16 February 2005

Into Orbit Via A Slingshot

by Kate Melville

The public's imagination may have been fired up in the past by proposals for "space elevators" - where satellites or space vehicles are carried up a 62,000 mile cable into space - but the technological challenges that exist for such a project make its implementation a futuristic dream. But another type of tether may hold more promise. NASA's In-Space Propulsion group has contracted with a Tennessee Tech University team to investigate a rotating tether concept that could be implemented using technology that should be available within the next five years.

Scientist Steven Canfield, who leads the Tennessee Tech team, is working on a tether project called momentum-exchange electrodynamic reboost tether technology - MXER for short. In the MXER tether approach, the tether can be imagined as a long space cable equipped with a catching mechanism to capture payloads. The rotating tether transfers some of its momentum to a satellite through a brief but controlled capture and release process. After this energy transfer, the tether uses the Earth's magnetic field to propel itself into an orbit ready to launch another satellite, all without the use of onboard propellants. "The tether moves like a wheel in orbit around the Earth, storing kinetic energy as it travels, much the same way ice skaters play 'crack the whip,'" said Canfield. "The payload, which could be a satellite or spacecraft, would be launched into low orbit to arrive for a rendezvous with the tether. The tether would snare the payload at the precise moment they both arrive at the same spot, then toss it into another orbit. The energy is then transferred from the tether to the payload."

MXER technology could dramatically reduce the cost of raising the orbits of spacecraft, including those destined for deep-space missions. The process could eliminate or reduce the need for a booster rocket, which is not reusable and carries high fuel costs. And because a MXER tether can reboost its own orbit with stored energy and no additional propellant, it could perpetually capture and toss payloads. NASA considers MXER technology "high-risk, high-payoff." Several key challenges must be resolved before MXER technology is feasible. Current materials available to make tethers have a hard time lasting in the space environment. Stability is also another key issue, along with reliable rendezvous and capture mechanisms.

The capture demands precise design and execution. For instance, unlike the deliberate crawl and delicate docking seen when the space station and a shuttle connect in orbit, this payload capture ideally would take place in less than a second. "Testing space devices on the ground is hard," said Canfield, "but tethers actually create their own 'artificial gravity' when they rotate, so we don't have as much trouble simulating how they would act in space. Think of the force you feel on a centrifugal ride at the fair; you are spinning around and feeling that force created by the motion pushing against you. It's the same concept."

More Info: mxer.tntech.edu