Propagation of pressure waves in low Earth orbit

Question

So in a Wikipedia article about space debris I read that:

At higher altitudes, where air drag is less significant, orbital decay takes longer. Slight atmospheric drag, lunar perturbations, Earth's gravity perturbations, solar wind and solar radiation pressure can gradually bring debris down to lower altitudes (where it decays), but at very high altitudes this may take millennia.

Since debris decays at lower altitudes (presumably LEO) there has to be a mildly dense medium of matter/gas in that altitude. So if there is a medium then energy can flow through it since it is not really vacuum.

Can pressure waves (like sound waves) flow in LEO?

Perhaps you should distinguish between "sound" and "pressure" waves. If you mean "sound" in the sense of a pressure wave in gas that might be audible to a human [or other animal's] ear, then this is one sort of question. If you mean "sound" as in any measurable pressure wave, this is another sort of question. (Both questions are very interesting I think! :) — Craig Constantine, Apr 29 '18 at 13:10
This is a different question, but there is a bit of overlap Can astronauts hear sounds during space walks for repairs? — uhoh, Apr 29 '18 at 16:05

score 13 · Accepted Answer · answered Apr 29 '18 at 12:58

13

Considering this answer that says

It's obviously not a sharp cut-off, but as a general guide sound waves cannot propagate if their wavelength is equal to or less than the mean free path of the gas molecules. This means that even for arbitrarily low pressures sound will still propagate provided the wavelength is long enough.

we may argue that a sound with low enough frequency may indeed flow in LEO.

answered Apr 29 '18 at 12:58

Adam Trhon

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@JohnRennie 's answers are always a great source for explanations. There's also some thoughts on the general subject here. – uhoh Apr 29 '18 at 16:07
Sound in a gas is a periodic variation of pressure. But in LEO it might be difficult to measure a periodic pressure variation within the noise of single gas molecules detected by the sensor. – Uwe Apr 29 '18 at 18:05
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Correct answer. I'd add that the criterion isn't really applicable in LEO anyway because above the mesosphere, the gas isn't neutral anymore but contains a significant amount of ionised particles – a plasma. And for a plasma, you get more wave phenomena than just sound, in particular you also get non-fluiddynamic modes such as kinetic Alfvén waves. – leftaroundabout Apr 30 '18 at 00:27
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@Uwe plasma waves are actually measured routinely by spacecraft in scientific missions. It can be done quite well with Faraday cup detectors or by measuring the associated electromagnetic fields. Obviously, none of this is interesting for most satellites in LEO though. – leftaroundabout Apr 30 '18 at 00:30

Propagation of pressure waves in low Earth orbit

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