Hubble Space Telescope be kept in a clean room, powered up and purged with nitrogen, until a launch could be rescheduled. This costly situation (about per month) pushed the overall costs of the project even higher. This delay did allow time for engineers to perform extensive tests, swap out a possibly failure-prone battery, and make other improvements. Furthermore, the ground software needed to control Hubble was not ready in 1986, and was barely ready by the 1990 launch. Eventually, following the resumption of shuttle flights in 1988, the launch of the telescope was scheduled for 1990. On April 24, 1990, successfully launched the
Space Shuttle program from the back of a specially modified Boeing 747, only for initial atmospheric landing tests (ALT). "Enterprise"'s first test flight was on February 18, 1977, only five years after the Shuttle program was formally initiated; leading to the launch of the first space-worthy shuttle "Columbia" on April 12, 1981 on STS-1. The Space Shuttle program finished with its last mission, STS-135 flown by "Atlantis", in July 2011, retiring the final Shuttle in the fleet. The Space Shuttle program formally ended on August 31, 2011. Before the Apollo 11 moon landing in 1969, NASA began early studies of space shuttle designs.