Natural Satellite

Natural Satellite, in astronomy, a celestial body that orbits a larger celestial body. The larger body is referred to as the satellite’s primary. Natural satellites that orbit planets are often called moons.


The best-known natural satellite is Earth's Moon. The Moon is unusually large relative to the size of its primary (Earth); in fact, it is significantly larger than the planet Pluto. The Moon’s surface, like the surfaces of most of the natural satellites in the solar system, is heavily cratered and geologically inactive.

Neither Mercury nor Venus has any natural satellites, but Mars has two small moons: Phobos and Deimos. Jupiter has more than 30 natural satellites, four of which are quite large: Io, Ganymede, Callisto, and Europa. Active volcanoes cover Io, and scientists believe that oceans of water may hide beneath the icy crusts of Ganymede, Callisto, and Europa. All four of these moons are larger than Pluto, and Ganymede is larger than the planet Mercury as well. Saturn has more than 20 natural satellites, the largest of which is Titan. Titan is bigger than Mercury, and is the only moon with a thick atmosphere. Enceladus, one of Saturn’s smaller moons, has an unusually bright, geologically young surface apparently composed of ice. Uranus has more than 20 moons, none of which are nearly as large as Earth’s Moon. Miranda, one of Uranus’s smaller moons, shows signs of terrific upheavals on its surface. Neptune’s largest natural satellite, Triton, is slightly larger than Pluto. Its surface appears to be continually reshaped by the freezing and thawing of nitrogen. Pluto’s single moon, Charon, is half as large as Pluto itself. Some astronomers consider the pair a double planet.

The motion of most of the solar system's natural satellites about their planets is direct: west to east, in the same direction as the rotation of their planets. Several small satellites of the large outer planets, however, revolve in the retrograde direction: east to west, opposite the direction of the rotation of their planets. These retrograde satellites tend to orbit far from their primaries and were probably captured by the planets' gravitational fields some time after the formation of the solar system.

Encarta has separate articles on most of the natural satellites that have been studied in detail. In addition, overviews of planets’ systems of moons appear in the planet articles.