Why does an astronaut float in space?

The word "weightlessness" is misleading. At an altitude of a few hundred kilometers, Earth still pulls very strongly on a spacecraft and its occupants: nothing escapes gravity in low orbit. If astronauts float, it's for an entirely different reason, far more interesting.

An orbit is nothing more than a fall that never lands. The spacecraft moves fast enough horizontally that, as it falls, the planet's curved surface keeps dropping away beneath it. It misses the ground, indefinitely. Since the astronaut's body, their seat, their tools and the walls all fall together at the same rate, nothing pushes them toward a floor anymore. The sensation of weight, which comes from contact with a support, disappears.

Engineers prefer to speak of microgravity, because tiny forces still remain: friction from the residual atmosphere, thruster maneuvers, vibrations, a slight difference in pull between the head and the feet. The same state occurs for a few seconds aboard aircraft that fly repeated parabolic trajectories, used for training and scientific experiments, and in any cabin in free fall.

The body reacts quickly to this absence of up and down. Fluids shift upward toward the chest and face, which puffs up the features and blocks the nose; the inner ear sends contradictory signals, causing space sickness in the first few days. Postural muscles and the skeleton, no longer working against gravity, weaken and lose bone density. This is precisely why daily exercise sessions, on a treadmill, bike and resistance machine, are part of the onboard program.