The first man-made object to be placed in orbit around the Earth was not, as one might innocently assume, the Soviet satellite Sputnik 1. It was rather 1957 Alpha 1 - the spent portion of the rocket that was to place Sputnik in orbit shortly afterwards.
Alpha 1 became the prototype for space debris - a collection of orbiting junk currently totalling some 3,000 tonnes, and ranging in size from dead satellites as big as buses to tiny particles of dust from the exhausts of rocket motors.
There are many sources of debris. Over 23,000 objects of significant size have been sent into space in the last 40 years. Of these, some 7,000 pieces still remain, although the number of operational satellites is fewer than 200; all the rest are obsolete. Smaller items of debris, such as bolts, clamps or other unneeded bits and pieces result from the separation of the various stages of a rocket or the deployment of the satellites themselves.
And items already in space disintegrate with time into a multitude of smaller pieces, either from attrition by solar radiation or similar causes, or other more catastrophic happenings. Altogether, there are known to be at least 100,000 objects between 1 and 10 cms in size in orbit around the Earth, and millions whose dimensions are of the order of a millimetre.
But space junk is not just untidy; it is also dangerous. Orbital debris moves at very high speeds relative to operational spacecraft. In so-called "low Earth orbits" - for example, at altitudes of less than 2,000 km - the average relative velocity in the case of impact is about 10 km/sec, or over 21,000 m.p.h. Even the smallest particles have lethal capabilities if intercepted at these cosmic speeds.
Currently, the only mechanism for the removal of space debris is for the tenuous drag at high altitudes gradually to cause objects to descend to lower and lower orbits, before eventually being burned up as their surrounding atmosphere becomes more dense. But this is only effective for relatively low orbits; at higher orbits, it takes hundreds or even thousands of years for objects to reenter, and so there is no effective way for debris to dissipate.
Since the beginning of the space age, the creation rate of debris has far outpaced its removal rate, leading to a net growth in the number of objects, from big to minuscule, of something like 5 per cent per year. Scientists are worried that unless spacecraft operators become more space-tidy and space-litter conscious, the problem may get entirely out of hand.