Users of the space "environment" have become so concerned about the problem of space junk that since 1994 space debris has become an official agenda item in the Scientific and Technical Sub-Committee of the UN Committee on the Peaceful Uses of Outer Space.
The term "debris" denotes any man-made, Earth-orbiting object which is of no use. It includes everything from an orbiting fleck of paint off a rocket to the giants of the space junk firmament, such as the US Skylab (77 tonnes), Salyut78 (40 tonnes) and - when its time comes - the Mir space station (120 tonnes). While we might worry about the big stuff, heavy enough to come crashing to Earth, countries launching people into space are more worried about the little stuff.
A bit of paintwork about a third of a millimetre across smacked into shuttle flight STS-7 in June 1983. Travelling at about 4km per second, the fleck chipped the Challenger shuttle's windscreen, forcing its replacement back on Earth. So far more than 50 shuttle windows have had to be replaced due to chips and abrasions caused by impacts.
On Earth something travelling at that speed would have vapourised within seconds, but at high attitudes there's very little atmosphere and nothing to slow down objects moving in orbit. Impacts in space therefore occur at "hypervelocity" - up to 50,000km per hour. At that speed even a pea could be lethal.
While working satellites account for about six per cent of the 9,000 large orbiting objects, the rest includes fragments left from explosions in space (42 per cent), defunct satellites (22 per cent) and ejected covers, a wandering screw driver, an orbiting Hasselblad camera and a glass and other mission-related objects (13 per cent) left behind by space crews.
Much more problematic are the estimated 100,000 to 150,000 peasized objects that bounce around in erratic Earth orbit. They are too small to be tracked but each packs a powerful punch.