In 1959, NASA started the Reduced Gravity Program and used the KC-135 aircraft's parabolic arcs to simulate zero gravity.
During each parabolic arc, approximately 40 each mission, there are 20 to 25 seconds of zero gravity. The KC-135 has
supported 80,000 parabolas in support of NASA's space program. NASA's Johnson Space
Center operates the KC-135 Reduced Gravity Research aircraft.