From the Economist
ARISTOTLE believed that the heavens were perfect. If they ever were, they are no longer. The skies above Earth are now littered with the debris of dead satellites, bits of old rockets and the odd tool dropped by a spacewalking astronaut. Such is the extent of the detritus that the first accidental collision between two satellites has already taken place. It happened in February 2009, when a defunct Russian Cosmos smashed into a functioning American Iridium, destroying both and creating even more space junk. To stop this sort of thing happening again Vaios Lappas of the University of Surrey, in England, has designed a system that will remove satellites from orbit at the end of their useful lives-and as a bonus will scour part of the sky clean as it does so.
Dr Lappas’s satellite-removal system employs a solar sail. As light from the sun hits the sail, it imparts a minuscule but continuous acceleration. When a satellite is first launched, the sail is angled in a way that causes this acceleration to keep the satellite in orbit. (Orbits gradually decay as a result of collisions with the small number of air molecules found even at altitudes normally classified as “outer space”.)
Solar sails have yet to be used widely to propel spacecraft in this way-several earlier versions came unstuck when the sails failed to unfurl properly-but doing so is not a novel idea in principle. The novelty Dr Lappas envisages is to change the angle of the sail when the satellite has become defunct. Instead of keeping the derelict craft in orbit, it will, over the course of a couple of years, drag it into the atmosphere and thus to a fiery end. Not only that, but the sail will also act like a handkerchief, mopping up microscopic orbital detritus such as flecks of paint from previous launches. A fleck of paint may not sound dangerous, but if travelling at 27,000kph (17,000mph), as it would be in orbit, it could easily penetrate an astronaut’s spacesuit.