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There have been a number of videos posted about trash being hand-pitched out the back of the ISS. This seems like a waste of potential reaction mass, especially since it cost $20,000/kg to get it up there.

Taking trash back to Earth sounds as boring as… well, taking out the trash. The ISS needs thrust to counter orbital decay. Trash has to be thrown out retrograde to prevent an unwelcome rendezvous. Why not throw it real fast and get a little delta-v for the ISS? This would also shorten time to re-entry for the trash

A trash compactor mated to a rail gun? A potato gun fueled by CH4 from the ECLSS?

Could ISS trash be usefully employed as reaction mass?

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Guns would not be practical--you get more boost by simply bringing up rocket fuel than with gun propellant throwing trash.

However, this does not preclude a mass driver approach. Break the trash into small enough pieces, put a piece in the mass driver and shove it retrograde. Current tech can very easily put the periapsis low enough it's not coming back. There are multiple big problems with this approach, though:

  1. The mass of the mass driver. For this to make sense the mass driver and associated equipment must weigh less than the rocket fuel it's replacing.

  2. The ISS is meant to function in microgravity. This substantially limits the permissible weight of the bucket on the mass driver + the mass being thrown. Yes, it can be made small enough, but that means it must be loaded and fired many, many times.

  3. How do you propose to reduce the trash to bits of the correct mass and load them into the bucket? This is a decidedly non-trivial problem, although for some stuff it might be possible to simply shred it and there's a cover on the bucket until launch.

  4. Devices like this cause things to flex. That's not a good thing on a big, flimsy structure like the ISS.

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Yeeeeees, it can.

For some very horrid variants of the word "can", which includes things like "Can students make a good bonfire? Yes they can, if you stack them high enough before setting them alight".

You could indeed, form mass pellets from the trash, and fire it using either an electrically or chemically powered gun.

If you did not mind disrupting the zero-g experiments on the ISS with the short but sharp thumps of your gun firing.
If you did not mind the short-term but extreme pollution of the LEO orbits behind you.

and if you did not mind the horrid press it would give you. Things like "ISS farts its trash all over the Earth, Greenpeace in Uproar" springs to mind.

Frankly, the hand-disposal of junk from the station also offends the sensibilities in a very similar way, and the mind boggles a bit that they allow it.

Now if they could get a way to transform the trash into something harmless, that also allows for the gentle, non-impact thrust that you want for a reboost maneuver, that might work. Say something like grinding it down to a fine powder, and using that powder as the fuel for a lowthrust electrical thruster like , say, a Hall-effect thruster?

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    $\begingroup$ I generally agree that it's impractical, but orbital (and earth-based) pollution isn't too much of an issue. The mass pellets would presumably be fired retrograde at firearm-or-greater speeds (>1km/s) and with the barrel in vacuum and without air resistance, any reasonable trash gun would shoot very, very fast. Considering that the ISS "only" orbits at 8km/s ish, any trash fired likely wouldn't even complete a single orbit before re-entering the atmosphere and burning up. $\endgroup$
    – Dragongeek
    Oct 31, 2021 at 10:32
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    $\begingroup$ Also, while predicting the public opinion of such a system is difficult, launching trash as reaction mass (aka "reusing" it) would likely have a far lower environmental impact than that of launching a rocket to deliver an equivalent amount fuel instead (disregarding the initial cost for the space gun + pellet factory). Furthermore, while some environmentalists occasionally take cracks at space exploration, those that do generally don't last long because earth observation/space science are cornerstones of modern environmental science and critical tools for any conservation effort. $\endgroup$
    – Dragongeek
    Oct 31, 2021 at 10:38
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    $\begingroup$ Vacuum only helps firearm speed a very small fraction, under 1 percent for any 'normal' gun. But yes, you do not need a great speed, even just 250m/s will suffice to ensure a low enough perigee to reenter within a day or so. The problem is the recoil/shock associated with a gun firing discrete particles, on a platform that is explicitly designed and purposed to perform zero-g experiments. $\endgroup$
    – CuteKItty_pleaseStopBArking
    Oct 31, 2021 at 10:38
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    $\begingroup$ A lot of the trash is organic. Heated sufficiently organics will generally decompose into stuff with a reasonably low boiling point. Feed that into an ion engine. $\endgroup$
    – Loren Pechtel
    Nov 1, 2021 at 1:34

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