The Shuttle Carrier Aircraft (SCA) are two heavily modified Boeing 747 Planes from NASA and used to transport the Space Shuttle orbiter. One of these is based on model 747-100, while the other is a short-range version named 747-100SR. SCAs are used to bring the Shuttle back to the Kennedy Space Center every time they are forced to land on a track that is not NASA Shuttle Landing Facility for operational reasons. In the first shuttle test flights, the SCAs were used to bring up the prototype of the orbiter and then release it to test the landing.
The Space Shuttle Getting a Ride on a NASA 747 Over Los Angeles