Rhea, one of the moons of Saturn, photographed by Voyager 1, 1980.
© National Aeronautics & Space Administration / Science & Society
Saturn has a large number of moons, 18 of which have been formally named. Rhea, the 14th, is also the second largest. It is composed of water ice and a proportion of denser material, probably silicate rock. Rhea has two very different hemispheres, similar to those of Dione, the 12th moon. The trailing hemisphere features bright radiating patterns, thought to be features caused by some ancient resurfacing proces, posibly caused by vulcanicity. On the leading hemisphere, they have been obscured by impact craters. NASA's two Voyager spacecraft were launched in 1977 to explore the planets in the outer solar system. Voyager 1 flew past Saturn at a height of 124,000 kilometres in November 1980, having earlier visited Jupiter and several of its moons.