New Moon for Uranus

An astronomer has discovered a small moon of the planet Uranus that was captured on photos by the probe Voyager 2 (see below) but overlooked for more than 13 years. The find raises the number of known uranian satellites to 18, and there may even be more.

Before the flyby of Voyager 2, only five uranian satellites were known. Voyager discovered a sixth in December 1985 and nine more in January 1986. Two more small satellites orbiting far from the planet were found by an Earth-based telescope in 1997. The latest moon is tucked inside the orbit of Miranda, the innermost of Uranus's five larger satellites.

Erich Karkoschka of the Lunar and Planetary Laboratory of the University of Arizona found the new moon when he was looking at some 300 Voyager images of the Uranus system and comparing them with new Hubble Space Telescope images, in an effort to calculate more accurate figures for the orbital periods of the satellites. On the old Voyager photos, he noticed an object that he couldn't find on star maps. The new satellite, temporarily designated S/1986 U 10 (it is the tenth uranian satellite found on photos made in 1986), has a diameter of some 40 kilometers and probably has an icy, cometlike composition. It orbits Uranus every 15 hours and 18 minutes at a distance of 76,416 km from the planet's center.

Mark Showalter of NASA's Ames Research Center, who discovered Saturn's 18th satellite, is surprised by the find: "If you told me you were about to search for undiscovered satellites in old Voyager photos, I would've said you're wasting your time," he admits. On the other hand, computers have made searching through large numbers of images a lot easier. Could even more moons be lurking in old Voyager photos of, for instance, Neptune? Says Showalter: "All bets are off at this point."

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