Why does an astronaut float in space?

Orbit is not a place where gravity stops. At the height the International Space Station flies, roughly 400 km (250 mi), Earth still pulls almost as hard as it does at sea level. What has changed is that the station is moving sideways at about 28,000 km/h (17,500 mph). It falls towards the planet continuously and continuously misses, because the surface curves away beneath it as fast as it drops. Newton described exactly this with a thought experiment about a cannon fired hard enough from a mountaintop.

Weight is not gravity; it is the sensation of something stopping you from falling — a chair, a floor, the ground. In orbit the crew, the walls, the tools and the water in a drinking bag all fall at the same rate along the same path, so nothing pushes against anything else. The result is the feeling of the half-second after stepping off a wall, extended indefinitely. The proper name is microgravity rather than zero gravity, because small residual accelerations remain: atmospheric drag, the crew moving about, the slight difference in pull between one end of the station and the other.

Deprived of a direction to resist, the body begins unlearning. Fluid that gravity normally keeps in the legs migrates to the chest and head, bones lose mineral and the postural muscles weaken, which is why crews spend hours a day strapped to treadmills and resistance machines. The inner ear, receiving no cue about which way is down, argues with the eyes for the first days in orbit.