Space Shuttle program March 25, 1979, and was first launched on April 12, 1981—the 20th anniversary of Yuri Gagarin's space flight—with a crew of two. "Challenger" (OV-099) was delivered to KSC in July 1982, "Discovery" (OV-103) in November 1983, "Atlantis" (OV-104) in April 1985 and "Endeavour" in May 1991. "Challenger" was originally built and used as a Structural Test Article (STA-099), but was converted to a complete orbiter when this was found to be less expensive than converting "Enterprise" from its Approach and Landing Test configuration into a spaceworthy vehicle. On April 24, 1990, "Discovery" carried the Hubble Space Telescope into space during
Space Shuttle program for frequent flights. Various shuttle concepts had been explored since the late 1960s. The program formally commenced in 1972, becoming the sole focus of NASA's manned operations after the Apollo, Skylab, and Apollo-Soyuz programs in 1975. The Shuttle was originally conceived of and presented to the public in 1972 as a 'Space Truck' which would, among other things, be used to build a United States space station in low Earth orbit during the 1980s and then be replaced by a new vehicle by the early 1990s. The stalled plans for a U.S. space station evolved into the International Space Station