Why does a bow shock produce less heating?

Imagine a solid cone with half-angle $theta$ moving point-first at a velocity $M_1$ with respect to a compressible ideal gas. If possible, this cone will create a conical attached oblique wave with a half-angle $beta$ that comes to a point at the point of the solid cone. The shock wave angle $beta$ is a function of the deflection angle $theta$, the Mach number $M_1$, and characteristics of the gas. The relation between the deflection angle and the shock wave angle is depicted below for a diatomic ideal gas for various values of Mach number.

Note that for a given Mach number there is a maximum possible deflection angle. An attached oblique shock wave cannot be formed if the deflection angle is larger than this maximum. The shock detaches and forms a bow shock.

The above assumes a cone that comes to a point, which is not physically possible, and assumes an ideal diatomic gas, which also is not physically possible. A physical object, even one with a highly aerodynamic shape, will have some bluntness to its tip. There will always be a tiny portion of the shock that is detached from the deflecting body.

In a real gas, heating in the shock can raise the temperature to such an extent that the gas dissociates. The region behind the shock will contain dissociated elements that recombine and release heat. Most of this recombination occurs very close to the shock. If the gap between the bow shock and the physical body is small, the recombination heating will result in significant heating of the physical body. But if the gap is large enough, that the recombination occurs somewhat remote from the physical body limits the heat transfer to the physical body.

This reduced heating to a blunt body is one of the two key reasons why a blunt body is preferred over a more aerodynamic shape. The other is that the higher drag slows the body down more than would happen with a more aerodynamic shape. This deceleration is a very desired effect for a reentering body.

A pointed tip will produce an attached, oblique shock.

A blunt tip will result in a separated (detached) shock.

While the peak temperature in the attached shock is lower than that in the center of the detached shock, the temperature gradient* between the hot air and the surface of the tip is much higher in case of the pointed tip, resulting in much higher energy transfer into the tip. The reason is the distance between the shock and the tip: As the name implies, the attached shock sits directly on the tip while the detached shock keeps some distance between the shock and the tip. And a flatter gradient means less heat energy is transferred into the blunt tip. Most of the heat generated in the detached shock remains with the air and is carried away with the flow.

* That a temperature gradient is maintained lies in the limits of structural materials: The temperature in a hypersonic shock exceeds the melting point of all practical materials, so continuously new, cooler material is exposed to the shock as the tip material melts away. In a blunt nose, thermal conduction or ablative cooling keeps the nose cool. The first reentry bodies of intercontinental ballistic missiles contained a copper heat sink while newer designs use a layer of material which vaporizes during reentry.