Why are astronauts weightless?

What we perceive as weight is not the attraction itself, but the resistance that the ground, a chair or the sole of a shoe exerts against the body. If that surface disappears, the sensation vanishes instantly, whether in a falling elevator or in orbit. Astronauts are not weightless because Earth no longer attracts them, but because they can follow that attraction without obstruction, together with the vehicle surrounding them. Earth's pull reaches far beyond the Moon's orbit.

Free fall can also be reproduced on Earth, though not for long. In a drop tower a capsule floats for seconds, a parabolic flight generates the sensation through a sequence of climbs and dives, and both methods serve research when no space is available aboard the station. In orbit, however, the state is sustained because sideways motion keeps prolonging the fall: the ISS flies at about 28,800 km/h at roughly 400 kilometers of altitude and takes about 90 minutes to complete one revolution.

For the organism, this permanent state is anything but neutral. Blood redistributes upward, faces look puffy in the first days, and the balance organ sends confusing signals while the brain cannot determine where up is, hence the space sickness at the start of many flights. Bones and muscles, no longer bearing load, deteriorate, which is why daily training is required. Back on Earth, the first steps are unsteady, and it takes weeks for circulation and the sense of balance to readapt to gravity.