NOW that the dreaded Chinese satellite is back to earth we can relax, secure for a while, we hope, from rogue missiles of any oriental kind. The incident, however, high lights a problem unique to the second half of the 20th century the dangers of debris in space, or "space pollution". There are currently about to "dead" satellites in orbit, spinning lifelessly around the earth, for every working spacecraft still in use. Each is a possible hazard, and sometimes these dead satellites may be struck by existing debris, resulting in a cloud of fragments which poses an even greater danger for spacecraft of a following generation.
Scientists are worried that in a couple of decades, regions in the vicinity of the more popular orbits will be so densely polluted with debris, that serious problems will arise. Apart from the financial penalties associated with the possible loss of unmanned spacecraft, it may also happen that the risks associated, for example, with exploratory missions to other planets, will be unacceptable.
Even now it is not unknown for a space shuttle to have to alter course to avoid a derelict chunk of man made space debris. And even the smallest particles, like flakes of paint that may have peeled from an old satellite, have lethal capabilities if collided with at cosmic speeds.
The Chinese event was an unusual one, in that the satellite concerned was sufficiently well insulated for it to survive the journey through the earth's atmosphere more or less unscathed. More commonly, unmanned spacecraft suffer a searing increase in temperature due to atmospheric friction on re entry, and therefore vaporise long before they might have reached the surface.
Indeed under controlled conditions, this is a very effective way of getting rid of old unwanted low altitude satellites - although it is expensive, since it means that fuel that might have been more productively employed has to be reserved to disturb the final orbits to allow re entry to take place.
High altitude satellites tend to be in "geostationary" orbit. They circle the earth above the equator at a height of 22,350 miles, at which altitude their speed in orbit exactly matches that of the rotating earth below, and so appear to an observer on the ground as fixed in space - an arrangement particularly useful for meteorology and communications. The preferred solution for these satellites, located where the cosmic - regions tend to be less cluttered, is to consign them to a kind of graveyard in the sky: the last dregs of fuel are used to hoist them to a slightly higher orbit - where they will circle the earth aimlessly, and with any luck, harmlessly, forever.