These two Boeing 747-100/-100SR are the aircraft that were used to transport the shuttle fleet across the nation during the shuttle program. N905NA was used for the approach and landing trials conducted by the shuttle Enterprise OV-101 and later converted to transport shuttles. It was a former American Airlines 747-100 that was acquired by NASA in 1974:
After the Challenger accident, one of the recommendations of the review board was to acquire a second Shuttle carrier aircraft. In 1988 NASA purchased a Japan airlines 747-100SR for conversion to become the second SCA. The Aircraft was given the designation N911NA and was first used to ferry Endeavour OV-105 from the Palmdale plant where she was assembled in California to the Kennedy Space center. I cannot seem to find this aircraft on the tracking list, as NASA911 pulls up a T-38 aircraft and not the Boeing 747-100SR.
These aircraft will be used for the final time to transport the orbiters to their final homes at museums. Space Shuttle Endeavour (OV-105) will be going to the California Science Center in Los Angeles, Space Shuttle Discovery (OV-103) will be going to the Steven F. Udvar-Hazy Center of the National Air and Space Museum in Reston, VA and the Shuttle Enterprise (OV-101) will go from the Udvar-Hazy to the Intrepid Sea-Air-Space Museum in New York City. These will represent the last flights of the shuttle carrier aircraft and will happen in roughly a year. The Space Shuttle Atlantis (OV-104) will remain at the Kennedy Space Center and will not require transport.