January 17, 2003
Tethers As Orbital Space Debris Cleaners

It is very expensive to launch propellants into orbit. So it would be prohibitively expensive for a spacecraft to move around in orbit to pick up space junk. Similarly, it would be too expensive to give each launched satellite enough propellant to deorbit itself at the end of its service life. However, a propellant-free way of moving objects around in orbit very slowly is under development. How a long tether propulsion system moves around in orbit:

It works as a thruster because a magnetic field exerts a force on the current-carrying wire. When electrical current flows through a through a tether connected to a spacecraft, the force exerted on the tether by the magnetic field raises or lowers the orbit of the satellite, depending on the direction the current is flowing. The current is extracted from the magnetic field of the Earth's ionosphere by the tether.

"The working principle of electrodynamic tethers is not new, but the application to space transportation will be revolutionary," said Les Johnson, principal investigator of the ProSEDS experiment. "Imagine driving your car and never having to stop for gas - that's what a tether does for a spacecraft in low-Earth orbit. Tether propulsion requires no fuel, is completely reusable and environmentally clean, and provides all these features at low cost."

There are a lot of small fragments flying around in low earth orbit. The number of fragments is growing in number and as they do they collide more often with satellites. Those collisions break pieces off of satellites and hence create new fragments that in turn can collide with still other satellites. Joseph Carroll of Tether Applications has proposed the use of space tethers as a cost effective way to collect up loose fragments in orbit.

His plan is to equip the tether with a roving sheepdog, a small vehicle that is released near a piece of debris to fly around it looking for a suitable point to latch onto. Once attached, it returns to the tether with its prize in tow. The tether then heads for another piece of junk and sets the sheepdog loose again. "A single tether could be reused up to 100 times, capturing a piece of junk many times its own mass each time, " he says.

The NASA ProSEDS project will soon test the first tether propulsion system.

The Propulsive Small Expendable Deployer system - called ProSEDS - is a tether-based propulsion experiment that draws power from the space environment around Earth, allowing the transfer of energy from the Earth to the spacecraft.

Inexpensive and reusable, ProSEDS technology has the potential to turn orbiting, in-space tethers into "space tugboats" -- replacing heavy, costly, traditional chemical propulsion and enabling a variety of space-based missions, such as the fuel-free raising and lowering of satellite orbits.

The flight of ProSEDS, scheduled for early in 2003, will mark the first time a tether system is used for propulsion. To be launched from the Cape Canaveral Air Force Station, Fla., ProSEDS will fly aboard an Air Force Delta II rocket and demonstrate an electrodynamic tether's ability to generate significant thrust.

"We achieved an important milestone with our tests in November," said ProSEDS project manager Leslie Curtis of the Marshall Space Flight Center's Space Transportation Directorate. "Using a vacuum chamber to represent the space environment, we successfully simulated the first 16 hours of the experiment's initial flight."

In orbit, ProSEDS will deploy from a Delta-II second stage a 3.1-mile-long (5 kilometers), ultra-thin bare-wire tether connected with a 6.2-mile-long (10 kilometers) non-conducting tether. The interaction of the bare-wire tether with the Earth's magnetic field and the ionosphere will produce thrust, thus lowering the altitude of the stage.

Although the mission could last as long as three weeks, the first day is the most critical, because the primary objective of demonstrating thrust with the tether should be achieved during the experiment's first 24 hours.

Tethers also look like a promising way to deorbit old satellites.

The Terminator Tether™ (TT) system will provide a lower mass and more reliable means of bringing old satellites out of orbit. The TT system will be a small package bolted onto the satellite. When the end of the satellite's useful life is reached, the TT system will deploy a several-kilometer length of conducting tether from the satellite. Because the satellite and tether are moving at great speed across the Earth's magnetic field, a voltage will be induced along the tether. This voltage will cause a current to flow along the tether. At the ends of the tether, the current will be transmitted to the thin space plasma present in low-Earth orbit.

The current flowing through the tether will cause power to be dissipated in the resistance of the metal in the tether. This power has to come from somewhere, and it comes out of the orbital energy of the satellite. As a result, the orbit of the satellite decays, and this decay can be very rapid. Calculations indicate that a tether massing as little as 2% of the satellite mass can bring a satellite out of some orbits in just a few weeks (compared to centuries without the Terminator Tether™).

The Tethers Unlimited Inc. Terminator Tether™does not require any propellant.

The Terminator Tether™ is a small device that uses electrodynamic tether drag to deorbit a spacecraft. Because it uses passive electromagnetic interactions with the Earth's magnetic field to lower the orbit of the spacecraft, it requires neither propellant nor power. Thus it can achieve autonomous deorbit of a spacecraft with very low mass requirements.

The tether is necessary because parking old satellites in "graveyard" orbits eventually results in the generation of smaller and more dangerous pieces of space debris as micrometeorites collide with the satellites.

Some organizations are currently planning on boosting their satellites to higher, "graveyard" orbits at the end of their missions. This also requires that the satellite's power, propulsion, and guidance be working at the end of the satellite's mission. Moreover, it doesn't really solve the problem - it just delays it, somewhat like a toxic waste dump. Recent studies have shown that satellites left in a higher graveyard orbit will slowly break apart as micrometeorites hit them, and the smaller fragments will filter back down to lower altitudes [2]. Thus satellites boosted to higher disposal orbits will eventually endanger operational satellites. Moreover, once the old satellites fragment into smaller particles, it will be nearly impossible to clean up the debris. Consequently, it will be much more cost effective in the long run to deal with the problem properly from the start, and deorbit all old spacecraft, rather than leaving them as a problem for our children to deal with.

Tethers are not going to exert a lot of force. Orbits will change only very slowly. But there's no rush when the cargos are under automated controls and there are no living passengers.

Share |      Randall Parker, 2003 January 17 07:22 PM  Space Exploration

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