Janus
Janus is an irregularly shaped moon, 181 kilometers across. These images were taken by Cassini in 2004 and 2005.
Janus shares an orbit beyond the F-ring with slightly smaller Epimetheus (out of the picture on the other side of Saturn when the images were taken).
Astronomers were astonished when this pair of orbit-sharing moons was discovered by the Voyager missions in 1980. Janus and Epimetheus move in almost identical orbits. The inner body moves slightly faster and after four years, laps the outer body. Their diameters exceed the distance separting their orbits, but the moons avert a collision by exchanging energy and switching orbits at the point of nearest approach. Slung ahead by the encounter, the other moon then becomes the pursuer, and so the cycle repeats. [after Lang and Whitney]
April 2005
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