In physics and materials science, a drop tower or drop tube is a structure used to produce a controlled period of weightlessness for an object under study. Air bags, polystyrene pellets, and magnetic or mechanical brakes are sometimes used to arrest the fall of the experimental payload. In other cases, high-speed impact with a substrate at the bottom of the tower is an intentional part of the experimental protocol.
Not all such facilities are towers: NASA Glenn's Zero Gravity Research Facility is based on a vertical shaft, extending to 510 feet (155 m) below ground level.