Why is there weightlessness on the space station?

Gravity does not disappear by climbing a few hundred kilometers. At the ISS's orbital altitude, Earth still attracts with almost the same force as at the surface. The difference lies in the motion: the station flies sideways at five miles per second and, in doing so, constantly falls toward Earth. Since the surface curves away beneath it, it never reaches it: it falls around the planet and takes about 90 minutes to complete one orbit.

Weight, strictly speaking, is nothing more than the pressure of a supporting surface. Standing on the ground, that ground prevents the body from following Earth's pull, and that is precisely what we perceive as weight. On the station, however, walls, floor, experiments and crew fall with exactly the same acceleration. Nothing presses against the body anymore, no joint bears load, and a scale would read zero. The same sensation occurs on a parabolic flight, though only for a brief instant.

Even so, not everything aboard is free of force. Residual atmosphere brakes the station, thrusters and docking maneuvers shake it, fans and treadmills generate vibrations, and parts farther from the center of mass behave slightly differently than it does. Tiny residual accelerations remain, hence the technical term microgravity rather than weightlessness. This difference matters for experiments: anyone growing crystals or studying flames without buoyancy must know about these disturbances, which is why some racks hang cushioned against vibrations.