Due to gravitational pull, natural satellites, like our moon, move in definite paths around the parent body. These paths are called their orbits. Artificial satellites too move around the earth in definite paths.
Depending on what their mission is, satellites are placed in one of three basic types of orbits.
* Geo-stationary orbits are used by satellites, which remain at a fixed point over the earth’s surface at all times. These satellites are used for communications, television broadcasts or as weather satellites. In order that the satellite moves in its orbit at a speed to match the speed of the earth, it must be placed at an altitude of around 35550 kilometres above the earth. Geo-stationary satellites move at speeds of around 11,000 kilometres per hour. Geo-stationary orbits are also called geo-synchronous orbits or simply synchronous orbits.
* Asynchronous orbits are lower than the geo-stationary orbits. The closer the satellite is to the earth, the faster it travels in its orbit. Since the speed is much faster than the orbital speed of the earth, the satellite passes the same point on the earth more than once a day. For instance, the Hubble Space Telescope, which orbits at a distance of around 600 kilometres, completes an orbit every 97 minutes.
* Polar orbits are usually low orbits in which the satellite passes over the Earth’s poles in each revolution. This type of orbit provides the maximum coverage of the land and therefore these satellites are used for mapping and for photography.
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