What goes up need not come down, unfortunately

WE HUMANS are a dirty species, seemingly determined to cover our whole planet with our pollution. Even in Antarctica, where the population consists of a few thousand environmentally-conscious weathermen and climatic scientists, industrial chemicals are turning up in the snow.

But our generosity with garbage extends even beyond Earth. Fifty years of space exploration has left a great cloud of litter in orbit containing everything from discarded rocket stages and flecks of paint to astronaut's toothbrushes and fragments of exploded satellites.

This stuff is dangerous as well as ugly. The objects whizz round the Earth at eight kilometres a second or more. At that speed a piece of debris the size of a coin packs as much punch as a bus moving at 100 kilometres an hour.

Worse still, nobody is sure exactly how much is up there. America's Space Surveillance Network tracks around 11,000 of the largest bits, but the small stuff―less than about 10cm across―goes unwatched. NASA reckons there are more than 100,000 of these smaller pieces, still big enough to damage badly or destroy anything they hit.

Don't think this just a theoretical risk. Holes thought to have been punched by small particles have been found in returning space shuttles. In 1991 the shuttle Discovery had to adjust its orbit to avoid a speeding fragment of an old Russian satellite. Five years later an errant chunk of Ariane 5 rocket smashed into a French satellite at around 49,000 kilometres an hour, destroying a stabilising boom and narrowly missing the satellite's main body.

Spacefaring countries have set up a body called the Inter-Agency Space Debris Coordination Committee, in the hope of ensuring that new space missions will avoid adding to the problem. New spacecraft are designed to commit suicide in the Earth's atmosphere once their missions are complete, says Richard Crowther of Britain's Rutherford Appleton Laboratory.

An errant chunk of Ariane 5 rocket smashed into a French satellite at around 49,000 kilometres an hour, destroying a stabilising boom and narrowly missing the satellite's main body

But still, according to a paper published last year by two NASA scientists, the total amount of orbital litter is likely to go on increasing for another couple of centuries even if rocket launches cease. That is likely to happen in part through accidents and decay. Left-over fuel in old rocket stages and satellites may explode, for example, scattering satellite parts far and wide.

Nor are all countries equally co-operative. Last month China tested an anti-satellite missile by―whoops―blowing up an old orbiting weather satellite. The test could well generate more than 800 substantial fragments, each in a slightly different orbit.

Mr Crowther worries that firms operating satellites in popular orbits―such as the geosynchronous orbit that keeps satellites above the same part of the earth―may be unwilling to shell out the cost of waste disposal, ensuring that the most useful orbits are also those most filled with rubbish. And, of course, orbital junk is a self-multiplying problem. Each collision produces more debris, adding to the chance that something else will be hit.

One suggestion is to send up robot-binmen to drag the debris down. But that would be risky and expensive, since a separate mission might well be needed for almost every piece of junk. There is talk, too, of using ground-based lasers to nudge debris back to earth, but such a scheme would take years to work.

in the mean time, it is possible to toughen up spaceships: the International Space Station is already armoured, for example. But that is only likely to be effective against the smallest particles; and tougher spacecraft are heavier, and so more expensive to launch.

Low-flying litter―anything below 800 kilometres or so―will fall out of the sky eventually as friction with Earth's atmosphere slows its orbit. But at least half the known debris is higher than that, and can stay up indefinitely. The consensus seems to be that it constitues a (literally) unavoidable danger of doing business in space.

“No single remediation technique appears to be both technically feasible and economically viable,” concludes a paper published last year. In other words, says Mr Crowther, “we're just going to have to live with it.”