Lowering the Cost of Space Flight.

New propulsion system harnesses Earth's magnetic field.

Launching a satellite or spacecraft into orbit isn't cheap: It can I cost up to $10,000 Per pound, plus an extra $7,000 per pound to maneuver the craft once it's up there. Considering that most of a craft's weight is fuel, there

Scientists and engineers at NASA's Marshall Space Flight Center in Alabama are working on a new propulsion system that uses no fuel, is environmentally friendly, potentially reusable, and could reduce the costs of maneuvering a spacecraft--once it is launched into orbit--to only hundreds of dollars per pound. The Propulsive Small Expendable Deployer System, or ProSEDS, deploys a 5-kilometer-long aluminum wire tether. This tether produces thrust when it interacts with the Earth's magnetic field and the ionized gases, or plasma, in the upper atmosphere.

The wire tether works on the same principle as an electric generator: As the tether moves across the Earth's magnetic field, a voltage is induced across its length. The upper end of the tether becomes positive, so electrons in the upper-atmosphere plasma, which carry a negative charge, are attracted to the upper end. By using a device to emit the electrons back into space at the lower end of the tether, a current can be made to flow along the length of wire.

When attached to a spacecraft, this system produces thrust because the Earth's magnetic field exerts force on a current-carrying wire. When the electron current flows from the upper end of the tether to the lower end, the pull from the Earth's magnetic field slows the spacecraft and causes it to fall into a lower orbit. But if electricity is artificially made to flow from the lower end to the upper end of the tether, the Earth's magnetic field pushes on it, speeding up the craft and raising its orbit.

The first demonstration of this system is planned for September 2000. In the experiment, NASA scientists will use a tether to slow down a spent rocket stage and lower its orbit. The ProSEDS experiment, as it is called, should be visible from the ground. NASA plans to develop a Web site to inform viewers where and when to look if they want to catch a glimpse of this string in the sky. (Visit www.msfc.nasa.gov for more information.) Future experiments are in the works to show that tethers can raise the orbit of a spacecraft as well.

NASA and the commercial space industry foresee many possible uses of this technology, such as reducing the ever-increasing amount of space junk. A satellite equipped with a small tether could deploy it near the end of the satellite's useful life; this would cause the satellite to lower its orbit and burn up as it reentered the Earth's atmosphere, rather than orbit as junk for a long period of time.

Tethers could also keep the International Space Station aloft. Using power from the space station's solar arrays, current could be made to flow from the lower to the upper end of a tether, resulting in an orbit-raising thrust. It might be possible to completely eliminate the need for chemical propellant to re-boost the station, thus potentially saving as much as $2 billion over 10 years.

Les Johnson is a scientist at NASA's Marshall Space Flight Center and a member of the World Future Society. His address is Mail Code TD15, NASA Marshall Space Flight Center. Huntsville, Alabama 35812.