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
Space Shuttle in an emergency, the orbiter was loaded atop the Shuttle Carrier Aircraft, a modified 747, for transport back to the Kennedy Space Center, landing at the Shuttle Landing Facility. Once at the Shuttle Landing Facility, the orbiter was then towed along a tow-way and access roads normally used by tour buses and KSC employees to the Orbiter Processing Facility where it began a months-long preparation process for the next mission. NASA preferred Space Shuttle landings to be at Kennedy Space Center. If weather conditions made landing there unfavorable, the Shuttle could delay its landing until conditions are favorable, touch down