Gemini astronauts training, 1960s. Historical footage of Gemini astronauts during centrifuge training and spacewalk weightlessness simulations. Centrifuges are used in astronaut training to simulate the large acceleration forces experienced in spaceflight. Project Gemini was an American human spaceflight programme which ran from 1961 to 1966. The Gemini spacecraft carried a two-astronaut crew, and ten crews flew low Earth orbit missions between 1965 and 1966.