It's natural to think astronauts float because there's no gravity in space. But here's the surprise: where the Space Station orbits, gravity is still about 90% as strong as it is on the ground. There's plenty of gravity up there. So why do the astronauts drift around? Because they are falling — and so is the station, at exactly the same rate. Imagine being in a lift when the cable snaps: for those few scary seconds you'd float inside it, because you and the lift fall together. The astronauts get that feeling all the time. The clever part is that they're also racing sideways so fast that as they fall, the round Earth curves away beneath them. They keep missing the ground, falling round and round. In the simulator, fire the cannonball and find the sideways speed that turns a crash into a clean orbit.
Most people think astronauts float because there is no gravity in space. In fact gravity at the Space Station is about 90% as strong as on the ground; they float only because they and the station are falling around the Earth together.
What's actually happening
Of all the phrases science has handed the public, "zero gravity" might be the most misleading. Watch footage from the Space Station and the astronauts drift through the cabin, hair fanned out, water curling into wobbling spheres, and it certainly looks as though gravity has been switched off. It hasn't. At the station's height, around 400 kilometres up, the Earth still pulls with about nine-tenths of the strength you feel standing in your kitchen. There is nothing weak about the gravity up there. So the floating needs a different explanation entirely.
The explanation is that everyone and everything aboard the station is falling. Picture the snapped-lift thought experiment: if the cable broke and the lift plunged, you inside it would float for those few terrifying seconds, weightless, your feet leaving the floor. Not because gravity vanished, but because you and the lift were falling together at the same rate, so there was nothing left to press you down. The astronauts live in a permanent version of that fall. They never hit the ground for one reason: they are also travelling sideways at an enormous speed, roughly 7.9 kilometres every second. As they fall toward the Earth, the planet's surface curves away beneath them just as quickly, so they keep missing it, looping round and round in what we call an orbit.
Isaac Newton saw this three centuries before anyone left the ground, with an imaginary cannon on a mountain so tall it poked above the air. Fire the cannonball gently and it arcs down and lands nearby. Fire it harder and it lands farther off. Fire it hard enough and its fall exactly matches the curve of the Earth dropping away — and now it never lands at all. It falls forever, all the way round, and comes back to where it started. That is an orbit, and the only special ingredient is sideways speed. Too slow and you crash; a touch too fast and you climb into a stretched loop; past about 11.2 kilometres a second you break free of Earth completely. The astronauts aren't escaping gravity. They're surfing it, falling endlessly and missing the planet every time.
Weightlessness is free-fall, not zero gravity: astronauts float because they and their craft fall around the Earth together, forever missing the ground.
- 1Poke a small hole low in the side of a plastic bottle and fill it with water. Held still, water squirts out of the hole — gravity is pushing it.
- 2Now drop the whole bottle (over grass or a sink) and watch the hole as it falls. The stream stops mid-fall.
- 3While the bottle is falling, the water and bottle fall together, so the water no longer presses out of the hole — your own tiny taste of weightlessness, with gravity still fully switched on.
Common questions
Yes. At about 400 km up, Earth's gravity is still roughly 90% as strong as on the ground. The astronauts are deep inside Earth's gravity, not beyond it — they float only because they are falling.
Because they are also moving sideways at about 7.9 km/s. As they fall, the curved Earth drops away beneath them just as fast, so they keep missing it, looping round in an orbit.
An orbit is falling sideways fast enough to keep missing the planet. Past escape speed, about 11.2 km/s, the path stops being a loop and opens out, and the craft leaves Earth for good.