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.
One of the perturbing forces is due to the fact that the Earth is not a perfect sphere. The slightly irregular mass distribution inside the Earth changes the gravity force at a geographical longitude of 75 deg East, shown here in yellow. The sum of the Earth's gravity and the centrifugal force does not vanish and instead a small force remains pulling the spacecraft towards the 75 deg meridian. If the satellite is not continuously pushed back, it will librate about this - or its antipodal - equilibrium point.
