<P> Spacecraft that land on a planet with an atmosphere, such as Earth, Mars, and Venus, currently do so by entering the atmosphere at high speeds, depending on air resistance rather than rocket power to slow them down . A side effect of this method of atmospheric re-entry is aerodynamic heating, which can be highly destructive to the structure of an unprotected or faulty spacecraft . An aerodynamic heat shield consists of a protective layer of special materials to dissipate the heat . Two basic types of aerodynamic heat shield have been used: </P> <Ul> <Li> An ablative heat shield consists of a layer of plastic resin, the outer surface of which is heated to a gas, which then carries the heat away by convection . Such shields were used on the Mercury, Gemini, and Apollo spacecraft . </Li> <Li> A thermal soak heat shield uses an insulating material to absorb and radiate the heat away from the spacecraft structure . This type was used on the Space Shuttle, consisting of ceramic or composite tiles over most of the vehicle surface, with reinforced carbon - carbon material on the highest heat load points (the nose and wing leading edges). Damage to this material on a wing caused the 2003 Space Shuttle Columbia disaster . </Li> </Ul> <Li> An ablative heat shield consists of a layer of plastic resin, the outer surface of which is heated to a gas, which then carries the heat away by convection . Such shields were used on the Mercury, Gemini, and Apollo spacecraft . </Li> <Li> A thermal soak heat shield uses an insulating material to absorb and radiate the heat away from the spacecraft structure . This type was used on the Space Shuttle, consisting of ceramic or composite tiles over most of the vehicle surface, with reinforced carbon - carbon material on the highest heat load points (the nose and wing leading edges). Damage to this material on a wing caused the 2003 Space Shuttle Columbia disaster . </Li>

What was the apollo heat shield made of