Volume 8, Number 3     May/June 2000

Removing Space Debris From LEO

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

Space debris is becoming a problem. Increased amounts of space debris, traveling at orbital speeds (7 to 8 kilometers per second), pose a significant threat to the Space Shuttle, the International Space Station and the many satellites in Earth orbit. The number of satellites (more than 8,000), countless smaller pieces of debris generated by spacecraft explosions and collisions between satellites, and other large objects orbiting Earth has grown quickly.

Until recently, it has been standard practice to put a satellite into orbit and leave it there. To prevent old satellites, spent rockets and the orbital debris they generate from making LEO unusable, satellite users must begin to provide a means of removing their old satellites and spent rockets from orbit.

A number of telecommunications firms have plans to deploy satellite constellations in LEO and mid-Earth orbit that will have tens to hundreds of satellites each. These satellites will have operational lifetimes of 5 to 10 years, and thus the constellations will require the constant replacement of old satellites. Satellites and other objects placed in LEO will remain in orbit for many years. How long depends on their perigee altitude, but objects in orbits above approximately 700 kilometers will stay in orbit for hundreds or even thousands of years.

Findings from recent studies have proven that LEO is not a limitless resource and must be managed carefully in the future. The studies of interactions 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 at high collision risk. Or, larger constellations of up to 350 satellites could be deployed in lower collision risk orbits as long as strict mitigation measures are implemented, such as explosion prevention and immediate satellite deorbiting upon end-of-life and failure.

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

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 kilogram 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 10 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 also requires that the satellite's power, propulsion and guidance be working at the end of the satellite's mission. However, this does not really solve the problemÐit just delays it, somewhat like a toxic waste dump.

The results of recent studies have indicated 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. Thus satellites boosted to higher disposal orbits could eventually endanger operational satellites and manned operations in LEO. 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 address the problem properly from the start and deorbit all spacecraft as soon as they complete their operational missions.

The Terminator Tether device will provide a lower mass and more reliable means of bringing old satellites out of orbit. The device will be a small package bolted onto the satellite. When the end of the satellite's useful life is reached, the Terminator Tether will deploy a several-kilometer length of conducting tether from the satellite. Because the satellite and tether are moving at great speed across 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 LEO.

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 with a mass of as little as two percent of the satellite mass can bring a satellite out of some orbits in just a few weeks (compared to centuries without the Terminator Tether). As an example of the potential mass savings, to deorbit a two-metric-ton satellite from a 1,000-kilometer orbit using chemical rockets would require approximately 200 kilograms of propellant. The Terminator Tether could do the same job, but it would require only 25 kilograms of propellant.

Under NASA SBIR Phase I funding with Marshall, Tethers Unlimited has developed a numerical simulation of electrodynamic tethers, which includes models for all of the orbital dynamics, tether dynamics, plasma physics and other physical phenomena relevant to the Terminator Tether. Using this simulation, Tethers Unlimited has studied the performance of the Terminator Tether for disposing of LEO constellation satellites and upper stages.

In April 1999, Tethers Unlimited signed a two-year SBIR Phase II contract to continue developing the patent-pending deorbit device. The company hopes to have a prototype Terminator Tether ready soon to demonstrate in a test flight.