An electrodynamic tether
(EDT) is a simple idea, but one with an amazing number of uses. An EDT system is
made up of two masses in orbit connected by a long, flexible, electrically
conductive cable; the tether is essentially a wire that moves through the
magnetic field of the Earth (or another planet or large body). An EDT takes
advantage of two basic principles of electromagnetism: current is produced when
a conductive wire moves through a magnetic field, and the field exerts a force
on the current. While it is not a colorful as the Rasta space tug from William Gibson's Neuromancer, it could
provide a technology that takes low-flying satellites to a new level.
An EDT can use these principles in two ways:
- An EDT can generate electric current flow towards
the planet; this can provide enough electricity to run experiments on board a
satellite; this also causes the tether to experience a force from the planet's
magnetic field that is opposite the tether's direction of motion. In other
words, it slows the EDT system down (produces drag), lowering the EDT's orbit.
- By adding a battery (or solar panel) to the EDT
circuit, the induced current is overcome, reversing the current direction; the
force experienced by the tether is now in the same direction as the EDT's
motion. In other words, this produces thrust, raising its orbital attitude.
A tether moving from west to east
through the Earth's northward-pointing magnetic field will experience a current
flow down the tether. The anode end of the tether collects electrons from the
ionosphere and ejects them from negatively charged cathode; the ionosphere is
electrically conductive and completes the circuit. Steady current for onboard
power results. A 20 kilometer tether in low earth orbit (LEO) could produce up
to 40 kilowatts of power; this is enough to run manned research facilities.
As discussed, this method of creating electrical
power has a serious side-effect, namely, that the spacecraft or satellite will
experience drag. That is, it will slow down and seek a lower orbit, eventually
crashing on the planet. However, there is a way to turn this "bug" into a
feature. One problem that is getting worse around Earth is that of discarded
"space junk" (see illustration below). This property of EDTs could be used to
bring down this junk earlier and in a controlled fashion. At the end of a
satellite's life (or a rocket stage, or anything else), it is given a signal to
release a long wire antenna. A current will flow in the wire, and the satellite
will begin to slow down, quickly heading for burn-up in the atmosphere.
The second use of EDT is really amazing. The
International Space Station will require over seventy tons of propellant over
the next ten years to keep its orbit from decaying. All of this reaction mass
must be hauled up the gravity well at a cost of $7000 per pound! A
properly deployed EDT, combined with another power source, could actually push
itself forward on the Earth's magnetic field, speeding the IST up without the
use of any propellant.
This will have profound implications on missions to
other planets, since satellite missions will no longer need to carry expendable
propellant to continue their missions. An EDT can be used to alternately boost
to a new location, and then reverse current and use the resulting drag force to
decellerate and fine-tune its orbit. This greatly extends the useful life of
these space probes.
So why aren't EDTs in widespread use? The biggest
problems are electromechanical; EDTs experience high voltages in space. Also,
EDTs are prone to vibrations that produce significant mechanical forces. Also,
development has been slow; a program planned for launch in 2004, ProSEDs, was
repeatedly postponed and ultimately canceled.
Science fiction author David Brin has made a good
story (from 1982) available; you might want to go read Tank Farm Dynamo. This story is a
poignant tale of what might have been in the last century of American space
travel. The story takes maybe fifteen minutes to read - it's well worth it! For
another spacetug story, see ConeXpress OLEV - Will A Good Tug Save Hubble?.
Long space tethers don't need to be conductive to
have unusual effects - see this article on generating artificial gravity with tethers.
A lot of the background information for this article
was found in Electrodynamic Tethers in
Space from the August issue of Scientific
American; unfortunately, it's not free. Thanks to yet another alert reader for
the scoop on this story.
(This Science Fiction in the News story used
with permission from Technovelgy.com - where science meets fiction.)
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