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 Bartb once every 90 minutes.
The father away the satellite, the longer the time it takes to go round the Barth. At distance of 36,000 km, it takes a satellite a day to go round the Barth. However, the Barth also rotates once in thia time so such a satellite appears to stay in one place above the Barth'a 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 thia way, it gats odere fron Barth and sends back its observations in the fora of radio signals. Power for the instruments is made by pa-
•els of solar cells ( ccumeiHHe oarapeB ). These are small cells
•ade of silicon that turn sunlight into electricity.
KINDS Of SATBLLITB Sputnik I sent information about conditions in apace around the Barth back to scientists. The first US satellite, Explorer I, discovered the great radiation belts that lie in space around the Berth. Astronomical satellites can
•end back Information about the Sun and stare.
ConuBunications satellites enable world-wide television broadcasts to be made and can tran"Bit international telephon* calls. Heather 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 ( ypoaafl ), and niineral and oil deposits.

BXOK n
Tepun" x reicTy TJ



STABS AND CONSTELLATIONS ( CoSBeaiUM ). On B clear night, away from the lights of a town, you can see about 2,000 atari with the naked ( HeBOOpyxeHHut ) eye. A telesoope can reveal