Gemini Capsule

Project Gemini bridged the gap between Mercury''s tentative steps and Apollo''s lunar ambitions. Named after the constellation of the twins (for its two-person crew), Gemini was designed to demonstrate that astronauts could work outside their spacecraft, rendezvous and dock with other vehicles in orbit, and endure flights lasting up to two weeks -- all essential capabilities for a Moon mission.

Ten crewed Gemini missions flew between March 1965 and November 1966, a pace of roughly one mission per month that has never been matched. Gemini 4 saw Ed White perform the first American spacewalk. Gemini 6A and 7 achieved the first orbital rendezvous, with the two spacecraft flying in formation just feet apart. Gemini 8 accomplished the first docking between two spacecraft, though the mission nearly ended in disaster when a stuck thruster sent the docked vehicles into a violent spin.

Gemini 7 set an endurance record of nearly 14 days in a capsule barely larger than the front seat of a Volkswagen Beetle. Astronauts Frank Borman and Jim Lovell endured the cramped conditions to prove that humans could survive in space long enough for a lunar mission. The data from their flight -- including the first evidence of bone density loss in microgravity -- informed medical planning for all subsequent long-duration missions.