Returning from orbit means hitting the atmosphere at roughly 7.8 kilometres per second. Friction and compression heat the air around the vehicle to thousands of degrees. A heat shield absorbs or sheds that energy so the structure underneath survives.

Materials vary by mission. Apollo used an ablative shield that burned away in controlled fashion, carrying heat with it. The Space Shuttle used reinforced carbon-carbon tiles and silica tiles that radiated heat away.

Common heat shield materials

• Ablative composites that char and erode
• Reinforced carbon-carbon for leading edges
• Silica tiles for reusable vehicles
• Phenolic impregnated carbon ablator for high-speed entries

Any breach is fatal. A damaged tile on Columbia's wing led to the loss of the vehicle and crew in 2003, which prompted major changes in inspection procedures.