Why is no gravity in space?

Gravity reaches everywhere; it only weakens with distance, and it does so slowly. Earth's pull at the surface is 9.80 m/s², and at the 400 km altitude of the International Space Station, a mere six per cent further from the planet's centre than the ground is, it remains about nine tenths of that. If gravity really switched off up there, the station would simply leave in a straight line instead of circling the planet.

An orbit is a fall that keeps missing. The station moves sideways fast enough that, as gravity bends its path downwards, the curve of the Earth drops away underneath at the same rate. Everything inside falls exactly as fast as the walls do: the astronaut, the camera, the coffee. With nothing pushing back against the body, no floor and no seat, there is no sensation of weight and none of the internal compression that bones and muscles read as gravity. The honest name for the condition is free fall; microgravity is used because the fall is not quite perfect.

Tiny residual accelerations remain. Parts of the station further from Earth's centre feel a slightly weaker pull than parts nearer to it, so a loosely floating object drifts across the cabin over hours. Thin traces of atmosphere drag on the structure, thrusters fire to hold the attitude, crew members push off walls. Together these amount to millionths of Earth gravity, which is where the term comes from. The distinction matters for the experiments flown up there, and it matters for the body: bone and muscle respond to the absence of load, not to the absence of a gravitational field.