There's a nice graph on Wikipedia about this, and what I didn't realise is that the pressure doesn't have a lot of influence on the speed, it's nearly all temperature-based. I would never have looked had you not made the comment above.

https://en.wikipedia.org/wiki/Speed_of_sound#Altitude_variat...

For practical engineering purposes, it is only dependent on temperature, proportional to the square root of the temp.

A mediocre intuition is that the speed of sound is governed by the speed of the individual gas molecules. Higher pressure and density mean there are more molecules hitting each other more often, but the speed of any individual molecule is the same and defined by temperature only. (it's only a mediocre intuition because the statistical dynamics that relate bulk properties to individual molecule interactions are complicated)

I'm curious if the practical approximation still roughly holds in the thermosphere at very high temps and very low densities, do you know?

So according to the above graph, supersonic speed in the ionosphere is ~280-300 m/s, lower than at sea level. Temps are lower on average too. But a plasma jet punching through the ionosphere at 10k C would mean the plasma itself is creating the condition for supersonic speeds on the order of 2000 m/s. (Not sure if the research is saying this, or saying the jets are moving at supersonic relative to the surrounding atmosphere...)

Just found this super-helpful link: http://www.sengpielaudio.com/calculator-speedsound.htm

I had forgotten about speed of sound being primarily governed by temperature until @djaychela reminded me. But I still find it completely unintuitive, even with your explanation... which is really interesting!

Depends on what you're trying to do with it.

Hot near-vacuum gases often need molecular approaches to make models. "Sound" itself needs careful definition in such atmospheres. They're talking about jets of air pushed around by huge electric fields. "Speed of sound" is just a good way to indicate scale, it probably doesn't have much other useful meaning.

> "Speed of sound" is just a good way to indicate scale, it probably doesn't have much other useful meaning.

Yeah, that's pretty much what I'm getting out of the article... the problem is I still have no idea what that scale is, do you? I'm sure the term 'supersonic' was meant to impress and sound fast, but there's at least an order of magnitude uncertainty.

> "Sound" itself needs careful definition in such atmospheres.

Nothing to say here except I love that sentence.

I could hazard a guess, but I doubt the scientists have particularly precise measurements (and if they did they'd be better found in a paper rather than my guessing). Having a number and concentrating on it misses the forest for the trees, it's a big number, but not the most interesting part by far.

Supersonic means there are shock waves; subsonic means there are no shocks.

Well. In layman terms, 1) Gas's thermal energy is: E=kT, k -some constant, T is a temperature in Kelvins 2) that thermal energy is roughly a kinetic energy at molecular level, i assume :), hence it is something like E = (mv^2)/2, m - molecule mass, v - its speed 3) so we have kT = (mv^2)/2 ==> ==> v = sqrt(2kT/m) this way it maybe more intuitive

Thermal energy is stored as kinetic energy in the form of vibration, rotation, and translation. That k will depend on the makeup of the gas. Monatomic gas can't store any energy in rotation whereas diatomic and larger molecules can. (This is why humidity matters, water has three atoms and thus different thermal energy characteristics)

How much thermal energy ends up as kinetic energy in translation effects the proportion of T to speed of sound.

The storage also depends on temperature, how much energy goes into each degree of freedom (rot, trans, vib) changes with temperature but is fairly static around the human experience scale of things.

Wow, I completely forgot the ranges of different atmosphere regions since I was in secondary school.

This picture here makes me wonder:

https://upload.wikimedia.org/wikipedia/commons/8/8c/Earth%27...

Did any spacecraft manage to fly through an aurora? Wouldn't it be cool if someone did that and shoot photos from within?

Check this out https://www.nasa.gov/content/flying-through-an-aurora

Also google image search for "space shuttle aurora"... spectacular pictures!

Ah, good point, I forgot that the ISS effectively passes through that region anyway.

Usually spacecraft are launched near the equator if possible, so aurorae aren't particularly prevalent there. Furthermore, since the atmosphere is ionized there, I'd guess spacecraft would avoid launching if it meant passing through it. Since atmospheric charges due to thunderstorms and clouds are already a concern I wouldn't expect an ionized part of the atmosphere to be any different.

Many spacecraft are launched into polar orbits. For the rest, it takes less fuel if you launch to the inclination of the launch pad. The ISS is inclined midway between the main US and Russian launch sites. Antares cargo rockets launch from a mid-Atlantic launch site because it's closer to the ISS' inclination. SpaceX's new Texas launch site is as far south as you can get in the continental US, and will be mainly used for GEO satellites that want to be on the Equator.

The article points out temperatures rise "up to almost 10,000°C"

I had no idea!

And if you were floating in it shaded from the sun you'd freeze to death. It's something like talking about the temperature of the air in a vacuum chamber. There is _always_ air in a vacuum chamber and it'll have a temperature, and that temperature will mean different things than normal intuition gives.

Similar to wind speeds on Mars, you read about hurricane speeds of 200mph+ then remember with the low density that would feel like a fairly strong breeze to a human standing there.

The problem with the storms is dust and blocking of the sun.

Planet wide storms have been recorded, that bodes poorly for solar powered installations without significant battery backup.

Considering how well the mars rovers have done with just a few panels the dust issue is definitely surmountable.

But that's partly luck: the panels get somewhat cleaned by passing dust devils. Didn't Spirit end up freezing to death, after it got bogged down?

Edit: https://en.wikipedia.org/wiki/Spirit_(rover)#Mars_winter_at_... yes: normally they'd park it on a south-facing slope to maximize insolation during the winter, but it was stuck on flat ground.

Well, there's always alternative power sources, like on Curiosity.

Really? I thought vacuum was an excellent insulator. Seems like the normal 75-100 watts should be plenty. After all a vacuum sealed thermos keeps hot coffee warm for days and has a much worse surface to volume ratio.