A Lightweight System for End-of-Life Deorbit of Low-Earth-Orbit Spacecraft

The TERMINATOR TETHER™

Working with Marshall Space Flight Center (MSFC) in a Small Business Innovation Research (SBIR) agreement, Tethers Unlimited, Inc., is currently developing a system called the "Terminator Tether™" that will provide a low-cost, lightweight, and reliable method of removing objects from low-Earth orbit (LEO).

Background: The Space Debris Problem

There are over eight thousand satellites and other large objects in orbit around the Earth, and there are countless smaller pieces of debris generated by spacecraft explosions and by collisions between satellites. Until recently, it has been standard practice to put a satellite into orbit and leave it there. However, the number of satellites has grown quickly, and, as a result, the amount of orbital debris is growing rapidly. Because this debris is travelling at orbital speeds (7-8 km/s!), it poses a significant threat to the space shuttle, the International Space Station, and the many satellites in Earth orbit.

Satellites and other objects placed in low-Earth orbit will remain in orbit for many years. How long depends upon their perigee altitude, but objects in orbits above about 700 km will stay in orbit for hundreds or even thousands of years.

Recent studies of the interaction of satellite constellations with the space debris environment have concluded that, without debris mitigation measures, "the debris environment cannot sustain the long-term operation of [large constellations but] . . . could sustain the long-term operation of medium-sized constellations of up to 100 satellites deployed in orbits associated with the highest collision risk, or alternatively larger constellations of up to 350 satellites deployed in lower collision risk orbits . . . provided that the constellations implement strict mitigation measures such as explosion prevention and immediate satellite deorbiting upon end-of-life and failure. These findings have proven that low-Earth orbit is not a limitless resource and must be managed carefully in the future."[1]

In order to prevent old satellites, spent rockets, and the orbital debris that they generate from making low-Earth orbit unusable, satellite users must begin to provide a means of removing their old satellites and spent rockets from orbit.

Currently, there is no law requiring that old satellites be removed from orbit, but NASA has recently implemented a guideline for NASA satellites, and it is likely that this guideline, or one like it, will become a law.

Conventional Disposal Method: Rockets and Graveyard Orbits

Satellites typically have some means of propulsion for orbit corrections. One method of removing a satellite from orbit would be to carry extra propellant so that the satellite can bring itself down out of orbit. However, this method requires a large mass of propellant, and every kilo of propellant that must be carried up reduces the weight available for revenue-producing transponders. Moreover, this requires that the rocket and satellite guidance systems must be functional after sitting in orbit for ten years or more. Often, this is not the case, and the satellite ends up stuck in its operational orbit.

Some organizations are currently planning on boosting their satellites to higher, "graveyard" orbits at the end of their missions. This requires that the satellite's power, propulsion, and guidance be working at the end of the satellite's mission; however, it does not really solve the problem ¾ 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. Also, 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.

The Terminator Tether™

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™).

Performance

Under Phase I NASA SBIR funding with Marshall, TUI has developed a numerical simulation of electrodynamic tethers that includes models for all of the orbital dynamics, tether dynamics, plasma physics, and other physical phenomena relevant to the Terminator Tether™. Using this simulation, TUI has studied the performance of the Terminator Tether for disposing low-Earth-orbit constellation satellites and upper stages. The figures below (figs. 3 and 4) show the predicted deorbit rate and time for a Terminator Tether™ massing just 25 kg deorbiting a satellite massing 1,500 kg from various altitudes and inclinations.

In April 1999, TUI signed a $600,000 two-year NASA SBIR Phase II contract to continue the patent-pending deorbit device.

If you are interesting in finding out more about Tethers Unlimited's Terminator Tether™ System, visit their Web address: www.tethers.com.

References:

[1] Walker, Stokes, & Wilkinson, "Long Term Collision Risk Prediction for Low-Earth Orbit Satellite Constellations", IAA Paper 99-IAA.6.6.04.

[2] See Fig. 4 in Krisko, et al., IAA Paper 99-IAA.6.6.05.