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.
There are two constellations of navigation satellites at an altitude of about 20000 km. One is the US American GPS constellation and the second one is the Russian Glonass constellation. Soon a third constellation will orbit above these two: the European navigation system Galileo.
