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Home , Caliban (moon), Hale Telescope, Hill sphere, Irregular moon, Sycorax (moon)

{ PLANETARY SCIENCE }

Vagabond

MOON-SIZE FRAGMENTS FLY as bodies in the collide. Some pieces will drift inward to become planetary satellites. DAN DURDA

46 /// may 05 Around each gas-giant dozens of small moons born elsewhere. These far-ranging bodies are beginning to give scientists clues to the dark corners of the . /// BY DAN DURDA AND DOUG HAMILTON

Late on a rainy Saturday evening in early October 1997, Brett Gladman carefully searched the star- filled CCD images he had recently helped a team of planetary scientists collect. The star fields surrounded the planet , and Gladman’s elusive quarry were new uran- Mountain in . They ian moons, small objects orbit- were hunting objects in the ing far from their giant Kuiper Belt, a band of small, icy blue-green parent and bound bodies extending outward from only weakly to it. ’s orbit, 30 times far- Other had ther from the than . sought new moons in the envi- The 200-inch Hale , rons of the gas-giant completed in 1948, was once the before, but their searches had world’s largest, but numerous 8- been sporadic and new discov- and 10-meter have eries were few. This time, how- long since eclipsed it. The vener- ever, Gladman spotted almost able instrument, however, still immediately two faint dots had some surprises in store. moving relative to the back- The first hour of the night

ground stars at nearly the same was unusable for the astron- COLUMBIA BRITISH OF UNIVERSITY GLADMAN, BRETT rate as Uranus. Subsequent omers’ Kuiper Belt program -FINDING, PART 1. Searching for faint moons around bright planets observations confirmed the because the star-crowded Milky resembles hunting fireflies near a bright lamp. In this mosaic of 72 seach objects, now named Way was passing overhead. So frames (above), lies outside the field of view at bottom. Some- and , were indeed new the team focused its attention, where within these images lurks jovian moon S/2003 J21, circled in the . and the telescope, on two handy three discovery frames below. His discoveries were the first targets: Uranus and Neptune. irregular satellites found using a Nicholson reasoned that even ground-based telescope in near- though they could search only a ly a quarter-century. (Irregular small region around each plan- moons are those having large, et, Palomar’s modern CCD highly eccentric, and often camera would capture objects 3

highly inclined, .) to 4 magnitudes fainter than the COLUMBIA BRITISH OF UNIVERSITY GLADMAN, BRETT The discoveries came from previous best attempts. an astronomical fishing expedi- Those were the images in tion. A team of astronomers which Gladman spotted Caliban including Gladman, Phil and Sycorax. His initial excite- Nicholson, and Joe Burns, all ment, however, was tempered then at , were with a fair amount of skepticism observing with the 200-inch when he told his collaborators (5-meter) telescope on Palomar about the result — not all BRETT GLADMAN, UNIVERSITY OF BRITISH COLUMBIA BRITISH OF UNIVERSITY GLADMAN, BRETT

MOON-FINDING, PART 2. Discovering moons means sifting through dozens 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.

www.astronomy.com 47 EVERY GAS-GIANT PLANET’S rules a volume of space called the , where the planet’s gravity is stronger than the JUPITER’S HILL SPHERE Sun’s. The size of the sphere (shown here to scale, with planet sizes enlarged 100 times) URANUS grows with distance from the Sun as solar gravity weakens. ASTRONOMY: ROEN KELLY

discoveries prove out in the end. However, the objects’ motion in the well as the wave of subsequent firefly. When images, plus calculations findings. Replacing photo- viewed from a by Nicholson and Brian graphic film with CCDs great distance, Marsden of the International (charge-coupled devices) radi- however, the light Astronomical Union’s Minor cally improved the sensitivity of from the planet and Planet Center, paid off when the astronomical observations. The the satellite cannot be team returned to the telescope recent advent of large CCD separated easily. Instead, later that . arrays with tens of millions of the bright object’s glare over- The two new moons were The next satellite discovered, pixels has made possible system- whelms the faint one because right where expected, slowly Saturn’s , was glimpsed atic mapping of large regions of they lie close together. swooping around the distant first by Christiaan Huygens with sky. Ground-based satellite Despite this, searches for planet. The discovery of two an improved telescope of his searches today use 10,000 satellites work best from large new uranian moons sparked a own design. Ever-increasing x10,000-pixel CCD arrays capa- distances. The person near the flurry of satellite hunts and telescope sizes, the use of photo- ble of covering a quarter- lamp must look in all directions finds over the next 6 years. By graphic film, and other techno- square-degree patch of sky to find nearby fireflies, while a now, these have pushed the logical improvements allowed (roughly equivalent to the Full more distant observer needs to number of known moons in the fainter and fainter satellites to be Moon) in a single exposure. look only in its general direction solar system to 140. spotted. This culminated in the Searches for satellites from to see fireflies circling the lamp. last ground-based detection of a the ground neatly dovetail with Similarly, telescopes on Mooning the planets small , Jupiter’s space-based ones. Where the lat- Earth need to map only a small The search for planetary satel- , in 1974. In the 1980s and ter are superior at finding small region of sky around a planet to lites began in 1610, when Galileo 1990s, spacecraft brought satellites close to their planets, find new moons rather than the Galilei announced the discovery advanced instruments to each of the former are better suited for full sky a spacecraft must search. of four large moons orbiting the giant planets in turn and efficiently searching regions far- In addition, large-aperture tele- Jupiter: , , , revealed a whole new class of ther away. scopes on Earth are immensely and . New discoveries moonlets circling close to their The reasons are geometric. A more powerful than the much often come from improvements parent planets in and among spacecraft flying close to a plan- smaller ones carried by space- in technology; indeed, the dis- planetary rings. et can focus its cameras on a craft. Better instruments and a covery of the four Galilean New technology drove the potential satellite while keeping smaller area of sky to search satellites directly resulted from discovery of new satellites by the much brighter planet out of from Earth more than make up the invention of the telescope. Gladman and his colleagues, as the field of view, much as a per- for the advantage in proximity son standing near a bright lamp that a spacecraft enjoys. Dan Durda (Southwest Research Institute, Boulder, Colorado) and Doug can turn away to spot a nearby But how much sky must be Hamilton (University of Maryland) are planetary scientists specializing in searched? The answer lies in wandering objects such as moons and — two objects that may — and the have lots in common, it turns out. work of American SATURN .

NEPTUNE JUPITER

URANUS SEEN FROM EARTH, Jupiter’s Hill sphere looms much bigger than Neptune’s — even though the latter extends more than twice as far from the planet in actual distance. ASTRONOMY: ROEN KELLY cles the Sun). These NEPTUNE techniques also outline the region around a planet where a moon can keep a stable orbit. Today, astronomers call this region the Hill NASA/JPL/SSI sphere. Far down inside a planet’s Hill sphere, a moon’s OBJECTS THAT HAVE LIVED a knockabout life usually wear their past on their faces, and is no exception. The heavy cratering it orbital motion is dominated by shows testifies to many billions of years of impacts as Phoebe jour- the planet’s gravitational influ- neyed from the Kuiper Belt, where scientists think it formed, to its ence. The Sun’s gravity is still present orbit among Saturn’s collection of moons. present, but its pull is minor compared to the more powerful Spheres of influence tug of the much closer planet. Searching space though. The moons of far-off Hill grew up on a farm in West Moons deep in a planet’s Hill As seen from Earth, the areas of planets shine more faintly and Nyack, New York. As a student sphere have stable orbits, while sky covered by the Hill spheres are more difficult to spot than of mathematical astronomy at those orbiting farther out feel of Jupiter, Saturn, Uranus, and those nearby because the sun- Rutgers College in the late more of the Sun’s gravitational Neptune are 48, 22, 6, and 7 light they reflect drops off rap- 1850s, he studied the classic influence and less of the plan- square degrees, respectively. idly with increasing distance works of celestial mechanics. et’s. Eventually, there comes a These are small compared to the from the Sun. For example, a These included Pierre-Simon point — the edge of the Hill 41,000-square-degree area of 1-mile-diameter moon orbiting Laplace’s Méchanique Céleste, sphere — where the Sun’s effect the full sky that an in situ space- Jupiter appears as bright as a 4- Joseph Louis Lagrange’s begins to dominate. Beyond this craft would have to search. mile moon at Saturn, a 16-mile Méchanique Analytique, and distance, a moon cannot stably But ground-based observers moon at Uranus, and a 36-mile what was perhaps the most orbit the planet. Instead, it will still have to stitch together a lot moon at Neptune. influential book in Hill’s young slip away and follow its own of quarter-square-degree images Of the 75 new moons dis- career, Charles-Eugènes path around the Sun. to cover a planet’s entire Hill covered over the last several Delaunay’s Théorie du The size of a planet’s Hill sphere. This is one practical rea- years, most are smaller than Mouvement de la Lune. sphere depends both on the son why Gladman’s team about 6 miles across. The largest While on the staff of the planet’s and its distance focused its initial satellite of the new satellites is 120-mile- American and from the Sun. A large planet like searches on the neighborhoods diameter (190 kilometer) Nautical Almanac,Hill worked Jupiter reigns gravitationally of Uranus and Neptune rather Sycorax orbiting Uranus, while for many years on accurately supreme over a vast region of than Jupiter and Saturn. the smallest new satellites, spot- calculating the Moon’s orbit and space, while a small planet such Investigating the most dis- ted at Jupiter, are barely half a its long-term stability. There, he as has a much lesser tant planets comes with a price, mile across. developed mathematical tech- sphere of influence. For planets niques for describing orbital with equal , those closer motions in a system of three to the Sun have smaller Hill NASA/JPL/SSI bodies (for example, a moon spheres than those orbiting orbiting a planet that itself cir- farther away.

CLOSE UP ON PHOEBE, Cassini’s camera imaged a battered landscape. Scientists are studying the moon’s crater population for clues to Phoebe’s history.

www.astronomy.com 49 Jupiter JUPITER’S MOONS wrap the planet in an ever-shifting orbital network. The orbits in red go retrograde — clockwise as seen here — unlike the regular moons (other colors) that orbit in the same direction Jupiter rotates. The innermost (pink) orbits belong to Io, Europa, NASA/JPL/GSFC NASA/JPL/UNIV. ARIZONA NASA/JPL/UNIV. Ganymede, and Callisto. PHOEBE’S SURFACE displays a COLD DOMINATES the surface ASTRONOMY: ROEN KELLY/SCOTT SHEPPARD, CIW variety of materials as mapped of Phoebe, which has a peak 10,000,000 miles by Cassini’s spectrometers. of 107 kelvins Exposures of water ice appear (–267° Fahrenheit), shown in Saturn blue, carbonaceous material is white. The coldest temperature red, and a second carbona- Cassini detected (purple) was ceous “mystery material” (not found on the nightside: 75 THE SATURNIAN IRREGU- yet identified) appears green. kelvins, or –324° F. LAR MOONS belong to several groups (coded by color). The outer- Patterns in the sky among these vagabond moons most pink orbit is that The flurry of new satellite dis- subsequently thinned their of . Note the large gap between coveries (75 in all) reveals some numbers and left the planets Iapetus and the inner- patterns that planetary scientists with orbiting fragments for most irregular moons. are using to probe the outer scientists to study today. solar system’s history. For example, at Jupiter, all Nearly two-thirds of the but one of the newly found 10,000,000 miles known moons, including all the irregular objects orbit retro- recent discoveries, are irregular grade. At Saturn, however, dis- Uranus satellites. These orbit far from tant prograde and retrograde their planets along highly tilted, moons appear in roughly equal THE IRREGULAR MOONS OF elliptical paths. (Many irregulars numbers. Furthermore, their URANUS weave a loose also orbit retrograde, opposite orbital tilts are not random: pattern around the regu- the direction followed by the They’re grouped into associa- lar moons, shown in planets themselves.) tions that imply a common pink. Although Uranus In particular, the orbital dis- origin for a group’s members. has only a sixth the mass of Saturn, its tributions of Jupiter and The associations resemble moon swarm reaches Saturn’s irregular moons hint at the minor-planet families in the about as far into space past interactions and collisions. main belt, which are because the Sun’s gravity When the giant planets were the remains of large parent bod- is weaker there. forming, dense envelopes of gas ies that broke apart ages ago. 10,000,000 miles surrounded each, feeding the Another pattern is that none growing bodies. Asteroidal of the 75 new moons circles a objects born elsewhere in the terrestrial planet. (One search of Neptune solar system flew closely past the Mars’ Hill sphere would have NEPTUNE’S MOONS include 7 embryonic planets. Encounter- found any undiscovered inner vagabonds and 6 regulars ing the gas, they were slowed by satellite bigger than 100 yards close to the planet. The friction, and at least some across.) This comes as no sur- small number comes became captured. Collisions prise. Terrestrial planets orbit from search difficulties caused by Neptune’s distance from Earth. Its Hill sphere spans more than twice Jupiter’s, and Neptune orbits near the Kuiper Belt of icy planetes- 10,000,000 miles imals. It could have many undiscovered moons.

50 astronomy /// may 05 the Sun more closely than the year “walkabout” through the gas giants (thus reducing their inner solar system. Hill spheres’ extent), and even the most massive terrestrial Close-up views planet (Earth) has only 7 per- To scientists, the wayward cent the mass of the lightest gas moons of the outer solar system giant (Uranus). Small masses have long been just featureless plus solar proximity equals points of light circling distant small Hill spheres. planets. Yet this picture began to NASA/JPL/SSI Yet the real picture is a little change June 11, 2004, when the GEOLOGICAL DETAILS APPEAR in this 8-mile-diameter (13 km) crater more complicated. Terrestrial Cassini spacecraft, en route to on Phoebe — building-size boulders litter the crater’s floor, while planets actually can have a kind arrive at Saturn 3 later, bright splashes mark recent impacts and landslides. Scientists hope of vagabond moon. Recent the- flew past the planet’s tiny, irreg- such details will help them better understand Kuiper Belt objects. oretical and computational ular moon Phoebe. work has turned up asteroids Cassini’s instruments that are not currently satellites revealed a unique and alien Some craters came from The New Horizons mission of the terrestrial planets — but world, potato-shaped and heavi- passing through the to and the Kuiper Belt is which have been in the past and ly crater-pocked. Dark organic saturnian system, while others scheduled to launch in January will be again in the future. Most material overlies a buried icy may have come from material in 2006. In February 2007, the of these are natural objects, layer partly exposed to view orbit around Saturn. The five spacecraft will attempt a close asteroids that wandered into the inside deep-penetrating craters. additional small moonlets in flyby of one of Jupiter’s small inner solar system and were Cassini spotted bright streaks Phoebe’s family came from at irregular satellites. If successful, temporarily captured. and rays associated with craters, least one large impact, evidence this will bring into close-up At least one, however, is and found long grooves and of which may still be visible on focus another distant point of almost certainly artificial. In other linear features. Phoebe’s surface. What fraction light in the sky. September 2002, amateur Phoebe’s density is 1.6 times of the moonlets created in such The irregular moons of the observer Bill Yeung discovered that of water, a low value imply- a titanic event came back one giant planets are the hoboes of an unusual, fast-moving object ing large amounts of ice and, — billions of years later — the Hill sphere — vagabonds near Earth that appeared to be perhaps, a somewhat porous to reimpact its parent satellite? that come and go as dictated by orbiting our planet about twice exterior. Oddly (and probably How many will do so in the dis- gravity. With origins far from as far away as the Moon. coincidentally), Phoebe’s rota- tant future? their present homes, they At first, it seemed J002E3 tion period, 9 hours and 16 Cassini won’t return to sur- intrigue scientists with the mes- was the only known case of a minutes, gives it a day not much vey Phoebe again — it lies too sages they bring from distant small asteroid captured into different than Saturn’s. far from the spacecraft’s trajec- parts of the solar system. orbit around Earth. But analysis Scientists looking to unravel tory for the rest of the mission. Studying them in detail will give of its pre-capture orbit and the origins of irregular satellites But scientists studying irregular us a better look at how the solar-’s effects are focusing closely on Phoebe’s moons have another target in planetary system we call our on its trajectory proved that crater population. These scars mind for an upcoming mission. own was born. X J002E3 is manmade. It’s the S- from ancient impacts give a IVB third stage from Apollo 12, glimpse into this mysterious Find out how planetary scientists study vagabond moons and their origins at www.astronomy.com/toc which returned home after a 33- object’s violent past.

/// MOONS, VAGABOND AND OTHERWISE

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

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