A satellite is a body that moves in orbit around a larger body. The moon is a natural satellite of Earth because it orbits around the Earth. All the planets, except Mercury and Venus, have natural satellites.
Today we also have artificial satellites. These ‘artificial’ satellites are man-made and launched into the space by powerful rockets. They orbit around Earth performing certain specified tasks.
A satellite orbiting the Earth is like a chest-nut being whirled on the end of a piece of string. The centrifugal force drives it outwards, but Earth’s gravity keeps it from moving away. These two forces balance it and the satellite continues to orbit around the earth. It moves without any resistance since there is no air in the space. It will keep on orbiting forever, unless the upper atmosphere of Earth drags on the satellite and slows it down. The satellites move in elliptical orbits and not in circles. The nearest point to Earth is called the perigee and the farthest, the apogee. Manned space craft’s are temporary satellites during a space mission, but most artificial satellites are unmanned. The geostationary satellites positioned at a height of about 36,000 km. have the advantage that they have a stable position in the space in respect to any point on Earth.
Hundreds of artificial satellites have been launched since Sputnik I which was the first satellite to be launched by the Soviet Union on October 4, 1957. Artificial satellites have become an integral part of our day to day life. They serve many useful purposes in different fields — communication, weather forecasting, geological survey, oceanography, astronomical experiments and observations etc. They also help in navigation and air traffic control. The satellites can be either unipurpose or multipurpose, depending on their service in either one specific area or more than one area. The Indian National Satellite (INSAT) series is a multipurpose one used for domestic telecommunications, meteorological observations, radio and TV broadcasting etc.