Which Boeing aircraft carried the first reusable Space Shuttle orbiter during its approach tests?

The Shuttle Carrier Aircraft carried the first reusable Space Shuttle orbiter during its approach tests.

The Shuttle Carrier Aircraft was a modified Boeing 747 used by NASA to transport Space Shuttle orbiters between facilities and to conduct early atmospheric flight tests. NASA converted two 747s for the role: one based on a 747-100 and another based on a 747-100SR.

In 1977, the aircraft carried the prototype orbiter Enterprise for the Approach and Landing Tests. Enterprise was released from the 747 at altitude so NASA could evaluate the orbiter’s unpowered glide, flight handling, and landing procedures. These tests helped validate the Shuttle’s ability to return to Earth and land like a glider.

The carrier aircraft was not itself a spacecraft and did not launch orbiters into space. Its job was atmospheric transport and test support. After Shuttle missions, the aircraft also moved orbiters between landing sites, processing facilities, and launch locations.