Nasa engineers examining the break-up of the space shuttle Columbia think a possible cause may have been the clutter of human debris in space.
US air force scientists who track objects in orbit spotted something moving slowly away from Columbia on January 17. It could, Nasa experts say, have been a meteor, a fragment of the shuttle itself, or ice from waste water dumped from the spacecraft. Or it could have been a piece of the 2,000 tonnes of man-made junk that has built up in orbit since the launch of Sputnik 1, more than 45 years ago.
Anything in orbit is potentially at risk - and itself a hazard. Long after space missions have ended, or commercial satellites have run out of fuel, the hardware stays on the gravitational merry-go-round. Screwdrivers, camera lens hoods, CD-roms, ballpoint pens, lumps of waste water, fragments of rocket casing and splintered satellites continue to orbit the planet at five miles a second, running the gauntlet of space pebbles and comet dust arriving at up to 20 miles a second. Collisions are frequent, and potentially catastrophic. By June 2001, space scientists had monitored 170 events in which objects had collided and broken up into fragments. The 10 largest of these events generated an average of about 150 lumps of flying metal each.
At orbital speeds, almost anything is lethal. Richard Crowther, a space scientist at Farnborough, calculated last year that a penny travelling at six miles a second would have the same destructive force as a small bus travelling at 60 miles an hour. And there could be 100,000 objects not much bigger than bullets - but travelling far, far faster - in the shooting gallery of near-Earth space.
But so far, researchers can only conjecture. The object may have been ice that formed on a waste water vent from the shuttle itself. Normally, the water showers into space as a spray of crystals. But on one mission in 1984, the water built up into a baseball-sized lump of ice on the shuttle's wing, and had to be dislodged by a robot arm. "The short answer is that we don't know what it is, but we are looking at it very closely," a Nasa spokesman said.
Researchers are also examining two key pieces of Columbia debris recovered from Texas. One is a 0.6-metre piece of the spacecraft's wing, including some of its thermal tiles, and the other is a 140kg cover over the landing gear - perhaps the site of a sudden temperature rise moments before the shuttle broke apart. More than 12,000 pieces of debris have been found in Texas and Louisiana - enough to begin only a very tentative reassembly of the sequence of disaster.
"There is certainly no way we are going to be able to reconstruct it," said the Nasa chief Sean O'Keefe. "The pieces are just absolutely mangled. It's an awful lot of tangled stuff."