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).
In order to eliminate collision risk, GEO satellites should be moved out of the geostationary ring at the end of their mission. It is recommended that their orbit should be raised by about 300 km, which is considered a safe distance to avoid future interference with active GEO spacecraft. The change in velocity that is required to raise the orbit altitude by 300 km is 11 meter/sec, and the required propellant corresponds to that necessary for three months of station keeping. This means spacecraft operators have to stop operations three months before the spacecraft runs out of fuel and give up considerable revenue in order to reorbit their spacecraft. However, today, this is the only possibility for preserving the unique resource of the geostationary ring.
