There's more than one way to make a person feel weightless. You can lower future astronauts into one of the Neutral Buoyancy Pools we just mentioned, or you give them a ride on a reduced-gravity air flight.
In 1959, NASA began working with the U.S. Air Force to modify ex-military planes so they could be used to train America's original class of astronauts (and test out space equipment). Thus began the saga of the lovingly nicknamed "Vomit Comets" [source: NASA].
With the right aircraft, it is possible to carry out flights that (briefly) subjected their riders to low-gravity conditions. To do this, a wide plane with padded walls and a roomy interior was required. For decades, NASA used KC-135 aircraft for this purpose. Passengers and trainees were loaded into the plane and then the pilots flew it in a wave-like ("parabolic") motion. That sent the plane through a series of upward climbs and rapid descents. When the trajectory was just right, people inside the aircraft would experience near-weightlessness for about 20 to 25 seconds on the downward falls [source: NASA].
As you can imagine, the experience made a lot of people nauseous. Estimates vary, but according to one article NASA ran in 2004, "about one in three first-time fliers" barfed on these rides. That's why astronauts traditionally called the planes "Vomit Comets."
Though NASA told us that its astronauts-in-training are no longer required to take microgravity flights, the astronaut candidate class of 2017 did get to experience one of these trips aboard a different specialized plane. The trip was made possible by a collaboration between NASA, the Canadian Space Agency (CSA) and Canada's National Research Council.