Space travel is a tremendously costly enterprise, largely because today's spacecraft use rockets to move around, and launching the significant amounts of fuel needed to propel those rockets is very expensive. For humankind to move beyond its current tentative foothold in low Earth orbit and begin frequent travel to the Moon, Mars, and other planets, the cost of traveling through space must be substantially reduced. To do this, it may be necessary to rely less on the pyrotechnics of rocket technologies and utilize simpler and less complex technologies. This could entail the use of long strings or wires to move payloads around in space without the need to burn large quantities of fuel.
A space tether can be used to move spacecraft through space through two different mechanisms. First, a high-strength string connecting two spacecraft can provide a mechanical link that enables one satellite to "throw" the other into a different orbit, much like casting a stone with a sling. Second, if the tether is made of conductive wire, currents flowing along the wire can interact with Earth's magnetic field to create propulsive forces on the tether. Both momentum-transfer and electrodynamic tethers can move spacecraft from one orbit to another without the use of propellant.
A number of tether experiments have been flown in space. In the early days of the space age the Gemini 11 and 12 missions (1966) used short tethers to connect two spacecraft and rotate them around each other to study artificial gravity and other dynamics.
In the 1990s the National Aeronautics and Space Administration (NASA) conducted two series of tether experiments. One series involved a large tether flown on the space shuttle that was called the Tethered Satellite System (TSS). Unfortunately, the TSS missions encountered well-publicized problems. In the 1992 TSS-1 mission, the TSS system attempted to deploy a spherical satellite built by the Italian space agency upwards from the shuttle at the end of a 20-kilometer-long (12 miles) tether made of insulated copper wire. A few hundred meters into deployment the spool mechanism jammed, ending the experiment.
In 1996 NASA repeated the experiment. As the tether approached its full length, the rapid motion of the orbiting tether through Earth's magnetic field generated a current of over 3,500 volts along the tether. The TSS system included devices that emitted electrons or ions at both ends of the tether, enabling the tether system to make electrical contact with the ionosphere . This allowed the induced voltage to drive a current along the tether, demonstrating that an electrodynamic tether could generate power by converting the shuttle's orbital energy into electrical energy.
A flaw in the insulation allowed an arc to jump from the tether to the deployment boom. The arc burned through the tether, causing it to part and effectively ending the electrodynamic tether part of the experiment. The break, however, showed that tethers could be used to move spacecraft to higher orbits. When the TSS tether was severed, the Italian satellite at the end of the tether was tossed 140 kilometers (87 miles) above the shuttle.
Despite the difficulties encountered in the TSS experiments, enthusiasm for tether missions remains high, largely because of the many successes of the second series of NASA tether experiments, which were based on a much smaller and less expensive system called the Small Expendable-Tether Deployer System (SEDS). Four highly successful SEDS tether experiments have been carried out as piggyback missions on upper-stage vehicles launching larger satellites. The SEDS-1 mission used a tether to drop a payload back down to Earth. The experiment showed that a spool of string could perform the same job that a rocket does. This technique could be used to drop scientific payloads from the International Space Station down to Earth. The 1993 Plasma Motor Generator mission used a modified SEDS system to deploy a 500-meter (1,640-foot) conducting wire to study electrodynamic interactions with the ionosphere. The SEDS-2 mission deployed a 20-kilometer-long (12.4 miles) tether below an upper-stage rocket and left it hanging to see how long it would survive in space. After only four days a micrometeorite or piece of space debris cut the tether, which was only about 0.8 millimeters (0.0315 inches) in diameter. This experiment showed that in order for tethers to be useful for long-duration missions in space, they must be designed to withstand cuts by micrometeorites and space debris.
Future Uses of Tethers
One way to solve this problem was demonstrated by the Tethered Physics and Survivability experiment, which was conducted by the Naval Research Laboratory. That experiment used the SEDS system to deploy a tether constructed as a hollow braid that had ordinary knitting yarn stuffed in the middle to puff it out. Launched on June 20, 1996, the 4-kilometer-long (2.5 miles), 2.5-millimeter-diameter (0.098 inches) tether has been orbiting in space uncut for more than five years.
Another method of ensuring that tethers can survive impacts with space debris may be to fabricate them as long, spiderweb-like nets rather than as single-line cables. Tethers Unlimited is developing a flight experiment to demonstrate this and other technologies.
Tethers also may provide a cost-effective means for removing spacecraft and space trash from orbit. In late 2001 NASA planned to fly the ProSEDS experiment to demonstrate that a conducting tether can be used to lower the orbit of a spacecraft by dragging against Earth's magnetic field.
In the future, long rotating tethers may be used to toss payloads through space. Tethers Unlimited has developed a design for a Cislunar Tether Transport System that could repeatedly transport payloads to the Moon and back, and other researchers have developed designs for tether systems to take payloads to Mars and back. In addition, tethers may provide a way to lower the cost of boosting payloads into orbit. In one concept a small hypersonic airplane could be used to carry a payload halfway into orbit, where a rotating tether facility already in orbit could pick it up and toss it into orbit.
Although a number of technical challenges have to be addressed before tethers can provide routine transport around and beyond Earth orbit, tethers have the potential to reduce the cost of space travel greatly and may play a key role in the development of space.
see also Accessing Space (volume 1). Getting to Space Cheaply (volume 1); Payloads (volume 3); Space Elevators (volume 4).
Robert P. Hoyt
Cosmo, M. L., and E. C. Lorenzini. Tethers in Space Handbook, 3rd ed. Prepared forNASA/MSFC by Smithsonian Astrophysical Observatory, Cambridge, MA, December 1997. <http://www.harvard.edu/spgroup/handbook.html>.
Hoyt, Robert P., and C. W. Uphoff. "Cislunar Tether Transport System."Journal of Spacecraft and Rockets 37, no. 2 (2000):177-186.