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MOONS

MOON-SIZE FRAGMENTS FLY as bodies in theKuiper Belt collide. Some pieces will drift inwardto become planetary satellites. DAN DURDA

Vagabond

{ PLANETARY SCIENCE }

Around each gas-giant planet orbit dozens of small moons born elsewhere.These far-ranging bodies are beginning to give scientists clues to the darkcorners of the solar system. / / / BY DAN DURDA AND DOUG HAMILTON

elusive quarry were new uran-ian moons, small objects orbit-ing far from their giantblue-green parent and boundonly weakly to it.

Other astronomers hadsought new moons in the envi-rons of the gas-giant planetsbefore, but their searches hadbeen sporadic and new discov-eries were few. This time, how-ever, Gladman spotted almostimmediately two faint dotsmoving relative to the back-ground stars at nearly the samerate as Uranus. Subsequentobservations confirmed theobjects, now named Sycoraxand Caliban, were indeed newmoons of Uranus.

His discoveries were the firstirregular satellites found using aground-based telescope in near-ly a quarter-century. (Irregularmoons are those having large,highly eccentric, and oftenhighly inclined, orbits.)

The discoveries came froman astronomical fishing expedi-tion. A team of astronomersincluding Gladman, PhilNicholson, and Joe Burns, allthen at Cornell University, wereobserving with the 200-inch (5-meter) telescope on Palomar

Mountain in California. Theywere hunting objects in theKuiper Belt, a band of small, icybodies extending outward fromNeptune’s orbit, 30 times far-ther from the Sun than Earth.

The 200-inch Hale telescope,completed in 1948, was once theworld’s largest, but numerous 8-and 10-meter telescopes havelong since eclipsed it. The vener-able instrument, however, stillhad some surprises in store.

The first hour of the nightwas unusable for the astron-omers’ Kuiper Belt programbecause the star-crowded MilkyWay was passing overhead. Sothe team focused its attention,and the telescope, on two handytargets: Uranus and Neptune.Nicholson reasoned that eventhough they could search only asmall region around each plan-et, Palomar’s modern CCDcamera would capture objects 3to 4 magnitudes fainter than theprevious best attempts.

Those were the images inwhich Gladman spotted Calibanand Sycorax. His initial excite-ment, however, was temperedwith a fair amount of skepticismwhen he told his collaboratorsabout the result — not all

MOON-FINDING, PART 2. Discovering moons means sifting throughdozens of long-exposure images looking for tiny specks of light mov-ing in just the right way. Here, moon-finder Brett Gladman (left) con-fers with codiscoverer Lynne Allen.

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Late on a rainy Saturday evening in early October1997, Brett Gladman carefully searched the star-filled CCD images he had recently helped a teamof planetary scientists collect. The star fieldssurrounded the planet Uranus, and Gladman’s

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MOON-FINDING, PART 1. Searching for faint moons around bright planetsresembles hunting fireflies near a bright lamp. In this mosaic of 72 seachframes (above), Jupiter lies outside the field of view at bottom. Some-where within these images lurks jovian moon S/2003 J21, circled in thethree discovery frames below.

discoveries prove out inthe end. However, theobjects’ motion in theimages, plus calculationsby Nicholson and BrianMarsden of the InternationalAstronomical Union’s MinorPlanet Center, paid off when theteam returned to the telescopelater that month.

The two new moons wereright where expected, slowlyswooping around the distantplanet. The discovery of twonew uranian moons sparked aflurry of satellite hunts andfinds over the next 6 years. Bynow, these have pushed thenumber of known moons in thesolar system to 140.

Mooning the planetsThe search for planetary satel-lites began in 1610, when GalileoGalilei announced the discoveryof four large moons orbitingJupiter: Io, Europa, Ganymede,and Callisto. New discoveriesoften come from improvementsin technology; indeed, the dis-covery of the four Galileansatellites directly resulted fromthe invention of the telescope.

The next satellite discovered,Saturn’s Titan, was glimpsedfirst by Christiaan Huygens withan improved telescope of hisown design. Ever-increasing telescope sizes, the use of photo-graphic film, and other techno-logical improvements allowedfainter and fainter satellites to bespotted. This culminated in thelast ground-based detection of asmall irregular moon, Jupiter’sLeda, in 1974. In the 1980s and1990s, spacecraft broughtadvanced instruments to each ofthe giant planets in turn andrevealed a whole new class ofmoonlets circling close to theirparent planets in and amongplanetary rings.

New technology drove thediscovery of new satellites byGladman and his colleagues, as

well as the wave of subsequentfindings. Replacing photo-

graphic film with CCDs(charge-coupled devices) radi-cally improved the sensitivity ofastronomical observations. Therecent advent of large CCDarrays with tens of millions ofpixels has made possible system-atic mapping of large regions ofsky. Ground-based satellitesearches today use 10,000x10,000-pixel CCD arrays capa-ble of covering a quarter-square-degree patch of sky(roughly equivalent to the FullMoon) in a single exposure.

Searches for satellites fromthe ground neatly dovetail withspace-based ones. Where the lat-ter are superior at finding smallsatellites close to their planets,the former are better suited forefficiently searching regions far-ther away.

The reasons are geometric. Aspacecraft flying close to a plan-et can focus its cameras on apotential satellite while keepingthe much brighter planet out ofthe field of view, much as a per-son standing near a bright lampcan turn away to spot a nearby

firefly. Whenviewed from agreat distance,however, the lightfrom the planet andthe satellite cannot beseparated easily. Instead,the bright object’s glare over-whelms the faint one becausethey lie close together.

Despite this, searches forsatellites work best from largedistances. The person near thelamp must look in all directionsto find nearby fireflies, while amore distant observer needs tolook only in its general directionto see fireflies circling the lamp.

Similarly, telescopes onEarth need to map only a smallregion of sky around a planet tofind new moons rather than thefull sky a spacecraft must search.In addition, large-aperture tele-scopes on Earth are immenselymore powerful than the muchsmaller ones carried by space-craft. Better instruments and asmaller area of sky to searchfrom Earth more than make upfor the advantage in proximitythat a spacecraft enjoys.

But how much sky must besearched? The answer lies incelestial mechanics — and thework of American astronomerGeorge William Hill.

SATURN

JUPITER’S HILL SPHEREURANUS

NEPTUNEJUPITER

EVERY GAS-GIANT PLANET’S GRAVITYrules a volume of space called the Hill sphere,where the planet’s gravity is stronger than theSun’s. The size of the sphere (shown here toscale, with planet sizes enlarged 100 times)grows with distance from the Sun as solargravity weakens. ASTRONOMY: ROEN KELLY

SEEN FROM EARTH, Jupiter’s Hill spherelooms much bigger than Neptune’s — eventhough the latter extends more than twice asfar from the planet in actual distance.ASTRONOMY: ROEN KELLY

URANUS

Dan Durda (Southwest Research Institute, Boulder, Colorado) and DougHamilton (University of Maryland) are planetary scientists specializing inwandering objects such as moons and asteroids — two objects that mayhave lots in common, it turns out.

SATURN

Spheres of influence Hill grew up on a farm in WestNyack, New York. As a studentof mathematical astronomy atRutgers College in the late1850s, he studied the classicworks of celestial mechanics.These included Pierre-SimonLaplace’s Méchanique Céleste,Joseph Louis Lagrange’sMéchanique Analytique, andwhat was perhaps the mostinfluential book in Hill’s youngcareer, Charles-EugènesDelaunay’s Théorie du Mouvement de la Lune.

While on the staff of theAmerican Ephemeris andNautical Almanac, Hill workedfor many years on accuratelycalculating the Moon’s orbit andits long-term stability. There, hedeveloped mathematical tech-niques for describing orbitalmotions in a system of threebodies (for example, a moonorbiting a planet that itself cir-

cles the Sun). Thesetechniques also outlinethe region around aplanet where a mooncan keep a stable orbit.

Today, astronomerscall this region the Hill

sphere. Far down inside aplanet’s Hill sphere, a moon’s

orbital motion is dominated bythe planet’s gravitational influ-ence. The Sun’s gravity is stillpresent, but its pull is minorcompared to the more powerfultug of the much closer planet.

Moons deep in a planet’s Hillsphere have stable orbits, whilethose orbiting farther out feelmore of the Sun’s gravitationalinfluence and less of the plan-et’s. Eventually, there comes apoint — the edge of the Hillsphere — where the Sun’s effectbegins to dominate. Beyond thisdistance, a moon cannot stablyorbit the planet. Instead, it willslip away and follow its ownpath around the Sun.

The size of a planet’s Hillsphere depends both on theplanet’s mass and its distancefrom the Sun. A large planet likeJupiter reigns gravitationallysupreme over a vast region ofspace, while a small planet suchas Mars has a much lessersphere of influence. For planetswith equal masses, those closerto the Sun have smaller Hillspheres than those orbitingfarther away.

Searching spaceAs seen from Earth, the areas ofsky covered by the Hill spheresof Jupiter, Saturn, Uranus, andNeptune are 48, 22, 6, and 7square degrees, respectively.These are small compared to the41,000-square-degree area ofthe full sky that an in situ space-craft would have to search.

But ground-based observersstill have to stitch together a lotof quarter-square-degree imagesto cover a planet’s entire Hillsphere. This is one practical rea-son why Gladman’s teamfocused its initial satellitesearches on the neighborhoodsof Uranus and Neptune ratherthan Jupiter and Saturn.

Investigating the most dis-tant planets comes with a price,

though. The moons of far-offplanets shine more faintly andare more difficult to spot thanthose nearby because the sun-light they reflect drops off rap-idly with increasing distancefrom the Sun. For example, a1-mile-diameter moon orbitingJupiter appears as bright as a 4-mile moon at Saturn, a 16-milemoon at Uranus, and a 36-milemoon at Neptune.

Of the 75 new moons dis-covered over the last severalyears, most are smaller thanabout 6 miles across. The largestof the new satellites is 120-mile-diameter (190 kilometer)Sycorax orbiting Uranus, whilethe smallest new satellites, spot-ted at Jupiter, are barely half amile across.

CLOSE UP ON PHOEBE, Cassini’s cameraimaged a battered landscape. Scientists are studying the moon’s crater populationfor clues to Phoebe’s history.

OBJECTS THAT HAVE LIVED a knockabout life usually wear their paston their faces, and Phoebe is no exception. The heavy cratering itshows testifies to many billions of years of impacts as Phoebe jour-neyed from the Kuiper Belt, where scientists think it formed, to itspresent orbit among Saturn’s collection of moons.

NEPTUNE

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Patterns in the skyThe flurry of new satellite dis-coveries (75 in all) reveals somepatterns that planetary scientistsare using to probe the outersolar system’s history.

Nearly two-thirds of theknown moons, including all therecent discoveries, are irregularsatellites. These orbit far fromtheir planets along highly tilted,elliptical paths. (Many irregularsalso orbit retrograde, oppositethe direction followed by theplanets themselves.)

In particular, the orbital dis-tributions of Jupiter andSaturn’s irregular moons hint atpast interactions and collisions.When the giant planets wereforming, dense envelopes of gassurrounded each, feeding thegrowing bodies. Asteroidalobjects born elsewhere in thesolar system flew closely past theembryonic planets. Encounter-ing the gas, they were slowed byfriction, and at least somebecame captured. Collisions

among these vagabond moonssubsequently thinned theirnumbers and left the planetswith orbiting fragments for scientists to study today.

For example, at Jupiter, allbut one of the newly foundirregular objects orbit retro-grade. At Saturn, however, dis-tant prograde and retrogrademoons appear in roughly equalnumbers. Furthermore, theirorbital tilts are not random:They’re grouped into associa-tions that imply a common origin for a group’s members.

The associations resemblethe minor-planet families in themain asteroid belt, which arethe remains of large parent bod-ies that broke apart ages ago.

Another pattern is that noneof the 75 new moons circles aterrestrial planet. (One search ofMars’ Hill sphere would havefound any undiscovered innersatellite bigger than 100 yardsacross.) This comes as no sur-prise. Terrestrial planets orbit

JUPITER’S MOONS wrap theplanet in an ever-shifting

orbital network. The orbitsin red go retrograde —clockwise as seen here— unlike the regularmoons (other colors)that orbit in the same

direction Jupiter rotates.The innermost (pink)

orbits belong to Io, Europa,Ganymede, and Callisto.

ASTRONOMY: ROEN KELLY/SCOTT SHEPPARD, CIW

THE SATURNIAN IRREGU-LAR MOONS belong toseveral groups (codedby color). The outer-most pink orbit is thatof Iapetus. Note thelarge gap between

Iapetus and the inner-most irregular moons.

THE IRREGULAR MOONS OFURANUS weave a loose

pattern around the regu-lar moons, shown inpink. Although Uranushas only a sixth themass of Saturn, itsmoon swarm reaches

about as far into spacebecause the Sun’s gravity

is weaker there.

NEPTUNE’S MOONS include 7vagabonds and 6 regulars

close to the planet. Thesmall number comesfrom search difficultiescaused by Neptune’sdistance from Earth. ItsHill sphere spans more

than twice Jupiter’s, andNeptune orbits near the

Kuiper Belt of icy planetes-imals. It could have many

undiscovered moons.

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PHOEBE’S SURFACE displays avariety of materials as mappedby Cassini’s spectrometers.Exposures of water ice appearblue, carbonaceous material isred, and a second carbona-ceous “mystery material” (notyet identified) appears green.

COLD DOMINATES the surfaceof Phoebe, which has a peaktemperature of 107 kelvins(–267° Fahrenheit), shown inwhite. The coldest temperatureCassini detected (purple) wasfound on the nightside: 75kelvins, or –324° F.

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the Sun more closely than thegas giants (thus reducing theirHill spheres’ extent), and eventhe most massive terrestrialplanet (Earth) has only 7 per-cent the mass of the lightest gasgiant (Uranus). Small massesplus solar proximity equalssmall Hill spheres.

Yet the real picture is a littlemore complicated. Terrestrialplanets actually can have a kindof vagabond moon. Recent the-oretical and computationalwork has turned up asteroidsthat are not currently satellitesof the terrestrial planets — butwhich have been in the past andwill be again in the future. Mostof these are natural objects,asteroids that wandered into theinner solar system and weretemporarily captured.

At least one, however, isalmost certainly artificial. InSeptember 2002, amateurobserver Bill Yeung discoveredan unusual, fast-moving objectnear Earth that appeared to beorbiting our planet about twiceas far away as the Moon.

At first, it seemed J002E3was the only known case of asmall asteroid captured intoorbit around Earth. But analysisof its pre-capture orbit andsolar-radiation pressure’s effectson its trajectory proved thatJ002E3 is manmade. It’s the S-IVB third stage from Apollo 12,which returned home after a 33-

year “walkabout” through theinner solar system.

Close-up viewsTo scientists, the waywardmoons of the outer solar systemhave long been just featurelesspoints of light circling distantplanets. Yet this picture began tochange June 11, 2004, when theCassini spacecraft, en route toarrive at Saturn 3 weeks later,flew past the planet’s tiny, irreg-ular moon Phoebe.

Cassini’s instrumentsrevealed a unique and alienworld, potato-shaped and heavi-ly crater-pocked. Dark organicmaterial overlies a buried icylayer partly exposed to viewinside deep-penetrating craters.Cassini spotted bright streaksand rays associated with craters,and found long grooves andother linear features.

Phoebe’s density is 1.6 timesthat of water, a low value imply-ing large amounts of ice and,perhaps, a somewhat porousexterior. Oddly (and probablycoincidentally), Phoebe’s rota-tion period, 9 hours and 16minutes, gives it a day not muchdifferent than Saturn’s.

Scientists looking to unravelthe origins of irregular satellitesare focusing closely on Phoebe’scrater population. These scarsfrom ancient impacts give aglimpse into this mysteriousobject’s violent past.

Some craters came fromcomets passing through the saturnian system, while othersmay have come from material inorbit around Saturn. The fiveadditional small moonlets inPhoebe’s family came from atleast one large impact, evidenceof which may still be visible onPhoebe’s surface. What fractionof the moonlets created in sucha titanic event came back oneday — billions of years later —to reimpact its parent satellite?How many will do so in the dis-tant future?

Cassini won’t return to sur-vey Phoebe again — it lies toofar from the spacecraft’s trajec-tory for the rest of the mission.But scientists studying irregularmoons have another target inmind for an upcoming mission.

The New Horizons missionto Pluto and the Kuiper Belt isscheduled to launch in January2006. In February 2007, thespacecraft will attempt a closeflyby of one of Jupiter’s smallirregular satellites. If successful,this will bring into close-upfocus another distant point oflight in the sky.

The irregular moons of thegiant planets are the hoboes ofthe Hill sphere — vagabondsthat come and go as dictated bygravity. With origins far fromtheir present homes, theyintrigue scientists with the mes-sages they bring from distantparts of the solar system.Studying them in detail will giveus a better look at how the planetary system we call ourown was born. X

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GEOLOGICAL DETAILS APPEAR in this 8-mile-diameter (13 km) crateron Phoebe — building-size boulders litter the crater’s floor, whilebright splashes mark recent impacts and landslides. Scientists hopesuch details will help them better understand Kuiper Belt objects.

PLANET REGULAR MOONS IRREGULAR MOONS TOTAL MOONSJupiter 8 55 63Saturn 19 14 33Uranus 18 9 27Neptune 6 7 13

/// MOONS, VAGABOND AND OTHERWISE

Find out how planetary scientists study vagabond moons andtheir origins at www.astronomy.com/toc

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