WASHINGTON — A year from now, the U.S. Navy hopes to launch a pair of small satellites that are able to harvest electrons from space and use them and the Earth’s magnetic field as a propulsion system.
“It’s like a giant electric motor in space,” said Shannon Coffey, who heads the Tether Electrodynamics Propulsion CubeSat Experiment at the Naval Research Laboratory. “What are the components of a motor? Current in wires and a magnet.”
In this case, the wire is a kilometer-long, 3-millimeter-thick tether strung between two tiny satellites. The Earth acts as the magnet. The result — it is hoped — will be limitless propulsion for maneuvering the satellites.
At present, satellites can maneuver only as long as they have fuel, which typically accounts for a good part of their launch weight. The errant U.S. spy satellite that the Navy shot down in 2008, for example, weighed about 2,250 kilograms; one-fifth of that was hydrazine fuel.
The need for fuel means satellites must be bigger and the rockets that launch them must be bigger, pushing costs up. And when the fuel runs out, the satellite is stuck in orbit until Earth’s gravity pulls it into the atmosphere, where it burns up.
Electrodynamic propulsion offers an alternative.
“We have a magnetic field around the Earth,” Coffey said. “Now all we have to do is figure out how to get a current to flow on a wire.”
And, he says, the Naval Research Lab has.
Ordinarily, to get electrical current to flow through a wire requires a complete circuit — a loop of wire, Coffey said. But that won’t generate propulsion in space.
When electricity flows through a wire loop in the presence of Earth’s magnetic field, the field will exert a force in one direction on one side of the wire and in the opposite direction on the other side, and the forces will cancel each other.
To get a force that pushes in only one direction, electricity must be made to move through the wire. But that means electrons have to come from somewhere and go somewhere.
“We complete the loop through the Earth’s plasma,” Coffey said.
Plasma is essentially a low-density cloud of electrons that exists in space, but only in useful quantity up to altitudes of about 1,000 kilometers. An electron collector at one end of the tether gathers electrons from the plasma. An electron emitter at the other end discharges electrons back into the plasma, completing the circuit.
“This way, we get a force on the wire,” Coffey said.
Small solar cells generate enough current to begin pushing electrons through the kilometer-long wire.
The electrical current then generates a force that pushes against the tether, and depending on the orientation of the tether to the magnetic field, the force will either speed up or slow the satellite, thus changing its orbit.
By reorienting the tether to the magnetic field, satellite operators can change the plane of the satellite’s orbit.
The power of the electrodynamic motor is limited, though.
Coffey said the demonstration planned for 2011 “should allow us to change altitude by about 5 kilometers per day.”
By contrast, a standard liquid-fueled thruster can push a satellite thousands of kilometers per hour, he said.
“So we use time to our advantage. It may take a month or more to change the inclination” of a satellite’s orbit, “but we’ve got time,” Coffey said.
Another problem to overcome is ensuring there are enough electrons in space for the electrodynamic motor to work. “It works best at 500 to 1,000 kilometers,” Coffey said.
Farther away from Earth than that, electrons become exceedingly scarce.
But that doesn’t preclude satellites from operating at high altitudes. They can establish elliptical orbits and use the motor only when the satellite is closest to Earth and electrons are relatively plentiful. Elliptical orbits offer important advantages. Satellites can dwell for hours at their apogee — the distance farthest from Earth. That is particularly useful for certain kinds of communications and surveillance satellites.
Electrodynamic propulsion might also make it possible to tackle another problem — removing orbiting junk from space.
“One idea is to use the propulsion system to rendezvous with debris, grab it and use the propulsion system to bring it down to an orbit where it would decay” and eventually burn up in the atmosphere, Coffey said.
Indeed, the U.S. Defense Advanced Research Projects Agency has enlisted Tethers Unlimited to develop tether-powered satellites that can capture orbital debris and even small asteroids and maneuver them out of orbit.
Electrodynamic propulsion systems are ideal for that work because they generate thrust over long periods without consuming propellant, said Robert Hoyt, a founder of Tethers Unlimited, Bothell, Wash.
Space debris — dead satellites, rocket parts, tools that astronauts dropped and the like — has increasingly cluttered space and threatens to damage satellites and other spacecraft.
Tether propulsion systems also could be used to power space tugs that would be used to drag satellites to higher or lower orbits and reposition them to different inclinations, Hoyt said.
Another possible use for the propulsion system is keeping satellites in low orbits of about 300 kilometers where they are subjected to substantial drag that would normally cause them re-enter the atmosphere.
Using electrodynamic propulsion, the satellites could overcome the effects of drag and remain in orbit to study the ionosphere, conduct low-altitude surveillance and perform other missions.
Hoyt even said an electrodynamic tether attached to the international space station could reduce or eliminate the need for the tons of fuel the station consumes to remain in its proper orbit.
“They have to launch several rockets a year” to take fuel to the space station, Hoyt said. “It costs hundreds of millions of dollars.” An electrodynamic propulsion system “would enable the space station to hang indefinitely in orbit, saving billions of dollars” over the space station’s lifetime, he said.
