One who travels in space (in the USSR the term cosmonaut is used). Due to a number of factors, the degree of skill and technical knowledge required, the physical and mental stresses involved in spaceflight, etc., extremely careful selection procedures and long training periods are required for prospective astronauts. In addition, additional training is required for specific missions; for example, each member of the US SKYLAB crew had to complete some 2150 hours of specific training, including simulator time, medical training and EVA mock-ups.
