A Brief Overview of Electrodynamic Tethers

Abstract

Tethers in space can be used for a wide variety of applications such as power generation, propulsion, remote atmospheric sensing, momentum transfer for orbital maneuvers, micro-gravity experimentation, and artificial gravity generation. These are only a few of the host of uses that have been envisioned and proposed for many years. In general, a tether is a long cable (even up to 100 km or more) that connects two or more spacecraft or scientific packages. Electrodynamic tethers are conducting wires that can be either insulated (in part or in whole) or bare, and that make use of an ambient magnetic field to induce a voltage drop across their length. The induced voltage is given by |vxB•L|, where v is the relative velocity to the ambient plasma (the orbital velocity less the earth’s rotational speed), B is the geomagnetic field strength (2−6×10^-5T), and L is the length of the tether. For a 20 km tether in low earth orbit (LEO), this voltage would generally fluctuate between 1500 to 5300 volts open circuit, depending on the orbital inclination. If a current is allowed to circulate through the tether and a load, substantial power on the order of 15-30 kW can be generated. However, this power is generated at the cost of orbital energy. Hence an electrodynamic force |ILxB|, on the order of a few newtons, is exerted on the tether, lowering the orbit. On the other hand, with a sufficiently large power supply onboard the spacecraft, the direction of the current can be reversed and the tether becomes a thruster, raising its orbital height. Thus a spacecraft can use an electrodynamic tether system as a pure power generator (with a small rocket to periodically make-up for the drag), as a pure thruster, or in a combination of both roles.