WESTERN LIMB OF THE SUN, OR A CRATER ON THE EASTERN LIMB OF THE MOON, ETC. AS THERE IS NO “UP” IN SPACE, HOW ARE NORTH, SOUTH, EAST, AND WEST DETERMINED? Tom Bennett Bagley, Wisconsin

A: For more than 350 years, Vast clouds called dark nebulae, such as SAC SL 17 in the constellation Scorpius, are known to contain high concen- cartographers drew lunar maps trations of dust that block out background starlight. You can find dust in every corner of the universe. FRED HERRMANN showing the edge of the Moon that was nearest the eastern of labeling directions on the among the stars of individual from such encounters. horizon as the eastern limb. face of our daytime star. This galaxies (interstellar dust), and Another contribution, albeit For north and south, they means that, for maps dated within our and other solar sys- minor, comes from comets. adopted the traditional naked- post-1961, the Sun is the only tems (interplanetary dust). Two Cometary dust is probably the eye orientation, with north up. solar system body on which a elements that arise from differ- most famous type in the inter- In 1961, Commission 16 of “north up” map will have east ent types of stars form interga- planetary dust cloud. Most of it the International Astronomical to the left. lactic and interstellar dust. lies dormant, frozen within the Union determined that there Michael E. Bakich Dust made primarily of sil- icy nuclei of comets. When one should be two types of lunar Senior Editor ica begins its life as particles of those objects nears the Sun, maps: those intended for explo- condensing out of the atmo- however, the ice in the comet ration (think astronauts) and spheres of cool red giant stars vaporizes and releases the dust those intended for use at the Q: ASTRONOMY MAGAZINE that contain lots of oxygen. all along the comet’s orbit. telescope. The first type labels HAS LOTS OF REFERENCES Carbon dust (which also Earth, in its yearly trip about east in the same direction as TO DUST CLOUDS. WHAT IS includes carbon compounds the Sun, passes through dozens Mare Crisium (Sea of Crises). “DUST” COMPOSED OF? such as silicon carbide) forms of these dust trails and the As we gaze at the Moon when Dan Baum in similar red giant stars that result in each case is a meteor it lies highest in the south, that Pendleton, Oregon are different in one way: Their shower. feature lies near the right edge. atmospheres are rich in the Michael E. Bakich So, for the past 57 years and on A: Three types of dust exist in element carbon. Another type Senior Editor into the future, east is to the space, and none of them of intergalactic/interstellar dust right, just as all terrestrial resembles what we call “dust” made of graphite comes almost maps label it. on Earth. Here, outdoor dust exclusively from supernovae. Send us your The second type, telescopic consists mainly of small par- Interplanetary dust contains questions maps, show south up, as it ticles of soil and rock carried some of the dust that floats Send your astronomy would be in a telescope that into the atmosphere by wind. between galaxies and stars, but questions via email to inverts the image. This type of Indoor dust is a combination of it has some local components [email protected], map doesn’t carry any “east” or soil attached to our shoes that as well. Asteroid and even or write to Ask Astro, “west” directions. we carry in, human and animal planetary collisions early in the P. O. Box 1612, Waukesha, The direction east is to the hair, tiny bits of paper and solar system’s history contrib- WI 53187. Be sure to tell us right on maps of all other other fibers, pet dander, and a uted to what astronomers call your full name and where objects in the solar system tiny percentage of human skin. the interplanetary dust cloud. you live. Unfortunately, we except one: the Sun. For what- Astronomers find cosmic Researchers are currently try- cannot answer all questions ever reason, astronomers have dust in the areas between gal- ing to figure out what percent- submitted. never changed the convention axies (intergalactic dust), age of the local dust results

WWW.ASTRONOMY.COM 45 The unsolved ICEmysteries GIANTS of the Geysers on Triton, furious storms on Neptune, and a seemingly serene Uranus that turned out to be a cosmic oddity. by Korey Haynes

When Voyager 2 explored Uranus in 1986, it found a bland, bluish world with little contrast. But the flyby occurred in the dead of summer when direct sunlight suppresses cloud formation. A decade later, Hubble began to reveal bright clouds. NASA/JPL-CALTECH

fter visiting Saturn, the twin planet. The secrets that Voyager teased out the sequencing teams for Uranus and Voyagers parted ways substantially remain the most up-close views that scien- Neptune. The group determined exactly for the first time. As Voyager 1 tists have to work with. when Voyager’s instruments should take Araced out of the solar system, data in order to return the information the Voyager 2 struck out on its Big blue nothing science team wanted. This meant under- own toward the last two unvisited giant Voyager 2 flew past Uranus on January 24, standing in minute detail how the planets planets: Uranus and Neptune. Smaller 1986, more than four years after the probe and their moons moved. The sequencing and more distant than Jupiter and Saturn, visited Saturn. Following the excitement team orchestrated the various instruments these ice giant worlds were better hidden at that ringed world (and Jupiter before it), to use every second of the precious flyby from Earth’s prying telescopic eyes, and scientists were eager to see what Voyager windows to image the most valuable tar- therefore more mysterious. And, to some would reveal at the more distant and enig- gets: the limb or edge of the planets, the extent, they remain so. No spacecraft since matic uranian system. terminators where day and night meet, the Voyager has visited them, and there are Suzy Dodd, the project manager for moons in their orbits, and the planets’ own no missions currently scheduled to either Voyager’s interstellar mission, worked on broad faces.

46 ASTRONOMY • OCTOBER 2017 Uranus’ menagerie of major moons Miranda

Voyager 2 captured the first good photos of Uranus’ ring system. These dark, faint rings glow in false color in this computer- enhanced image, which reveals the nine rings known before the spacecraft’s arrival. (The fainter pastel lines are processing artifacts.) Scientists have since discovered four more rings. NASA/JPL

After the rich and complex atmospheres of Jupiter and Saturn, Uranus seemed pretty bland, Dodd recalls. “You didn’t get all the great storms you got at the other planets,” she says. Instead, Uranus “looked like a fuzzy, blue tennis ball.” Voyager did reveal a previously unde- tected magnetic field around Uranus, com- parable in strength to Earth’s. Due to its nearly 90° axial tilt, Uranus rolls around its orbit like a ball. And while Earth’s orbital and magnetic fields are offset by roughly Ariel Umbriel Oberon Titania 12°, Uranus’ are 60° apart. This results in a The planet’s Frankenstein moon, Miranda, looks like a mad scientist stitched it together from corkscrewing magnetic field trailing mil- unrelated parts. Some regions feature heavily cratered ancient terrains while others are younger lions of miles behind the planet. and display canyons, cliffs, and steep ridges. Craters dominate Ariel’s surface, though this moon What’s more, scientists still aren’t sure also shows some major canyons and ridges. Heavily cratered Umbriel is the planet’s darkest large moon; the white ring at top resides on a crater’s floor. Bright rays surround several of the big why the magnetic field exists at all, since craters that dot the surface of Oberon. Abundant craters also cover Uranus’ largest moon, Titania, Uranus lacks the standard liquid metallic though several canyons and steep cliffs break up the monotony. MIRANDA: NASA/JPL/USGS; ALL OTHERS: NASA/JPL inner layer that powers such fields on other planets. Voyager also revealed intense radi- ation belts around the planet, similar to those seen at Saturn. but the star’s repeated appearance and dis- the rings are probably made mostly of ice, But the highlights, Dodd says, were appearance before it slid out of view behind like Saturn’s, but covered in organic mate- centered on Uranus’ rings and moons. the planet made astronomers realize that a rial such as methane, and then baked dark Compared with Saturn’s rings, visible since series of rings surrounded our far-off by the planet’s radiation belts. the earliest days of the telescope, Uranus’ neighbor. The flyby was a chance to inves- Uranus’ moons, too, camouflage were still recent discoveries, and scientists tigate them up close. well against the dark of space. When were eager to learn more. Voyager imaged the ring system for the Voyager left Earth, astronomers knew of Astronomers at Cornell University dis- first time, informing astronomers of its only five satellites around the planet. From covered Uranus’ ring system in early 1977, detailed structure. The spacecraft also dis- observations during its brief visit, the just before Voyager’s launch. The sighting covered two entirely new rings. The close- spacecraft tripled that number, yielding was a happy accident, when a chance align- up views confirmed that Uranus’ subtle 10 new moons. ment carried Uranus in front of a distant bands are not like Saturn’s bright icy rings; “I really think the satellites were the star. Scientists had planned to use the they are dark and reflect little light, mak- highlight of Uranus,” says Dodd. Voyager occultation to study Uranus’ atmosphere, ing them difficult to see. Scientists think images lent the five larger known moons

WWW.ASTRONOMY.COM 47 Above: Voyager captured Neptune’s three brightest rings after it flew past the planet. (The dark bar blocks the ice giant’s crescent.) The outer ring’s clumpy nature doesn’t show up well from this angle. NASA/JPL

Left: Neptune’s dynamic atmosphere surprised scientists, who expected the planet to resemble its dull cousin, Uranus. The Earth-sized Great Dark Spot lies at center; several smaller storms also appear. NASA/JPL detail and character, telling varied stories reasonable proxies for Uranus’ system, More surprises waited on Triton, of violent pasts. Chancia explains. So he and Hedman Neptune’s biggest moon. Triton was Two of the new moons, Cordelia and adopted techniques that planetary scientist already a hotbed of intrigue; it’s by far Ophelia, were identified as shepherd Mark Showalter used to find the moonlet the solar system’s largest retrograde satel- moons. They orbit on either side of Uranus’ Pan in Voyager 2’s observations of Saturn’s lite, meaning it orbits in the direction outer Epsilon ring, and their gravitational ring system. opposite to its planet’s rotation. This is pull herds the small particles in that ring They found distinct patterns in Uranus’ usually a sign of a captured object, but along their orbital path and keeps them rings consistent with “wakes” carved by most other retrograde moons are small, from dissipating into space. Uranus’ rings moonlets circling a planet within a ring misshapen asteroids. Triton is three- are uncommonly narrow; without shep- system. The predicted moonlets are tiny, quarters the size of our Moon, and sur- herd moons, the small particles would dis- only 2 to 9 miles (4 to 14 kilometers) vived its capture intact. Scientists wanted perse over long timescales. across. And they are likely dark, like close-up views of the satellite, and since it Over the years, astronomers have used the rest of the moons and ring system. was the last target, they were free to adjust imagery, mostly from Hubble, to add more Confirming the moonlets will be a chal- Voyager’s trajectory as needed. So the moons to Uranus’ count, which currently lenge. But even 30 years later, Voyager is spacecraft swooped only 3,075 miles stands at 27. But Cordelia and Ophelia still helping to crack Uranus’ secrets. (4,950km) above Neptune’s north pole — remain the only observed shepherds of the its closest approach to any object during ring system. Astronomers have long won- Final surprises the mission — and flew toward its encoun- dered whether more moons are hiding from Voyager’s last planetary encounter came ter with Triton. view, or whether other forces are at work. August 24, 1989. Far from Uranus’ “fuzzy The last world Voyager 2 visited Last year, astronomers from the tennis ball,” Neptune was alive with storms stunned scientists. The moon boasted a University of Idaho revisited the Voyager and bright, quick-moving clouds, delight- thin atmosphere, polar caps, and active data. Thirty years after the flyby, they ing unsuspecting astronomers. Clouds not geysers that spewed icy material miles found evidence for two more tiny moonlets only appeared clearly in Voyager images, high. The active cryovolcanism puts Triton shaping Uranus’ rings. “Nobody — or not but they also cast shadows on deeper cloud in a select group of satellites, in the com- many people — had looked at this in a very layers, allowing scientists to measure the pany of other dynamic moons such as long time,” says Robert Chancia, who led planet’s atmosphere in great detail. The Europa and Enceladus. the investigation. In fact, the Voyager data “Great Dark Spot,” as astronomers termed Voyager also discovered six new moons were taken before he was born. Chancia the largest tempest, was as big as Earth, orbiting Neptune and delivered clear pic- and his adviser, Matthew Hedman, usually swirling in Neptune’s southern hemi- tures of its ring system for the first time, study Saturn’s rings. But recent discoveries sphere and boasting wind speeds as high revealing the rings to be clumpy but com- by the Cassini spacecraft have added as 750 mph (1,200 km/h). In the decades plete, unlike those at Uranus. greatly to astronomers’ understanding of since, that storm has died, while new And as it did at Uranus, Voyager dis- planetary rings. So Chancia and Hedman storms have risen in its place. covered that Neptune’s magnetic pole is decided to take another look at the Voyager “That was a bit of a surprise to me misaligned from its rotational pole, causing findings, applying new theories to old data. when you consider the Great Red Spot on extreme variations in its magnetic field as “There are several narrow ringlets Jupiter has been going on for 400 years,” the planet rotates. Furthermore, both plan- within the rings of Saturn” that provide Dodd says. ets’ magnetospheres are offset from center

48 ASTRONOMY • OCTOBER 2017 by a large fraction: about one-third the To mark the final flyby, NASA’s Jet planet’s radius for Uranus, and nearly half Propulsion Laboratory hosted a special a radius for Neptune. Both planets could event celebrating Voyager’s journey and have oceans of conductive icy slush that accomplishments. Scientists shared images perform the work of the liquid metallic with the public, and rock-’n’-roll legend cores at Earth and Jupiter, but inconclusive Chuck Berry, whose music lives on as part models and observations have left scientists of Voyager’s Golden Record, played in a with little more than guesswork as to what special concert. exactly drives the magnetic fields that At the edge of our planetary system, Voyager observed. 2.75 billion miles (4.43 billion km) from Voyager also detected aurorae on Earth, Voyager turned its cameras back for Neptune. Due to the strange and complex a last look, imaging farewell shots of a cres- nature of the planet’s magnetic field, these cent Neptune. Dodd recalls her reaction to aurorae don’t occur only at the poles; the images: “Wow. The planetary mission instead they are scattered across Neptune’s is done. We’re going off into the deep dark upper atmosphere. and cold realms of space. Who knows how long the mission will last?” End of an era Voyager also closed a contentious chapter Epilogue Voyager 2 captured a crescent Neptune in astronomy history by revising Neptune’s When she left her position with the as it sped away from its final planetary encounter. Now, 28 years later, the mass downward by around half a percent Neptune team, Dodd says, no one then spacecraft continues to explore the — or roughly the mass of Mars. This mis- imagined Voyager would continue as long outer realm of the solar system. NASA/JPL calculation had sent astronomers on a wild as it has. She returned to Voyager’s inter- goose chase through the years as they tried stellar mission in 2010, 21 years after she to make sense of Uranus’ and Neptune’s left the project. In many ways, she admits Voyager continues to measure magnetic orbits, usually by invoking the existence of that the spacecraft is an artifact — memory fields, charged particles, plasma density, a mysterious Planet X tugging on both of and power limited, with many of its spe- and more as it cruises the solar system’s them. (Pluto was found as a direct result cialists long since retired or passed on. hinterlands, teaching scientists about the of this hunt, but its small size was never Since Voyager’s departure from Neptune, subtle edges of the solar system’s boundar- enough to resolve the initial problem.) many of its instruments have gone quiet. ies. Voyager 1 has passed beyond the reach Voyager settled the issue, as Neptune’s There is no need for imaging cameras in of the solar wind, and thus is sampling smaller mass means it and Uranus orbit the dark void of space. But that does not aspects of interstellar space, though it still just as they should. mean the project is defunct. lies well within the Sun’s gravitational influence. Voyager 2, following a slower trajectory from its two-planet detour, tags behind, still sampling the solar wind. From their distance, it takes more than 15 hours for their signals to reach Earth. Sometime in the next decade, the space- craft will lose power and begin to shut down. Dodd’s team will turn the Voyagers’ heaters off first, and one by one, the science instruments will succumb to the cold of space. But the spacecraft themselves and their Golden Records will journey on, car- rying humanity’s imprint into the cosmos. It will be years before any spacecraft retreads Voyager’s path to Uranus or Neptune. With at least half a century of technological advances behind it, any future craft will undoubtedly revolutionize our understanding of the ice giants all over again. But it’s safe to say that nothing will match Voyager for sheer adventure and scope. Decades after its primary mission, Voyager continues to teach, to inspire, and to explore. Neptune’s largest moon, Triton, boasts some of the solar system’s most unusual landscapes. The unique “cantaloupe terrain” in the top half of this image is riddled with crevices and depressions but few impact craters. The south polar region at bottom shows dark streaks Korey Haynes is a contributing editor to deposited by huge geysers that were active during the Voyager 2 flyby. NASA/JPL/USGS Astronomy. You can find her on Twitter @weird_worlds.

WWW.ASTRONOMY.COM 49

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The author uses this 14-inch Celestron Schmidt-Cassegrain telescope for many of the images he has produced during the past several years.

hotographing the planets has the best film photographs ever obtained telescope can record considerable detail on long been a mainstay of both with the world’s largest telescopes. Jupiter, while larger apertures are needed amateur and professional astrono- for Saturn, Uranus, and Neptune. mers. It wasn’t so long ago that Telescopes for Regardless of the type of telescope you images from Voyager were wowing planetary imaging decide to use, there are several key points astronomers and scientists while Telescopes come in many designs, shapes, to consider from a telescope/hardware Pamateurs struggled with photographic and sizes, and selecting the right one for perspective: film to capture clear images of our nearest high-resolution imaging is important. In • Collimation of the optics. This is neighbors. It’s fair to say they were falling general, almost any good-quality telescope vital, especially with reflectors. Many well short of not only the images space- can produce great images, but certain guides are available online and are often craft were taking, but even the sketches designs are more favored than others. provided with the purchase of a new tele- visual observers were recording with pen- Today the most popular choices among scope. Also keep in mind that it is a good cil and paper. experienced amateurs are large-aperture idea to check the telescope collimation reg- Since then, a revolution has occurred Newtonian or Schmidt-Cassegrain tele- ularly, especially if you store it indoors and within this field of astronomy. Gone are scopes. These provide plenty of light- must move it outside each time. the days of blurry and fuzzy photographs gathering power at affordable prices and, • Thermal equilibrium. Allow the tele- showing little fine detail. Major leaps in in experienced hands, can produce highly scope as much time as possible to reach the technology have enabled today’s amateur detailed images. same temperature as the outside air. This is astronomers to produce images better than With modern cameras, even a 6-inch especially important for larger apertures.

52 ASTRONOMY • OCTOBER 2017 With the right camera and a small telescope, you can go on the same planetary “Grand Tour” that the Voyagers took. IANT planets Text and images by Damian Peach

On March 18, 2016, the author imaged Jupiter through an 889-nanometer methane band filter. Methane absorbs sunlight, producing the dark regions. Brighter areas, such as the Great Red Spot, contain little methane and probably represent high-altitude ammonia clouds.

Above: Several storms are visible at different latitudes in this pair of Saturn images, taken during a period of excellent seeing June 11, 2016. The author captured the left image at 3h28m18s UT and the right one at 4h20m12s. North is up. Left: A wealth of detail is visible across Jupiter’s disk. The author took this image March 18, 2016, at 3h18m UT through his 14-inch telescope.

• Focusing. Do not fight an inadequate A dispersion corrector can remove the del- experienced observers, a monochrome focusing mechanism, such as the standard eterious effects and help sharpen the view. camera with a set of filters will provide focusing knob on a Schmidt-Cassegrain. higher-quality results. A wide range of Invest in a decent motorized system that Cameras, filters, camera choices that perform well is avail- allows fine focusing adjustments without and software able today from several different manufac- touching the telescope. Camera technology is a fast-moving field turers. Companies such as ZWO Optical • Usability. Overlook this at your peril! and is primarily responsible for the vast and Point Grey Research have led the way Nothing is worse than ending up with a leaps in image quality over the past decade. in recent years, but other companies, such telescope that is difficult and cumbersome Gone are the slow transfer rates of old as Imaging Source and Celestron, also pro- to use. Choose one that you think you’ll be CCD cameras, when a photographer could duce an excellent range of cameras for able to use easily and frequently. obtain perhaps only one image every few planetary imaging. • Focal length. Invest in a high-quality seconds. The best cameras in use by ama- Any serious planetary observer should Barlow lens to increase your telescope’s teurs today can shoot at rates of more than own a set of filters to image in different focal length and image scale. Aim for an 100 frames per second (fps). All typically wavelengths. Longer-wavelength filters, f-ratio between f/20 and f/30. operate via similar software packages that such as infrared (IR), allow us to see deeper • Dispersion. Planets low in the sky are allow control over important settings such into planetary atmospheres, while shorter- affected by an atmosphere-induced effect, as exposure, frame rate, and gain. wavelength filters focus on the high-alti- known as optical dispersion, that causes For those starting out, I recommend a tude regions. Also available are specialized their light to spread out into a spectrum. one-shot color camera. For more filters that focus on specific absorption

WWW.ASTRONOMY.COM 53 The author considers this image of Saturn — taken June 18, 2016, at 3h41m UT — the finest he’s ever captured with his 14-inch telescope. Note the great detail visible within the ring processing routines that allow imagers to cameras, the procedure is far less frantic. system and on the planet’s globe. compensate for rapid planetary rotation, Using image de-rotation in WINJUPOS thereby creating higher-quality images. allows a much longer capture window (http://jupos.org/gh/download.htm) overall (up to 15 minutes), making the pro- bands. By far the best for planetary imag- cess much easier. ing is the 889-nanometer methane band Image the giant In terms of video frame rate, shooting filter. This narrowband IR filter focuses on The solar system’s largest planet is the jewel through the R and G filters at around sunlight absorption due to methane in the of the night sky. Jupiter is one of the few 80 fps should work well. You’ll have to use atmospheres of the giant planets and is truly dynamic astronomical objects that a slower rate when you image through a B especially useful for Jupiter and Saturn. can be studied closely with amateur equip- filter, though. Because most camera chips One particularly nice aspect of plan- ment. Its huge size means it presents a large are more sensitive to red light than blue, etary imaging today is that many software apparent disk as seen from Earth, and even your frame rate in blue may need to be as packages you’ll need are available for free. small telescopes can show considerable much as one-half that as with the other Below you’ll find a few of the key titles: detail. It also rotates rapidly — a jovian filters. Finally, you’ll need to stack at least a • Firecapture. This program is by far day lasts 9 hours, 55 minutes — so you can thousand frames to create a reasonably the most popular for planetary camera see much of the planet during just a single noise-free result that will allow sharpening. operation. It supports almost every model night under the right circumstances. Narrowband methane filters also can and has an advanced and well-designed Jupiter’s rapid rotation makes it chal- help bring out lots of detail. Such filters are user interface. (www.firecapture.de) lenging to photograph. The time window typically quite expensive but a worthwhile • Autostakkert! For aligning and available before rotation begins to smear investment for those keenly interested in stacking the frames taken by today’s plan- detail is quite short compared with the imaging Jupiter with the highest quality. etary cameras, this software leads the way. other planets. For those using large aper- (www.autostakkert.com) tures in good seeing conditions, 60 seconds Image the ringed world • Registax. This program has gone is probably the upper limit per single cap- With its captivating ring system, Saturn hand in hand with the advancements we ture. For an RGB sequence, 3 minutes is is without question one of the most spec- have seen in planetary imaging over the the upper limit before too much rotation tacular sights visible through any telescope. past decade or so. It allows both image occurs between the color channels. Larger amateur scopes can capture a alignment and processing of image data. Having a smooth routine in place is wealth of detail across the globe and rings. (www.astronomie.be/registax) really important to use every second for Minor storms appear as small bright • WINJUPOS. Originally intended to RGB imaging of Jupiter, and this only spots and are typically quite frequent. measure the position of atmospheric fea- comes from practice. Such a routine also Larger storms also can erupt, such as dur- tures within the jovian atmosphere, this helps render the tones of the planet accu- ing the 2011 apparition when observers program now contains sophisticated rately. Of course, for those using color were treated to a planet-encircling storm,

54 ASTRONOMY • OCTOBER 2017 23h13m 23h55m 0h43m

Distant Uranus showed some belts and bands in its atmosphere October 7/8, 2015. The author took these shots through a near-infrared filter and then processed them to give the planet the correct color appearance based on how it looks to the eye. At the time of these images, the planet’s diameter was only 3.68".

The author captured Neptune and Triton September 19, 2015, at 23h15m UT (left) and five days later on the 24th. The earlier shot was a 50-minute exposure, and the later one was 25 minutes. Both images show bright storms. Amateur astronomers have discovered several such bright spots in recent years. the largest seen on Saturn in more than 20 with rotation, because sometimes these 15 minutes are usable for single captures, years. The ring system also shows many features are not immediately apparent. and you should try to stack plenty of data interesting details, such as brightness varia- into each single image — up to 30 minutes’ tions, and occasionally you can image even Image the ice giants worth works well. Bright storms have been the famous spoke features. When the rings The distant ice giants have been two largely detected on both planets in recent years, are presented edge-on to Earth, large neglected worlds in terms of amateur study, most of them discovered by amateurs. scopes will allow you to capture shadow although this has started to change. At transits of the saturnian moons easily. their great distances, they present only tiny Not just pretty pictures Photographically, Saturn is challenging. disks even through large telescopes. Both While the majority of deep-sky work is Although it has a large angular diameter, planets exhibit interesting activity with primarily aesthetic in its appeal, this is not its low surface brightness means that larger occasional bright storms, though any hope the case with planetary images. Amateur scopes (8 inches or more) are needed to get of capturing such events is reserved for work on the outer planets remains impor- detailed results, and the frame rates achiev- those using large apertures. tant to professional researchers, and in able are much lower compared with its Both planets are also challenging recent years many discoveries have been larger and brighter neighbor. Using typical objects to photograph well at high resolu- made using amateur data. Science based RGB filters, frame rates will range from tion. RGB or color imaging serves little on amateur images is also more detailed around 20 to 40 fps. Color cameras work purpose with these planets because much and credible than it has ever been. Image well with the planet, and around 50 fps of the transient activity and even the belt quality today is good enough that detailed should be achievable. During RGB imag- and zone patterns are best seen in near- papers about the atmospheric dynamics of ing, you can speed things along by taking a infrared wavelengths. Jupiter have been written based purely on luminance image, as the frame rates are An important filter to use on these the analysis of amateur data. much higher and the resulting shorter planets is a near-IR 600nm long-pass filter. With recent advances in camera tech- exposure time can help produce a sharper For larger apertures, a 700nm IR filter nology, we have moved even further, dis- result. (which passes somewhat less light) works covering storms on the distant worlds of Saturn rotates more slowly than Jupiter, well. These filters stand the best chance of Uranus and Neptune. and at only half the apparent angular picking up atmospheric detail while still Indeed, amateurs continue to make sig- diameter (not including the rings), a far delivering a bright enough image that is nificant discoveries and contributions to more generous capture window is usable. realistically usable with amateur-size tele- studying the four planets of Voyager. For a single capture, 3 to 4 minutes is scopes. All of the most detailed images of usable, and for a total RGB capture, twice these planets produced in recent years have Damian Peach is one of this planet’s finest this length works fine. It can help to take a been through IR filters. Because astroimagers. He collects photons from around series of images over a 30-minute period to both planets are so distant and rotate the world and processes them from his home show any small spots or features that move fairly slowly, time windows of around in Selsey, England.

WWW.ASTRONOMY.COM 55 HOW AN INTERPLANETARY

Voyager 1 and 2 revealed much about the cosmos — and left a giant mark on pop culture.

story of Voyager while visiting the Arecibo Observatory in Puerto Rico. He starts the episode, “On August 20 and September 5, 1977, two spacecraft were launched from the Kennedy Space Flight Center, Florida. They were called Voyager. Each one carries a message. A gold-plated record depicting images, music, and sounds of our planet, arranged so that it may be understood if The once-in-more-than-a-lifetime mis- “The main thing people reference when ever intercepted by a technologically mature sion was born from a rare alignment of the they talk about Voyager in the media is extraterrestrial civilization.” planets that happens only every 167 years. Star Trek,” says Dodd. “V’ger, V’ger, they Mulder is searching (as always) for evi- With the right amount of planning and always say.” dence of extraterrestrial life, and the Golden engineering, NASA realized it could send When NASA designed Voyager, they Record containing a message from Earth, not just one, but two identical spacecraft on knew that someday it would leave the solar calling attention to our own existence, is a a journey from Earth to visit the four giant system entirely and might be visible to alien fitting parallel to his journey. In the epi- planets in the outer solar system. civilizations — if there were any out there sode, a classical music selection from the At the time of their flybys, both craft to intercept it. In Star Trek, the fictional Golden Record plays while Mulder has a brought incredible clarity to Jupiter and Voyager 6 has encountered a sentient bizarre alien encounter in Arecibo. Saturn and their moons, which had previ- artificially intelligent species that gave The Golden Record in and of itself is a ously been visited by Pioneers 10 and 11 the spacecraft the gift of self-awareness. By vessel for a mixture of popular culture and and their more primitive cameras. At the the time the crew of the Enterprise meet general documentation of what it’s like to be time the Voyagers launched, there were no V’ger 6, it’s likely the craft had encountered an earthling. There were musical selections close-up images of Uranus and Neptune; several civilizations. from the likes of Chuck Berry and two of our outermost planets were myster- After the discovery of the parked Beethoven; rights management prevented ies. The world waited for the spacecraft to spacecraft, Spock decides to venture inside “Here Comes the Sun” by the Beatles from fly past each planet and send back whatever and find out more about what exactly V’ger being placed on the record. (The Fab Four treasures they found. Glimpses of the light is. As he’s flying through the ship, he says, were in favor of the move; their record label, teal of Uranus, the bright blue of Neptune, “Curious. I’m seeing images of planets, which owned the rights to the recording, and those textured rings of Saturn infil- moons, stars, whole galaxies all stored was not.) The record also contained 116 trated the nightly news and the collective here, recorded. It could be a representation photos and images of scientific concepts consciousness. of V’ger’s entire journey. But who or what and daily life worldwide; a “hello” in 55 lan- “At the time, the mission was all over the are we dealing with?” What Spock is guages; a recording of human brainwaves; news,” explains Voyager Project Manager seeing is a recorded database of V’ger’s various sounds of our planet, both human Suzy Dodd. “In those days, the press came encounters. Before the missions had even and geologic; and countless other time-cap- in person, and we would have these live reached Saturn, people were already look- sule sights and sounds. Etchings of Earth’s press conferences every day from JPL.” ing to Voyager as our future interstellar position, guided by the position of local Each week, people watching the news emissary. pulsars, were included on the outer cover. would see the planets grow in size as each In perhaps the perfect blending of real Carl Sagan, who was a member of the spacecraft got closer, and the public fol- life and science fiction, Star Trek’s Voyager Voyager imaging team at the time, found lowed loyally along with the mission. 6 encounter is what scientists had the right colleagues to work on the record. Earthlings suddenly knew more about dreamed of when they launched Voyager Jon Lomberg, an astronomical artist, chose their home solar system, and that 1 and 2 into space. the images. Linda Saltzman-Sagan, a writer excitement bled into popular culture, and artist, compiled the language greetings; especially science fiction. Just two years Ann Druyan, an artist, handled the every- after the launch of the Voyagers, Star Trek: Star Trek wasn’t the only sci-fi hit to jump day sounds; Rolling Stone editor Timothy The Motion Picture was released, featuring on the Voyager bandwagon. In a 1994 epi- Ferris lent his particular support on the a familiar spacecraft. In the film, they sode of The X-Files titled “Little Green musical selections; and SETI founder Frank called it “V’ger.” Men,” FBI agent Fox Mulder narrates the Drake lent to the project his vast knowledge

56 ASTRONOMY • OCTOBER 2017 Y MISSION

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M Candy Hansen at Sagan’s I of trying to home in on the best way to T S M A communicate with alien species. request, no doubt changed E R D Since the 1970s, the symbolism of the the world and how we saw | IM D S S record has shown up in TV shows, film, ourselves. As one of its final O J ©

and even clothing. The record even images, Voyager 1 pointed its

recently became popular again when a cameras toward Earth and snapped Kickstarter campaign to reissue the full one final shot before the cameras were Golden Record set received over $1 million shut down to save power. in funding from 10,000 people wanting Sagan would later say the dot repre- their own copy of history. sented the human experience: “Look again grand vision of humans wandering from at that dot. That’s here. That’s home. That’s the cradle of our planet into the wider us. On it everyone you love, everyone you expanse of our solar system and, eventually, know, everyone you ever heard of, every the universe. While working on the Voyager mission, human being who ever was, lived out their The closest thing to the Voyager mission Sagan and Druyan created a TV show lives. The aggregate of our joy and suffer- in the modern age may be the New called Cosmos. The PBS show, which ing, thousands of confident religions, ide- Horizons flyby of Pluto. Today, we can debuted in 1980, not only highlighted the ologies, and economic doctrines, every search every major body in the solar system beautiful intricacies of Earth, but it gave hunter and forager, every hero and coward, online and find an image of that object, but viewers a perspective about our place in the every creator and destroyer of civilization, it wasn’t always that way. In 2015, the previ- cosmos, coinciding with the mission’s jour- every king and peasant, every young couple ously blurry dot we called Pluto became ney toward Saturn. (Sagan and Druyan met in love, every mother and father, hopeful crisp with color, geological features and while working on the Golden Record and child, inventor and explorer, every teacher even a heart-shaped glacier, as the space- married in 1981.) of morals, every corrupt politician, every craft undertook a six-month reconnaissance The impact of the show on popular cul- superstar, every supreme leader, every saint of the distant planet and its moons. Like the ture is so great, it’s hard to quantify, though and sinner in the history of our species Voyagers, New Horizons’ discoveries help one benefit is that Cosmos ultimately lived there — on a mote of dust suspended us understand Earth’s place in the cosmos. inspired an entire generation of scien- in a sunbeam.” “For the first time, we have the power to tists. Jim Bell, a planetary scientist at As photos of Earth taken from space decide the fate of our planet and ourselves. Arizona State University who worked had done in the 1960s, the Pale Blue Dot This is a time of great danger, but our spe- on Voyager at the beginning of his career put humanity’s place in perspective. We cies is young and curious and brave. It and would ultimately memorialize the mis- weren’t just one species uniting for our shows much promise,” Sagan says in the sion in the oral history book The Interstellar planet — we also weren’t the rulers of the first episode of the original Cosmos. “In the Age, credits Cosmos with inspiring his universe, but rather an “insignificant planet last few millennia, we have made the most career path. of a humdrum star lost in a galaxy tucked astonishing and unexpected discoveries “Voyager launched back when I was in away in some forgotten corner of a universe about the cosmos and our place within it.” high school, and the show Cosmos by Carl in which there are far more galaxies than Thanks to the Voyagers and our contin- Sagan was enormously impactful,” he says. people,” Sagan says. ued exploration of the solar system, this “It was the first time that science was on TV “To my mind, there is perhaps no better statement still rings true. We know more and being communicated by a real profes- demonstration of the folly of human about our place in space and can imagine sional communicator who could speak the conceits than this distant image of our other civilizations having an encounter language of science and translate it.” tiny world,” Sagan says in Cosmos. “To with an artifact of humanity. Sometime me, it underscores our responsibility to deal around 2025, both Voyagers will go dor- more kindly and compassionately with one mant. But they will always carry a cultural another and to preserve and cherish that pale legacy that few robotic missions have before — or may ever again. Perhaps more than anything else though, blue dot, the only home we’ve ever known.” the biggest influence the Voyager mission The photo inspired Sagan’s book Pale had on popular culture came in the form of Blue Dot, which, in turn, would go on to Shannon Stirone is a Bay Area freelance a single photo. The “Pale Blue Dot,” taken inspire short films like Wanderers and tele- writer who covers NASA, space exploration, February 14, 1990, by Carolyn Porco and vision shows like The Expanse with its and space policy.

WWW.ASTRONOMY.COM 57 TALK ASTRONOMY with your un-cosmic friends Here’s a concise guide to avoid going over the heads of your science-avoiding peeps. Text by Michael E. Bakich • Illustrations by Roen Kelly

hat’s new in astron- subject while I was in Arizona in Dave White. He and his wife, Sunni, omy these days? February 2016 for the fourth annual graciously hosted me for the week. Why does the Moon Tucson Star Party. This event runs White is a machinist and a look bigger when it’s from 9 a.m. to 9 p.m. on a Saturday, designer of parts for large mining Wnear the ground? and it’s grown in popularity each equipment, but his interests lie in What star is that? year. The East Campus of Pima other places as well. Like space. As Such questions are rare when I Community College hosts the gath- he and I enjoyed Tucson’s marvelous meet with astronomy hobbyists. ering, members of the Tucson warm, clear evenings, he would ask When I’m with family, non-science- Amateur Astronomical Association questions about the sky, and I would oriented friends, or the general pub- set up telescopes for the public to reply. His follow-up questions clued lic, however, these are the only types gaze through, and Astronomy publi- me in to whether or not he under- of questions I get. And now, after cizes and sponsors it. stood my answers, and that made more than four decades in astron- After conducting interviews, me actually think about them. omy education and popularization, being the subject of interviews, and Does that sound funny? Who I’m wondering if I’ve been giving making certain everything was gives an answer without thinking good answers. ready for the daylong event, I relaxed about it? Often, it’s an expert — I started to think about this under the stars with my friend someone who has dealt with a

58 ASTRONOMY • OCTOBER 2017 Resulting Seeing the light Prism acting as spectrum spectroscope

White light Refracted light

Hydrogen gas

Absorption lines characterizing gas

Spectroscopy is the most important tool used by astronomers. Our job is to find a simple way to explain what they discover with it.

Strange orbits

Comet dust trail

Mars Earth

Sun Mercury

Venus

Jupiter Saturn

Comet 55P/Tempel-Tuttle Uranus’ orbit © DENISBELITSKY | DREAMSTIME.COM | DENISBELITSKY © A simple way to describe how meteor showers work is to explain that passing comets leave dust trails that Earth runs into around the same time each year.

Up above . . . 10 In transit Stratosphere

Tropopause 12 3

5 Wind flow around dome Troposphere

1 3 Miles

Heat from 2 Brightness within dome Time

“Why do stars twinkle?” is one of the most common questions This simple diagram illustrates how planets are often found, asked by newcomers to amateur astronomy. This illustration as the light we receive from a star dips when an exoplanet demonstrates a few atmospheric reasons. passes in front of it.

WWW.ASTRONOMY.COM 59 The key here is to focus on the Comet question and not to stray too far A rocky entrance A block of frozen gases and dust; afield. It’s tough — believe me. forms a coma as it Often I keep talking about a subject approaches the Sun because I have subconsciously made the judgment that the other person should be as interested about it as I Asteroid am. It’s worse, however, if other Stony or metal-rich rock in space; smaller experts are present. Then what than a planet and would be a simple explanation often bigger than a meteoroid transforms into a knowledge com- petition. In the meantime, I’ve lost Meteoroid my target audience. Stony or metallic debris in space; can range up Bolide Lack of examples to about 10 meters A fireball that explodes in Consider the question (asked in a Earth’s atmosphere way a non-astronomer would phrase it), “How do you know the power of a telescope?” Most of us in the know would reply that anyone can calcu- Meteor late the magnification of a telescope/ The streak of light in Earth’s eyepiece combo by simply dividing skies formed by infalling debris burning up the focal length of the telescope by the focal length of the eyepiece. Simply? Even if we take the time to care- Fireball Meteorite fully explain what focal length is, it’s An exceptionally The solid portion of a bright meteor that meteoroid that lands kind of an abstract concept. I suggest can cast a shadow on Earth’s surface that we offer more. Provide the lis- tener with an example. If you’re just chatting and no telescope is around, One easy jargon-buster is clarifying the difference between meteoroids, meteors, and meteorites. have the questioner imagine one with a focal length of 100 millime- ters. Then say, “If we put a 25mm particular subject matter for a long Often, then, a simple question eyepiece in, the power would be 4 time. Unfortunately, most experts like, “Why does the Moon have (100 divided by 25).” You could even answer questions the same way no phases?” morphs into a full-blown continue by asking, “What would matter who is asking them. Years of discussion of planetary motion. And happen if we swapped the 25mm dealing with their chosen subject you know what? It doesn’t have to. eyepiece out for a 10mm one?” In offer little versatility in the replies You can show how the Moon’s such a case, the power would they give. phases work with three rocks — or increase to 10 (100 divided by 10). In my opinion, many creden- preferably two rocks and a flashlight. If, on the other hand, a telescope tialed experts make three mistakes: They give too much information, they provide no examples, and they liberally use jargon. Weighing in Mass creates gravity. Because planets in our solar system have 56.8 different masses, a 200-pound Too much information pounds person would weigh different The dreaded “TMI” usually occurs amounts on each of them. 76.0 in social situations (the kind that pounds often emerges in childhood or in a comment about my bathroom 181.4 185.0 158.8 habits), but it’s equally possible Mercury 200.0 pounds 225.0 pounds pounds pounds when an amateur astronomer talks 501.2 pounds to friends. Except in such cases, Mars pounds the “too much” is factual in nature rather than personal. Venus Saturn Uranus Earth Neptune Michael E. Bakich is a Senior Editor at Jupiter Astronomy.

60 ASTRONOMY • OCTOBER 2017 Measure up Dome globe lights Street non-full-cutoff lights Square full-cutoff lights Background stars

Distance star appears to move

Star Line of sight from Earth

Light pollution can be easier to explain once you’re familiar with the different types of shielding that manufacturers make for lights. Parallax angle

1 parsec (3.26 ly) 1 arcsecond Eclipsing Penumbra

Earth Sun Partial eclipse begins 1:58 A.M. EDT When someone asks you, “How do Umbra astronomers know how far away stars Totality ends are?” you’ll have to introduce them 4:25 A.M. EDT to tiny angles and the word parallax. Penumbral eclipse ends 6:38 A.M. EDT Penumbral eclipse begins is present, use its focal length and 12:54 A.M. EDT that of the eyepiece currently in it Totality begins to figure out the power. That will 3:07 A.M. EDT bring home the concept in a more Partial eclipse ends concrete way. 5:33 A.M. EDT An illustration explaining mag- nification could start with the This chart demonstrates how lunar eclipses occur in an easy-to-follow format. statement, “A power of X is like being X times closer to something.” Explain that 7x binoculars make to a non-astronomy-savvy friend or, if we must use strange terminol- objects appear seven times closer. who just asked you to define right ogy, we’ll provide definitions, often Then pick an object. Select a big ascension (probably because you in the same sentence. one but not an immense one. So, used it in a sentence without think- Oh, and in case you’re wonder- pick a car but not a building, a bush ing about it). How much detail ing, after many years of confusing but not a giant sequoia. Stand next would you go into? people with definitions, here’s how to your chosen thing and — with Would you point out that I’d define right ascension to a new- your friend — pace out 10 steps astronomers usually measure right bie now: It’s a coordinate system and turn around. Gauge the appar- ascension in hours, minutes, and just like longitude on Earth, but it ent size of the object. Then pace out seconds? That it’s one of the two moves with the stars. five steps toward the object and spherical coordinates used to mea- In the early 1960s, American examine it again. Boom! You’ve just sure the positions of objects in the social scientist and Nobel laureate doubled the magnification. sky? That John Flamsteed, the Herbert Alexander Simon wrote Astronomer Royal of England, first that something is simple or complex Jargon used this coordinate in a star cata- depending on the way we choose to Newcomers to our hobby aren’t log? That 0 hours marks the spot of describe it. As a group of hobbyists alone in dealing with a huge num- the vernal equinox? That preces- who desire to grow our field of ber of strange terms. A person sion causes the right ascensions of interest, let’s err on the side of sim- could be listening to an economist celestial objects to change in a plicity. And we’ll all have lots of talk about game theory, a computer roughly 26,000-year cycle? chances quite soon. The astronomi- specialist praise the latest GUI, or Merriam-Webster defines jargon cal event of our lives — the great an amateur astronomer detail his several ways. My two favorites are American total solar eclipse on mirror’s Strehl ratio. In all these “confused unintelligible language” August 21, 2017 — just took place. If examples, a better than average and “obscure and often pretentious you weren’t barraged with questions chance exists that a word or two language marked by circumlocu- about it, it’s probably because people will slip by undefined. tions and long words.” Here at the don’t know you’re an amateur Try this: Pretend you’re talking magazine, we try to avoid jargon, astronomer. Tell ’em.

WWW.ASTRONOMY.COM 61 Memories from a BACKYARD OBSERVER

Decades-old observing journals recall a time when Voyager utterly transformed what we know about the outer planets. by Raymond Shubinski

Valentine’s Day 1990, Voyager 1 sent a loving and last portrait of the solar system it had begun to explore 13 years earlier. Its camera was turned ON backward to catch a final glimpse of a home now billions of miles away. For nearly a decade and a half, the cameras of Voyager 1 and 2 had looked outward, deliver- ing images of the outer solar system as never seen before. Voyager 2 launched August 20, 1977, and its sister, Voyager 1, departed 16 days later. By mid-1979, both spacecraft had made close approaches to the giant of the solar system, Jupiter, and then journeyed on to worlds so distant, our minds fail to comprehend the immensity of that space. For all of us sitting back on Earth, this was an incredible time of discovery only dreamed of a few years earlier. A few months after the Voyagers left Earth, I was working with current Astronomy Senior Editor Michael E. Bakich at a small planetarium in Lafayette, Louisiana. We delivered program after program on the Voyager mission. Schoolchildren and the public alike were thrilled with every detail of this unfolding adventure. We also ventured out every possible night to observe the targets of all this excitement: Jupiter, Saturn, Uranus, and Neptune. Blast from the past Recently I rediscovered some of those memories in my observing Always spectacular in backyard telescopes, Jupiter shows notes and journals. An early morning rendezvous with Jupiter its cloud bands, Great Red Spot, and other features and Saturn, for example, on October 21, 1978, recalls that both CHRISTOPHER GO explored in such depth by Voyager. planets were well placed in the east an hour or so before sunrise. We were observing with a 6-inch refractor mounted on an old surveyor’s tripod. My drawings show one cloud band of Jupiter as

62 ASTRONOMY • OCTOBER 2017 The iconic planet image for amateur astronomers, Saturn features a yellow- golden planetary sphere surrounded by the most famous rings in the universe. DAMIAN PEACH

dark and a bit ragged on the upper edge with a hint of a “bulge” Voyagers launched, the world learned that Uranus, like Saturn, on the lower section. The other band was fainter, and we could has a system of rings. MIT astronomer Jim Elliott made the dis- see little detail on the cloud tops of Jupiter. Europa and Callisto covery while watching Uranus occult, or pass in front of, a star, to appeared as two starlike objects. Saturn was lower in the sky, but study the planet’s atmosphere and other features. To his surprise, was beautiful as always. The rings were not edge-on, but were the star faded in and out before it disappeared behind the planet, partly closed. Unsurprisingly, we couldn’t make out any cloud providing evidence for the rings. Then, in 1979, Voyager 1 details with a scope of this size. revealed that the king of planets, Jupiter, is also surrounded by The 6-inch refractor we used that morning in 1978 was pretty four thin dust rings. typical of the kinds of scopes many amateurs used in the 1960s, Flipping through my observing journals, I came across an ’70s, ’80s, and beyond. Browse through the ads of any astronomy unusual night several years later, on July 17, 1985. I was observing magazine of the time, and you will see ads for the Criterion both Jupiter and Saturn. The Voyager rendezvous with these two Dynascope RV-6 reflector, Unitron 2.4- and 3-inch reflectors, and gas giants was over, and Voyager 1 was on its way out to the edge the orange squat tube of an 8-inch of the solar system. That night, I was Celestron telescope. using a Celestron 8-inch telescope and a Our proverbial socks were range of eyepieces. At about 250x, both Resetting the solar system planets held up well with clear details. I What we didn’t appreciate fully quite about to be blown off as could see at least five bands on Jupiter yet, in 1978, was that all of us who were Voyager 1 made its closest and the shadow of the moon Io as it pro- into amateur astronomy were about to gressed across the disk of Jupiter. have our proverbial socks blown off as approach to Jupiter. Saturn was gorgeous, as always. My Voyager 1 made its close approach to drawing shows the rings almost wide Jupiter in March 1979 and Voyager 2 in July of the same year. open. I noted one cloud band and the Cassini Division as quite The astounding detail of the images returned by these spacecraft clearly visible. As I watched Saturn drift by in my field of view, I forced us to totally rethink not only what we knew about this thought of all we had learned about the ringed planet during the giant planet, but the solar system as a whole. past few years. In the C8, I could easily see features that had been In the early years of my planetarium career, I used astronomy observed for the past three centuries. Now we knew that Saturn’s books written by Isaac Asimov as an authoritative source for rings were far more complex than what any ground-based tele- school and public programs. One of those was Jupiter, The Largest scope could show. In the high-resolution images returned by Planet. According to Asimov, Jupiter had 13 moons. The Voyagers Voyager 2, astronomers discovered hundreds of thin rings, some added three more moons to the realm of Jupiter. Today we count only a few feet thick. 51 moons with names, and another 18 that orbit anonymously around the giant planet. Imaging the outer planets Planetary discoveries were happening even before the space- I have always enjoyed drawing what I see in the eyepiece. Many of craft departed. On March 10, 1977, just months before the my contemporaries, however, have produced images of the outer

WWW.ASTRONOMY.COM 63 and an excited participant on a television panel of experts talk- ing about Voyager 2’s recent close encounter with Neptune. On August 24, Voyager made its closest approach to this distant, shimmering, blue ice giant. Sitting next to me was Carolyn Porco, a key member of Voyager’s imaging team. I felt out of place among the professional astronomers from the university and their years of experience and knowledge they represented. I was, however, the only one at this KUAT public television event who observed the sky with small Neptune’s dynamic weather telescopes and regularly brought these exciting events to the pub- was a surprise for Voyager lic through planetarium shows. scientists. In backyard One of the most exciting discoveries discussed was the existence Uranus is easy to see as a scopes, it appears as a small of massive storms in the upper atmosphere of Neptune. Since the bluish-green disk, but it is but bright bluish disk, here essentially featureless in visible with its brightest planet is nearly 3 billion miles from the Sun, it was assumed that backyard scopes. DAMIAN PEACH moon, Triton. DAMIAN PEACH it’d be far too cold to have any kind of weather. Passing just 3,000 miles (4,800 kilometers) above the planet’s cloud tops, Voyager 2 discovered what was dubbed the Great Dark Spot. Voyager also planets using backyard scopes and cameras. added six new moons, recorded the fastest winds on any planet in For many years, my go-to book on astrophotography was Outer the solar system, and uncovered a set of planetary rings. Space Photography for the Amateur by Henry E. Paul, published in When the moderator, John McClury, turned to me and asked, 1960. The book still provides a gold mine of information, covering “Can the average amateur astronomer expect to see any of these everything from lenses and telescopes to home observatories and new discoveries in their telescopes?” I had to answer, “None of it.” sky conditions. Paul recommends using a standard 6-inch f/8 These discoveries are truly beyond not only any amateur equip- reflector or a 3-inch f/15 refractor, with a preference for the reflec- ment, but also the greatest telescopes on Earth. “You may not be tor. Obviously, a telescope needs a motor drive to compensate for able to see these discoveries for yourself, but knowing about them Earth’s rotation. But the revolution in CCD imaging and the use of makes your own observations so much more interesting,” I said. digital cameras has made the techniques described in this book Thinking about what the Voyager mission meant to astronomy obsolete. Even films recommended by Paul, such as Kodak’s High enthusiasts, I found another old notebook entry. This one was Contrast Copy Film and Plus-X, have long become extinct. from August 19, 1976. I was observing with the 24-inch Ritchey- In the section on planetary photography, Paul says, “The plan- Chrétien telescope at Michigan State University with Dave ets are difficult subjects for the amateur Duzinski, a fellow grad student. We had photographer because of their small angu- been taking photographs of Jupiter. After lar size.” A number of illustrations in this “This is the time when we were done and Dave had gone into the book show Jupiter, Saturn, Uranus, and humans have begun darkroom, I had the scope to myself and Neptune. Paul provides a group of draw- began observing the planet visually. ings of Saturn next to a few photographs. to sail the sea of space,” It was about 3:30 a.m., and the sky was Every one of the drawings is better and steady and clear. The view of Jupiter was shows more detail than the photos. There wrote Carl Sagan. one of the best views of the planet I had is a universal graininess to the photos that ever seen. My drawing shows five distinct was impossible to avoid for ground-based imagers. The Voyager 2 bands with a few bulges and delicate features. Io’s shadow was images were astounding compared with any images we could cap- slowly progressing across the giant’s face, and the star field in ture from telescopes on Earth. which it was embedded was beautiful. It turned out that all the In the 1980s, Voyager 2 undertook flybys of Uranus and photographs Dave and I took were overexposed, and my drawings Neptune, the ice giant planets. My ever-present notebook has an became the only record of that night. entry for June 9, 1981, featuring an evening search for Uranus. I was using a 44-inch rich field telescope, a sheet from Atlas of the A unique and epic voyage Heavens, and a finder chart from the Abrams Planetarium Sky All these memories and musings have brought me back to Carl Calendar. It was easy to spot Uranus’ blue-green disk among a Sagan’s book Cosmos, published in 1980. The chapter “Travelers’ number of stars that formed a loose triangle with 41 and Kappa Tales” compares the adventures of the newly launched Voyagers to Librae. Needless to say, Uranus appeared as only another bright the early discoveries of the 18th century. “star” among this fairly bright group. Even with my 3-inch “This is the time when humans have begun to sail the sea of Unitron and Celestron 8, I could never see any details on this space,” Sagan writes. “The modern ships that ply the Keplerian planet. Both Paul’s book and the Jupiter book by Asimov have trajectories to the planets are unmanned. They are beautifully photographs of Uranus and Neptune made with large ground- constructed semi-intelligent robots exploring unknown worlds. based telescopes. Because of atmospheric turbulence, no details . . . We have embarked on epic voyages.” can be seen in these photos — just a bright, blurry disk with a few Not only was our understanding of these outer worlds about to of its moons present. change forever, but so was the view in our modest telescopes. Sharing the excitement of Voyager By the end of Voyager’s Grand Tour, in August 1989, I was direc- Raymond Shubinski is a contributing editor of Astronomy who has tor of the Flandrau Planetarium at the University of Arizona, been observing the sky for many decades.

64 ASTRONOMY • OCTOBER 2017 Breakthrough Science for Curious Minds

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Subscribe online at DiscoverMagazine.com Or call 800-829-9132 P30498 SECRETSKY BY STEPHEN JAMES O’MEARA Hello, Moon Our nearest neighbor in space doesn’t just shine at night.

ost people a sense, biblical: “Let your eyes associate the look straight ahead; fix your Sun with day gaze directly before you” and the Moon (Proverbs 4:25). My gaze with night — remained relatively focused Mand many mythologies tell us forward with occasional why this is so. But myths are sweeps from side to side. myths, and perhaps they are I don’t own a smartphone. the reason why some non- So, I tend to keep my head level astronomers think that some- as I walk, so that my field of thing is wrong if they see the vision is partly on the ground The Moon appears above Grand Canyon National Park in this image taken November 9, Moon during the day. and partly on the sky. 2013. The photographer was backpacking at the time and set up his camera near the In fact, the Moon is quite Research has shown that if Hermit Rapids area of the Colorado River. WOLFGANG GOLSER noticeable at various times on we have such a horizontal line most days throughout its of sight, we see (on average) Hide-and-seek wonder it’s a surprise when monthly cycle (except for about more Earth than sky: roughly Moon non-astronomers do see it. I two to three days before or 35° below the horizontal The previous figures suggest once received a call from some- after New Moon). (Earth), and about 25° above that as we go about our day, we one who feared that the Moon the horizontal (the sky). My do not see the Moon unless it was going to collide with Earth Scanners own observations support happens to be within 25° of the because it was near the horizon Once the Sun rises, humans these figures. We can extend horizon and we happen to be in the daytime. seem to be naturally pro- grammed to look (literally) See the light down and out, but not neces- With only a little thought During the night, there is sarily up. This is mainly a pro- it becomes apparent just how difficult not much of interest on the tective mechanism due to the ground, which under natural immense brilliance of the solar it is to notice the daytime Moon. conditions is swathed in dark- disk. But up is where you’d have ness. So we look up where to direct your gaze most of the all manner of celestial lights time to see the daytime Moon. our sky coverage to approxi- looking in that direction. This caress our eyes. I tried being a daytime sky- mately 50° by simply rolling usually occurs in the middle We notice the Moon, mainly watcher for a couple of weeks, our eyes up while keeping a to late afternoon after First because it is the brightest noting where my vision takes level head, but that probably Quarter and before Full Moon, object in the night sky. During me naturally throughout the requires a reason to do so, such or in the early to late morning the day, however, the Moon day. What I discovered was, in as when a bird flitters by. after Full Moon and before hands over its crown to the Sun Last Quarter. and then gracefully slips into Around these times, the the background. Moon is also at or near its If you happen to notice the brightest (between magnitudes Moon in a blue sky while –10 and –13), so you’re more you’re with other people, ask likely to notice it. Add possible them if they’ve noticed it today. horizon obstructions such as Then listen to their replies, and trees and buildings, and the perhaps learn something new fact that many of us are work- about human perception. As ing or equally occupied for always, send your thoughts to large chunks of the day. [email protected]. With only a little thought it becomes apparent just how Stephen James O’Meara difficult it is to notice the day- is a globe-trotting observer When the Moon is 1 percent illuminated, as in this image taken May 26, 2017, only time Moon — unless you’re who is always looking for the dedicated amateur astronomers will search for it. At this time, our lone natural satellite next great celestial event. was a scant 19.2 hours old and appeared only 15° east of the Sun. CHRIS SCHUR specifically looking for it. No

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WWW.ASTRONOMY.COM 67 BINOCULARUNIVERSE BY PHIL HARRINGTON Exploring Capricornus The dazzling globular star cluster M30 glows at 7th magnitude and is Two double stars and a great globular the finest deep-sky object in the constellation Capricornus. DANIEL VERSCHATSE cluster highlight this dim star group. Beta Capricorni consists of an unequal pair of 3rd- and 6th-magnitude suns that float together in this sketch made with a 6-inch f/8 reflector at 240x. JEREMY PEREZ

apricornus the Sea In October’s evening sky, we space. The yellow giant south- (ζ) Capricorni. Center Zeta in Goat is one of the find Capricornus fin-deep in eastern star, known as Alpha2, your binoculars, and then look faintest constella- the area of sky known as the is 109 light-years from us, while toward the eastern edge of the tions along the eclip- “wet quarter.” There, we see Alpha1, a yellow supergiant, is field. A 5th-magnitude star, 41 tic. With none of its several constellations that are nearly seven times farther away. Capricorni, should just be com- Cstars shining brighter than 3rd associated with water in one The second star, Dabih or ing into view. Hold on 41, and magnitude, many suburban star- way or another. Besides Beta (β) Capricorni, is just 2° then glance just to its west. Can gazers often bypass it in favor of Capricornus, there’s Aquarius south of Algedi. It easily fits you see a dim, round patch of brighter regions. Yet for those the Water-bearer; Piscis into the same field of view. grayish light surrounding a who persevere, Capricornus Austrinus the Southern Fish; While Algedi’s two stars appear brighter core? That’s M30. holds a few hidden gems visible Cetus the Whale; and Pisces the identically bright, Dabih’s two- Messier discovered M30 through our binoculars. Fish. You may need to travel to a some looks noticeably different. the night of August 3, 1764. So, what exactly is a Sea site that is free of light pollution The brighter sun, called Dabih- Like most globulars in his cata- Goat, and who concocted and horizon-hugging haze to Major, shines at 3rd magnitude, log, it was described as a round that image? Despite its faint- make them out, since faint stars while its companion, Dabih- nebula free of any stars. How ness, Capricornus is one of comprise nearly all of them. Minor, is 16 times fainter at 6th wrong he was. We now know the oldest constellations in Binoculars, however, come to magnitude. Both form a physi- that faint patch is actually a the sky, tracing its origin back our rescue when viewing under cal system 330 light-years away, swarm of 100,000 or so indi- to ancient Sumeria. As one less-than-ideal circumstances. with each separated from the vidual suns lying an estimated culture was assimilated into Begin by drawing an imaginary other by about a third of a light- 27,100 light-years away. But another, Capricornus became line from Vega southeastward year. It takes approximately unlike most globulars associ- associated with Pan, the Greek to Altair, and then keep going 700,000 years for the pair to ated with our galaxy, M30 god of nature who was half about an equal distance farther. orbit one another. appears to be orbiting through goat and half human. In one The area may look barren by There is much more to Dabih the Milky Way’s inner galactic tale, Pan helped Zeus battle the eye, but aim your binoculars than meets the eye. Studies halo backward, or in retro- Titans. During an especially there and you should spot two show that both stars are them- grade. This suggests that, harrowing part of the clash, conspicuous stars separated by selves accompanied by fainter unlike most other globulars, it Pan barely escaped Typhon, about 2°. Both lie at the north- companions unseen through may have been stolen from a the deadliest creature in Greek western corner of Capricornus’ binoculars. Dabih is at least a satellite galaxy rather than mythology, by giving himself a triangular form. quintuple system. having been formed from the fish’s tail and jumping into the Let’s first examine the north- Our final stop within protogalactic cloud that begat Nile River. ernmost of those stars, Algedi, Capricornus is the Sea Goat’s the Milky Way. or Alpha (α) Capricorni. sole contribution to the Messier Do you have a favorite bin- Algedi is made up of two 4th- catalog. The 7th-magnitude ocular object? I’d love to hear magnitude suns that are easy globular cluster M30 lies in about it and feature your to resolve with the smallest the constellation’s barren observations in future col- pocket binocular, and even southeastern corner. To find umns. Drop me a line through with the unaided eye given dark it, slowly scan eastward from my website, philharrington.net. skies. They look like heavenly Algedi along the top side of the Until next month, remember headlights spaced 6' apart. But Capricornus triangle to Deneb that two eyes are better than looks can be deceiving. It turns Algedi (Delta [δ] Capricorni) one. out the pair is strictly a chance, at the triangle’s eastern corner. line-of-sight coincidence. Algedi Now, shift 8°, or a little more Phil Harrington is a longtime is a so-called “optical double,” than a typical binocular field, Alpha Capricorni’s twin 4th-magnitude contributor to Astronomy and stars are visible in this sketch made with two stars that are actually to the southwest. There you the author of many books. a 6-inch f/8 reflector at 240x. JEREMY PEREZ nowhere near each other in should see 4th-magnitude Zeta

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1. COLORFUL SPIRALS NGC 4151 (left) is a Seyfert galaxy 45 million light-years away. It hosts one of the brightest active galactic nuclei known at X-ray wavelengths. NGC 4145 is a million light-years closer and is considered “anemic” because not much star formation is occurring in it. • Mark Hanson

2. CHROMATIC SPLENDOR The region around the star Rho (ρ) Ophiuchi is one of the most colorful in the sky. The interstellar clouds of gas here contain both red emission and blue reflection nebulosity. And then there’s the dust, which pep- pers the region with dark nebulae. • Miguel Claro

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70 ASTRONOMY • OCTOBER 2017 3. PTOLEMY’S CLUSTER Open cluster M7 in the constellation Scorpius the Scorpion is the eighth- brightest such object in the sky. Shining at magnitude 3.3, it’s an easy catch with naked eyes from a dark site. This object lies about 980 light-years away. It carries the common name “Ptolemy’s Cluster,” although when that philosopher observed it, he classified it a nebula. • Dan Crowson

4. SUNBEAMS Crepuscular rays form when sunlight streams through breaks in distant clouds, creating shafts of alternating sunlit and shadowed air. If you trace all the rays back to their origin, you’ll arrive at the Sun. The word crepuscular comes from the Latin for “twilight,” named that because we see them most often at sunset. • Jared Bowens

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WWW.ASTRONOMY.COM 71 5. SPIN DOCTOR The Propeller Nebula (DWB 111) is a small part of a much larger emission nebula in the constellation Cygnus. It’s also faint. To create this image, the photographer combined 16 hours of exposures. • Rodney Pommier

6. COSMIC BALLOON Planetary nebula NGC 7094 lies in the constellation Pegasus not far from the famous globular cluster M15. Observers using 11-inch or larger tele- scopes can see some details in it, along with its 13th-magnitude central star. • Warren Spreng

7. YOU IN? Irregular galaxy Sextans B lies 5 4.4 million light-years away. Because of its distance, astronomers are unsure if it belongs to our Local Group. And although it appears as only stars, it contains several planetary nebulae and a large globular cluster. • Dan Crowson

8. GASEOUS ERUPTION The photographer captured this pair of huge prominences dancing above the Sun’s surface July 8, 2017. The large one arced some 43,400 miles (70,000 kilometers) above the solar limb. • Paco Bellido

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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 [email protected]. 8

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WWW.ASTRONOMY.COM 73 BREAK THROUGH A colorful crustacean

The Lobster Nebula (NGC 6357) harbors at least three clusters of hot young stars. In visible-light photos, these clusters often hide inside the dense clouds of gas and dust that gave birth to them. Yet X-rays from the massive suns penetrate the shrouds and reveal these luminaries. In this composite image, X-rays appear purple, infrared radiation glows orange, and visible light shows blue. The purple clouds highlight the youthful clusters and their embed- ded stars, whose stellar winds and radiation are carving cavities into their surroundings. The Lobster Nebula lies some 8,000 light-years from Earth in the constellation Scorpius. X-RAY: NASA/CXC/PSU/L. TOWNSLEY ET AL.; VISIBLE: UKIRT; INFRARED: NASA/JPL-CALTECH

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