SATELLITE - 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.

All the planets in the solar system except Mercury and Venus have natural satellites. More than 100 such objects have so far been discovered, with Jupiter and Saturn together contributing about two-thirds of the total. The planets' natural satellites vary greatly in size. Some of them measure less than 10 km (6 miles) in diameter, as in the case of some of Jupiter's moons. A few are larger than Mercury, as for example Saturn's Titan and Jupiter's Ganymede,each of which is more than 5,000 km (about 3,100 miles) in diameter. The satellites also differ significantly in composition. The Moon, for example, consists almost entirely of rocky material. On the other hand, the composition of Saturn's Enceladus is 50 percent or more ice. Some asteroids are known to have their own tiny moons.


The first artificial satellite to be placed in orbit was Sputnik 1, launched on October 4, 1957, by the Soviet Union. Since then, thousands have been sent into Earth orbit. Various artificial satellites have also been launched into orbit around Venus, Mars, and Jupiter, as well as around the Moon and the asteroid Eros. Spacecraft of this type are used for scientific research and for other purposes, such as communication, weather forecasting, Earth resources management, and military intelligence (see space exploration).

SATELLITE - man-made object launched into a temporary or permanent orbit around the Earth. Spacecraft of this type may be either manned or unmanned, the latter being the most common.

The idea of an artificial satellite in orbital flight was first suggested by Sir Isaac Newton in his book Philosophiae Naturalis Principia Mathematica (1687). He pointed out that a cannonball shot at a sufficient velocity from atop a mountain in a direction parallel to the horizon would go all the way around the Earth before falling. Although the object would tend to fall toward the Earth's surface because of gravitational force, its momentum would cause it to descend along a curved path. Greater velocity would put it into a stable orbit, like that of the Moon, or direct it away from the Earth altogether. On Oct. 4, 1957, nearly three centuries after Newton had proposed his theory, the Soviet Union launched the first Earth satellite, Sputnik I. Sputnik circled the Earth every 96 minutes, and its simple radio signal was heard by scientists and radio operators across the world. The United States orbited its first satellite, Explorer 1, three months later (Jan. 31, 1958). Explorer, though much smaller than Sputnik, was instrumented to detect radiation and discovered the innermost of the two Van Allen radiation belts, a zone of electrically charged solar particles that surrounds the Earth.

Since these initial efforts, more than 5,000 Earth satellites have been orbited by at least 15 different nations. The satellites vary widely in size and design, ranging from a tiny sphere of several pounds equipped with only two radio transmitters to heavily instrumented space laboratories weighing many tons. They are equally diverse in function. Scientific satellites are chiefly used to collect data about the Earth's surface and atmosphere and to make astronomical observations. Weather satellites transmit photographs of cloud patterns and measurements of other meteorological conditions that aid in weather forecasting, while communications satellites relay telephone calls, radio and television programs, and data communications between distant parts of the world. Navigation satellites enable the crews of oceangoing vessels and airplanes to determine the position of their craft in all kinds of weather. Some satellites have distinctly military applications, such as reconnaissance and surveillance.

Satellites can be placed in any number of different orbits. The particular path selected is largely determined by the function of the spacecraft. Most weather and reconnaissance satellites, for example, are fired into a polar orbit in which the Earth's polar axis is a line on the orbital plane. Because the Earth rotates under polar-orbiting satellites, they pass over its entire surface within a given time period, providing full global coverage. Communications satellites, on the other hand, are generally placed into an equatorial orbit, which enables them to circle the most densely populated regions of the Earth from west to east. Moreover, communications satellites comprising a network or system are nearly always launched to a distance of 22,300 mi (35,890 km) above the Earth. At this altitude, the motion of a satellite becomes synchronized with the Earth's rotation, causing the craft to remain fixed over a single location. If properly positioned, three communications satellites travelling in such a synchronous orbit can relay signals between stations around the world. See also spacecraft.