NASA has operated six space shuttles, beginning with the prototype orbiter, which was flown in Earth's atmosphere during landing and flight tests. Two of the shuttles, Columbia and Challenger, were lost in accidents, and the rest have been retired to museums.
The space shuttle was conceived as a reusable space craft, with the prototype orbiter Enterprise undergoing its first test flight in 1977. The space shuttle fleet was made up of Columbia, Challenger, Discovery, Atlantis and Endeavour. Two of the surviving shuttles and the prototype are located in museums, and the third is on display at its launch and landing site in Florida.