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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.

  1. 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?
  2. 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.

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  1. Yes.

    In 1950 Wayne Proell proposed the use of "fuelless box-ramjets" to harness the energy of recombination of atomic oxygen at about 100 km altitude to propel space stations and satellites. (The Significance of Monatomic Gases in Planetary Space Operations, J. of Space Flight, V.2, no.7, Sept. 1950 pp.1-8, V.2, no.8, Oct.1950, pp.1-9.)

    In 1958 Lionel Baldwin and Perry Blackshear from the Lewis lab (now GRC) published a report with detailed computations on the feasibility of this approach.

    Another variation on the topic is, of course, Bussard ramjets.

  2. Baldwin and Blackshear's conclusion was that the contraption is of marginal utility. Too large, too inflexible, too costly to develop.

EDIT: Transforming the ramjet into a fuel scoop would negate all the thrust advantage, since it would no longer be ejecting anything. To counteract drag you will be forced to spend lots of fuel from the Earth. Essentially, producing oxygen and hydrogen here is much more economical (due to abundance of energy and raw inputs) than hauling the machinery into space and keeping it from falling on our heads.

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