[redpoint5]

I often have physics questions but fail to explore them before I forget.

Driving home I started thinking about heat generation on spacecraft reentry vehicles. The Gemini and Apollo reentry vehicles used ablative heat shields to deflect and dissipate heat. The shuttle used tiles to insulate against the heat. I don't know what strategies are used in current reentry vehicles.

My question is, what is the primary cause of heating on the surface of the reentry vehicle? We know that compressing air increases the temperature of it, but I've heard that air friction on the surface is what causes the extreme temperatures. Which action is most responsible for heating the surface of the object? How much does air compression contribute to heating, if at all? Are there other significant sources of heating that I'm not thinking of?