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?)