This left-hand hatch was part of the Gemini IV spacecraft, which flew a four-day mission in space, June 3-6, 1965. The astronauts were James McDivitt, command pilot and Edward White, pilot. McDivitt sat on the left side of the cockpit and this hatch covered his ejection seat. It was not opened in space, unlike White's, which he used to exit the spacecraft for the first American EVA (spacewalk).
Gemini hatches were designed to open extremely rapidly during an emergency ejection, the astronauts' escape method from a failing launch vehicle on the pad or in the lower parts of the Earth's atmosphere. Otherwise, the hatches were only open before launch, for the insertion of the astronauts, during a mission for an EVA by the pilot (right side only), or at the end of the mission.
NASA Manned Spacecraft Center gave this Gemini IV hatch to the Smithsonian in 1967.
Transferred from the National Aeronautics and Space Administration