AN AEROSPACE company has come up with a bizarre way to deal with space junk. Instead of waiting for it to break up into a huge number of fragments, it is proposing to send up what it calls a 鈥渟pace sheepdog鈥� to usher the junk safely out of orbit. It could even put its spare momentum to good use.
Space junk poses an increasingly serious danger to spacecraft. A head-on collision with a centimetre-sized piece of junk would release a similar amount of energy to a collision with a bowling ball at 100 kilometres per hour. Low-Earth orbits are now littered with around 1900 tonnes of debris.
Most of the junk is accounted for by a relatively small number of large items such as spent launchers and dead satellites, which are easy to track and avoid. But that could change. In some orbits, a chain reaction is under way: fragments from past collisions are becoming involved in more collisions, generating more fragments, and so on. 鈥淚t鈥檚 a very slow process but at some point we will have to tackle this problem,鈥� says Joe Carroll, an aerospace engineer at Tether Applications, a space technology company based in California.
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One way to clear the skies is to attach rocket motors to the largest objects and send them crashing to Earth. But this requires large amounts of fuel to put the rockets into orbit and power them when they get there. So Carroll suggests a more elegant solution: a reusable solar-powered craft that manoeuvres using forces generated when an electric current interacts with the Earth鈥檚 magnetic field. NASA鈥檚 Institute for Advanced Concepts is funding a feasibility study into his idea.
The main component of Carroll鈥檚 vehicle is a conducting wire several tens of kilometres long, known as an electrodynamic tether, carrying an electric current. As the tether sweeps though the Earth鈥檚 magnetic field, the current interacts with the field, raising or lowering the craft鈥檚 orbit. 鈥淚t鈥檚 a bit like tacking in a sail boat. You push and pull against the field until you get where you want,鈥� says Carroll.
His plan is to equip the tether with a roving sheepdog, a small vehicle that is released near a piece of debris to fly around it looking for a suitable point to latch onto. Once attached, it returns to the tether with its prize in tow. The tether then heads for another piece of junk and sets the sheepdog loose again. 鈥淎 single tether could be reused up to 100 times, capturing a piece of junk many times its own mass each time,鈥� he says.
To send pieces of junk out of orbit would be relatively straightforward 鈥� they simply need decelerating to sub-orbital speed. But Carroll has a more ambitious idea. He suggests using the momentum of a large mass of collected junk to boost the orbit of a working spacecraft by transferring momentum from the junk to the spacecraft.
In Carroll鈥檚 scheme, the junk would act as ballast at one end of the tether with the spacecraft at the other end. By carefully controlling the forces produced on the tether, the two masses would be set rotating about each other. Releasing the spacecraft at the appropriate moment would send it to a higher orbit 鈥� while the junk ends up in a lower orbit. The junk鈥檚 orbit can then be restored slowly using the tether until it is ready for its next launch.
