VERY HIGH SPECKS

Tin foil was the simple tool that helped track cosmic dust in space

DR JOHN Zarnecki's cosmic dust experiment was supposed to circle the Earth for a year. Due to the Challenger space shuttle disaster, it stayed up for five-and-a-half. When it was eventually retrieved, it showed for the first time the alarming degree of orbiting pollution. "The real killer is the intermediate stuff," says Zarnecki. "Something 1cm across is too small to track, but it would go through your space suit and cut you in half."

Cosmic dust is overwhelmingly made up of hydrogen and helium, with a rare speck of some heavier element, fused in an ancient star and blown into the far reaches of space when that star went super nova. "Sometimes cosmic dust particles enter the Earth's atmosphere and create shooting stars," says Zarnecki. "Occasionally they reach the ground and end up in the British Museum with the label `meteorite'. Once in a while the bigger pieces cause craters."

He wanted to examine cosmic dust motes before they hit the atmosphere. His experimental tool was simple: a sheet of aluminium and brass foil, 1m square, unfurled from the space shuttle and left to float like a glittering target. By the time a later shuttle returned to pick it up, the foil was perforated with 5,000 holes, an average of more than two impacts a day. In some size ranges, particularly around a micron (one millionth of a centimetre) in diameter, most of the particles were artificial. "Thirty years ago they'd almost all have been natural."

How do you tell if a hole is natural or man-made? Three clues helped categorise the particles hitting the foil. One was the angle of impact. Those that came from outside Earth's gravitational pull are natural. Those that were in orbit are almost certainly artificial. The second clue was the speed at which the particles were moving when they hit. By examining damage done to the foil, it is possible to deduce whether the particles were travelling at relatively slow orbital speeds, or the faster rates, more than 20km per second, common to interstellar dust.

Thirdly, tiny residues are left around the rims of the holes. Cosmic dust was expected to leave hydrogen and helium. "We found loads of titanium," says Zarnecki.

Most impacts detected by the experiment were far too small to be seen by Earth-based observatories, but not too small to cause problems for voyagers in space. If a space ship and a bit of debris are in the same orbit, moving in the same direction, their relative speed will be close to zero. If they are moving in the opposite direction, each at 15km per second, the impact velocity will be about 30km per second. The kinetic energy released is equal to one half mass times velocity squared, so a collision with even a small particle could have a dramatic effect. "If you've got sensitive equipment up there, this could be very damaging," says Dr Zarnecki. "It's like sand-blasting." PR