What primarily causes the intense thermal energy at the stagnation point during re-entry?

Massive, localized compression of air creating a superheated shock wave.

The intense heating experienced at the front of the spacecraft, known as the stagnation point, is primarily caused by adiabatic compression. When the vehicle slams into the atmosphere at orbital speeds, the air directly ahead cannot move away fast enough. This rapid deceleration of the air molecules results in massive, localized compression, generating a shock wave that stands off from the vehicle's surface. This process converts kinetic energy into thermal energy far more effectively than simple friction alone, leading to temperatures reaching thousands of degrees Celsius. While friction contributes, the compression shock wave is identified as the dominant factor in generating this extreme thermal load.