Why don't astronauts' vessels burn up while re-entering the atmosphere?

shutterstock

Currently, the astronauts and cosmonauts return to Earth in a Soyuz capsule. The Soyuz capsule has a curved ablative heat shield that is used to prevent heat from building up and penetrating the crew cabin.

As the capsule plummets through the atmosphere it slams into molecules of air. These bounce off of the capsule and in turn slam into the air molecules that were behind them. This creates a shockwave in front of the vehicle. The greatest temperature is at the shockwave, so forming the shockwave farther from the vehicle surface reduces the amount of heating the vehicle experiences. This displacement is related to the shape of the capsule and explains the curvature.

Some heat still makes it to the vehicle surface, and that’s where the ablative heat shield comes into play. As the outer molecules of the heat shield absorb heat, they reach a point where they vaporize and leave the surface of the vehicle. When they do this, they carry away the heat they have absorbed. This phenomenon is called pyrolysis. The next layer then gets hotter and hotter until it vaporizes and takes its heat away. Eventually, the vehicle has slowed to the point where heat is no longer significant and what is left of the outer surface will be relatively cool.

So, the idea is to create a heat shield that has the right curvature, ablative properties, and thickness such that significant heat never makes it to the crew compartment or vital equipment.