'Space Sheepdogs' Could Round Up Space Junk
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'SPACE SHEEPDOGS' COULD ROUND UP SPACE JUNK
By Justin Mullins, Boston
January 15, 2003
An aerospace company is proposing to deploy what it calls a "space sheepdog"
to usher space junk safely out of orbit. It could even put the junk's spare
momentum to good use.
Space junk poses an increasingly serious danger to spacecraft. A head-on
collision with a centimetre-sized piece 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 about 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.
"It's a very slow process but at some point we will have to tackle this
problem," says Joe Carroll, an aerospace engineer at Tether Applications, a
space technology company based in California.
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's magnetic field. NASA's Institute for Advanced Concepts is
funding a feasibility study into his idea.
The main component of Carroll's 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's magnetic field, the current
interacts with the field, raising or lowering the craft's orbit. "It's a bit
like tacking in a sail boat. You push and pull against the field until you
get where you want," 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. "A single tether could be reused up to 100 times,
capturing a piece of junk many times its own mass each time, " he says.
To drag pieces of junk out of orbit would be relatively straightforward --
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's 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 -- while the junk ends up in a lower orbit. The junk's orbit
can then be restored slowly using the tether until it is ready for its next
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