How does weightlessness work?

Weightlessness doesn't mean gravity has disappeared. At the altitude of the International Space Station, about 400 kilometers, Earth pulls on a body almost as strongly as it does on the ground. What's missing is resistance: on Earth, the ground pushes back against us from below, and it's precisely that counterpressure that creates the sensation of weight. If the surface falls along with us, that sensation vanishes too. That's why the term zero gravity is misleading; free fall describes the state much more accurately from a physical standpoint.

The ISS flies around Earth at about 28,000 km/h and takes about 90 minutes to complete one revolution. What matters is the sideways speed: the station continually falls toward the center of the Earth, but moves so fast sideways that the ground beneath it keeps curving away. The station, the crew, and any pencil on board all fall at exactly the same rate; that's why they float relative to one another, even though all of them are traveling at tremendous speed.

On board, one is never completely free of forces. Residual air drags on the station, pumps and fans vibrate, the crew pushes off as they move, and because the station is large, slightly different attractive forces act at its ends. Sensitive experiments with crystals, flames, or liquids respond to this, which is why they're placed in vibration-damped racks. For the body, though, the difference remains drastic: without weight, fluids shift upward, and muscles and bones stop being loaded and deteriorate without daily training.