Is there zero gravity on the International Space Station?

Every video from the International Space Station looks like proof that space has no gravity. Astronauts somersault in mid-air, water forms floating blobs and nobody ever needs a chair. So it is surprising to learn that, up there, gravity is almost as strong as it is in your kitchen.

The short answer

The space station orbits only about 400 kilometres above Earth’s surface. At that height, according to NASA, gravity is still about 90 percent as strong as at sea level, roughly 8.8 metres per second squared instead of 9.8. Astronauts float because they are in free fall. NASA’s own training material is blunt about it: microgravity is not zero gravity.

The real story

The station is falling. Right now. All the time. The secret is that it is also moving sideways at tremendous speed. As it falls towards Earth, the curved surface of the planet drops away beneath it at the same rate, so it never actually gets any closer. It keeps falling around the Earth instead of into it. That endless fall is what we call an orbit.

In other words, the station is not floating above gravity’s reach. It is held firmly in gravity’s grip and is simply moving sideways fast enough that the grip bends its path into a loop. Popular Science made the same point years ago about the space shuttle: everyone on board was only apparently weightless, because the whole spacecraft and everything in it kept falling around the Earth rather than straight down.

Now imagine you are inside something that is falling. You, the walls, your sandwich and your toothbrush are all falling at exactly the same rate. Nothing pushes against anything else. The floor does not press up on your feet, so you feel weightless. NASA defines free fall as accelerating at the rate of gravity, and says that in this state the forces that normally push back on objects are greatly reduced. That is why the scientific term is microgravity: the effects of gravity seem tiny, even though gravity itself is very much present.

You can get a brief taste of this on Earth. The stomach lurch at the top of a roller-coaster drop, or the moment a lift starts heading down quickly, is a tiny moment of partial free fall.

Why people get it wrong

The phrase zero gravity is everywhere, in films, adverts and even the names of rides, so it feels like official science. And the footage seems to settle the matter. If people are floating, surely there is no gravity pulling on them.

There is also a language problem. In everyday speech, weight and gravity sound like the same thing, so feeling weightless seems to mean there must be no gravity. Scientists separate the two: gravity is the pull, while the weight you feel is really the floor pushing back up against you. Take away the floor’s push by letting everything fall together, and the sensation of weight disappears even though the pull remains.

But think about it this way. If there were no gravity at the space station, nothing would keep it near Earth. It would drift off in a straight line into deep space. The very fact that it keeps circling our planet shows gravity has a firm grip on it.

“So astronauts aren’t floating, they’re falling with great confidence. Mate, that’s been my whole approach to life.”