Space stations like the ISS orbit at an elevation that puts them squarely in the Thermosphere, and while this has extremely rarefied gas (which is probably more accurately a plasma at many times), it still suffers orbital decay due to drag.
- Has anyone seriously proposed a way to compress and sequester the gas at such an orbit, so that it could be used as propellant or for other applications?
- Is there anything that clearly rules the idea out?
It seems like a notable absence in places like the NASA Institute for Advanced Concepts. It's quite common to see proposals for novel launch systems for delivery to propellant depots, but sequestering gas in orbit would accomplish the same thing. You could produce cryogenic fuels right there in orbit, without the need for launches at all.
The best counter-argument might be momentum balance. If each molecule of gas you sequester hits you at 7.9 km/s, then your thrusters have to exceed that. But ion thrusters can do 50 km/s. So that doesn't seem to kill the concept right away.