SATELLITE, NATURAL

celestial body orbiting a planet, asteroid, or star of a larger size. The most familiar natural satellite is the earth's moon; thus, satellites of other planets are often referred to as moons. Within the solar system, Pluto's moon, Charon, and the earth's moon are the largest satellites in relation to their planets, although neither is the largest in actual size. Ganymede of Jupiter is 3,268 mi (5,262 km) in diameter and is larger than the planet Mercury. In comparison, some satellites are quite small, e.g., Deimos, the outer satellite of Mars, is c.4 mi (6 km) in diameter. Neither of the inferior planets, Mercury or Venus, has a known satellite; all of the superior planets (those whose orbits lie beyond the orbit of the earth) except Pluto have at least two known satellites each (Mars, 2; Jupiter, 39; Saturn, 30; Uranus, 21; Neptune, 8; Pluto, 1). A number of satellites, e.g., Phoebe of Saturn, Triton of Neptune, and most of the small outer moons of Jupiter, have retrograde motion and may be asteroids that were captured by the planet's gravitation. The asteroid Ida has a tiny moon, Dactyl, that is about a mile (1.6 km) in diameter and orbits about 60 mi (97 km) above Ida's surface.

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