Then the engines are briefly cut back, the airplane slows, and the nose is pitched down to complete the parabola. As the plane is tracing the parabolic arc, microgravity conditions are achieved for 20-25 seconds. As many as 40 arcs can be flown on a typical flight so that scientists can conduct several experiments or can repeat short runs of one experiment many times. An experiment is placed on the rocket, which is launched and then freefalls back to Earth. A sounding rocket follows a parabolic arc, like the aircraft, but it goes above the Earth's atmosphere, where air drag does not disturb microgravity conditions.
Experiments experience several minutes of microgravity before the rocket reenters the atmosphere.
The shuttle can stay in orbit around the Earth for up to 17 days, which is ideal for experiments that need longer periods in microgravity to be successful. Using this facility, a scientist can conduct an experiment over several months and leave experiment equipment on the station permanently, rather than exchanging it with each mission, as is done with the space shuttle.