{ Planetary Science }

{ PLANETARY SCIENCE } Vagabond MOONS MOON-SIZE FRAGMENTS FLY as bodies in the Kuiper Belt collide. Some pieces will drift inward to become planetary satellites. DAN DURDA 46 astronomy /// may 05 Around each gas-giant planet orbit dozens of small moons born elsewhere. These far-ranging bodies are beginning to give scientists clues to the dark corners of the solar system. /// 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 Uranus, and Gladman’s elusive quarry were new uran- Mountain in California. 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. Neptune’s orbit, 30 times far- Other astronomers had ther from the Sun than Earth. sought new moons in the envi- The 200-inch Hale telescope, rons of the gas-giant planets 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 telescopes 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 MOON-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 Sycorax Way was passing overhead. So frames (above), Jupiter lies outside the field of view at bottom. Some- and Caliban, were indeed new the team focused its attention, where within these images lurks jovian moon S/2003 J21, circled in the moons of Uranus. 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, orbits.) 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 Cornell University, 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 moving 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 GRAVITY rules a volume of space called the Hill sphere, 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 SATURN 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 month. 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 Titan, 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 irregular moon, Jupiter’s space-based ones. Where the lat- Earth need to map only a small The search for planetary satel- Leda, 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: Io, Europa, Ganymede, revealed a whole new class of ther away. scopes on Earth are immensely and Callisto. 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 asteroids — two objects that may celestial mechanics — and the have lots in common, it turns out. work of American astronomer SATURN George William Hill. 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 Phoebe 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.

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