Janus (jāˈnəs) [key], in astronomy, one of the named moons, or natural satellites, of Saturn. Also known as Saturn X (or S10), Janus is an irregularly shaped (nonspherical) body measuring about 122 mi (196 km) by 119 mi (192 km) by 93 mi (150 km); it orbits Saturn at a mean distance of 94,120 mi (151,472 km), and has equal orbital and rotational periods of 0.6945 earth days. The French astronomer Audouin Dollfus is credited with the discovery of Janus in 1966. However, in 1978, the American astronomers Stephen M. Larson and John W. Fountain determined that there were two moons orbiting Saturn at a distance of about 94,000 mi, and it was not until 1980 that the Voyager 1 space probe provided sufficient data to enable Janus to be distinguished from Epimetheus; today it is difficult to say which one Dollfus really discovered. Janus and Epimetheus are co-orbital, that is, they share the same average orbit; about every fourth year—at closest approach—the lower, faster satellite overtakes the other, they exchange angular momentum, and the lower one is boosted into a higher orbit while the higher one drops to the lower orbit. The two moons may have formed from the disruption of a single satellite early in the formation of Saturn's satellite system. Janus's surface is extensively cratered, with several larger than 18 mi (30 km); however, there are few linear features.