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An electromagnetic tether is a tether (perhaps a hoytether) for use in low earth orbit that can be used for propulsion without requiring propellent to be expended.

The idea is that a large current is passed through a many kilometer long tether, and this current interacts with the earth's magnetic field uses the standard physical principle of the motor rule to give thrust.

Normally a current flows in a wire loop of some kind. In this application a loop cannot be formed of wire, otherwise the forces would cancel. Instead, an electron gun at one end and positive charge generator at the other spit positive and negative charges at each end and permit a current to flow, and the charges mill around in space until they finally cancel each other somewhere in space and close the loop.

Electromagnetic tethers can also generate electricity. When used in this way some of the kinetic energy of the vehicle gets converted into a current in the tether; and the vehicle slows. This is useful as a way of leaving orbit; and has been proposed as a way of deorbiting spacecraft at the end of their life and minimising the problem of space junk.

Electromagnetic tethers work best in low earth orbit; in higher orbits the earths magnetic field is weaker and they are less effective, however they have also been proposed for use around Jupiter which also has a massive field.

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