The Reduced Gravity Program was started in 1959 to investigate human and hardware reactions to operating in a weightless environment. The reduced gravity environment is obtained with a specially modified KC-135A turbojet transport, which flies parabolic arcs to produce weightless periods of 20 to 25 seconds. The KC-135A can also provide short periods of lunar (1/6) and Martian (1/3) gravity. Approximately 80,000 parabolas have been flown in support of the Mercury, Gemini, Apollo, Skylab, Space Shuttle, and Space Station programs.

Over the decades there have been five, KC-135 aircraft with the last one being retired on October 25, 2004. It is being replaced by a C-9 aircraft acquired by NASA from the Navy.

The aircraft is officially referred to as the "Weightless Wonder" but is commonly known (affectionately) as the "Vomit Comet".