What the numbers mean
Gravity is still strong. The whole station is falling together.
At 400 kilometres up, our calculation gives about 89% of Earth's surface gravity. Astronauts float because they and their spacecraft are in free fall together, moving sideways fast enough to keep falling around Earth.
Read the sourcesFloating does not mean gravity has vanished
NASA explains that the station, its crew and objects inside are falling together. With no floor holding a floating astronaut up, they feel weightless relative to the cabin. Gravity still acts on all of them. It is the force that continually bends their path into an orbit around Earth. [1]
Measure from Earth's centre
The distance in the gravity calculation starts at Earth's centre, not the ground. A point 400 kilometres above the surface is about 6,771 kilometres from the centre in our model. Compared with Earth's 6,371-kilometre radius, the extra height is relatively small. Crossing into space does not produce a sudden switch-off of gravity. [2]
Twice as far means a quarter of the pull
Newton's inverse-square relationship says that gravitational strength falls with the square of distance from the centre. Double that distance and the strength becomes one quarter. Double only the altitude above the ground and something different happens, because Earth's radius is still part of the total distance. The starting point of a measurement matters. [3]