Plugged in to space
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A team of engineers and scientists at NASA's Marshall Space Flight Center has proposed flying a Propulsive Small Expendable Deployer System - ProSEDS - that would "plug in" to the same physics principle that runs electric motors.
Right: Artist's concept depicts how a propulsive tether system works. Links to. Credit: Dan Holland, NASA/Marshall Space Flight Center.
"This would be the first demonstration of a propellant-free propulsion system," said Les Johnson of NASA's Marshall Space Flight Center. "We're trying to reduce the cost of space transportation."
It's not exactly something for nothing - we'll explain that shortly - but it's reasonably close. The story starts in 1831, when Michael Faraday and Joseph Henry demonstrated that moving a wire through a magnetic field produced an electrical current, and then running a current through a wire produced a magnetic field. The principle is the basis of electrical motors, generators, even computer disks.
Fast forward to 1966 when the Gemini 11 and 12 manned spacecraft each attached a tether to a rocket stage and demonstrated that Earth's gravity would stabilize the two.
The electrical and tether principles were first demonstrated together in space by the Tethered Satellite System on the Space Shuttle in 1992 and 1996. The second flight yielded a surprise for plasma physicists.
Before the flight, the models predicted that the tether would produce 0.5 amp (0.5 A) under ideal conditions. Instead, it produced more than 1 amp under less than ideal conditions. About the same time, Dr. J. R. Sanmartin of the Polytechnic University of Madrid, Spain, predicted that a bare wire will produce more current that the insulated wire-plus-large sphere design used by the Tethered Satellite System.
"If this new bare wire tether works as advertised," Stone said. "it would allow us to collect considerably more current for a given length of tether." As a result, shorter tethers could be used for propulsion or to generate electrical power.
In the ProSEDS flight, the satellite actually would be an expended second stage from a Delta II rocket. It will be in an orbit 375x414 km (233x257 mi) after launching a pair of Air Force navigation satellites (a third stage will take the satellites to a higher orbit). Normally, these stages slowly spiral back to Earth over a half year as atmospheric drag nibbles away at their speed. By generating an electrical current, ProSEDS will turn itself into an electromagnetic brake making the stage re-enter Earth's atmosphere in about 14 days.
The International Space Station (ISS) will be the largest spacecraft ever assembled - including several hundred square meters of solar arrays and module surface. This large area combined with the minute drag of Earth's atmosphere will slowly drag the station back towards Earth. NASA plans to reboost the station several times a year with propellant supplies brought up from Earth, an expensive proposition. The cost of propellant resupply could be eliminated by using a propulsive tether - for a system mass of less than 200 kg a conducting tether could be 10 km long.
"With a relatively low development and operations cost of less than $50 million, a tether reboost system on the ISS could potentially save the program up to $2 billion over 10 years," wrote Johnson and Melody Hermann, another space engineer at NASA/Marshall. The tether would also increase the time available for microgravity experiments, a key justification for ISS, and cancel aerodynamic drag that would upset the more sensitive of those experiments. Although using the same principles as the ProSEDS demonstration, a propulsive tether on ISS would be powered by electricity from the ISS's solar arrays: 5 kW of electricity would produce a steady push of 0.5 Newton (about one-eighth of a pound).
It doesn't sound like much, but atmospheric drag in space also is small. Balancing one against the other would ensure a long, healthy orbit for the ISS.
Details of this plan are available in the International Space Station Electrodynamic Tether Reboost Study (NASA TM -1998-208538, July 1998) by Johnson and Hermann.
"Our goal is to show an orbital decay of at least 5 km (3 mi) a day," Johnson explained. "It's not quick compared to a retrorocket, but it is compared to natural decay. And its speed will be consistent and it will be being done without the use of any propellant." It would also be a great boon to the space business.
It may sound odd but outer space is becoming littered. While space is infinite, most human activity is in a finite region near Earth. A lot of satellites go into the same region of low Earth orbit on their way to higher orbits. In addition, a number of Earth observation and scientific satellites and - increasingly - specialized communications satellites also operate at intermediate orbits.
A lot of spent rocket stages get left behind, causing headaches for launch planners who have to make sure that they avoid collisions.
Virtually every scheme to de-orbit satellites relies on complex systems that would have to work after five years of storage. These impose extra weight and costs on the craft.
Propulsive tethers will do the job for a small cost and a weight less than 50 kg (110 lbs). In the ProSEDS demonstration, it's a 15 km(9.3 mi) tether (shown at left) unwound upward from the Delta II rocket stage. The upper 10 km (6.2 mi) are nonconducting, and the lower 5 km (3.1 mi) are a bare wire to make an electrical connection with the ionized gases - plasmas - of space.
The slight decrease in Earth's gravity across the length of the tether actually pulls the two ends apart, keeping the tether stretched out. Once unreeled, the tether moves across the Earth's magnetic field lines and, in "generator mode," produces an electrical current through the tether. A hollow cathode then ejects electrons back into space, thus completing the circuit - and putting the brakes on the rocket.
"The next step would be another demonstration that shows orbit raising by using the tether in the 'motor mode.' Rather than generating power, the tether system could use electrical power from solar arrays to generate thrust and boost the satellite's altitude," Johnson continued. That probably would require a dedicated spacecraft for the demonstration. But it would be a worthwhile investment that could lead to a savings of up to $2 billion in space station operations costs, power space tugs taking satellites to higher orbits, and possibly reduce the weight and cost of probes to Jupiter and its moons.
With its great gravity and magnetic field, Jupiter would seem a natural place for a propulsive tether system to move a space probe into orbit around the planet then tour the moons and even power the spacecraft. NASA/Marshall investigated just such an intriguing possibility, but the answer was a surprising "maybe" rather than a resounding "yes."
"The use of EDTs in the Jovian system presents entirely new challenges and opportunities," wrote Johnson and Dr. Dennis Gallagher, a scientist at NASA/Marshall's Space Sciences Laboratory. If anything, the tether would produce more electricity than the spacecraft would need.
While Jupiter has a strong magnetic field, the gravity gradient - in effect, its "steepness" - is not strong enough to keep the tether straight as it pushes the probe.
With some engineering - such as a rotating, two-part spacecraft that uses centripetal force to keep the tether straight, plus advanced control electronics - it should be possible.
Details of are available in the Electrodynamic Tether Propulsion and Power Generation at Jupiter (NASA TM -1998-208475, June 1998) by Johnson, Gallagher, J. Moore of SRS Technologies, and F. Bagenal of the University of Colorado at Boulder.Web Links Up, up, and away (bit by bit)
Tethers in space describes the past, present, and promising future of tether applications in space, including technical details.
High wire act may be best way to explore Europa A March 13, 1998, story describing the use of propulsive tethers to explore Jupiter.
Tethers Unlimited is a Clinton, Wash., company partnering with NASA/Marshall on ProSEDS and developing advanced tethers for other space applications.
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