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BUILDING a space station in orbit is a risky business. One false move while
tightening a bolt and an astronaut can go spinning off into the void, 200 miles
above the Earth. So the hunt is on for technologies that will let structures
assemble themselves.

One idea is, quite literally, repulsive. The idea is to use the mutual
repulsion of superconducting cables to hold a structure rigid. James Powell of
Plus Ultra Technologies, of Shoreham, New York, believes his company鈥檚 brainwave
will not only speed up construction times but also make space structures
cheaper.

Plus Ultra鈥檚 plan is to ship coils of floppy cable into orbit on a space
shuttle, and then slowly play it out into space, allowing magnetic forces to
make it rigid as it does so. This structure will provide a stiff skeleton for a
spacecraft or space station.

Each truss or beam in Powell鈥檚 scheme would consist of four superconducting
cables arranged in a square
(see Graphic). The cables get their power from a
solar panel on the satellite. They are wired up so that the current in each
cable flows in the opposite direction to that of its immediate
neighbour鈥攃reating repulsive magnetic fields between the cables. This
forces the four cables apart, but to ensure that they don鈥檛 simply float away
from each other, tethers hold them in a square formation.

鈥淚t creates a very strong, stiff structure that is resistant to bending,鈥�
says Powell.

But the structure will only stay rigid if enormous magnetic forces push the
cables apart, requiring huge currents鈥攈undreds of thousands of
amps鈥攖o flow in the cables. Only resistance-free superconductors can
handle such currents.

Each of the cables has to be cryogenically cooled in order to superconduct.
This is done while the four cables are coiled tightly together inside the
cryogenic container. The low temperature is then maintained by pumping cooled
helium through the cables.

On Earth, one of the problems with superconducting cables is creating and
maintaining a vacuum around the cable to preserve the temperature鈥攂ut at
least in space the vacuum comes for free.

As the solar panel starts pumping current through the cables, they gradually
begin to uncoil and expand to their full size. It鈥檚 a bit like a giant Indian
rope-trick. 鈥淚n principle, you can launch each of these trusses into space and
have them deployed without any human intervention,鈥� says Powell.

Although the cables require constant current to keep them apart, you don鈥檛
need a lot of energy, Powell says. As long as the cables stay supercold, they
can sustain currents big enough to do the job indefinitely, with only a tiny
trickle current from the solar panels.

The advantage of using magnetically inflated cables would be that the long
trusses are easy to deploy鈥攔ather than the current method of attaching
shorter pieces of pre-assembled trusses together鈥攕aving time and reducing
the amount of time astronauts spend on dangerous spacewalks.

But unfolding the cables in space will be a major challenge. The history of
such experiments is not a happy one鈥攅nergy-gathering tethers have failed
to unfold, robot arms have developed rheumatism, and space mirrors have stayed
resolutely stowed. Alan Thirkettle, head of crewed space flight at the European
Space Agency, says it鈥檚 not going to be easy to prove that the magnetic cables
will unfurl as intended. 鈥淚f you are launching something that鈥檚 stowed you have
to make sure it will deploy successfully and reliably,鈥� he says.