Reentry heating differs from the normal atmospheric heating associated with jet aircraft, and this governed TPS design and characteristics. The skin of high-speed jet aircraft can also become hot, but this is from frictional heating due to atmospheric friction, similar to warming one's hands by rubbing them together. The orbiter reentered the atmosphere as a blunt body by having a very high (40°) angle of attack, with its broad lower surface facing the direction of flight. Over 80% of the heating the orbiter experiences during reentry is caused by compression of the air ahead of the hypersonic vehicle, in accordance with the basic thermodynamic relation between pressure and temperature. A hot shock wave was created in front of the vehicle, which deflected most of the heat and prevented the orbiter's surface from directly contacting the peak heat. Therefore, reentry heating was largely convective heat transfer between the shock wave and the orbiter's skin through superheated plasma. The key to a reusable shield against this type of heating is very low-density material, similar to how a thermos bottle inhibits convective heat transfer.
