The electrodynamic tether is an aluminum wire orbiting the Earth. Electrons are extracted from the adjacent ionosphere, enter the wire at one end, and leave it at the other end. The Earth magnetic field generates voltage in the wire. The electrodynamic tether is similar to an electric motor; it can either convert its orbital energy to electric energy (generator), or it can convert electric energy to orbital energy (propulsion). The energy is provided by solar panels. When the tether is on the dark side of the Earth, the solar panels are useless, so the tether is passive unless it has another source of energy: either a battery or a winch which transforms the tether into a flywheel. The electrodynamic tether is the most efficient when it operates in the area of the highest conductivity of the ionosphere: the altitude of 350 km and the sunny side of the Earth. The conductivity drops to 20% of the maximum value at the altitude of 1500 km. Specific thrust of the electrodynamic tether is about 0.005 N/kg. The electrodynamic tether is unstable unless it spins or is attached to a rigid structure with both ends.
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