Astronauts are persons trained to fly or operate systems aboard a spacecraft. "Astronaut" is the term typically applied to those who fly on U.S. spacecraft, whereas "cosmonaut" refers to crewmembers who have flown on Russian space vehicles. The National Aeronautics and Space Administration (NASA) selected the first American astronauts in 1959 to pilot the single-seat Mercury spacecraft. These "Original 7" were all chosen from the ranks of military test pilots.

Qualifications for these first astronauts were extremely high. Not only were the Mercury and Gemini astronauts professional test pilots, but they also had to meet strict standards for eyesight, health, and physical size (because of the tight confines of the spacecraft cockpits). In advance of the Apollo Moon landings, six scientists were selected for astronaut training in 1965, but only one made it to the lunar surface (compared with eleven former test pilots) before the Apollo...