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.
80% of all catalogued objects are in Low Earth Orbit (LEO) which extends to 2000 km above the earth's surface. To observe the Earth spacecraft must be at such a low altitude and furthermore access for satellites as well as astronauts is easier. The spatial density of objects increases at high latitudes.
