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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
Division of Astronomical Sciences
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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
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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.
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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
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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
or call 1-800-533-6644
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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10/8010 ASTRONOMYt NOVEMBER 2013
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
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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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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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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)
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
www.facebook.com/AstronomyMagazine
REGISTER TODAY!Go to www.Astronomy.com/registerfor access to bonus articles, photos, videos, and more.
WEBTALK
MOBILE UPDATESScan the code to access the
latest news and observing infofrom your mobile device.
COMETIMAGES:NASA/ESA/
THEHUBBLEHERITAGETEAM
STSCI/AURA
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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
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AESAREUM
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Viewpoint A Viewpoint B
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EarthB
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ight
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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