Rhea, in astronomy, one of the named moons, or natural satellites, of Saturn. Also known as Saturn V (or S5), Rhea is 950 mi (1530 km) in diameter, orbits Saturn at a mean distance of 327,487 mi (527,040 km), and has equal orbital and rotational periods of 4.517 earth days. The second largest of Saturn's moons, Rhea was discovered by the Italian-French astronomer Gian Domenico Cassini in 1672. Rhea's density of 1.3 means that it is composed primarily of water ice, rocky material making up about a third of its mass. Its leading hemisphere is highly reflective and heavily cratered, strongly resembling the cratered highlands of the moon, although Rhea's craters do not have high walls or a towering central peak; the trailing hemisphere is darker with bright wispy streaks and few impact craters. In 2010 the space probe Cassini discovered that Rhea has an extremely thin atmosphere of oxygen and carbon dioxide.