To survive the reentry into the Earth's atmosphere at a velocity of over 17,000 mph, all human spacecraft until the space shuttle used ablative heat shields based on technology developed for nuclear missile nosecones. The blunt end created a shock wave that held off the majority of the heating, but the ionized gases behind the shock wave nonetheless heated the shield to a temperature of several thousand degrees. This heat was carried away by the ablation (charring and evaporation) of the heat shield material. This sample shows the construction of the Gemini heat shield, and how the surface charred as it ablated. It apparently comes from the Gemini 7 spacecraft in which astronauts Frank Borman and James Lovell spent 14 days in space from December 4 - 18, 1965.
Transferred from NASA
A heat shield protected the Gemini spacecraft against the enormous heat generated by reentry into the atmosphere at more than 27,500 kilometers (17,000 miles) per hour. Like those of other early American and Soviet manned spacecraft, Gemini's heat shield derived from ballistic-missile warhead technology. The dish-shaped shield created a shock wave in the atmosphere that held off most of the heat. The rest was dissipated by ablation--charring and evaporation--of the heat shield's surface. Ablative shields were not reusable.
At the time of reentry, the spacecraft would be maneuvered to the appropriate orientation and the equipment adaptor section would be detached and jettisoned, exposing the retrorocket module. The retrorockets consisted of four spherical-case polysulfide ammonium perchlorate solid-propellant motors mounted near the center of the reentry adaptor module, each with 11,070 N thrust. They would fire to initiate the spacecraft reentry into the atmosphere, with attitude being maintained by a reentry control system of 16 engines, each with 5.2 N thrust. The retrorocket module would then be jettisonned, exposing the heat shield at the base of the reentry module. Along with the ablative heat shield, thermal protection during reentry was provided by thin Rene 41 radiative shingles at the base of the module and beryllium shingles at the top. Beneath the shingles was a layer of MIN-K insulation and thermoflex blankets. At an altitude of roughly 15,000 meters the 2.4 meter drogue chute would deploy from the rendezvous and recovery section. At 3230 meters altitude the the drogue is released which extracts the 5.5 meter pilot parachute. The rendezvous and recovery section is released 2.5 seconds later, deploying the 25.6 meter main ring-sail parachute which is stored in the bottom of the section. The spacecraft is then rotated from a nose-up to a 35 degree angle for water landing. At this point a recovery beacon was activated, transmitting via an HF whip antenna mounted near the front of the reentry module. The Gemini reentry module consisted of the pressurized cabin which held the two Gemini astronauts. Instrumentation pallets holding cameras, accelerometers, batteries, and other devices were situated in the astronaut area.
Separating the reentry module from the retrorocket section of the adaptor at its base was a curved silicone elastomer ablative heat shield. The module was composed predominantly of titanium and nickle-alloy with beryllium shingles. At the narrow top of the module was the cylindrical reentry control system section and above this the rendezvous and recovery section which holds the reentry parachutes. The cabin held two seats equipped with emergency ejection devices, instrument panels, life support equipment, and equipment stowage compartments in a total pressurized volume of about 2.25 cubic meters. Two large hatches with small windows could be opened outward, one positioned above each seat.