A NEW module on the International Space Station will be wrapped in cavity wall insulation鈥攂ut it鈥檚 not designed to keep out the cold. It鈥檚 a novel way to buy astronauts enough time to evacuate a stricken module after a collision with space junk or a micrometeoroid. It鈥檚 secret? Balls.
The threat is clear: there are around 35 million shards of debris the size of sugar lumps hurtling through space in low Earth orbit. And another 100 000 as big as tennis balls. Even the smallest particles can cause serious damage to a poorly protected spacecraft because they travel at 2 to 15 kilometres per second (between 4500 and 33,500 miles per hour).
Space agencies have developed different types of shield but most rely on a thin but brittle layer of either aluminium mesh or ceramic fibre. This smashes the incoming particle into a harmless cloud of finely distributed debris. A second tough layer of aluminium or Kevlar, the stuff of bullet-proof vests, absorbs the debris cloud.
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But these shields only work if the particles are around a centimetre in diameter, says Hedley Stokes, who works on satellite debris shields at QinetiQ, a British defence research establishment. Larger debris over 10 centimetres is routinely tracked by radar and the ISS regularly makes precautionary manoeuvres to avoid it. And NASA has developed a puncture repair kit for larger holes, effectively a glue-on hub cap (快猫短视频, 1 April 2000, p 11).
What about particles larger than a centimetre across but smaller than 10 centimetres? Hidehiro Hata at the Kyushu Institute of Technology says his priority is to give astronauts enough time to evacuate a module before it depressurises and they suffocate.
Hata鈥檚 invention is essentially a two-layered shield filled with air and polystyrene balls. It is designed to sit behind a standard debris shield. If a piece of space junk or a micrometeoroid pierces the first layer of his shield the pressure difference between the cavity and the vacuum outside sucks air out鈥攁nd plugs the hole with one of the balls. If the wall of the spacecraft is punctured and the cabin starts to depressurise the plug will slow the leak sufficiently to let the astronauts get out.
To test the idea, Hata fired metal projectiles at a pressurised container fitted with his shield. Inside the shield鈥檚 cavity, Hata suspended one of the balls on a thread. As the projectile, which was travelling at 6710 miles per hour, penetrated the shield, the escaping air forced the ball to swing and block the hole. Hata has now coated the polystyrene balls with metal foil to protect them from heat, after finding that the temperature produced by the impact is enough to melt and fuse the balls together.
Hata鈥檚 shield is being designed for Japan鈥檚 Kibo research module, which is due to be fitted onto the ISS in 2004. To see if the shield would give astronauts more time to evacuate a punctured module, he measured the drop in air pressure after firing projectiles at containers with different amounts of shielding. With no debris shielding, the pressure dropped to ambient levels within a second. The same pressure drop took around three minutes with the shield fitted.
Having more time to evacuate could be vital. Even if an astronaut isn鈥檛 struck by an incoming particle, the confusion following an impact could slow them down considerably, says Stokes. 鈥淥ne of the many problems is the light flash caused by the particle鈥攊t can be enough to blind the astronauts,鈥� he says.
