The Shuttle Carrier Aircraft (SCA) are two extensively modified Boeing 747 airliners that NASA uses to transport a space shuttle orbiter. One is a 747-100 model, while the other is a short range 747-100SR.
The SCAs are used to ferry space shuttles from landing sites back to the launch complex at the Kennedy Space Center, or more specifically, the NASA Shuttle Landing Facility, and to and from other locations too distant for the orbiters to be delivered by ground transport. The orbiters are placed on top of the SCAs by Mate-Demate Devices, large gantry-like structures which hoist the orbiters off the ground for post-flight servicing, and then mate them with the SCAs for ferry flights.
In early test flights, the shuttle was released from SCA during flight, and the shuttle glided to a landing under its own control.