An artificial satellite is a man-made spacecraft circling the earth.
Such satellites are sent into space for many purposes.
Satellites can be any size from tiny packages of instruments to huge ballballoonsoon. They can weigh a few kilos or many tonnes. They can be any shape-balls, hatboxes, tin cans, bell buoys, and cigar boxes.
Some satellites have orbits around the earth as near as 110 miles away. Some travel 22,300 miles from earth. A satellite's orbit is chosen by scientists in advance, according to the task the satellite must per-
form. All satellites need electrical power to operate their equipment.
The main source of this power is the sun. Satellites carry many solar cells on their outside surface. A solar cell is a device that uses sunlight to generate electricity; this electricity keeps the satellite's batteries charged.
Radio and television signals can be sent from one continent to another by means of communications satellites. Most communications satellites have receivers and transmitters. The receivers pick up radio and television broadcasts from a ground station.
Electronic devices then increase the strength of the broadcast signals. The transmitters send the broadcasts to a distant ground station, which may be on another continent. An example of such a satellite is Telstar. Telstar I was launched by the United States in July 1962. Direct television transmission between the United States and Europe was first made possible by Telstar I.
One type of communications satellite has a "stationary"
• orbit
around the earth. It is at a distance of 22,300 miles above the earth and completes one orbit in 24 hours, the same time it takes the earth to rotate on its axis. Thus the satellite is always in the same place above the earth. The Early Bird satellite is this kind of satellite. Because its orbit is so high, it can transmit signals over very great distances.
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