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Is it possible to create thrusters for station keeping that require no fuel? Are there ideas similar to solar sails which can help in station- keeping and make use of a natural phenomenon? Except the EM drive.

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    $\begingroup$ Air breathing Ion thruster's have been developed lately (article from 5 March 2018). They use the thin air on low earth orbit as propellant. $\endgroup$ – Christoph Apr 9 '18 at 14:02
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    $\begingroup$ Then there's the photon propulsion, which unfortunately provides way too little thrust for station-keeping. $\endgroup$ – SF. Apr 9 '18 at 18:11
  • $\begingroup$ +1 for "Except the EM drive." $\endgroup$ – uhoh Apr 10 '18 at 2:04
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In theory an Electro Dynamic Tether (https://en.wikipedia.org/wiki/Electrodynamic_tether) could be used to impart momentum to a satellite or station for station-keeping maneuvers, converting electricity to momentum.

See this paper: http://issfd.org/2015/files/downloads/papers/111_Oliveira.pdf

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The EM drive is currently the only concept under testing that I'm aware.

If by fuel you mean some consumable that is loaded before lunch, and if by "Is it possible" you are proposing a more theoretical rather than readily practical question, I would argue that it should be possible to create some kind of particle acceleration technique such that regular atoms and electrons found in the space environment could be accelerated or decelerated thus exchanging momentum with the spacecraft. This would require electrical power, but no consumable fuel. Also, in theory light carries momentum, so emitting a laser is also a way of exchanging momentum.

Solar Sails are an interesting concept but they also would cause drag, in this sense if the station-keeping over long term is meant to compensate drag effects, I strongly doubt they could be effective. They could be used for deep-space missions, such as voyager, where the goal is to get away from the sun instead of remaining in a controlled neighborhood of a planet.

Note however that station keeping requires some specific minimum amount of delta-V provided over a finite windows of time, so any futuristic method needs to comply with real-world restrictions in order to be actually useful. Think about the Carnot Engine, that while it's theoretically the most efficient would take infinite time to run a single cycle.

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