Why do astronauts float in space?

Weightlessness aboard the International Space Station is not caused by the absence of gravity: at that altitude Earth still pulls with roughly 90% of the force felt at sea level. The station is simply travelling sideways fast enough — about 28,000 km/h (17,500 mph) — that the curve of its fall matches the curve of the planet beneath it. A lap takes some 90 minutes, and the ground never gets any closer. Orbit is often described as a balance between speed and gravity, but it is better understood as a throw that never lands: launch a projectile fast enough and the surface drops away beneath it exactly as quickly as it falls.

What we call weight is the sensation of a floor, a seat or a pair of boots pushing back. Take that contact force away and the body loses its main cue for up and down: fluids shift towards the head, faces puff, legs thin out, and the balance organs of the inner ear stop reporting any direction for gravity. Astronauts often describe the first hours as a feeling of endless falling, and space adaptation syndrome — nausea and disorientation — is common until the brain learns to trust what the eyes see instead. Anything released in mid-air stays put, so tools, food and laptops must be strapped, clipped or velcroed down.

Parabolic aircraft flights reproduce identical weightlessness for seconds at a time, a few kilometres above the ground, where gravity is essentially unchanged. Drop towers manage a shorter burst. The physics is the same; in orbit the only difference is that the fall never ends, because sideways speed keeps the ground from arriving. That is why zero gravity is a misleading label: there is plenty of gravity up there, it just has nothing to press the crew against.