How does a heat shield protect a returning spacecraft?

A returning spacecraft has a large amount of kinetic energy relative to the atmosphere. As it slows, energy is transferred into the surrounding gas and vehicle. Ahead of a blunt capsule, a strong shock rapidly compresses and heats the gas. That hot gas transfers energy toward the surface through convection and, under some entry conditions, radiation. The heating is more complex than simply rubbing two solid surfaces together.

An ablative heat shield manages this energy using a material designed to change during entry. Its outer region heats, decomposes and sheds material. Gas and removed material carry energy away from the vehicle. The remaining shield and supporting layers also limit how rapidly energy passes inward. A hot outside surface can therefore coexist with a much cooler interior.

The supplied energy account uses a 1000-kilogram capsule. At 4000 meters per second, its kinetic energy is eight gigajoules; doubling that speed to 8000 meters per second gives thirty-two gigajoules. The factor of four follows from the square of speed. These totals are not the amount of heat absorbed by the shield.

The atmosphere carries away much of the energy, and surface heating depends on speed, air density, shape and the path through the atmosphere. The illustration explains shock heating and ablation without calculating a heat flux, shield thickness or cabin temperature. Other spacecraft can use different thermal-protection designs.