Since the launch of Sputnik on 4 October 1957, more than 4,200 launches have placed some 5500 satellites into orbit. Currently about 700 satellites are used operationally for science and other applications. Space debris comprise the ever-increasing amount of inactive space hardware in orbit around the Earth as well as fragments of spacecraft that have broken up, exploded or otherwise become abandoned.
A geostationary satellite needs 23 hours, 56 minutes and 4 seconds to orbit the earth, the same time as the earth needs to rotate once around its axis. Thus the satellite appears to stay above the same point on the equator.
