NASA uses two modified Boeing 747 jetliners, originally manufactured for commercial use, as Shuttle Carrier Aircraft (SCA). One is a 747-100 model (NASA 905), while the other is a short range 747-100SR (NASA 911). The two aircraft are identical in appearance and performance.
The 747 series of aircraft are four-engine intercontinental-range, swept-wing "jumbo jets" that entered commercial service in 1969. The two SCAs are under the operational control of NASA's Johnson Space Center, Houston, TX.
The SCAs are used to ferry Space Shuttle orbiters from landing sites back to the Kennedy Space Center launch complex in Florida and also to and from other locations too distant for the orbiters to be delivered by ground transportation.
An orbiter is placed atop the SCA by a Mate-Demate Device (MDD), a large gantry-like structure that hoists the orbiter off the ground for post-flight servicing, and then mates it with the SCA for ferry flights.