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.
To compensate for the perturbations of the orbit due to gravitational and other forces, satellites must perform manoeuvres in regular time intervals. Whenever a satellite diverges too much from its nominal position its thrusters are fired. Here a so-called North-South station keeping manoeuvre is shown, whereby the orbit inclination is put back to zero degrees.
