Their sequence was:
1). Mercury
(one astronaut in the capsule, in Earth orbit)
2). Gemini
(two astronauts in the capsule, in Earth orbit)
3). Apollo
(three astronauts in the capsule, in Earth orbit, then to the Moon)
4). Space Shuttle
(crew of six, in Earth orbit; carried other satellites and put them into orbit;
carried the Hubble Space Telescope and put it into orbit;
carried materials and began construction of the International Space Station;
ferried astronauts and supplies to and from the ISS.)
