The world entered the space age on 4 October 1957, when the first artificial satellite, Sputnik 1, was launched by the Soviet Union.
Since then, thousands of satellites have joined our natural satellite, the Moon, in the sky.
	ORBITING TUB EARTH. An artificial satellite is sent hurtling around the world by a rocket. 
It Bust be high enough to be free of the atmosphere, otherwise the air will slow down and it will fall back to Earth. 
Satellites therefore orbit at height of at least 160 km, circling the Earth once every 90 minutes.
	The father away the satellite, the longer the time it takes to go round the Earth. 
At distance of 36,000 km, it takes a satellite a day to go round the Earth. 
However, the Earth also rotates once in this time so such a satellite appears to stay in one place above the Earth's surface. 
These are called synchronous satellites and are very useful because they stay in continual contact with a ground station.
	A satellite does not contain any people. 
It has instruments for taking readings from up in space and for receiving and sending radio signals from and to ground stations. 
In this way, it gets oders from Earth and sends back its observations in the fora of radio signals. 
Power for the instruments is made by panels of solar cells. 
These are small cells made of silicon that turn sunlight into electricity.
	KINDS Of SATELLITE Sputnik 1 sent information about conditions in apace around the Earth back to scientists. 
The first US satellite, Explorer I, discovered the great radiation belts that lie in space around the Earth. 
Astronomical satellites can send back Information about the Sun and stars.
Communications satellites enable world-wide television broadcasts to be made and can transmitt international telephone calls. 
Weather satellites send back pictures of the Earth and the clouds, which help us to oake accurate weather forecasts. 
Other satellites can photograph the land, showing up all sorts of detail valuable to scientists: forests, crops, and mineral and oil deposits.