If a longer period of microgravity is needed, NASA uses a specially equipped jet airplane for the job. Most of the plane's seats have been removed and the wall, floor, and ceiling are covered with thick padding similar to tumbling mats. One of the advantages of using an airplane to do microgravity research is that experimenters can ride along with their experiments. A typical flight lasts 2 to 3 hours and carries experiments and crew members to a beginning altitude about 7 kilometers above sea level. During the pull up and pull out segments, crew and experiments experience a force of between 2 g and 2.5 g.
Small rockets provide a third technology for creating microgravity.
A sounding rocket follows a parabolic path that reaches an altitude hundreds of kilometers above Earth before falling back. The experiments onboard experience several minutes of freefall. The microgravity environment produced is about equal to that produced onboard falling packages in drop towers. Although airplanes, drop facilities, and small rockets can be used to establish a microgravity environment, all of these laboratories share a common problem. After a few seconds or minutes of low-g, Earth gets in the way and the freefall stops.