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The ISS uses a Sabatier reactor to recover oxygen from carbon dioxide. This has an output product of methane.

I would imagine that most methane that enters our atmosphere at ground level only reaches a maximum altitude. Assuming the ISS is above that altitude, I am curious to how methane introduced into our atmosphere, at the ISS's altitude, interacts with our atmosphere differently?

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  • $\begingroup$ Do you have a source that details how much methane is expelled by the ISS? $\endgroup$
    – Jens
    Mar 30, 2018 at 21:30
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    $\begingroup$ A somewhat related question, you may find some of the discussion there interesting; Does “What happens beyond Kármán, stay beyond Kármán”? $\endgroup$
    – uhoh
    Mar 30, 2018 at 22:53
  • $\begingroup$ I have removed attempts to answer the question in the comments. $\endgroup$
    – called2voyage ♦
    Apr 2, 2018 at 18:08
  • $\begingroup$ I have edited my question to explain how ISS generates methane. Also I'm a scientist that ponders cause and effect, and underlying systems. This helps me build mental experiments in my head. I'm not debating the causes of the greenhouse effect and human's contribution to climate change. I'm a systems thinker that is curious to what atmospheric and chemical systems might play a causal role. $\endgroup$
    – Gabriel Fair
    Apr 2, 2018 at 18:38

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Assuming the methane stays in orbit around the Earth as indicated by uhoh, it will then not do much else. Wikipedia has a fairly extensive article about methane in the atmosphere, and it mentions that the primary reaction of methane in the atmosphere is oxidation by hydroxyl radicals. Because the hydroxyl radicals come from water and water in our atmosphere is primarily confined to the troposphere, methane molecules have an average lifetime of about 10 years in the troposphere ... but 120 years in the stratosphere.

Methane produced even higher up by the ISS is therefore going to hang around for quite some time, not reacting with much of anything. As it does so it could contribute a greenhouse effect, as methane is 84 times as an equal mass of powerful as carbon dioxide.

We should keep this all in perspective, however. This question cites NASA as reporting that "1,000 pounds per year" are produced from the ISS. That's about 500 kg -- versus the teragrams per year methane sources and sinks given here and reproduced in the above referenced Wikipedia article (1Tg = $10^9$ kg). In the end, the methane from the ISS, even if it is relatively long-lasting, is just a tiny blip in the larger scheme of things.

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    $\begingroup$ The thermosphere is not a happy place for molecules. N2 and O2 disassociate pretty rapidly, such that at 400km there are orders of magnitude more single-atom O than O2, and even N2 is a factor of 10 rarer than N. (See ccmc.gsfc.nasa.gov/modelweb/models/nrlmsise00.php for some fun data on this) CH3 will probably behave similarly, though I haven’t seen data on disassociation spectra. $\endgroup$
    – Bob Jacobsen
    Jun 19, 2019 at 4:11
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The atmosphere is thin at 400 km, but it's still there and the ISS is in it.

The velocity of a circular orbit at that altitude is about 7700 m/s and escape velocity is the square root of 2 larger than that. So unless the gas was vented at a velocity of 3000 m/s relative to the ISS (which it certainly isn't), it would remain in orbit around the Earth.

Over time, through collisions, it would equilibrate with the other gas molecules in the upper atmosphere, and slowly diffuse and mix with lower layers.

In the mean time the ISS will occasionally run into a few of those molecules, which along with the others, will be a slow but inexorable source of drag, slowing the ISS down.

There is some similar discussion in this answer to the somewhat similar question Does “What happens beyond Kármán, stay beyond Kármán”? though that one is about exhaust that really could be emitted at escape velocity.

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    $\begingroup$ This is a good answer for the general mechanical effects, but I think the OP was asking specifically about methane as a chemical. E.g. would emitting methane and say (LiH)₂ have different effects even though they have the same molecular weight? $\endgroup$
    – Ray Butterworth
    Jun 16, 2019 at 13:31

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