Satellites may travel in orbits of various kinds (A). Some move in the plane of the equator (1), others have inclined orbits (2), and some use polar orbits (3). For a geostationary satellite (b) the orbital period is exactly one day. Its distance from Earth is 35,900 km ((22,300 miles), so that is appears to hang motionless in the sky.
Satellites may be natural or artificial.
A natural satellite, also known as a moon, is a natural body that revolves around a larger body, normally either a planet or an asteroid. For example, Europa is a satellite of Jupiter, and Dactyl is a satellite of the asteroid Ida. Since 1957 the term has also been applied to human-made objects, so that a distinction is now drawn between natural and artificial satellites. Several moons are larger than Pluto and two (Ganymede and Titan) are larger than Mercury.
An artificial satellite is a human-made object placed in orbit around Earth or some other celestial body. Satellites can perform many tasks and can send back data or pictures to Earth. They may study the atmosphere, or photograph the surface for scientific or military purposes. Communications satellites relay radio, television, and data signals from one part of the Earth to another. navigation satellites transmit radio signals that enable navigators to determine their positions. The Global Positioning System (GPS) uses satellites in this way. Geodesy satellites are used to make accurate measurements of the Earth's size and shape.
Sputnik 1, launched by the Soviet Union on October 4, 1957, was the first artificial satellite. Telstar, launched by the US on July 10, 1962, was the first active communications satellite. The first satellite in geosynchronous orbit, Early Bird, was launched in 1965.
Geosynchronous satellites circle Earth at a height of 35,900 km (22,300 miles), enabling them to complete one revolution every 24 hr. Satellites that orbit Earth at heights less than about 160 km (100 miles) are slowed by friction with the atmosphere and eventually spiral in and burn up. In space exploration, a spacecraft may be put into orbit around a planet or a moon and thus become an artificial satellite of that body.
A satellite constellation is a set of satellites arranged in orbit to fulfill a common purpose. In the case of GPS (Global Positioning System), the full operational constellation is composed of six orbital planes, each containing four satellites.
Satellite mass categories
There are many different ways to classify artificial satellites – by function, type of orbit, cost, size, and so forth. Classification by mass is useful because it has a direct bearing on the launcher vs. cost tradeoff. The table below illustrates a scheme for classifying satellites in terms of deployed mass that has been generally adopted in recent years. The masses listed refer to in-orbit fully-fueled ("wet mass"). Within this scheme the term "small satellite" is used to cover all spacecraft with in-orbit masses of less than 500 kg. Particular agencies and organizations may have their own names for certain categories. For example, small satellites are referred to by the US Defense Advanced Research Projects Agency as LightSats, by the US Naval Space Command as SPINSat's (Single Purpose Inexpensive Satellite Systems), and the US Air Force as TACSat's (Tactical Satellites).
|category||mass range (kg)|
|large satellite||> 1,000|