<!--intro-->Tiny particles of dust pose a more serious risk to satellites than huge lumps of space junk, according to a team of British scientists who have analysed damage to solar panels from the Hubble Space Telescope.<!--/intro--> <center ><a HREF='http://www.newscientist.com/'><img SRC='http://www.newscientist.com/ads/people_why.gif' BORDER=0></a></center> Most bits of space junk, even pieces as small as a few centimetres across, are tracked by the US Air Force. This allows any spacecraft that is on a collision course to be moved out of the way. But some particles are too small to be tracked by radar. This week scientists from the Open University and Oxford Brookes University will tell the Hypervelocity Impact Symposium in Galveston, Texas, that these particles, just a few micrometres across, pose a serious threat. They believe that dust grains vaporise on impact, creating a hot, conducting plasma that can induce currents that seriously perturb electronic systems on spacecraft, potentially rendering them helpless. One way to assess the risks posed by space dust is to examine the tiny craters which pepper the surface of space shuttles after orbital missions. But this is often impossible, as telltale residues in the craters are usually burnt away as the shuttle re-enters the Earth's atmosphere. So the British team examined solar cells removed from Hubble at the same time as its famously defective mirror was replaced. The panels were brought down in the shuttle's hold, so the residues remained intact. The researchers found that the craters (see inset Photograph) on the solar cells contain enough residue from impacting dust particles to identify the culprits, say Giles Graham at the OU and Anton Kearsley at Oxford Brookes. They found traces of iron, nickel and magnesium in the craters. This suggests the damaging particles were asteroids or comet debris. "We also found residues of aluminium and titanium from space debris in the smallest craters," says Graham. Neil McBride of the OU says the findings highlight the risk posed to satellites by the numerous particles of space dust orbiting Earth: "Natural particles can be travelling up to seven times faster than space debris particles, which means they produce over a thousand times more plasma on impact." The warning is timely, because on 17 November the Earth will pass through the tail of Comet Tempel-Tuttle, and the debris from it will rain down on Earth--producing the display better knows as the Leonid meteors. Next year's Leonid shower is expected to be the strongest since 1966. The stream of debris will be rich in micro- particles. "There's a very serious risk to satellites," says David Asher of the Armagh Observatory in Northern Ireland. McBride points out that a satellite called Olympus suffered an electrical malfunction at the peak of the 1993 Perseid meteor shower, which eventually led to its loss. "We can't say that it was definitely hit by a particle, but it seems likely," he says. Eugenie Samuel From New Scientist magazine, 11 November 2000.