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'Space Sheepdogs' Could Round Up Space Junk

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    NHNE News List Current Members: 760 Subscribe/unsubscribe/archive info at the bottom of this message. ... SPACE SHEEPDOGS COULD ROUND UP SPACE JUNK By Justin
    Message 1 of 1 , Jan 17, 2003
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      By Justin Mullins, Boston
      New Scientist
      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.

      Electrodynamic tether

      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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