User blog comment:Riley Heligo/Seriously EA?/@comment-4118072-20120217142609/@comment-4800118-20120217170520

Friction is a good explanation. You can of course take the explanation several magnitudes in complexity, and talk about dynamic pressure, shape of the surface, the airspeed of the air going past this surface. But a good basic explanation is the friction created by the dynamic pressure of the vehicle flying at fast speeds. That is not to say the surface of the vehicle does not get hot. It certainly does. It is built to withstand those temperatures, at least for the amount of time required until the vehicle slows down enough that the dynamic pressure falls.