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I am wondering - what is the nature of the atmosphere in and around the transition to space at the Karman line?

It is not difficult to find references using terms like "in the atmosphere" and "out of the atmosphere" as if there is some fairly clear border. For example, this spaceflight101.com article about the booster underperformance issue for the Atlas V launch of the Cygnus spacecraft for OA-6 says "...Centaur was holding a pitched-up attitude for much longer in Tuesday’s mission just to keep out of the atmosphere." This draws the picture in my mind of a fairly clear border.

Obviously, the atmosphere is gaseous, so there is no surface tension and no surface - so therefore the boundary cannot be as sharp as being "in the ocean" and "out of the ocean" - but is that actually a fairly reasonable analogy? Do all the heavier gases in the atmosphere sort of tend to pool such that there is a fairly sharp cut-off point, above which the density falls precipitously? E.g., below altitude X, the atmospheric density is approximated by function 1, whereas above altitude X, the density is approximated by function 2.

On the other hand, the Wikipedia Article about the Karman line clearly states "An atmosphere does not abruptly end at any given height, but becomes progressively thinner with altitude." Again, obviously since it's a gas and there is no surface tension, there is no true "surface" - but for the purposes of spaceflight, is there something relatively close to this? (And is the Karman line a pretty close approximation to where this would happen?)

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There is no border at all, just progressively thinner air. The Kármán line is an arbitrary reference point, at which an aircraft can't produce enough lift to stay in the air at less than orbital speed.

There are different layers of atmosphere with somewhat different emergent characteristics, but those don't greatly change the overall progressive thinning of air with altitude, and don't have a large effect on the performance of rockets passing through.

From this image you can see there's a bit of a knee in the density curve at 80-90km, but note the log scale on the left: density is practically zero there anyway.

enter image description here

The use of the binary distinction of "in the atmosphere" and "out of the atmosphere" in discussion of launchers is informal. In a typical two-stage-to-orbit launcher, the first stage will take you from sea level to high altitude; this has significant effects, so we say the stage must be "designed for flight in the atmosphere". The upper stage design may not be able to ignore atmosphere completely, but it will do most of its work in microbar pressure or less, so we say it's "designed for flight out of the atmosphere". The rocket as a whole will spend only a few minutes in the transition.

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  • $\begingroup$ Thanks! So it appears that up until the 80-90km range, the mass density is exponentially decreasing, and at that point, it changes to something else. Looking it up further this appears to be in the vicinity of mesopause (the boundary between the mesosphere and the thermosphere) and turbopause (the point below which turbulence continuously mixes the atmosphere resulting in a relatively constant mixture of N2 and O2). $\endgroup$
    – orulz
    May 12, 2017 at 17:42
  • $\begingroup$ I'd say it's still exponentially decreasing, but with a different exponent. ;) $\endgroup$
    – Russell Borogove
    May 12, 2017 at 18:15
  • $\begingroup$ @orulz: While some changing properties of atmosphere may be observed, the concept of Karman Line is derived from aviation, as per Russel's first paragraph - as weight of airplane in flight is offset by aerodynamic lift + centrifugal force, both proportional to speed (but only lift to air density), at that altitude air density drops to the point where centrifugal force would begin to account for 100% of force counteracting weight. $\endgroup$
    – SF.
    Sep 18, 2017 at 23:04
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Contrary to Russell Borogove's answer, there is something very significant about the Kármán line. This is more or less where the upper (and more significant) minimum that roughly separates the mesosphere from the thermosphere (the mesopause), and not uncoincidentally, the surface that distinguishes whether turbulent mixing or molecular diffusion is the dominant atmospheric process (the turbopause), and also not uncoincidentally, where temperature once again starts increasing with increasing altitude.

While the 100 km mark is a bit arbitrary, that a perhaps fuzzy boundary does exist is not. You can certainly see that boundary in the graph of temperature in Russell Borogove's answer. His graph happens to show a marked temperature change at an altitude of 90 km. (Where this occurs varies, with time of year, latitude, and paper author.)

The changes in temperature gradient and in atmospheric behavior both have a marked impact on low-orbiting vehicles. A vehicle is essentially orbiting above this line. (Maybe not for long, but it's orbiting.) Below it, it's entering the atmosphere. The exponential increase in pressure means that drag rather suddenly dominates over gravitation somewhere near this point.

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