Here's a surprise: a satellite isn't held up by anything at all. There's no string, no rocket pushing constantly, nothing propping it up. In fact it's falling toward Earth the whole time, exactly like a dropped ball. The trick is that it's also racing sideways incredibly fast. So as it falls, the round Earth is curving away underneath it just as quickly — and it keeps missing the ground, falling and falling around the whole planet without ever hitting it. That endless missing is what we call an orbit. In the simulator you can set the sideways speed: too slow and the satellite spirals down and crashes, too fast and it shoots off into space, but at just the right speed it sweeps round in a perfect circle.
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 together.
What's actually happening
The phrase 'astronauts float in zero gravity' is one of the most misleading in all of science. On the International Space Station, about 400 km up, Earth's gravity is still roughly 90% as strong as it is on the ground. There is nothing weak about the gravity up there. So why do astronauts and their satellites float instead of plummeting? Because they are plummeting — all of them, the station included, are falling toward Earth constantly. They just never arrive.
Isaac Newton explained it three centuries ago with a thought experiment. Imagine a cannon on an impossibly tall mountain, firing a ball horizontally. Fire it gently and it arcs down and lands a short way off. Fire it harder and it lands farther, the arc stretching out. Now fire it so hard that, as it falls, the curve of its fall exactly matches the curve of the Earth bending away beneath it. The ball is still falling, gravity is pulling it down the whole time, but the ground keeps dropping away at the same rate, so it never gets any closer. It falls forever, all the way around the planet, and comes back to hit the cannon from behind. That is an orbit, and the only special ingredient is sideways speed. For a low orbit you need to be moving sideways at about 7.8 kilometres every second, fast enough to cross a city between heartbeats.
This is why orbital speed is so unforgiving, and the simulator makes the knife-edge obvious. Move sideways a bit too slowly and you don't fall straight down — you trace a long curve that dips back into the atmosphere and burns up or crashes. Move a bit too fast and you climb away into a stretched-out loop. Move faster still, past about 11.2 km/s, and you break free of Earth entirely on a path that never returns — that's how we send probes to other planets. The hard part of getting to space was never going up; a balloon can go up. The hard part is going sideways fast enough that you keep missing the planet you just left.
An orbit is just falling sideways fast enough to keep missing the planet, which is why getting to space is about speed, not height.
- 1Tie a small soft ball (or a bunch of keys) to a length of string and whirl it in a horizontal circle above your head.
- 2Feel the steady inward pull on your hand — that is gravity in this model, constantly tugging the ball toward the centre while its sideways motion keeps it circling.
- 3Now let go: the ball flies off in a straight line, not outward. That straight escape is what 'too fast' looks like, and the constant tug-versus-sideways-speed balance is exactly an orbit.
Common questions
No. At the International Space Station, about 400 km up, Earth's gravity is still roughly 90% as strong as on the ground. Astronauts float only because they and the station are falling together at the same rate.
Too slow and it traces a curve that dips back into the atmosphere and burns up or crashes. Too fast and it climbs into a stretched loop; past about 11.2 km/s it escapes Earth entirely on a path that never returns.
Isaac Newton, three centuries ago, with a thought experiment about a cannon on a tall mountain. Fire the ball fast enough and its fall matches the curve of the Earth, so it never lands.