Vomit Comet is a nickname for any airplane that briefly provides a nearly weightless environment in which to train astronauts, conduct research, and film motion pictures. Versions of such airplanes have in the past been operated by NASA's Reduced Gravity Research Program since 1973, where the unofficial nickname originated. NASA then adopted the official nickname Weightless Wonder, for publication.
The airplane produces weightlessness by following a parabolic vertical flight path. A parabolic flight path is the same path that would be taken by an object in free fall, such as a cannonball fired into the air. As a result, the aircraft does not exert any g-force on its contents, so the contents have zero apparent weight relative to the aircraft.
The aircraft heads upward at an angle of 45 degrees. As soon as the pilot begins the rotation into the parabolic trajectory, weightlessness is achieved. This lasts all the way "up-and-over the hump", until the craft reaches a declined angle of 30 degrees. At this point, the craft has lost a significant amount of altitude, and must begin to pull into a hard upward turn. The forces are then roughly twice that of gravity on the way down, at the bottom, and up again. This lasts all the way until the aircraft is again halfway up its upward trajectory, and the pilot again initiates the zero-g parabola.
In general, this aircraft is used to train astronauts in zero-g maneuvers, giving them about 25 seconds of weightlessness out of 65 seconds of flight. This often produces nausea due to airsickness, especially in novices, giving the plane its nickname.
Twin KC-135 Stratotankers were used until December 2004 and have since been retired. One, a KC-135A known as NASA 930, was also used by Universal Pictures and Imagine Entertainment for filming scenes involving weightlessness in the movie Apollo 13; that airplane was retired in 2000 and is now on display at Ellington Field, near the Johnson Space Center. It is estimated to have flown over 58,000 parabolas. The other (N931NA or NASA 931) made its final flight on October 29, 2004, and is permanently stored in the Pima Air & Space Museum in Tucson, Arizona.
In 2005, NASA replaced the airplane with a McDonnell Douglas C-9B Skytrain II that was formerly owned by KLM Royal Dutch Airlines and the United States Navy That same year, the Zero-Gravity Corporation, a commercial parabolic flight operator which offers parabolic flights to both researchers and adventure tourists, began flying parabolic flights for NASA with Boeing 727 jets.
Since 1984, the ESA and the CNES have flown similar reduced-gravity missions in a variety of airplanes, including NASA's KC-135, a Caravelle, an Ilyushin IL-76 MDK, and, most recently, an Airbus A300 known as the Zero-G, which is flown out of the Bordeaux-Mérignac airport in France.
The first zero G plane in Latin America is a T-39 Sabreliner nicknamed CONDOR, operated by the Ecuadorian space agency and the Ecuadorian Air Force. It has operated since May, 2008. On June 19 2008, this plane carried 7 year old Jules Nader as he set the Guinness world record for the youngest human being flying in microgravity. Nader worked on an hydrodynamics experiment designed by his brother.
Commercial operator Zero Gravity Corporation claims their tourist rides with up to 15 parabolas are much less traumatic compared to the typical research flight with 40–80 parabolas.