Science: Sucked from space

Hurtling towards you at 20,000mph, even the tiniest flake of paint becomes a deadly bullet. And with tons of debris now orbiting Earth at these speeds, the race to dream up a space-age dustcart is on. Roger Dobson reports

JEANNE LEE CREWS is eager to point out that while her ambitious plan for cleaning up space may be out of this world, it is not in any way wacky. "It might be expensive and a technical challenge, but it's sure not crazy," says Lee Crews, an expert in hyper-velocity impact at Nasa, as she explains her blueprint for the extraterrestrial garbage collector she's designed to zap much of the large amounts of man-made debris currently circling the Earth.

The popular perception of the space around our planet is of a huge black void, free from human pollution, where the peace is disturbed only by the glint of reflected light from the occasional passing satellite.

But "near" space (up to 25,000 miles away) is no longer the virgin territory it once was. Nasa's latest research shows that there are more than 35 million pieces of junk in orbit, with a combined weight of more than 3,000 tons. There are 9,000 pieces longer than 10cm, nearly 150,000 between 1cm and 10cm, and 35 million bits of debris smaller than 1cm.

And one doomsday scenario suggests that unless something drastic is done about all this high-speed waste, it will keep on circling and colliding and breaking down into more (and smaller) chunks until eventually a giant, self-perpetuating and uncontrollable cloud of tiny metalic particles will cast a shadow over the Earth which may even interfere with our sunlight and climate.

Dr Lee Crews' brainchild, a 10km-wide balloon that floats in space and acts as a giant "shock-absorber", is not the only extraordinary idea on the table as scientists around the world debate what to do about the growing problem of junk in space. Other proposed solutions include a giant foam ball to "mop up" smaller debris, an inflatable wall, salvage rockets to tow satellites into graveyard orbits, and a giant laser to knock down the bits and pieces.

There is now so much debris from previous space journeys that every time the Shuttle goes up to its 400-mile-high orbit, it collides with some of the tiny bits of wreckage that travel at speeds of up to 30,000km an hour. The junk appears to be spread throughout the space around the Earth, locked in orbits as close as 200 miles and as far as 20,000 miles away (which is one-eleventh of the distance to the moon). So the chances of spacecraft being hit are fairly high.

"The Shuttle is struck every time it flies and the five largest im-pacts we've had so far were between 1mm and 3mm in size," says Dr Jeff Thealle, an orbital debris specialist for Nasa who works at the Johnson Space Center.

Precisely how many hits the spaceship takes each journey is not known, because on re-entry its outer coating of ceramic tiles goes through an annealing process as a result of the high temperatures, which removes much of the evidence of impact from small pieces of debris. "We review the risk each time the Shuttle goes up and each time it comes back we go over it with a fine-tooth comb and we do find some of the signs of impact. The windows are the easiest place to spot this. Every 10 days of orbit we take one hit large enough to mean we need to replace a window," says Dr Thealle.

"Typically, this kind of small debris comes at an angle of 30 to 45 degrees and with an average collision speed of about 10km a second, but because of the Shuttle's design it is pretty resistant to what would be the catastrophic penetration of the crew compartment."

But it is these smaller pieces, the ones less than 10cm in length, which are potentially the most dangerous: while the movement of larger bits can be tracked from the Earth, so allowing for avoiding action, the smaller debris is invisible right up to the point of impact.

Much of the junk is the wreckage of satellites that have disintegrated for one reason or another in space - nearly 160 in 35 years - and includes old fuel tanks, the occasional tool lost from a manned spacecraft, drops of coolants from a spent nuclear reactor, and millions and millions of copper needles from an aborted weather experiment that was launched when the Beatles were still waiting to hit the big time.

"There are all kinds of things out there, and they're travelling at extremely high speeds," says Dr Thealle. "There are dead rocket bodies, fragments of spacecraft that have broken up, aluminium rocket parts, pieces of copper from wires, circuit boards, bits of plastic, carbon-fibre rods from the Pegasus break-up, and copper needles in an orbit 6,000km away that were put up there years ago with the aim of reflecting radio waves off them."

More bizarrely still, there are also pieces of titanium paint hurt-ling around the Earth. As sections of wrecked spacecraft have degraded, the paint has come off. One tiny flake of paint travelling at 10km per second - more than 20,000mph - has enough momentum to penetrate a radiator tube on the Shuttle.

It is against this background that engineers have been looking at various ways of ensuring that there is no further build-up of space- polluting junk. One idea already under consideration by the world's space agencies is that each satellite should be brought back to Earth within 25 years of the end of its useful life. Once steered back into the Earth's atmosphere, they would burn up harmlessly. Another proposal suggests that damaged or redundant satellites should be towed by special salvage crafts into new "graveyard" orbits which would be clearly sign-posted for future space travellers.

But some scientists, Dr Jeanne Lee Crews among them, believe that something must also be done about the rubbish that is already out there. "Debris in space is a big problem and we have to do something," says Dr Lee Crews. "Just as with the rivers and the lakes on Earth, space is rapidly becoming polluted, almost before we realise it."

The Nasa Project Orion, for example, is examining the idea of using ground-based lasers to "de-orbit" the pieces of space-junk that are larger than 1cm in diameter. The theory here is that one part of the object would be heated by a pulsed laser beam, a process that would cause the material to liquefy at one end, which would in turn unbalance it. Within a few revolutions, the debris would be nudged out of orbit and would fall to Earth.

Another idea involves the use of giant foam balls to catch and "smother" smaller bits of debris. The foam would be contained within a spacecraft for launching but once in orbit and chemically activated, it would expand into a large ball - not unlike the concept of silly string. Few of the details have yet been worked out, but the spacecraft would need to be fairly large, and would keep to a known orbit in order to avoid any collisions with working satellites. When it had done its filtering job it would fall to Earth and burn up on re-entry.

Dr Lee Crews, who designed some of the shields for the new international space station, has drawn up plans for a huge balloon that would mop up the junk. Between 1km and 10km in diameter, the balloons would be made from the same ceramic material that is currently used in some of the new-generation spacecraft. The principle, similar to that of the space station's shield, depends on the ability of this ceramic material to produce shock effects in whatever hits it.

Dr Lee Crews explains: "Ceramics are excellent at producing shock effects. In this application you want something that will shock whatever has struck the craft so much that it is vaporised. If you have four or five sheets, one behind the other, you shock it repeatedly and raise energy levels so much you get more vaporisation and it gets smaller and smaller."

The balloons, complete with collapsible framework, would be launched into space uninflated and once in their target orbit would be filled with gas. The balloon would have four or five smaller balloons inside it, each separated by about 20ft. "With this balloon in orbit, most of the smaller debris that hit it would be vaporised and the rest contained inside the balloon. The balloons would act as a multi- shock shield." They would be left in space for some time to collect debris and when full, would be nudged out of orbit and returned to Earth. The balloon could then either be scrapped, or the holes caused by impact repaired and the whole thing re-used.

"My biggest fear," says Dr Lee Crews, "is that with everyone wanting satellites these days, the problem is going to get worse and worse. We can track everything over 10cm, and we can shield against anything under 1cm, but with all the rest you just have to hope and pray, and I don't much like that." !