Space Shuttle landing at Edwards, the orbiter was flown back to the KSC on the Shuttle Carrier Aircraft, a specially modified Boeing 747. The first orbiter, "Enterprise", was built in 1976, used in Approach and Landing Tests and had no orbital capability. Four fully operational orbiters were initially built: "Columbia", "Challenger", "Discovery", and "Atlantis". Of these, two were lost in mission accidents: "Challenger" in 1986 and "Columbia" in 2003, with a total of fourteen astronauts killed. A fifth operational (and sixth in total) orbiter, "Endeavour", was built in 1991 to replace "Challenger". The Space Shuttle was retired from service upon the conclusion
Criticism of the Space Shuttle program and payloads into low Earth orbits. It would afford the opportunity to conduct science experiments on board the shuttle to be used to study the effects of space flight on humans, animals and plants. Other experiments would study how things can be manufactured in space. The shuttle would also enable astronauts to launch satellites from the shuttle and even repair satellites already out in space. The Shuttle was also intended for research into the human response to zero-g. The Shuttle was originally billed as a space vehicle that would be able to launch once a week and give low launch