China's FSW1 satellite is due to hit the Earth sometime in the next four weeks, after becoming the source of considerable embarrassment to its makers during the two years since it was launched.
Designed to carry out a series of experiments in the low-gravity conditions 100 miles in orbit - and also, say unofficial sources, to take spy photographs of enemies' military installations - the FSW1 is in a "decaying" orbit, out of control, after its rockets misfired early in its mission and altered its orbit from a stable, circular one to an unstable, elliptical one.
"We have known for nearly two years that it was going to come down," said Richard Tremayne-Smith, of the British National Space Centre, yesterday. "And we knew it was going to come down out of control."
The satellite, which does not contain any nuclear materials, is being tracked by at least five organisations - the Defence Research Agency in Farnborough, Hampshire, the US Space Command in Colorado, the European Space Agency in Darmstadt, Germany, and by the Russian and Chinese space agencies. But none is able to say for sure where it will finally crash, because the rate of decay of the orbit depends on the thickness of the Earth's atmosphere. Friction with the atmosphere slows the satellite, allowing gravity to pull it downwards. But precise calculations are impossible for an object travelling at 18,000 mph, in an orbit which makes it wander over every point on the globe between 56 degrees north and 56 degrees south.
The impact will be tremendous. FSW1 weighs just under a ton, and will be red-hot from falling through the atmosphere. When it reaches Earth it will be travelling at about 1,000 mph. Nick Johnson, a space junk specialist at NASA, said it could leave a crater 30 feet wide and 20 feet deep on impact. But the likelihood is that it will land in the ocean.
Space junk is an increasing hazard for spacecraft. According to NASA estimates, there are at least 7,000 "substantial" objects such as satellites in low earth orbits. But almost all are either in stable orbits, or designed so that if they fall to Earth they will be vapourised by the tremendous heat generated on re-entry to the atmosphere.
NASA maintains an electronic newsletter, called Spacewarn, to alert the authorities about satellites and rocket fragments expected to fall to Earth.
FSW1 is unusual in that it is designed to withstand re-entry, because the Chinese wanted its onboard films and experimental results. The only other large man-made items that have hit Earth have been Russian and American satellites, including the US space station Skylab, parts of which landed in a remote part of the Western Australian outback in July 1979. Most of its 850 tons burnt up on re-entry. Last month, an experimental Russian moon landing vehicle fell into the Pacific Ocean after 20 years in orbit.Reuse content