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![]() Space is big, but not big enough EUROPEAN SPACE AGENCY SCIENCE RELEASE Posted: September 28, 2002 According to Douglas Adams, in his famous book The Hitch-Hikers Guide to the Galaxy, space is big. However, it seems near-Earth space is not big enough. In December 2001, the Space Shuttle pushed the International Space Station away from a discarded Russian rocket booster that was due to pass uncomfortably close. Space litter is a growing problem but smarter satellite design may help in the future.
ESA's European Space Operations Centre (ESOC) in Darmstadt, Germany, tracks space litter. It estimates that over 23 000 objects larger than 10 centimetres have been launched from Earth. Of these, about 7500 are still orbiting - only a very small proportion of them (6%) is operational. Half of all the objects are inoperable satellites, spent rocket stages, or other large space litter; the remaining 44% is debris from explosions and accidents in space. To make things worse, there are an estimated 70 000 to 120 000 fragments smaller than 1 centimetre and the amount of space debris increases by about 5% every year. Tiny fragments, such as paint flecks, moving at very high velocities of around 6 kilometres per second can create problems for the spacecraft and for astronaut. One way to lower the threat is to remove satellites from orbit at the end of their working lives. If we force satellites down through the Earth's atmosphere, they burn up. However, this is more complicated if the satellite is so large that parts of it are liable to survive reentry and strike the ground. This is the case for some Earth observation satellites, for example, which are very big and heavy. When removing a (dead) satellite from orbit is too difficult, it is simply left in orbit. However ESA is developing a new technology for its Darwin mission. This technology may allow smaller, more easily disposable satellites to replace often enormous relics in the future and may improve spacecraft control.
Another improvement is to position satellites further away, to reduce 'traffic jams' in near-Earth space. "This is a better position for Earth observation anyway," says Fridlund, "Because, in low-Earth-orbit, the satellite orbits every 90 minutes, it is only over each spot on Earth for a short period of time. In geostationary orbit, however, the satellite would be looking at one whole hemisphere continuously, so you could just point the array to wherever you are interested." Moreover, we could use Darwin's formation-flying technology to equip every satellite with a collision-avoidance system. Unrelated satellites would communicate with their neighbours and take corrective action if they began to drift together. Of course, nothing will completely remove the threat of space litter. However, if we can use advanced technology to remove unwanted hardware from orbit, space will definitely become safer.
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