Natural object (moon) or spacecraft (artificial satellite) orbiting a larger astronomical body. Most known natural satellites orbit planets; the Earth's Moon is the most obvious example and was the only one known until the discovery of the Galilean satellites of Jupiter in 1610. All the solar system's planets except Mercury and Venus have moons, which vary greatly in size, composition (from rock to mostly ice), and activity (from cold and inert to volcanic). Some asteroids are also known to have their own moons. The first artificial satellite, Sputnik 1, was launched into orbit around Earth in 1957. Since then, thousands have been sent into orbit around Earth as well as the Moon, Venus, Mars, Jupiter, and other bodies. Artificial satellites are used for scientific research and other purposes, such as communication (see communications satellite), weather forecasting, Earth resources management, and military intelligence. See also Landsat.

This entry comes from Encyclopædia Britannica Concise.
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