While the word "satellite" simply means some object or person that is attendant to another more important object or person, in astronomy it has taken on a much more specific meaning. Here the term refers to any object that is orbiting another larger more massive object under the influence of their mutual gravitational force. Thus any planetary moon is most properly called a satellite of that planet. Since the word is used to describe a single object, it is not used to designate rings of material orbiting a planet even though such rings might be described as being made up of millions of satellites. In those rare instances where the mass of the satellite approaches that of
the object around which it orbits, the system is sometimes referred to as a binary. This is the reason that some people refer to Pluto and its moon Charon as a binary planet. This description is even more appropriate for some recently discovered asteroids which are composed of two similar sized objects orbiting each other.
In this century we have launched from the Earth objects that orbit the Earth and other planets. A tradition has developed to refer to these objects as man-made satellites to distinguish them from the naturally occurring kind. Surveillance satellites orbiting the Earth have been used to measure everything from aspects of the planet's weather to movements of ships. Communications satellites revolve about the earth in geostationary orbits 25,000 mi (40,225 km) above the surface and a recent generation of navigation satellites enables one's location on the surface of the earth to be determined with errors measured in centimeters.
Surveillance satellites have been placed in orbit about the Moon, Mars, and Venus to provide detailed maps of their surfaces and measure properties of their surrounding environment. This program will soon be extended to Jupiter and Saturn.
Spacecraft missions to other planets in the solar system have revealed the existence of numerous previously unknown natural satellites. In addition, the nature of many of the planetary satellites has become far clearer as a result of these voyages. It is said that more information concerning the four major Galilean Satellites of Jupiter was gained from the first flyby by Pioneer 10 then had been gained since the time of Galileo. The knowledge gained from the satellites in our solar system have revealed considerable insights into their formation and evolution. As we continue to probe the solar system, there can be little doubt that our knowledge of the satellites of the planets will continue to broaden our understanding of planetary moons and the nature of the solar system as a whole.