The fifth-largest satellite of Saturn, average diameter 1062 km, sharing the same orbit as the much smaller moons Calypso and Telesto; also known as Saturn III. Its orbital distance is 294 672 km, and its orbital period 1.888 days, the same as its period of axial rotation. Tethys was discovered in 1684 by G. D. Cassini. It is densely covered with impact craters; one of these, Odysseus, is 445 km across, nearly half the satellite’s diameter. Another prominent feature is Ithaca Chasma, a valley over 1200 km long, up to 100 km wide, and up to 3 km deep. The density of Tethys is 0.97, slightly less than that of water, suggesting that it is composed largely of ice.