Bare tethers for electrodynamic spacecraft propulsion

Electrodynamic tether thrusters can use the power provided by solar panels to drive a current in the tether and then the Lorentz force to push against the Earth's magnetic field, thereby achieving propulsion without the expen diture of onboard energy sources or propellant. Practical tether propulsion depends critically on being able to extract multiamp electron currents fro m the ionosphere with relatively short tethers (10 km or less) and reasonab ly low power. We describe a new anodic design that uses an uninsulated port ion of the metallic tether itself to collect electrons. Because of the effi cient collection of this type of anode, electrodynamic thrusters for reboos t of the International Space Station and for an upper stage capable of orbi t raising, lowering, and inclination changes appear to be feasible. Specifi cally, a 10-km-long bare tether, utilizing 10 kW of the space station power could save most of the propellant required for the station reboost over it s 10-year lifetime, The propulsive small expendable deployer system experim ent is planned to test the bare-tether design in space in the year 2000 by deploying a 5-km bare aluminum tether from a Delta II upper stage to achiev e up to 0.5-N drag thrust, thus deorbiting the stage.