A heat shield protected the two-man Gemini spacecraft against the enormous heat of reentry into the atmosphere beginning at a velocity of more than 27,500 kilometers (17,000 miles) per hour. Like those of other early human 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 dissipated by ablation: charring and evaporation of the shield's surface. Ablative heat shields are not reusable.The ablative substance of the Gemini heat shield is a paste-like silicone elastomer material which hardens after being poured into a honeycomb form. This heat shield may have been installed on Gemini 2, launched on an unmanned test in January 1965. The circular marks show where samples were removed afterward for testing, and the resulting holes repaired. In 1970 McDonnell-Douglas gave this object to the Smithsonian on behalf of NASA.