Why does an astronaut feel weightless in space?

What a body senses as weight is not gravity itself but the floor pushing back. Stand still and the ground resists your fall; the pressure on your feet, the load on your spine and the fluid in your inner ear all report it. Take the floor away, by jumping off a diving board or riding a lift whose cable has been cut, and the sensation disappears at once even though Earth's pull has not changed at all. Astronauts live permanently in that condition.

An orbiting spacecraft is not parked beyond gravity's reach. At the International Space Station's altitude of about 400 km, Earth still pulls almost as hard as it does at sea level. What the station has is sideways speed: roughly 28 000 km/h, fast enough that as it falls, the curve of its path matches the curve of the planet and it never arrives. Newton described the idea with a cannon on a mountain firing ever harder until the ball comes all the way round. Crew, station, tools and food fall together along the same trajectory, so nothing presses on anything else and nothing has weight.

The correct term is microgravity, because the cancellation is never perfect. Residual atmospheric drag, the slight difference in gravitational pull between one end of the station and the other, thruster firings and crew members pushing off walls all leave tiny accelerations behind. Experiments in fluid physics or crystal growth have to be isolated from them. The same free-fall trick can be bought briefly on Earth: aircraft flying parabolic arcs give their passengers a few weightless seconds per dive, long enough to test hardware and train crews, and drop towers do the same for shorter intervals still.