Hyperion hīpēr´ēən [key], in astronomy, one of the named moons, or natural satellites, of Saturn. Also known as Saturn VII (or S7), Hyperion is the largest highly irregular (nonspherical) body in the solar system, measuring about 255 mi (410 km) by 160 mi (260 km) by 135 mi (220 km); it orbits Saturn at a mean distance of 920,310 mi (1,481,100 km) in 21.277 earth days—its rotational period is chaotic. It was discovered in 1848 independently by the American astronomer George Phillips Bond and the English astronomer William Lassell. Hyperion is believed to be composed primarily of water and carbon dioxide ice with only a small amount of rocky material; pictures taken by the space probe Cassini show a honeycombed surface that resembles that of a sponge. Hyperion differs from Saturn's other moons in that its surface features are relatively uniform, while the others have distinctly different leading and trailing hemispheres. The largest crater on its surface is c.75 mi (120 km) in diameter and 6 mi (10 km) deep. Hyperion forms a satellite pair with Titan; that is, the two moons interact gravitationally.
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