Propellant-free propulsion might sound like somebody is trying to string you along. And you're partially right. The difference is that the "string" is 9.3 miles (15 kilometers) of tether line.

The Propulsive Small Expendable Deployer system -- or ProSEDS for short -- is built to reel out into space a long tether. What scientists and engineers are eager to test is the interaction of the bare-wire part of the tether as it sweeps through the Earth's magnetic field. That hardware will fly aboard a Delta 2 rocket as a secondary payload. The booster's primary duty, however, is to loft an Air Force Global Positioning System (GPS) satellite into orbit.

What ProSEDS hopes to demonstrate is electrodynamic tether propulsion.

Similar to the principle of an electric motor in many household appliances, as well as automobile generators, when a wire moves through a magnetic field, an electrical current results. As this current flows through the wire, it experiences a push from any external magnetic field. For a space tether, that magnetic field is found naturally around the Earth. The force exerted on the tether by the magnetic field can be used to raise or lower a satellite's orbit, depending on the direction of the current's flow.

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	Artist's concept of NASA's Propulsive Small Expendable Deployer System - called ProSEDS - portrays the system as the tether is deployed in space. Inexpensive and reusable, Propulsive Small Expendable Deployer technology has the potential to turn orbiting, in-space tethers into "space tugboats" -- replacing costly traditional chemical propulsion and enabling a variety of space-based missions. Credit: NASA
	Les Johnson, a scientist at NASA's Marshall Space Flight Center in Huntsville, Alabama inspects the nonconducting part of a tether as it exits a deployer similar to the system to be used in NASA's Propulsive Small Expendable Deployer (ProSEDS) experiment. Inexpensive and reusable, ProSEDS technology draws power from the space environment around Earth, allowing the transfer of energy from the Earth to the spacecraft. Credit: NASA
	Artist's concept of an orbiting space vehicle in the Jovian system using an electrodynamic tether propellantless propulsion system. Electrodynamic tethers offer the potential to greatly extend and enhance future scientific missions to Jupiter and the Jovian system. Like Earth, Jupiter posses a strong magnetic field and a significant magnetosphere. This may make it feasible to operate electrodynamic tethers for propulsion and power generation. Credit: NASA
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Once in space, ProSEDS will deploy from the Delta 2's second stage the lengthy two-part tether: a 3.1-mile-long (5 kilometers), ultra-thin bare-wire tether connected with a 6.2-mile-long (10 kilometers) non-conducting tether. Thanks to touchy-feely physics, the tether will produce thrust, forceful enough to lower the altitude of the spent Delta 2 upper stage.

This propellant-free propulsion idea not only draws power from the space environment, it could open up the possibility of using orbiting in-space tethers as "space tugboats", or enabling fuel-free raising and lowering of satellite orbits. Furthermore, ProSEDS and the tether experiment open the door for a variety of concepts, from catching and tossing payloads from one orbit to another to elevators that offer "next floor, space" service from Earth.

Long in coming

July 25, 2002 is the latest estimate from the USAF for a ProSEDS launch, said Les Johnson, manager of in-space transportation at the NASA Marshall Space Flight Center in Huntsville, Alabama.

"ProSEDS will be successful if we demonstrate that the bare tether collects current and we get significant, measurable thrust," Johnson told SPACE.com . "That's the whole objective. We know that the tether physics is good…that it will collect current…but we've never demonstrated thrust."

The experiment will take place at about 224 miles (360 kilometers) above the Earth. That altitude was recently picked to reduce the likelihood that the tether could impact the International Space Station, Johnson said.

"Even though it was a very small probability," Johnson said, "the station is such a valuable asset you don't want to take any risks at all."

The ProSEDS tether has been hand crafted by Joe Carroll of Tether Applications Inc. of Chula Vista, California.

The tether try out in space could last as long as three weeks. But the first few orbits of the equipment are expected to yield all the data needed, and show that thrust was produced at predicted levels.

If the ProSEDS experiment works as planned, Johnson said that tethers are likely to find a special niche in future space operations. "We're demonstrating how to deorbit a spacecraft. I think that commercializing this for use to bring down satellites at their end of life…is the first, logical commercial application for what we're demonstrating with ProSEDS."

Beyond ProSEDS

Johnson explained that keeping an object in Earth orbit for an extended period of time is a candidate use for space tethers. "Doing so can save you a lot of money and a lot of weight over a conventional chemical propulsion system."

The International Space Station could benefit by a tether satellite reboost system, Johnson said, although a detailed design study has yet to be done. "If we can find a way to have minimum impact on the station itself, and yet provide the reboost capability, it might be something worth considering," he said.

Tether aficionados have started looking at a Momentum Exchange, Electrodynamic Reboost tether propulsion system, or MXER. For this idea, a 93-mile-long (150 kilometers) tether would use electrical current and gravity to put itself into a tight spin. As the tether rotates through space in an elliptical orbit, payloads launched from Earth rendezvous with MXER. The tether snags payloads via a net-like catch, then hurls them toward their final destination.

Using MXER as a momentum machine, shooting payloads beyond low-Earth orbit can be done cheaply and efficiently. No need to fuss with expensive, one use only, toss away upper-stage rockets to chuck spacecraft high above Earth, or even to the Moon, Mars, and the outer planets.

Next floor, please!

Tether advocate, Jerome Pearson is president of Star Technology and Research, Inc., of Mount Pleasant, South Carolina. "ProSEDS is the next significant step on the way to proving the usefulness of tethers in space. It will demonstrate the use of an electrodynamics tether to decay the orbit of a spent upper stage for the first time. That will open the way for the use of tethers to de-orbit debris," he noted.

Pearson said he is working on the EDDE spacecraft, standing for ElectroDynamic Delivery Express. EDDE is designed to demonstrate that an upper stage propulsion system can perform large changes in a spacecraft's orbit without use of propellant.

Taking tether work to a new level is Brad Edwards, a visionary engineer based in Seattle, Washington. He is busily scripting a futuristic space elevator.

"ProSEDS along with past and future tether experiments are great for learning about handling tethers in space, tether deployment dynamics, and to find out what nature has in store for us," Edwards said. Deploying the ProSEDS tether, he added, is good first practice for unleashing a 62,000-mile (100,000-kilometer) long ribbon as part of the researcher's elevator to space project.