Rhea, Saturn's second largest satellite after Titan, was discovered by Giovanni Cassini in 1672. Its diameter is 1,529 km (about half that of the Moon) and its average distance from the planet is about 527,000 km. Its orbit is almost circular and slightly inclined with respect to Saturn's equator, and it takes Rhea about 4.5 days to complete it. It is an icy satellite with a density of approximately 1.24 g / cm³, which indicates that Rhea probably has a rocky core, which would constitute a third of its mass, the rest being a combination of water-ice. Rhea's physical characteristics resemble Dione's, with different hemispheres, suggesting similar compositions and stories. Rhea has the only known ring system around a satellite.
