Heat shield is a covering on a spacecraft or on a rocket nose cone. It protects astronauts and instruments from the intense heat produced during high-speed flights through the atmosphere. Such heating occurs when a spacecraft descends from an earth orbit. Atoms and molecules of air generate heat by friction against the surface of the craft. The greater the craft’s velocity, the more intense the heat. A common type of heat shield is made up of heat sinks. Heat sinks are devices that absorb great amounts of heat and thus prevent heat from reaching delicate parts of the spacecraft. Early spacecraft used ablative shields, which absorbed heat by melting or burning off, layer by layer, and vaporizing. The air stream carried the molten particles and hot vapor away from the craft. Materials commonly used in heat shields include graphite, graphite composites, quartz composites, and porous ceramics.
