[ajuc]

Tangentially related question - what happens to the gas used as a reaction mass in thrusters in orbit - when it's used to speed up I guess it falls down cause velocities mostly cancel out, but when it's used to slow down the ship, and engines are fired retrograde - the reaction mass has orbital velocity, right?

Does it stay in some orbit forever, like a solid object would? Can it cause gas "Kessler syndrome", with gas rings around Earth's most common reaction mass orbits?

If we choose our orbits and burn times so that this gas piles up in particular place on particular orbit, can we then reuse that as "air" for these engines from the article?

[Armisael16]

The exhaust velocity in a xenon ion thruster is 20-50 km/s. Most of the time it’s on an earth escape trajectory (~11 km/s in low orbit).

[Symmetry]

The smaller an object is the less time it takes for drag from the super tenuous atmosphere up in orbit to slow it down so it falls back to Earth. Gas molecules are very low mass and I expect that the exhaust for any given thruster will be gone quickly, even in the higher levels of LEO.