By convention, an astronaut is an astronaut unless employed by the Russian government, in which case he or she is a cosmonaut. The term "taikonaut" is sometimes used for (still-hypothetical) astronauts from China.
NASA Astronauts of the Apollo Era
During the Apollo program, (1961-1975), the United States launched a total of 31 manned missions: 6 in the Mercury program, 10 in the Gemini program, 11 in the Apollo program, 3 in the Skylab? program, and 1 in the Apollo-Soyuz? Test program. These 31 missions provided 71 individual flight opportunities: 6 in Mercury, 20 in Gemini, 33 in Apollo, 9 in Skylab, and 3 in Apollo-Soyuz. These 71 positions were filled by 43 individuals. Of these 43, 4 flew a total of 4 flights, 3 flew a total of 3 flights, 10 flew twice, and the remaining 26 flew only once. Thus only 17 flew more than once, and only seven flew more than twice. (Several later made additional flights on the Space Shuttle.)
Of the 31 Apollo era flights, 2 were suborbital and 9 were lunar missions. The remaining 20 were earth orbital flights. The 9 lunar flights provided 27 individual lunar flight opportunities. These were filled by 24 individuals. Only 3 flew to the moon twice. The 6 successful lunar landing flights provided 12 individual lunar landing opportunities. These were filled by 12 individuals. No one landed on the moon twice. Of those who landed on the moon, 2 had already flown to the moon once, 5 had made previous non-lunar flights, and 5 had no previous spaceflight experience.
All 6 Mercury flights and 3 of 10 Gemini flights had all-rookie crews, as did 1 of the 3 Skylab flights.. All the Apollo missions included at least 1 veteran astronaut. Only 2 flights, the lunar landing mission and its dress-rehearsal, had all-veteran crews.
The Mercury Astronauts
The first group of American astronauts was selected for the Mercury program by NASA in April 1959. This group. known as the "Mercury Seven", included: [Scott Carpenter]?,[Gordon Cooper]?,[John Glenn]?,[Gus Grissom]?,[Wally Schirra]?, [Alan Shepard]? and [Deke Slayton]?. All seven were military test pilots, a requirement specified by President Eisenhower to simplify the selection process.
All seven of the first group of astronauts eventually flew in space, although one, Deke Slayton, did not fly a Mercury mission due to a medical disqualification. He eventually flew on the [Apollo-Soyuz mission]?. The other six each flew one Mercury mission. For two of these, Scott Carpenter and John Glenn, the Mercury mission was their only flight in the Apollo era (Glenn later flew on the space shuttle). Three of the Mercury astronauts, Gus Grissom, Gordon Cooper and Wally Schirra, also each flew a mission during the Gemini program. Alan Shepard did not fly a Gemini mission due to a medical disqualification, but later did fly an Apollo mission. He was the only Mercury astronaut to go to the moon. Wally Schirra also flew on Apollo, as well as Mercury and Gemini, the only astronaut to fly on all three types of spacecraft. (Gus Grissom was scheduled to fly the first Apollo flight, but died in a fire on the launch pad during training.)
The Gemini Astronauts
A second group of nine astronauts was selected by NASA in September 1962. This group included: Neil Armstrong, [Frank Borman]?, [Charles Conrad]?, [Jim McDivitt]?, [Jim Lovell]?, [Elliott See]?, [Tom Stafford]?, [Ed White]? and John Young. All of this group flew missions in the Gemini program except Elliott See, who died in a flight accident while preparing for his Gemini flight. All of the others also flew on Apollo, except for Ed White, who died in a fire on the launch pad during training for the first Apollo flight. Three of this group, McDivitt?, Borman and Armstrong, made single flights in both Gemini and Apollo. Four others, Young, Lovell, Stafford and Conrad, each made two flights in Gemini and at least one flight in Apollo. Young and Lovell both made two Apollo flights. Conrad and Stafford also made second flights in Apollo spacecraft, Conrad in Skylab and Stafford in Apollo-Soyuz. Six of this group, Borman, Lovell, Stafford, Young, Armstrong and Conrad, made flights to the moon. Lovell and Young went to the moon twice. Armstrong, Conrad, and Young walked on the moon. John Young also later flew on the Space Shuttle.
Five members of the third group of astronauts, selected by NASA in October 1963, also flew missions during the Gemini program. They were [Buzz Aldrin]?, Gene Cernan, Michael Collins, [Richard Gordon]? and [David Scott]?. Each flew a single Gemini mission and at least one mission in the Apollo program. Scott and Cernan both flew a second Apollo mission. All of this group went to the moon, Cernan went twice. Aldrin, Scott and Cernan walked on the moon.
U.S. Space Shuttle astronaut
Public domain picture from NASA
Astronauts include: