Mimas (mĪˈmăs) [key], in astronomy, one of the named moons, or natural satellites, of Saturn. Also known as Saturn I (or S1), Mimas is 244 mi (392 km) in diameter, orbits Saturn at a mean distance of 115,275 mi (185,520 km), and has equal orbital and rotational periods of 0.942 earth days. Mimas was discovered by the English astronomer Sir William Herschel in 1789. Its low density (1.17) indicates that it is composed mostly of water ice with only a small amount of rocky material. The surface is saturated with impact craters. The largest by far—called Herschel—has a diameter of more than 80 mi (130 km), almost a third the diameter of the entire satellite; the walls of this crater are about 3 mi (5 km) high, parts of the floor are about 6 mi (10 km) deep, and the central peak towers almost 4 mi (6 km) above the floor of the crater. Fractures can be seen on the opposite side of Mimas that may have resulted from the same impact. Mimas forms a satellite pair with Tethys; that is, the two moons interact gravitationally.