Electrodynamic Tethers: Getting into the Swing

The Momentum-Exchange/Electrodynamic-Reboost (MXER) Tether System could pick payloads up from a reusable launch vehicle in low Earth orbit and toss them to geosynchronous orbit. This idea could act as a Hypersonic Airplane Space Tether Orbital Launch system, or be utilized to handle other launch systems other than hypersonic airplanes. CREDIT: Tethers Unlimited, Inc.

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.)