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    The worlds best-selling astronomy magazine

    The science

    of cometsp. 24

    OBSERVE AND IMAGE COMET ISONAT ITS BRIGHTEST!

    Comet ISONBLAZES INTO GLORY

    November

    www.Astronomy.com

    Comets throughoutHISTORYp. 30AMAZING

    LLUSTRATION:

    Anatomy of a cometp. 44

    20brightcomets ofhe past 50 years p. 46

    SPECIAL ISSUE

    omet Hale-Bopp was thest Great Comet witnessed

    y Northern Hemisphere

    bservers. Will Comet ISONrpass it in magnificence?

    PrevieNovemberAfrican tot

    solar eclips

    BONUSONLINE

    CONTENT

    CODE p. 4

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    GIVE US YOUR

    BEST SHOT!Submit stunning imagesof Comet ISON forimpressive cash prizes!Astronomers predict ISON may become

    one of the best comets in decades.

    To commemorate this grand sky event,

    head out this fall to capture your finest

    images of Comet ISON, and you couldwin cash prizes and have your photo

    featured in Astronomymagazine.

    ENTER TODAY!Register and submit your

    Comet ISON images to

    www.nsf.gov/comet.

    The photo contest runs

    October 15, 2013, through

    January 15, 2014.

    See www.Astronomy.com/ISONphotosfor imaging guidelines and official contest rules.

    ENTER AND YOU COULD WIN $2,500From the National Science Foundation, Division of Astronomical Sciences

    Piggybackcamera

    Cameraand

    tripod

    Throughthescope

    National Science Foundation

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    CONTENTS

    4 ASTRONOMY t NOVEMBER 2013

    24 COVER STORYThe science of cometsScientists have sent 12 missions

    to explore comets. Heres whattheyve learned about these relics

    from the early solar system.

    MATTHEW KNIGHT

    30Comets: Fromsuperstition to scienceFor thousands of years, scientiststhought comets lived within

    Earths atmosphere, and nearlyeveryone believed these celes-

    tial visitors delivered death anddestruction with their tails.

    GARY KRONK

    36The Sky this Month

    ISON at its brightest.MARTIN RATCLIFFE AND

    ALISTER LING

    38StarDome andPath of the PlanetsRICHARD TALCOTT;

    ILLUSTRATIONS BY ROEN KELLY

    44Illustrated: Anatomy

    of a cometSlip into a coma, sweep up a dusttail, and brace yourself against

    the solar wind. SARAH SCOLES;

    ILLUSTRATIONS BY ROEN KELLY

    4620 bright comets of thepast 50 yearsEach of these cosmic visitors

    of the past half-century told aunique story and captured the

    attentions of observers and

    imagers. KARRI FERRON

    52Ask AstroPredicting a comets path.

    54Comet ISON blazesinto gloryis frozen ball of gas and dust

    puts on a show in the predawnsky. RICHARD TALCOTT

    60Totality crosses AfricaAn eclipse shadow, exotic loca-tions, and safaris will lure travel-

    ers to this event November 3.

    JOEL K. H ARRIS

    64A field trip to thestratosphereAssociate Editor SARAH SCOLEStakes a ride with SOFIA.

    COLUMNSStrange Universe 10BOB BERMAN

    Observing Basics 14GLENN CHAPLE

    Secret Sky 18STEPHEN JAMES OMEARA

    QUANTUM GRAVITYSnapshot 9

    Breakthrough 11

    Astro News 12

    IN EVERY ISSUEFrom the Editor 6

    Letters 10, 14Web Talk 22

    New Products 67

    Advertiser Index 71

    Reader Gallery 72

    Final Frontier 74

    FEATURES

    Astronomy(ISSN 0091-6358, USPS 531-350) is pub-lished monthly by Kalmbach Publishing Co., 21027Crossroads Circle, P. O. Box 1612, Waukesha, WI531871612. Periodicals postage paid at Waukesha,WI, and additional offices. POSTMASTER: Sendaddress changes to Astronomy, 21027 CrossroadsCircle, P. O. Box 1612, Waukesha, WI 531871612.Canada Publication Mail Agreement #40010760.

    NOVEMBER 2013VOL. 41, NO. 11

    The Skythis Week

    A daily digestof celestial

    events

    Trips & ToursTravel theworld withthe staff ofAstronomy

    SeasonalObserving

    Find great tar-gets in yourautumn sky

    Picture ofthe Day

    Astroimagesfrom around

    the world

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    Go towww.Astronomy.comfor info on the biggest news and

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    VisitAstronomy.com/tocfor bonus material itsexclusive toAstronomymagazine subscribers.

    24

    ON THE COVERA bluish tail of ionized gas and ayellow-white dust tail highlightedComet Hale-Bopp in 1997. Observ-ers hope Comet ISON puts on asimilarly dazzling show this month.

    Online Content Code: ASY1311Enter this code at: www.astronomy.com/code

    to gain access to web-exclusive content

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    OPT Telescopes 800.483.6287 www.opttelescopes.com

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    www.twitter.com/AstronomyMag

    www.facebook.com/AstronomyMagazine

    FollowAstronomy

    6 ASTRONOMYt NOVEMBER 2013

    B Y D AV I D J . E I C H E R

    FROM THE EDITOREditor David J. EicherArt Director LuAnn Williams Belter

    EDITORIALSTAFF

    Managing Editor Ronald KovachSenior Editors Michael E. Bakich, Richard TalcottAssociate Editors Liz Kruesi, Sarah ScolesAssistant Editor Karri FerronEditorial Associate Valerie Penton

    ARTSTAFF

    Senior Graphic Designer Chuck BraaschIllustrator Roen KellyProduction Coordinator Jodi Jeranek

    CONTRIBUTINGEDITORSBob Berman, Glenn F. Chaple, Jr., Martin George,

    Tony Hallas, Phil Harrington, Ray Jayawardhana,David H. Levy, Alister Ling, Steve Nadis, Stephen JamesOMeara, Tom Polakis, Martin Ratcliffe, Mike D. Reynolds,Sheldon Reynolds, John Shibley, Raymond Shubinski

    EDITORIALADVISORYBOARD

    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,Stephen P. Maran, Brian May, S. Alan Stern, James Trefil

    Kalmbach Publishing Co.President Charles R. CroftVice President, Editorial, Publisher Kevin P. KeefeVice President, Advertising Scott StollbergVice President, Marketing Daniel R. LanceCorporate Art Director M aureen M. SchimmelManaging Art Director Michael SolidayProduction Manager Helene TsigistrasCorporate Circulation Director Michael BarbeeGroup Circulation Manager Ken MeisingerSingle Copy Sales Director Jerry BursteinCirculation Specialist Sarah Zemplinski

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    changes. Subscription rate: single copy: $5.99; 1 year $42.95, 2 years (24issues) $79.95, 3 years (36 issues) $114.95; Canadian price: 1 year $50.95, 2years $95.95, 3 years $138.95 (Canadian price includes additional postageand GST, payable in U.S. funds.) International price: 1 year $58.95, 2 years$111.95, 3 years $162.95 (International price includes additional postage,payable in U.S. funds.) Expedited Delivery Service Surcharges: Domestic 1stClass $30 per year, Canadian air $30 per year, International air, $60 per year.BN 12271 3209 RT. Not responsible for unsolicited materials.

    s we enjoy Comet ISON

    (C/2012 S1) at its best

    this month, compari-

    sons will be made to

    the last really stun-

    ning comet, Hale-Bopp(C/1995 O1). On July 23,

    1995, while observing the

    globular star cluster M70, two

    observers in the southwestern

    United States each stumbled

    on a fuzzy blob near the clus-

    ter in their eyepieces. In

    Cloudcro, New Mexico,

    astronomer Alan Hale saw the

    comet, while 440 miles (710

    kilometers) away in Stanfield,

    Arizona, omas J. Bopp also

    spied it. Following confirmingobservations and the calcula-

    tion of an orbit by Brian

    Marsden, Comet Hale-Bopp

    (C/1995 O1) was born.

    Incredibly, at its discovery,

    the comet was a staggering

    700 million miles (1.1 billion

    km) away between the

    orbits of Jupiter and Saturn.

    Although at this distance

    most comets would appear as

    tiny specks, Hale-Bopp

    already showed a coma.

    Scottish-Australian astrono-

    mer Robert McNaught, at the

    Anglo-Australian Telescope

    at Siding Spring, found a

    1993 image of the comet

    showing a coma at the great

    distance of 1.2 billion miles

    (2 billion km) between the

    orbits of Saturn and Uranus.

    At that distance, Halleys

    Comet would have been more

    than 100 times fainter than

    Hale-Bopp.

    Astronomers, professional

    and amateur, would be able to

    view Hale-Bopp for a longtime before it moved into the

    inner solar system. Scientists

    were immediately excited by

    the fact that Comet Hale-

    Bopp was likely to brighten

    substantially, reaching a

    perihelion distance of 85 mil-

    lion miles (137 million km)

    April 1, 1997, and swinging

    past Earth at a distance of 122

    million miles (197 million

    km) March 22.

    No one knew it quite yet at

    discovery, but Hale-Bopp was

    destined to become the

    brightest comet of its era and

    was perhaps the most widely

    observed comet in history,

    given the huge numbers of

    the general public who looked

    at it with unaided eyes, bin-

    oculars, or telescopes. Calcu-

    lations showed the comet

    would peak shining brighterthan any star in the night sky

    except for Sirius, the brightest

    star. Starting May 20, 1996,

    Hale-Bopp would remain

    visible to the naked eye for

    569 days more than 18

    months, the longest period of

    unaided visibility of a known

    comet. That record would

    double the previous nine-

    month record set by the Great

    Comet of 1811.

    Now the stage was set forComet Hale-Bopp to put on

    its spectacular show. The

    incredible display of this

    amazingly bright comet and

    its prominently fanned gas

    tail awed millions of people

    across the globe, and the

    media coverage of the comet

    was enormous. The comet

    became one of the most

    beloved of all time. Comet

    Hale-Bopp became one of

    historys greatest comets, a

    path we all certainly hope

    Comet ISON will follow.

    Yours truly,

    David J. Eicher

    Editor

    The last

    really GreatComet

    Comet Hale-Bopp (C/1995 O1)wowed observers in 1996 and 1997,becoming the most observed cometin the 20th century. BILL AND SALLY FLETCHER

    A

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    Comet ISON

    THEDEFINITIVE GUIDE TO

    Monday Friday, 8:30 a.m. 4:30 p.m. CDT. Outside the United

    States and Canada call 262-796-8776, ext. 661. Please have your

    credit card ready.

    The Great Comet of 2013 arrives in early October 2013.

    19556 Background: Comet McNaught (C/2006 P1) by Martin Moline

    The Great Comet of 2013gets

    you ready for the spectacular

    appearance of what could be

    the centurys brightest comet!

    Trust the experts atAstronomymagazine

    to bring you Comet ISONs complete story,

    including:

    OPTIMAL VIEWING PERIODS: Discover the

    ideal time for your location

    OBSERVING TIPS: Get day-by-day info for

    October through January

    EQUIPMENT ADVICE: Find out whats

    best to view the comet

    IMAGING HOW-TOs:Learn how to

    successfully capture Comet ISON

    And MUCH MORE!

    ORDER NOWtoSAVE $1and receive

    FREE SHIPPINGon this new special issue!

    www.Astronomy.com/CometISON

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    A3B

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    QG

    QUANTUMGRAVITY

    EVERYTHING YOU NEED TO KNOW ABOUT THE UNIVERSE THIS MONTH . . .

    WWW.ASTRONOMY.COM 9

    CONTEST

    Give us yourbest shotWhether Comet ISON is aonce-in-a-lifetime event, agreat performer, or a run-of-the-mill comet, your pictureof it could win a $2,500 prize.

    Comet ISON looks to be the most

    publicized comet of our lifetimes

    and perhaps the most photo-graphed. Because so many peopleare planning to snap pictures of it,

    the National Science FoundationsDivision of Astronomical Sci-

    ences, Discovermagazine, andAstronomyare co-sponsoring the

    Comet ISON Photo Contest.The contest is open to amateur

    and professional photographersworldwide. You can submit

    images online at www.nsf.gov/comet beginning October 15,

    2013, through January 15, 2014.

    Judges will consider images inthree categories: 1) Cameras andtripodswithout the use of track-

    ing or telescopes; 2) Piggybackcamerasriding atop a tele-

    scope or motorized mount;3) Through-the-scopeimages

    where the telescope acts as thecameras lens.

    First prize in each category is$2,500. Second prize is $1,000. In

    addition to the six prize winners,

    website visitors will choose an

    additional Peoples Choiceaward worth $1,500.

    Winners will be notified beforethe public announcement is made

    in April 2014. Winning images

    will appear in print inAstronomy

    and online at Astronomy.com,Discover.com, and on the NSF

    website at www.nsf.gov/comet.For complete rules, eligibility,

    judging criteria, frequently asked

    questions, and to learn how tosubmit your images, go to www.

    Astronomy.com/ISONphotos.Your picture of Comet ISON

    may turn out to be a once-in-a-lifetime image. Michael E. Bakich

    HOT BYTES >>TRENDING

    TO THE TOP

    SUNSYSTEM TAIL

    Researchers reported July10 that the Suns protectivemagnetic bubble, theheliosphere, has a trailingheliotail that is flatter andbroader than expected.

    GUSHING GAS

    Data from NASAs Spitzertelescope show thatComet ISON emitted 2.2million pounds (1.0 mil-lion kg) of carbon dioxidegas per day in June.

    HABITABLE HOSTS?

    Binary stars might hosthabitable planets if theyorbit each other with aperiod that synchronizedto 15 to 30 days in thesuns first billion years.

    On September 30, 2006, astroimager Gerald Rhemann captured Comet SWAN (C/2006 M4). Can you take a better shot of Comet ISON?

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    utumn brings ever-expanding night. For

    many, this is notpleasant. Hallow-

    een unites classicdarkness terrors with deliberate

    spookiness. And November inmost of the United States and

    Canada is when suddenly bare,twisted branches and stronger

    gusts create a mood of icy men-

    ace. Outdoors, lifes colors andscents vanish. Even the birdspack it up and flee.

    Most astronomers welcomethese expanding eyepiece

    opportunities especiallywhen our sister galaxy, And-

    romeda, is overhead and weget our first peek at the return-

    ing brilliance of Orion andhis friends. But even backyard

    stargazers sometimes look overtheir shoulders when cold winds

    howl. It can be creepy out there.We glorify space travel and

    the notion of human colonies onother worlds. But astronauts will

    tell you: The starry realm is notbenign. Death lurks beyond the

    crew compartments metal walls.An unprotected person acci-

    dentally hurled out a spaceshipsairlock, that common sci-fi

    scene, would stay conscious forno more than 20 seconds. Com-

    posed mostly of water, a humanbody would quickly revert to

    the universes commonest formof H

    2O a block of ice. If sub-

    sequently struck by a meteor-

    oid, the body would shatter intopieces, perhaps dividing itself

    along internal fault structuresdefined by organs. If the expe-

    rience occurred near Jupiter,radiation would sterilize all liv-

    ing tissue. Near white dwarfs orneutron stars, tidal forces would

    rip a skeleton to pieces.

    In Earths vicinity, the side

    of the unprotected astronautsbody facing sunward would

    rapidly heat like a microwave

    to some 250 Fahrenheit (120Celsius), above waters boiling

    point. Searing solar ultravioletrays including the fearsome

    UVC never experienced onEarths surface would deliver

    a painful burn in 20 seconds. A

    three-minute exposure wouldcook skin to carbon. Mean-while, being in a vacuum would

    nullify ones health insurance.The pressure differential would

    force body gases outward; ear-drums and eyeballs would pop.

    We are creatures of Earth.We carry our planet inside us,

    fashioned as we are of terrestrialcomponents like oxygen, which

    alone constitutes 65 percent ofour bodies by mass. The more

    typical stuff of the cosmos hydrogen and helium offers

    no nourishment or support onits own. Fear of the sky thus has

    a rational basis.For some reason, children

    below the age of 2 are rarelyafraid of the dark. But they

    soon make up for this slowstart, since it eventually afflicts

    a majority of kids. If severeenough, a fear is a phobia,

    and if the enemy is darkness,the condition is variously called

    nyctophobia (from the Greekword for night), achluophobia,

    scotophobia, or lygophobia. Theonly phobia with four different

    names, it afflicts 11 percent ofU.S. adults, according to the

    National Institute of MentalHealth. It edges out acrophobia

    (fear of heights) but, in preva-lence, ranks below fear of public

    speaking (glossophobia), fear ofdeath (necrophobia), and fear

    of spiders (arachnophobia). Imnot sure where fear of dropping

    an expensive eyepiece fits in.For kids, its not the actual

    dark that scares them, but itsunseen terrors. Ghosts dont

    emerge until nightfall, andthat shirt slung over a chair

    surrenders its monster shape

    only with the dawn. Even many

    non-phobic adults retain a life-long nocturnal discomfort. The

    cause? Well, most ancient cul-tures feared darkness because

    of tigers and such, since thesepredators often boast superior

    low-light vision. They have the

    edge after nightfall.The strange netherworld of

    dreams doesnt help. Despite

    modern dream labs andsleep experiments, the little we

    know is more mystifying thanclarifying. No one can explain

    why dreams get longer as nightpasses or why early dreams

    relate to recent events whilelater ones tend to be more sur-

    real and mysterious in content.

    Beyond the psychological

    explanations, might night sweats

    stem from actual dangers posedby the starry canopy? Do weharbor genetic memories of

    celestial cataclysms like meteoricdestruction? Probably not. The

    sky has rarely been a sourceof injury. Only one person in

    recorded history was definitelystruck by a meteorite, and that

    was by day. In Sylacauga, Ala-bama, Ann Hodges suffered a

    bruised thigh in 1954 after achondrite came through her roof

    and ricocheted off a radio. Manyothers (in 1992 and in 1996, for

    example) have had close callswhen meteorites plowed into

    their lawns just as they walkedby again, all by day.

    So, away from city lights,the nocturnal realm is gener-

    ally friendly, with many ruralmeadows filled with nothing

    more menacing than garden-destroying deer. A nightly stroll

    under benignly wheeling starsis a low-risk activity. Indeed,

    we astronomers should perhapsquote Shakespeare. It was he, in

    Romeo and Juliet, who foresaw atime when all the world will be

    in love with night. And pay noworship to the garish sun.

    So let the darkness blossom.Nyctophobia begone!

    STRANGEUNIVERSE

    Astronomers must overcome the ancient human

    fear of darkness.

    B Y B O B B E R M A N

    Night sweats

    A

    FROM OUR INBOX

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

    Contact me aboutmy strange universe by visiting

    http://skymanbob.com.

    EVEN BACKYARD STARGAZERS

    SOMETIMES LOOK OVER THEIR SHOULDERS

    WHEN COLD WINDS HOWL.

    In remembrance of a loyal readerMy late husband was a charter subscriber to Astronomywhen it

    was first published. His name was Howard Duncan, and his

    dream was to afford a Meade telescope.He passed away in 2000, without ever realizing that dream,

    but he looked forward toAstronomymagazine every month. Ihavent been able to afford the subscription, but I am always in

    awe of the refinement of the telescopes and the clarity of thephotos that await the seekers in the universe.

    Thank you for giving my late husband so much to look for-ward to! Doris McClellan,Henderson, Nevada

    We welcome your comments at AstronomyLetters, P. O. Box 1612,

    Waukesha, WI 53187; or email to [email protected]. Please

    include your name, city, state, and country. Letters may be edited for

    space and clarity.

    QUANTUMGRAVITY

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    A galacticincubatorCollisions between galax-ies are more of the cosmicnorm than an aberration,but few such interactionshave the visual appealof the pair known col-lectively as Arp 142. Inthis celestial impact, tidalforces unleashed by thegravity of elliptical galaxyNGC 2937 (bottom) distortthe previously normal spi-ral NGC 2936 (just aboveit). The spirals arms anddark dust lanes now splayhaphazardly across thatgalaxys disturbed disk,

    while bright blue knotstrace the sites of ongoingstar formation. Amateurastronomers have dubbedArp 142 the Penguinbecause it resembles theflightless bird (NGC 2936)safeguarding an egg (NGC2937). NASA/ESA/THE HUBBLE HERITAGETEAM STSCI/AURA

    BREAKTHROUGH

    WWW.ASTRONOMY.COM 11

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    ASTRONEWS

    Successfularrival

    Launched 2 4 6 8 10 120

    2

    4

    6

    8

    10

    12

    Operational lifetime (years)

    Numberofmissions

    1960s

    1970s

    1980s

    1990s

    2000s

    2010s

    FASTFACT

    A

    12 ASTRONOMY t NOVEMBER 2013

    ENERGETIC EVENT. For the first time, the Chandra and XMM-Newton telescopes witnessed a planet passing in frontof its parent star and blocking its X-rays, a result published in the August 10 issue of The Astrophysical Journal.

    BRIEFCASE

    ORBITS AND AGES

    Globular cluster 47 Tucanae contains two separate popula-tions of stars, say astronomers in the July 1 issue of The

    Astrophysical Journal Letters. One holds younger, bluer starswith elliptical orbits concentrated near the core; membersof the other are about 100 million years older, redder, and

    have circular orbits farther out from the clusters center.

    tQUASAR SEARCHLIGHTS

    Two teams of scientists used two different quasars theextremely bright centers of active galaxies as backlightsto study unrelated galaxies closer to Earth. A July 5 Sciencepaper describes researchers observations of gas outside agalaxy moving inward to feed star formation. In an August21 Monthly Notices of the Royal Astronomical Societyarticle,astronomers report finding that gas at a galaxys center isjust as rich in oxygen as gas in its outer regions. Both gal-

    axies studied existed some 11 billion years ago.

    tMAGNETIC MOVEMENT

    The Suns visible surface has a temperature of about 5800kelvins, while its outer atmosphere is at least 1 million K;

    what heats this corona is a long-standing question in

    astronomy. Solar scientists from Columbia Universityannounced July 9 at the meeting of the Solar Physics Divi-

    sion of the American Astronomical Society in Bozeman,Montana, that their recent observations suggest magnetic

    Alfvn waves under the Suns surface carry energy beyondthe surface and deposit it into the corona. Liz Kruesi

    25 years agoinAstronomyIn the November 1988issue ofAstronomy, GerritVerschuur wrote A NewYardstick for the Uni-verse about an earlygravitational lens discov-ery. As he described lens-ing, If a distant galaxylies directly behind a

    massive galaxy closer tohome, the light from thedistant galaxy should bebent and refocused bythe interloper.

    The discoverer, Jac-queline Hewitt of theHaystack Observatory,hoped lensing wouldallow scientists to quan-tify dark matter. Scien-tists would not discoverthat dark energy is caus-ing the universes expan-sion to accelerate foranother 10 years.

    10 years agoinAstronomyIn the November 2003issue ofAstronomy,Michael S. Turner com-posed Absurd universeabout how little of thecosmos is regular matter.He based his write-up onthe first results from theWilkinson Microwave

    Anisotropy Probe(WMAP): 0.5 percent reg-ular matter, 33 percentdark matter, and 66 per-cent dark energy.

    In March 2013,WMAPs sensitive succes-sor, Planck, released itsfirst results, and scientistsnow estimate that theuniverse is a little bit lessabsurd, containing 4.9percent regular matter,26.8 percent dark matter,and 68.3 percent darkenergy. Sarah Scoles

    stronomers continually seek observa-

    tions that will support their leadingmodels of galaxy evolution andanswer remaining questions. Based

    on the lack of extremely massive galaxiesfound in the universe, they know some-

    thing must hinder a galaxys star produc-tion to keep it from continuing to grow at a

    vigorous rate. The models suggest ongoingstar formation could be the cause as the

    extreme stellar winds and ultimate explo-sions of massive stars fling molecular gas

    the fuel for future suns out of a gal-axy. But astronomers lacked observational

    proof to support the theory until now.Using the Atacama Large Millimeter/

    submillimeter Array (ALMA) in Chile,astronomers gained their first direct evi-

    dence of molecular gas streaming awayfrom an active region of star formation in

    the heart of the Sculptor Galaxy (NGC 253).

    With ALMAs superb resolution and

    sensitivity, we can clearly see for the firsttime massive concentrations of cold gasbeing jettisoned by expanding shells of

    intense pressure created by young stars,says Alberto Bolatto of the University of

    Maryland, College Park, lead author of theNGC 253 study published July 25 in Nature.

    The amount of gas we measure gives usvery good evidence that some growing gal-

    axies spew out more gas than they take in.In fact, Bolatto and his team determined

    that the total mass-outflow rate is more thanthree times the measured star-formation

    rate. At that pace, NGC 253 would run outof gas in as few as 60 million years. Still,

    future ALMA observations are needed todetermine if the gas completely escapes the

    Sculptor Galaxy to starve future star forma-tion or if it lingers in NGC 253s halo and

    eventually falls back in. Karri Ferron

    ALMA ANSWERS KEY GALAXYEVOLUTION QUESTION

    STARBURST

    STARVATION.By mapping the distri-bution of cold carbonmonoxide gas in NGC253 (from less intenseemissions in red tobrighter radiation inpink), astronomers

    were able to identifythe gas emerging fromthe galaxys centralstar-forming region.Such an outflow couldhinder future star for-mation. ALMA ESO/NAOJ/NRAO/E. ROSOLOWSKY

    A HARD PLANET TO REACH.Mars has been a tempting goal forNASA and other space agencies since the early 1960s.Yet of the 42 missions launched toward Earths neigh-

    bor, fewer than half reached their objective andreturned significant data. The plot shows the results foreach decade. The 2000s saw the most success. Of its six missions, all survived longerthan expected and four Mars Express, Mars Odyssey, Mars ReconnaissanceOrbiter, and Opportunity continue to function. ASTRONOMY: RICHARD TALCOTT AND JAY SMITH

    Every mission inthe 1960s was an

    attempted flyby; alllater flights soughteither to land on or

    orbit the Red Planet.

    TARGET: RED PLANET

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    ASTRONEWS

    WWW.ASTRONOMY.COM 13

    Cassinicatches Earthon camera

    A cloud of dust and gas with threetimes Earths mass is making itsclosest approach to the black holeat the center of the Milky Way, thesupermassive Sagittarius A*,according to a paper in the Febru-ary 1 issue of The AstrophysicalJournal. The cloud, called G2, willnot fall into the black hole, but itwill come so close that extremegravity will stretch it out in a pro-cess termed spaghettification.

    Already, the head of the cloudis pulling away from the tail. Theorbit is taking it around the other

    side of the black hole at more than6 million mph (10 million km/h).The tail soon will follow, as gravityrenders G2 longer and thinnerwith each passing second.

    This chance meeting betweenG2 and Sagittarius A*is a rare

    opportunity for astronomers to geta peek at our surprisingly calmgalactic center. Although the blackhole is 4 million times as massiveas the Sun, no one has ever seen iteat. Black holes are as their namessound black, hole-like butmaterial that approaches themheats up, speeds up, and thenemits radiation that telescopes cansee, as G2 is now doing.

    Luckily for astronomers, thecloud and its encounter will bevisible for a while. The cloud is sostretched that the close approach

    is not a single event, says G2s co-discoverer Stefan Gillessen of theMax Planck Institute for Extrater-restrial Physics in Garching, Ger-many, but rather a process thatextends over a period of at leastone year. S. S.

    QUICK TAKES

    SWITCH HITTER

    Star Tau Botis magnetic fieldflips once a year, 11 times moreoften than the Suns, accordingto a presentation July 4 at the

    National Astronomy Meeting ofthe Royal Astronomical Society.

    tIN THE CLOUDS

    Habitable zones around reddwarfs expand if the planets

    have clouds, which reflect radia-tion and heat to space, says a

    paper in the July 10 issue of TheAstrophysical Journal Letters.

    tMOON MONIKERS

    On July 2, the InternationalAstronomical Union officially

    recognized new names for Plu-tos two smallest moons. P4 andP5 have become Kerberos and

    Styx, respectively.

    tEXTREME MAKEOVER

    Results from NASAs MESSEN-GER mission, published July 4 in

    Nature, show that Mercuryreceived a face-lift 4.04.1 bil-

    lion years ago, when volcanoesresurfaced the planet.

    tROCKET SCIENTISTS

    The European Space Agencyannounced July 9 that Britishcompany Thales will be the

    prime contractor for Euclid, a2020 mission to study dark mat-

    ter and dark energy.

    tBIG BABY

    Scientists found a still-formingstar already 500 times as mas-sive as the Sun the largestknown in our galaxy, says a

    paper forthcoming inAstron-omy & Astrophysics. S. S.

    SAY CHEESE.NASAs Cassinispacecraft snapped this photo ofEarth as seen from Saturns rings.The image shows our planet as itappears from 898 million miles(1.44 billion kilometers) away: apale blue dot dwarfed by Saturnsdark side and its nearby immenserings. This image is one of 323 thatmake up a complete three-colormosaic of the ring system. In NASAsJuly 22 release of the image, theagency stated that this was the firsttime that inhabitants of Earth knewin advance that their planet wasbeing imaged. S. S.

    G2 gas cloud stretched as it

    rounds Milky Ways black hole

    NASA/JPLCALTECH/SPACESCIENCEINSTITUTE

    CLOSE CALL.The cloud of gas and dust called G2 has begun its close encounterwith the black hole at the center of the Milky Way. As the two continue to interact,the black holes gravity will stretch G2 dramatically, as shown in this still from asimulation. ESO/S. GILLESSEN/MPE/MARC SCHARTMANN

    Earth

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    14/8014 ASTRONOMYt NOVEMBER 2013

    For many years, one of

    the biggest stumblingblocks I faced at star

    parties was the confu-sion that arose when-

    ever I tried to show attendeesthe locations of notable stars

    and constellations or the where-abouts of the current telescopic

    target. Except for those standing

    directly behind me, few, if any,could see where I was pointing.

    The laser pointer came to the

    rescue. Here is a small, inexpen-sive device whose bright green

    beam, when directed skyward,illuminates minute dust particles

    in the air. The result is what lookslike a Star Warslight saber that

    seems to stretch all the way to thestars themselves. No sooner did I

    learn of this valuable tool than Iordered one for myself.

    What a difference it madeat star parties! I could now

    point out objects in the sky toan audience of dozens, confi-

    dent that all eyes were directed

    exactly where I wanted. Nextto the telescope, a laser pointer

    has become my most-used starparty accessory.

    As with any innovation,though, there is a downside. In

    the right hands, a laser pointeris a useful device; in the wrong

    hands, it becomes at the veryleast a public nuisance, at worst

    a dangerous weapon.The most common nega-

    tive use comes from seeminglyharmless pranks by those who

    have little or no knowledge ofthe power of even the smallest

    laser pointers. They flash themat each other, at neighborhood

    houses, passing vehicles, evenscreens at movie theaters.

    More serious is the purpose-ful misuse of laser pointers at

    outdoor concerts and athletic

    events. Last year, a visiblyangered Kanye West interrupteda performance to confront an

    audience member who hadaimed a green laser at him. A

    OBSERVINGBASICS B Y G L E N N C H A P L E

    Laser pointer

    safetyfan was arrested at an August

    2012 CardinalsGiants baseballgame after shining his laser at a

    Giants pitcher. The problem isglobal, with spectators flashing

    laser pointers at players duringinternational soccer matches.

    Perhaps the most dangerousmisuse of laser pointers is the

    deliberate targeting of mov-

    ing vehicles and aircraft. LastApril, I received an email fromReserve Deputy Robin Price of

    the Los Angeles County Sher-iff s Department. Laser hits on

    police cruisers and aircraft havebecome a serious problem in

    L.A., recently reaching epidemicproportions. Knowing that laser

    pointers are popular with ama-teur astronomers, he asked me

    to alert the readers ofAstron-

    omyto the crisis and refer them

    to the departments YouTubevideo Laser Strike (http://

    youtu.be/2RrR0Tc1w90).When I first heard of reports

    that a laser pointer could tempo-rarily blind operators of moving

    vehicles or aircraft, I was skepti-cal. How could the beam enter

    their quarter-inch-wide pupiland remain there long enough to

    cause visual impairment? LaserStrike set me straight. The beam

    doesnt have to hit the pilot ordriver directly in the eye. A sim-

    ple glance off a windshield cre-ates a blinding flash that floods

    the vehicle or cockpit. While itstrue that some of these hits are

    accidental, many are intentional.Criminals shine green laser

    pointers at police cruisers andhelicopters as a means of inca-

    pacitating the occupants longenough to make an escape.

    Amateur astronomers cantalone stop the invasion of the

    green laser, but we can help by

    making sure were not part ofthe problem. Star parties should

    not be set up within 2 miles (3kilometers) of an airport, and

    we should never let any attendeetry our laser pointers. Because

    even the most knowledgeableskygazer has mistaken the land-

    ing light of an approaching

    airliner for a bright planet or star(cmon, you knowyouve doneit!), the website Laser Pointer

    Safety (www.laserpointersafety.com) advises us to NEVER

    point directly at a dim orunknown star. Instead, move

    in a circular motion around theobject. When doing the circular

    motion, or when drawing outa constellation, keep the beam

    moving and keep it away fromany stars. The stars may be air-

    craft. As well as allowing us todemonstrate sensible use of laser

    pointers, a star party is a per-

    fect forum for enlightening thecommunity about the hazards

    inherent in the misuse of theseseemingly harmless instruments.

    Besides educating the publicabout the problem, we need

    to report any illegal use oflaser pointers. Laser Pointer

    Safety and the Federal AviationAdministrations Laser Safety

    Initiative (www.faa.gov/about/

    initiatives/lasers) provide guide-lines for reporting such abuse.Both sites, plus the Laser Strike

    video clip, are must-see sourcesfor anyone seeking information

    on laser pointer safety.Questions, comments,

    or suggestions? Email me [email protected]. Next

    month: my December 2012Prime 9 sky objects revisited.

    Clear skies!

    FROM OUR INBOX

    Cosmosinduces awe and horror

    Your story about Carl Sagan brought back memories. I am a

    great grandmother now, but I remember watching each episodeof Cosmos. When Sagan explained how we are all made of star

    stuff, I got goose bumps. It was an idea I had not heard before.

    I read the bookCosmos, too, and it sent me off in multipledirections of study. It really opened the universe to me andchanged the way I thought about everything. I have all of Sagans

    books and have read them more than once. He left us way toosoon. Ann Freeman,Cheyenne, Wyoming

    I enjoyed the article about Carl Sagan and the television pro-

    gram Cosmos. However, I noted with horror that the goldplaques sent along with the Pioneer probes include Pluto as

    a planet. Does NASA have any plans to send a correction?

    Tom Dempsey,Chadds Ford, Pennsylvania

    BROWSE THE OBSERVING BASICS ARCHIVE AT www.Astronomy.com/Chaple.

    When not used properly, geen laser pointers turned skyward can flood an airplane orhelicopter cockpit with blinding light. COURTESY LASD

    Know how to properly use this

    helpful star party tool to avoidharming others.

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    ASTRONEWS

    WWW.ASTRONOMY.COM 17

    Astronomers find thata planet is truly blueAstronomers have been able to determine themasses, sizes, orbits, and atmospheric composi-tions of planets in other solar systems for years.But on July 11, NASA announced that for thefirst time, they know what the true color of aplanet is: HD 189733b, located 63 light-yearsaway, is cobalt blue.

    Although its hue may make the planetappear Earth-like, it is anything but. Its surfacereaches 2000 Fahrenheit (1090 Celsius), and a4,500 mph (7,200 km/h) wind tosses tiny dropsof glass around. These glass raindrops scatterblue light more than red light, a phenomenonthat gives the world its tint. S. S.

    CERULEAN SPHERE.Researchers used the HubbleSpace Telescope to discover that the exoplanet HD189733b is blue. This is the first time they havededuced the color of another world. NASA/ESA/G. BACON STSCI

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    18/8018 ASTRONOMY t NOVEMBER 2013

    This November, go outafter sunset and look

    in the western sky forDeneb, the Alpha ()star of Cygnus the

    Swan. If youre under dark skies

    with the Milky Way blazing, lookfor the patch of light that con-

    tains the emission nebula NGC

    7000, more commonly known asthe North America Nebula.

    This 2 by 1.5 island of misty

    radiance is an enormous region

    filled with Hydrogen-II plasma charged hydrogen found in

    areas of recent star formation. Itis some 2,000 light-years distant

    and contains the fainter PelicanNebula (IC 5070). The complex

    lies 3 east of Deneb, just north-west of 4th-magnitude Xi ()Cygni. Ultraviolet radiation from

    a massive O-type star strips elec-trons from the local hydrogenatoms. But after these electrons

    recombine with the atoms, theyemit Hydrogen-alpha light,

    which appears red. You seemostly this light when you look

    at the North America Nebula.NGC 7000s radiance resem-

    bles its continental namesakeonly by coincidence: Fortu-

    nately placed intervening dust

    clouds, which block emission,create its shape and outline it indarkness. The most prominent

    cloud appears to trace the east-ern seaboard before curling

    into the Gulf of Mexico like theletter J.

    Commonly thought of as aphotographic object, the nebula

    resides in a star cloud that is afine binocular target. Some

    keen-eyed observers even havedetected the nebula itself with-

    out optical aid or have they?When I was younger, I, too,

    used to challenge myself to see

    SECRETSKY

    A continent

    withinThe North America Nebula is a

    celestial continent embeddedinside another continent.

    the North America Nebula, andsee it I did or so I thought.

    When I moved to the dark skiesof Hawaii, I realized my mistake.

    Pseudo-continent

    What I thought for years was theNorth America Nebula actually

    was the larger star cloud. Inimages, this complex appears

    divided into at least four seg-ments because of its size. It mim-

    ics the shape and orientation ofNGC 7000, but the real North

    America Nebula lies within,forming a wedge of starlight

    between the star clouds pseudo-Bangor, -Houston, and -Miami.

    In the pseudo-continent,

    Deneb sits near the position ofHalifax, Nova Scotia. The entirewestern edge of NGC 7000

    marks the Gulf Coast, from thesouthern tip of Texas to Tampa,

    Florida, and beyond. The darkgulf itself embays Xi Cygni,

    while the star 68 Cygni, 2 to theeast, lies near the position of

    Mexico City. The West Coast ofthe United States runs 4 to the

    northwest, where the star 63Cygni lies on the border of

    Washington state and British

    Columbia. The pseudo-NorthAmerica measures a healthy 5across, making it more than two

    times larger in extent than thereal North America Nebula.

    The real dealWhat we see unaided at NGC

    7000s location is mostly a vaststar cloud atop the fainter neb-

    ula. When stars crowd together,they appear fuzzy, as the eye has

    difficulty resolving the multi-

    tude; thats why the naked-eyeMilky Way looks, well, milky.

    Its hard to tell how much ofNGC 7000s nebulosity we see, if

    any. To find the real nebula, look

    for an obvious wedge of brightMilky Way in the aforemen-tioned location. First identify

    Deneb and Xi Cygni, and thenlook between these stars for an

    isolated triangular patch ofMilky Way. The dark nebulae in

    the region will help you separateit from the rest of the surround-

    ing stars. Remember to useaverted vision. Staring at it for a

    prolonged time (which youreusually told to avoid) enhances

    dark regions as your eyesbecome fatigued.Viewing through small bin-

    oculars under dark skies helpsbring out the true continental

    form quite strikingly. Still, itsdifficult to tell whether youre

    seeing nebulosity or just con-gested starlight.

    The use of a nebula filter willmake the ghostly glow stand out,

    albeit faintly. The view changesdramatically through larger bin-

    oculars, however, when NGC

    7000s brightest regions standout independently of the sur-rounding patch of starlight,

    etched ever so cleanly by riversof darkness. This view holds true

    if you switch to looking at thenebula through a telescope with

    a wide field of view. But considergoing naked-eye first!

    As always, let me know whatyou see (or what you dont see)

    at [email protected].

    B Y S T E P H E N J A M E S O M E A R APseudo-North America Nebula

    BROWSE THE SECRET SKY ARCHIVE AT www.Astronomy.com/OMeara.

    The real North America Nebula (NGC 7000) shows its coasts prominently and stands outfrom the surrounding stars, but only in a photographic view. ADAM BLOCK/NOAO/AURA/NSF

    After kids inNew Delhi find aspace rock, pub-

    lic educatorSachin Bahmba

    says, Indian stu-dents have

    beaten studentsall over the

    world in asteroiddiscoveries. Butwhos counting?

    Keeping score

    COSMIC WORLDA look at the best and the worst that astronomy and

    space science have to offer. by Sarah ScolesCold asspace

    Supernovahot

    Grand CentralPublishing

    releases TheAstronaut Wives

    Club, the storyof the women

    married to Mer-cury 7 astro-

    nauts, a.k.a. thereal housewives

    of low Earthorbit.

    Science sorority

    Venus windspeeds shot upfrom 186 to 249mph (300 to 400

    km/h) duringthe Venus

    Express mission.The planet mayhave watchedone too many

    episodes ofBreaking Bad.

    Amped up

    PlanetaryResources (PR)

    nets $1.5 millionon Kickstarterfor the crowd-

    controlled Arkydtelescope. Awe-

    some! Exceptthat PRs inves-tors are worth50,000 times

    that. Dont tell.

    Cashteroid

    ESAKEEPINGSCORE;NASACASHTEROID;ESA/MPS/DLR/IDAAMPEDUP;NASASCIENCESORORITY

    NGC 7000

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    ASTRONEWS

    S/2004 N1

    Galatea

    Despina

    Larissa

    20 ASTRONOMY t NOVEMBER 2013

    Six years ago, astronomers detected a bril-liant burst of radio waves that lasted a few

    milliseconds, but they couldnt associate theblast with a specific object. In the July 5 issue

    of Science, Dan Thornton of the Universityof Manchester and colleagues reported see-

    ing four other mysterious radio bursts.These had similar energies to the 2007 dis-

    covery and also appeared just once. Whilethe researchers still dont know what causes

    such an intense signal, they can now say for

    sure that the first detection wasnt a fluke.In the most recent study, scientists were

    scanning the sky with the 64-meter Parkes

    radio telescope in New South Wales, Aus-tralia. They were looking specifically for the

    rapidly spinning compact remnants of oncemassive stars, called neutron stars, and

    other transient radio sources. The fournewly observed bursts occurred in the high

    latitude of the Southern Hemisphere, some40 south of the Milky Ways disk.

    To determine how far away these fourbursts originated, the astronomers analyzed

    the signals. As radio radiation travelsthrough gas and dust, it bounces off elec-trons, which smears the signal. These four

    bursts emissions showed such a smearing,indicating the bursts were from space and

    not from Earth. But because they didntoccur in the galaxys disk (which is where

    material in the Milky Way is densest), the

    astronomers think these blasts originatedfrom outside the Milky Way. They calculated

    how much material the radiation had totravel through space outside the galaxy

    also contains gas and dust to get to the

    Parkes telescope on Earth, and that corre-sponds to the bursts distances: betweenabout 5.5 and 10.5 billion light-years.

    In just a few milliseconds, each of thesefour blasts released more energy than the

    amount the Sun puts out in 3,000 years.Narrowing down whats causing these blasts,

    however, has been difficult. Thorntons teamdidnt find any gamma-ray, X-ray, or optical

    radiation at the same positions as the radiobursts. The astronomers say in their Science

    paper that the most convincing cause of theradio bursts involves extremely magnetic

    neutron stars called magnetars. However,future all-sky radio surveys set to come

    online in the next decade should helpastronomers find many more such radio

    blasts, which will allow them to figure outwhat causes them. Thornton and colleagues

    suggest that 1,000 of these bursts occuracross the sky every day. L. K.

    SPACE SCIENCE UPDATE

    MYSTERIOUSRADIO BURSTSSPOTTED

    Scientist discovers new Neptune moonSMALL SATELLITE.The most distantplanets family has grown by one member,

    as NASA announced July 15 the discoveryof a 14th moon orbiting Neptune.The small satellite, which scientists

    designate S/2004 N1, showed up on archi-val photos from the Hubble Space Tele-scope. While studying Neptunes ringsegments, SETI Institutes Mark Showalternoticed a white dot about 65,400 miles(105,250 kilometers) from the planet thatappeared in more than 150 images takenbetween 2004 and 2009. By plotting itschanging position, he found that it com-pletes a revolution every 23 hours.

    Scientists estimate that the new moonis no more than 12 miles (19km) across,making it the smallest of Neptunes knownsatellites. K. F.

    BRIGHT BLAST.Astronomers using the Parkes64-meter radio telescope detected four extremelyluminous radio bursts one such blast is at theupper left of this photo illustration. They arent surewhat causes these signals, which might originatesome 11 billion light-years away.

    NASA/ESA/M.

    SHOWALTERSETIINSTITUTE

    SWINBURNEASTRONOMYPRODUCTIONS

    ROLLING STONE. Martian meteorite NW 5298 arrived 20 million years4 billion years later than scientists thought, according to the July 25 Natu

    Rings

    Rings

    Thalassa

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    Electronneutrino

    Muonneutrino

    Tauneutrino

    ie i+

    io

    WWW.ASTRONOMY.COM 21

    Particle conversion confirmedGhostly low-mass neutrinos interact weakly withother matter (they can pass through Earth withoutcolliding with other mass). These particles come inthree types: one associated with the electron andtwo associated with the electrons more massivecousins, the muon and tau. Some 15 years ago, sci-entists found that neutrinos could morph from onetype to another, a process called oscillation. And onJuly 19 at the European Physical Society meeting inStockholm, physicists with the Tokai to KamiokaExperiment announced the direct observation and

    confirmation that muon neutrinoshad oscillated into electron neutrinos.

    They sent beams of muon neutrinosfrom Tokai on the east coast of Japan to theSuper-Kamiokande underground detector locatedat Kamioka on the countrys west coast, some 183miles (295 kilometers) away. The team detected 28electron neutrinos, whereas it had expected about4.6. The random chance of the scientists finding somany more of these particles is less than one in 1trillion, which confirms it as a discovery. L. K.

    Scientists first discovered neutrinos in 1956. Sixyears later, they found that nature creates mul-

    tiple types, or flavors, of these particles.

    FASTFACT

    FLAVOR FLIP.Ghostly low-mass neutrinos come in three

    flavors associated with threeheavier particles: electrons,muons, and taus. Scientistsannounced in July that theydirectly observed muon neutri-nos morph into electron ones.

    ASTRONOMY:ROENK

    ELLY

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    FOLLOWASTRONOMY

    O B S E R V I N G T O O L S

    C O M M U N I T Y

    22 ASTRONOMYt NOVEMBER 2013

    Whats new at Astronomy.com.BY KARRI FERRON

    Year of the Comet

    Reader Photo GalleryBrowse thousands of beautiful astroimages like this oneof Omega Centauri by Samuele Gasparini. Submit yourown images at www.Astronomy.com/readergallery .

    Community Events

    Find star parties or lectures in your area or post an invite to your astron-omy club event at www.Astronomy.com/events. Its a great way to getto know others who share your passion about our night sky or to introduceyour local community to the hobby.

    StarDomeAstronomys interactive star chart is a great tool for planning your night ofobserving or for simply checking when a specific planet or deep-skyobject will appear in your sky. Customize the map for the time of day, andStarDome will show you what constellations, planets, comets, and moreare visible from your location. The program holds a database exceeding

    2,500 objects, including each targets celestial coordinates and currentmagnitude. Get acquainted with the night sky or plan your next observingsession at www.Astronomy.com/stardome.

    A dazzling comet can ignite a viewers passion better thanalmost any other celestial object. And those flames will burn

    bright this year as 2013 promises what could be the most magnifi-cent comet of the 21st century thus far. As Comet ISON (C/2012 S1)

    makes its closest approach to the Sun on November 28,it could shine at a brilliant magnitude 4.5 equalingthe brightness of Venus.

    To make sure readers have all the best observinginformation and can experience the beauty of thecomet for themselves and through astroimagers moststunning photographs and videos, Astronomys editorsare devoting a whole section of Astronomy.com to the

    Year of the Comet. Head to www.Astronomy.com/ISONfor findercharts, images, observatory news, prediction updates, observing tips,

    videos, and more of Comet ISON as it makes its first visit to the inner solarsystem. And we want your involvement to help usmake the most of this potentially historic event. Ifyou have images, observing reports, planned star

    parties, or suggested updates to share about CometISON, send them to Senior Editor Michael E. Bakich [email protected]. This fall should be a greattime for amateur astronomy.

    Your home for Comet ISON information

    www.twitter.com/AstronomyMag

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    REGISTER TODAY!Go to www.Astronomy.com/registerfor access to bonus articles, photos, videos, and more.

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    MOBILE UPDATESScan the code to access the

    latest news and observing infofrom your mobile device.

    COMETIMAGES:NASA/ESA/

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    Comets capture our attention today just as they did inages past even if the reasons are perhaps different.A bright comet makes people gaze into the heavens inwonder, and that ability has not dwindled in the currentera of television, smartphones, and the Internet. If any-thing, these technological advances make it easier forpeople to learn about comets and see them.

    These objects have drawn both the scrutiny and fascination ofmankind for thousands of years, and our understanding of cometshas evolved over that time. What weve discovered about cometarycompositions and origin locations is surprisingly recent, and thisknowledge has advanced tremendously with the launch of space-

    craft to study them up close.

    Ancient historyWhile the first proto-humans who started paying attention to theheavens undoubtedly noticed comets, one of the oldest survivingrecords dates to around 300 .. A silk unearthed from a HanDynasty tomb at Mawangdui in Changsha, China, displays stylizeddrawings of 29 comets, implying that Chinese astronomers hadbeen carefully noting comet appearances for hundreds of yearsbefore documenting them on this fabric (see p. 26). Over the ensu-ing centuries, nearly every culture around the world recordedcomets. While most surviving records lack the details of modernobservations, they continue to be useful to us in linking comets of

    today with those of the past.Historically, observers did not know the nature of comets, and

    thus their unexpected appearances often were considered harbin-gers of doom. The unusual motions of comets across the sky devi-ated from the predictable patterns of the stars and planets, so many

    Scientists have sent 12 missions to explore comets. Heres what the

    believed theseobjects were atmo-spheric phenomena.As the scientific methoddeveloped, careful observations even-tually revealed that comets orbit the Sun.Further study yielded an improved understand-ing of their compositions and behaviors, but thefirst comprehensive and correct model of comets didnot arrive until the middle of the 20th century. Not until thepast few decades, when spacecraft actually visited comets, have webeen able to confirm their true natures.

    Dirty snowballsIn 1950, Fred Whipple of Harvard University in Cambridge, Mas-sachusetts, proposed that comets are icy conglomerates, or dirtysnowballs. The basic idea is that a chunk of ice and dust thenucleus approaches the Sun, and the rising temperaturescause ice near the surface to turn directly into gas, or sublimate.The escaping gases drag small grains of dust off the nucleus tocreate a diffuse cloud of material, called the coma, surroundingthe nucleus. The Suns wind of particles and radiation sweeps away

    Matthew Knight researches comets primarily using Lowell Observatorys

    42-inch Hall Telescope and its new 4.3-meter Discovery Channel Telescope,

    both located at dark-sky sites near Flagstaff, Arizona.

    Planetary science THE

    COMETISSUE

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    arned about these relics from the early solar system.by Matthew Knight

    Since 1985,nine spacecrafthave visited sevencomets, providingscientists a tremendousamount of informationabout these objects.ASTRONOMY: ROEN KELLY

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    between the present distances of Jupiter and Pluto, so at least 5astronomical units from the Sun (1 AU is the average Earth-Sundistance). Comets also must have resided beyond this distanceuntil recently for the ices to survive.

    The giant planets appear to have migrated early in the solar sys-tems lifetime, scattering the smaller objects, including the proto-comets. Gravitational interactions flung some into the inner solarsystem, and a substantial number impacted Mercury, Venus, Earth,Mars, and the Moon. We can still see the record of this Late Heavy

    Bombardment in the increased number of impact craters on theMoon that date to between 3.8 and 4.1 billion years ago. Othersmall objects remained in the plane of the solar system but werepushed outward beyond Neptune. These objects have remained instable orbits between 30 AU and 50 AU in what is now known asthe Kuiper Belt, the most famous member of which is Pluto.

    The remaining proto-comets moved outward to greater dis-tances from the Sun. Many had enough velocity to escape our starsgravity and now drift between stars as interstellar comets. Thosemoving just a bit too slow to escape ended up in randomly inclinedorbits at the farthest reaches of the solar system. They make up ashell of icy bodies some 2,000 AU to 100,000 AU from the Sunknown as the Oort Cloud. While we havent observed an OortCloud object at these huge distances, the statistics of comet orbitsclearly indicate a population of trillions of frozen objects. The proto-

    comets perturbed with even less energy were scattered into orbitsstretching 10s to 100s of AU with a large range of inclinations. Wehave discovered a handful of these scattered disk objects (SDOs),also called hot Kuiper Belt objects, notably the dwarf planet Eris.

    Collectively, we call the icy objects orbiting beyond the eighthplanet trans-Neptunian objects (TNOs). A gravitational pertur-bation such as a passing star or galactic tides may occasion-ally dislodge a TNO from its long, frigid orbit and move it closer tothe Sun. If it reaches small enough distances, the Suns radiationwill cause its ices to sublimate and the object may become observ-able from Earth and thus be discovered as a comet.

    We now recognize distinct classes of comets whose membershave similar orbital characteristics and compositions. These groups

    trace back to different regions of origin and therefore give us clues tothe conditions in the solar nebula out of which the planets formed.

    Best known of these classes is the Jupiter family comets (JFCs);Hartley is a member. These objects typically have low-inclinationorbits with periods of less than 20 years. JFCs are recent interlopersinto the inner solar system from the scattered disk, and Jupitersgravity trapped them into small orbits that extend to near thatplanet. JFCs show more surface evolution from solar radiation thanother comet classes, which reflects the increased time they havespent in the inner solar system.

    Centaurs, icy bodies in orbits between Jupiter and Neptune, arerelated to JFCs. These originated in the scattered disk, but theirorbits have not evolved enough to reach the inner solar system, so

    Fred Whipple proposed that comets are con-glomerates of ice and dust. Modern observationscontinue to support his theory 63 years later.

    In 1950, Jan Oort suggested that comets originate from a cloud extending to perhaps 150,000 timesthe Earth-Sun distance. This Oort Cloud, as astronomers now call it, is still thought to be a major cometreservoir, even though it hasnt been directly observed yet. THE LEIDEN OBSERVATORY

    FAYFOTO,

    BOSTON/AIPEMILIOSEGRVISUALARCHIVES

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    they are intermediaries between SDOs and JFCs. Centaurs are icy,but they never get close enough to the Sun for the ices to sublimate,so these objects generally appear inactive.

    The remaining classes of comets resided in the Oort Cloud untilsome other object perturbed them, f linging them into the innersolar system. Long-period comets (LPCs) like the one currentlymaking headlines, Comet ISON (C/2012 S1) have orbits withrandom inclinations and periods spanning thousands to millionsof years. Many LPCs that reach the planetary region for the firsttime are so weakly bound to the Sun to begin with that they areejected from the solar system by their gravitational interactionswith the giant planets. Occasionally, interactions with the planets

    perturb an LPC into a smallerorbit spanning thousands ofyears; this is the case withComet Hale-Bopp (C/1995

    O1). This type of comet willtravel well beyond the KuiperBelt at its farthest distancefrom the Sun but will notreturn to the Oort Cloud.With the right series of per-turbations, some LPCs orbitalperiods shorten dramatically,

    to decades or centuries, and become Halley-type comets namedfor (you guessed it!) the first comet shown to have an orbit thatbrings it by the Sun on a set period, Comet Halley.

    When is a comet not a comet?To distinguish comets and asteroids, we typically define the formeras active, diffuse, and/or having a tail, while the latter appearpointlike and inert. Modern observations, however, have revealed anumber of objects that blur the line between comet and asteroid.One example is a class called main-belt comets (MBCs), which areobjects on classically asteroidal orbits that exhibit cometary char-acteristics. We think impacting bodies on these objects create thecomet-like behavior in some cases by exposing small regions ofsubsurface ice, which leads to activity analogous to that seen intraditional comets; in a few cases, the impacts might generateshort-lived clouds of dust that look like comas. In both cases,MBCs appear to have formed with the rest of the asteroids andnot from the comet reservoirs described earlier.

    We also know of apparent asteroidal objects on comet-likeorbits. Many are likely dead or dormant comets that have becomeinactive due to the complete loss of volatile ices at or near the sur-

    face, or from dust that built up to prevent sunlight from reachingthe ices below. Because their small sizes and dark surfaces makethem hard to detect when far away, dormant comets are foundmost frequently among the population of asteroids that come closeto our planets orbit, known as near-Earth objects. However, theylikely exist throughout the solar system.

    Another surprising discovery of the past couple decades is theexistence of many thousands of small sungrazing objects. Theseare on highly eccentric orbits that pass extremely close to the Sun just a few solar radii and are generally too faint to see withouta space-based solar telescope. Research has shown that most ofthese objects are fragments of larger known comets. However, wehave been unable to link some to any known comets, which raises

    the possibility that they are actually asteroids that have been per-turbed into sungrazing orbits. Due to the extreme radiation envi-ronment close to the Sun, rocks and metals that are ordinarily

    Scientists have sent 12 missions to investigate comet nuclei. Heres how someof those dirty snowballs compare. NASA/JPL/UMD 9P/TEMPEL; NASA/JPL 19P/BORRELLY; NASA/JPLCALTECH 81P/WILD; ESA/MPAE 1P/HALLEY; NASA/JPLCALTECH/UMD 103P/HARTLEY

    Even as comets

    can be a threat to lifetoday, they may have

    helped life take hold

    on ancient Earth.

    The European Space Agencys Giotto probe was the first to photograph a comet nucleus up close. In 1986, it imaged Comet 1P/Halley at a closest approach ofjust 370 miles (596 kilometers). Four other spacecraft visited the comet that same year. ESA/MPAE LINDAU

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    extremely stable in space will sublimate, and thus even an asteroid

    completely devoid of ices might appear cometary.

    Facilitators of life and deathComets eccentric orbits often cross Earths path, and while thechance of any given comet colliding with our planet is remote,given enough time and enough comets, collisions inevitably occur.No such collisions have happened in recorded history, but the Feb-ruary 2013 fireball over Chelyabinsk, Russia, serves as a reminderof the destruction that such an impact could cause. Preliminaryresearch suggests that the meteoroid was initially 60 feet (18meters) wide about the size of a house. An impact by a typicalcometary nucleus would cause orders of magnitude more damage.In fact, the impact of a comet or asteroid about 6 miles (10km)

    across a rather average size for a comet appears to havecaused the extinction of the dinosaurs around 65 million years ago.

    Impacts are an unavoidable part of living in the solar system,but fortunately theres little chance of a devastating one. Small dustgrains collide with our planet all the time; we observe these asshooting stars in the night sky as they burn up in Earths atmo-sphere. Impacts by objects the size of the Chelyabinsk meteoroidoccur much less often, probably every few decades. Collisions withobjects large enough to cause significant damage across a state-sized region are even more infrequent, likely millions of years.

    Yet even as comets can be a threat to life today, they may havehelped life take hold on ancient Earth. This is another of the pri-mary reasons we study them. Comets carry many of the basic

    ingredients we think are necessary for life in the form of ices anddust grains containing large amounts of carbon, hydrogen, oxy-gen, and nitrogen called CHON particles. During the LateHeavy Bombardment, the rain of small rocky objects, includingcomets, onto an early Earth delivered large quantities of thesematerials and likely brought enough water to fill our oceans.Comets may have helped transform a dry, barren planet into theblue marble we now occupy. Consequently, continued investiga-tions of comets are highly relevant to the burgeoning study ofplanets around stars other than the Sun as we seek the holy grailof astronomy: life beyond Earth.

    We continue to research comets to answer the mystery of how lifegot to Earth and to learn valuable clues about how the solar systemformed and evolved. While we now understand far more about com-ets than ever before, that knowledge does not diminish their gran-deur. And if comets happen to put on a show while teaching us moreabout our place in the cosmos, then thats just icing on the cake.

    VISIT www.Astronomy.com/tocTO WATCH A VIDEO OF WHAT THE EPOXI SPACECRAFT SAW AS IT FLEW BY COME T 103P/HARTLEY.

    The surface of Comet 9P/Tempel exhibits a scar from the Deep Impact probes2005 collision. The image at left shows the comets surface as the probeneared its final destination, while the image at right was captured more thansix years later as the Stardust-NExT (New Exploration of Tempel) craft flew bythe comet. NASA/JPLCALTECH/UNIVERSITY OF MARYLAND/CORNELL

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    Early comet chroniclesObservations of comets extend far into recorded history. The oldestaccounts are inscribed on the oracle bones of ancient China. Thiscollection contains 160,000 bones and turtle shells dating back to

    the Shang Dynasty, which existed from the 16th to 11th centuries.. Some mention hui, the Chinese term indicating a comet with atail. One particular bone references appeasing a queen: By makinga sacrifice to Pi-keng, will the comet disappear? Another bone callsattention to the changing shape of a strange comet and asks thequestion: Will it bring darkness upon us?

    This fear of comets inspired the Chinese to catalog them. His-torical texts contain details such as the date of a comets firstappearance, the constellations through which it moved, its charac-teristics, and the date of its final visibility. Although this a ll wouldprove valuable to future astronomers, the Chinese were working in

    the name of astrology, tying celestial to earthly events, especiallywars, plagues, and the deaths of rulers.

    Science versus superstitionThe ancient Greeks were first to try to explain comets scientifically.The mathematician Pythagoras (ca. 570495 ..), for instance, saidonly one comet existed. This object, he thought, was a planet in acircular orbit that occasionally brought it into view near the hori-zon. Philosophers Anaxagoras (ca. 500428 ..) and Democritus(ca. 460370 ..) said comets were close conjunctions of planets.Astronomer Hippocrates of Chios (ca. 470410 ..) believed, likehis contemporaries, that comets were planets, but he said the tailswere moisture drawn from Earth to comets and reflecting sunlight.

    Aristotle (384322 ..), however, did not care for any of thesetheories. He believed the heavens were perfect, with the planets

    By the time Comet Lovejoy (C/2011 W3) appeared late in 2011, people did not consider comets to be harbingers of disaster. It swung just 0.0056 AU from the Sun,peaked at magnitude 2.9, and, unsurprisingly, caused no damage to Earth. WAYNE ENGLAND

    TO SCIENCE

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    moving in circular orbits along the ecliptic, the yearly path the Suntraces across our sky. Comets were not planets, he said. They couldmaterialize anywherein the sky, and planetary conjunctions didntalways result in comets. Aristotle suggested that exhalations rosefrom Earth into the atmosphere, where they ignited. If it extendsequally in all directions, he said, it is called a comet or long-hairedstar, if it extends lengthwise only it is called a bearded star.

    A few other Greek and Roman philosophers offered competingtheories during the next 400 years. Seneca (4 .... 65), for

    example, pointed out that wind did not affect comets, which there-fore could not be in the atmosphere. But so respected was Aristotlethat his view, though groundless, held sway for some 2,000 years.

    With the Aristotelian views entrenched, phi-losophers hardly debated what comets were andinstead discussed what they represented. Manypeople, including the Roman poet Virgil (7019..), believed comets were warnings. Upon thedeath of Julius Caesar (44 ..), Virgil wrote,Never did fatal comets shine more often. TheGreco-Roman astronomer Ptolemy (ca. ..100170) agreed with Aristotle that comets wereearthly phenomena and with Virgil that they

    were portents. In his astrological work, Tetra-biblos, he said that comets positions and orien-tations point out the regions or places liable tobe affected by the events which they threaten;and the form of the signs indicates the qualityand nature of those events, as well as the genus,class, or kind, on which the effect will fall.

    Sky scapegoatDuring the next several centuries, people con-tinued to deem significant comets responsiblefor negative events, and scientists of the age

    made few rebuttals. Several Roman writers said the comet of 12 ..(Halleys Comet) marked the death of General Marcus VipsaniusAgrippa. Next, the comet of .. 54 supposedly killed the Romanemperor Claudius Caesar. The subsequent appearance of CometHalley in .. 66 foretold Jerusalems destruction in .. 70, whileanother visit from Halley in 451 announced the death of Attila theHun in 453. A comet in 565 forecast Byzantine emperor Justinianthe Greats demise that year. And the Nuremberg Chronicleevenblamed Halleys presence in 684 for three months of heavy rain and

    lightning that led to the deaths of people, animals, and crops.Perhaps the most noted cometary appearance of the Middle

    Ages was Comet Halleys in 1066. It popped up April 1 andremained visible until June 7. The Chinesemeticulously described its movement, while theItalian text Ex Regula Canonicorumlikened it toan eclipsed moon whose tail rose like smokehalfway to the zenith. The famous Bayeux Tap-estry illustrates the events leading up to theNorman conquest of England later in 1066,depicting the comet as a bad omen for KingHarold. It appeared almost three months afterhe became king, and he died at the Battle of

    Hastings in October of that year. Chroniclersdrew a link between Halleys presence and Har-olds demise, a negative astrological viewpointthe likes of which continued during the nextcenturies as bright comets came and went andbad events continued to transpire on Earth.

    The superstitious views of this epoch wereunaffected by an apparent sungrazing comet the Great Comet of 1106 (X/1106 C1) whose arrival did not correspond to a majorearthly upheaval. The comet was visible inbroad daylight about a degree from the Sun on

    Aristotle believed comets were atmospheric phenomena, an idea that reigned for 2,000 years despite

    the evidence against it. The philosopher thought of the universe as it appears above in Figure of theHeavenly Bodies (1568). A comet in a highly elliptical orbit did not fit within this worldview.BARTOLOMEU VELHO

    The earliest recorded observations of comets areinscribed on bones and shells dating back from the16th to 11th centuries B.C.The Chinese wrote detailed

    descriptions of comets positions, shapes, and visiblelifespans to determine how the heavenly visitorsnegatively affected the course of history.

    In the late 13th century, Peter of Limo-ges used a torquetum, illustrated here,to measure the position of the Cometof 1299 (C/1299 B1). A user couldadjust this instruments settings toaccount for latitude and date and then

    measure a celestial objects altitudeagainst a plumb line.

    MUSEUMO

    FTHEINSTITUTEOFHISTORYANDPHILOLOGY/ACADEMIASINICA

    ASTRONOMICUMC

    AESAREUM

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    Comet

    Viewpoint A Viewpoint B

    Sun

    Background stars

    EarthA

    EarthB

    Line

    ofs

    ight

    from

    Earth

    WWW.ASTRONOMY.COM 33

    February 2, 1106, and had moved to the western sky by February 10.Once again, the Chinese, who were meticulous but only interestedin astrological thought, provided most of the details. The Armenian

    text Chronicle, which author Matheos Urhayetzi finished around1137, said, An awful, big and amazing comet appeared, whichfrightened those who saw it. Wise people said that it is a king sign,and that this year a king should be born who will conquer all ofmankind. Nothing came of this prognostication.

    Some semblance of scientific thought began to return in the 13thcentury. Aegidius of Lessines (ca. 12301304) made observations ofthe Great Comet of 1264 (C/1264 N1), reporting that it was first inthe evening sky but as the days passed dropped into twilight anddisappeared. He said the same comet then appeared in the earlymorning sky. As simple as the inference may seem, Aegidius real-ization that the two sightings were a single comet was revelatory.

    But pseudoscience was still winning the battle against science.

    Peter of Limoges (died 1306) wrote a treatise on Comet C/1299 B1,providing the first measurements of position. He made them with atorquetum, a medieval astronomical instrument. Although he wasscientific, Peter of Limoges said Venus and Jupiter seemed to beinfluencing the comet, which may mitigate the ill effects of thecomet, or God may show mercy, if we live justly.

    While Peter of Limoges may have taken observation a step for-ward, interpretation remained firmly in its backward place, aswritings about Comet C/1402 D1 show. It arrived in the sky in earlyFebruary 1402 and appeared over Europe and Asia. It moved to thewestern sky, displaying a tail that curved toward the zenith. TheWelsh text Chronicle of Adam of Uskdescribed the comet as

    dreadful and said it forecast the death of the duke of Milan,providing no scientific input. Although astronomers made moreanalytical observations as time went on, the myth that comets pos-sessed undue influence over Earth persisted.

    Back to science classDuring the 15th century, two observers attempted to determine thedistances to comets. Austrian astronomer Georg von Peuerbach

    (14231461) measured a comets parallax, or how much the back-ground behind it changed when he observed it from different loca-tions. The more the background changes, the higher the parallax isand the closer the object. Two position measurements separated byseveral hours allowed Peuerbach to calculate the distance to CometHalley (1456), which many thought was a heavenly comment onConstantinoples fall in 1453. His measurements incorrectly indi-cated the comet was in the atmosphere, as did those of his studentRegiomontanus (14361476). He measured the parallax of CometC/1471 Y1 and found the comet to be nine Earth radii away, a dis-tance he thought placed it in the atmosphere. People continued toperceive comets to be Earth-centered phenomena.

    The next big breakthrough relied on observation but did not

    prove comets were in space. Peter Apian (14951552) outlined themotion of the Comet of 1531, which happened to be another returnof Halley. His graphic was the first to illustrate how a comets tailalways points away from the Sun.

    Peter Apian used a torquetum to measure Comet Halleys position at variouspoints during 1531, making him the first to demonstrate that a comets tailalways points away from the Sun. THE GALILEO PROJECT

    By observing how a nearby objects position against a background appears tochange with the location of the observer, astronomers can determine how faraway that object is. The more the background changes, the higher the paral-lax is and the closer the object. Georg von Peuerbach first used parallax toattempt to find the distance to a comet in the 15th century. ASTRONOMY:ROEN KELLY

    Tycho Brahe, whoseobservatory is illus-trated on this pagefrom a 17th-centuryatlas of Amsterdam,was the first toaccurately measurethe parallax of acomet, prove that itwas far from Earthsatmosphere, andsuggest that it was

    in an elliptical orbitaround the Sun.WILLIAM BLAEU

    Parallax measurements

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    Visible to the naked eye

    Visible with binoculars

    Visible with a telescope

    MARTIN RATCLIFFEand ALISTER LINGdescribe thesolar systems changing landscape as it appears in Earths sky.

    November 2013:ISON at its brightest

    The month of the comet

    has finally arrived. The

    long-awaited predawnappearance of Comet

    ISON (C/2012 S1),potentially one of the bright-est such objects in many

    decades, no doubt will leavemany observers bleary-eyedas they go about their normal

    daytime tasks.The solar systems