A NASA team is preparing a bid to test a chemical-free propulsion system that taps the power of Earth's magnetic field to move satellites and spacecraft in orbit.

Think of the force holding together or repelling two magnets. A similar energy is generated when an electrically charged wire encounters a magnetic field, such as what envelopes Earth.

Space tethers collect current when they fly near the ionosphere -- the charged, upper layer of atmosphere -- and magnetic field. The current flowing through the wire will be pushed on by Earth's magnetic field, creating a force that can be used to raise or lower a spacecraft's orbit.

Satellites circling Earth have to periodically re-boost themselves to compensate for the degradation of their orbits due to friction from colliding with atmospheric particles that have escaped into space. The satellites have a limited supply of fuel for thruster rockets. When the gas runs out, the spacecraft's days are numbered. A tether likewise could be used to lower a satellite's orbit so that it could, for example, more quickly re-enter Earth's atmosphere at the end of its operational life and avoid becoming another piece of space debris.

"You can move around by pushing against the Earth's magnetic field. That's the real benefit," Les Johnson, deputy manager of the Advanced Concepts Office at NASA's Marshall Space Flight Center in Huntsville, Ala., told Discovery News.

NASA tested the concept of space tethers during shuttle missions in 1992 and 1996, and had plans to launch a small, operational model on an unmanned rocket in 2003. Fallout from the shuttle Columbia accident, however, spurred NASA to cancel the flight.

Now, with a renewed emphasis on technology demonstrations at NASA, Johnson and colleagues are girding for a potential new flight opportunity. This time, however, they would like to fly a full-size, operational system.

"If we can't show how this can be used by people to do their new missions, then we don't want to do it," Johnson said. "We want to demonstrate a tether propulsion system that someone can just go and copy it, and fly it."

The team expects some fierce competition -- provided Congress agrees to fund the proposed technology demonstrations. The Obama administration has requested $75 million for the program for the year beginning Oct. 1.

NASA's chief technology guru Bobby Braun says tethers are among a broad array of space technologies he expects will be vying for funds.

"We really are interested in casting a wide net. We're not going to specify the precise technology to be demonstrated," Braun told Discovery News.