Astronauts float because they and their spacecraft are falling around Earth together, not because gravity is missing — at the ISS, gravity is still about 90 percent as strong as on the ground.
Videos from the International Space Station show astronauts drifting and water forming floating balls. It looks like there is no gravity. There is plenty.
NASA explains that at the station's height, Earth's gravity is about 90 percent of what it is on the surface. The astronauts float because they are in free fall.
Imagine standing on a weighing scale in a lift. If the lift cable snapped, you, the scale and the lift would all fall at the same rate. You would not press on the scale, so it would read zero. You would float inside the lift.
An orbiting spacecraft is like that lift, except it never hits the ground. NASA describes the spacecraft, its crew and everything aboard as all falling towards, but around, Earth. Because they all fall together, nothing presses on anything else.
Gravity at about 400 km up is roughly 8.7 m/s², against 9.82 m/s² on the ground. An astronaut of 70 kg is still being pulled by about 70 × 8.7 ≈ 610 newtons, compared with about 690 newtons at home.
But the station, the crew and their equipment all accelerate towards Earth at that same 8.7 m/s². The floor falls away from the astronaut's feet exactly as fast as the astronaut falls towards it. A bathroom scale on board would read zero.
Scientists call this condition microgravity. NASA notes that small amounts of gravity are everywhere in space. Even on the ISS — about 420 tonnes, with a crew of seven on board — gravity is far from zero. Everything on it is simply falling together, so the crew feel only tiny leftover forces.
The Altitude card reads about 400 km — at that height the crew are still pulled by about 90 percent of Earth's gravity, yet they float. Open the ISS tracker
What a scale measures is not gravity but the contact force between you and the floor — your apparent weight. In a frame accelerating downward with acceleration a, Newton's second law for a person of mass m gives
mg − N = ma → N = m(g − a)
On the ground, a = 0 and N = mg. In a lift accelerating down at 2 m/s², N = m(9.82 − 2): you feel lighter. In orbit, the station's acceleration is the local gravitational acceleration itself, a = g, so N = 0.
At the ISS (r = 6,771 km from Earth's centre, mean radius 6,371 km): g = 398,600 ÷ 6,771² ≈ 0.008694 km/s² = 8.69 m/s², or 88.5 percent of 9.82 m/s². That matches NASA's "about 90 percent".
Why microgravity? Two leftover effects stop N being exactly zero everywhere:
Both effects are of order 10⁻⁶ to 10⁻⁵ g, which is where the word microgravity comes from.
No. Gravity at the ISS is about 90 percent of its surface value. Astronauts feel weightless because they and the station are in free fall together, so nothing presses on them.
Zero. The person and the scale accelerate down at the same rate, so there is no contact force between them.
g = 398,600 ÷ 6,771² ≈ 0.00869 km/s² ≈ 8.7 m/s².