Quote:
Originally Posted by Piwoslaw
It's mostly the friction: The spacecraft is going VERY fast (appr 30k km/h) when reentering the atmosphere, so that is the relative speed with which air molecules are hitting the surface. I won't go into equations on how much heat energy each collision produces, but as the craft falls deeper into the atmosphere it becomes denser, so there are more and more collisions.
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Actually, adiabatic compression heats up the skin of the re-entering vehicle. Consider that the re-entering vehicle initially enters the atmosphere at some 25 times the speed of sound. The shock wave generated is actually a wave of compressed air that is adiabatically compressed.
Requesting an in depth explanation of heat created during atmospheric reentry