Tethys , in astronomy, one of the named moons, or natural satellites, of Saturn . Also known as Saturn III (or S3), Tethys is 659 mi (1060 km) in diameter, orbits Saturn at a mean distance of 183,093 mi (294,660 km), and has equal orbital and rotational periods of 1.8878 earth days. Tethys was discovered by the Italian-French astronomer Gian Domenico Cassini in 1684. Tethys's low density (1.2) implies that is composed primarily of water ice. The heavily cratered satellite has two dominant surface features: one is an impact crater—called Odysseus—about 250 mi (400 km) in diameter that is quite flat and without the high wall and central peak common on the moon; the other a huge valley—called Ithaca Chasma—that is 63 mi (100 km) wide, 2.5 mi (4 km) deep, and 1,250 mi (2,000 km) long, stretching three quarters of the way around Tethys's circumference. Tethys is co-orbital with two other moons; that is, they orbit Saturn at the same distance as Tethys but precede ( Telesto ) and follow ( Calypso ) Tethys by about 60°.