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.
The Sun, Moon and Earth's oblateness lead to long-periodic perturbations of the inclination of geostationary satellites. The period is 54 years and the maximum inclination is 15º. Thus, uncontrolled satellites build up an inclination of 15º after 27 years. They keep crossing the geostationary orbit twice a day with a velocity of 3000 kilometers per hour with respect to the controlled geostationary satellites.
