Between the launch of Sputnik on 4 October 1957 and 1 January 2008, approximately 4600 launches have placed some 6000 satellites into orbit, of which about 400 are travelling beyond geostationary orbit or on interplanetary trajectories.
Today, it is estimated that only 800 satellites are operational - roughly 45 percent of these are both in LEO and GEO. 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. About 50 percent of all trackable objects are due to in-orbit explosion events (about 200) or collision events (less than 10).
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, for instance near 75 degrees East, as shown here in yellow. The sum of the Earth's gravity and the centrifugal force does not vanish. Instead a small force remains, pulling the spacecraft towards the 75 degree meridian. If the satellite is not continuously pushed back, it will librate about this - or its antipodal - equilibrium point. Note that there are stable points at 75 degrees East and 105 degrees West.
