Why don't satellites fall down?

The International Space Station zooms around Earth about 400 kilometres up. Gravity is still pulling on it up there. In fact, gravity at that height is almost as strong as it is on the ground, about 90% as strong.

So why doesn't the Space Station come crashing down?

Here is the surprise: it is falling right now. It just never lands.

Throw a ball sideways

Imagine you stand on a very tall mountain and throw a ball straight ahead. The ball moves forward, but gravity pulls it down, so it lands on the ground.

Throw it harder and it flies farther before it lands.

Now throw it super hard. Here is the trick: Earth is round. The ground curves away below the ball. If the ball goes fast enough, the ground curves away as fast as the ball falls. The ball keeps falling, but it never gets any closer to the ground.

That is an orbit. An orbit is a fall that keeps missing.

Try it yourself

Each button below launches a satellite from 400 km up, moving sideways. The only thing that changes is the speed. Try all four.

What did you see?

• 6.5 km/s: too slow. The satellite curves a little, but gravity wins. It falls back and hits Earth.
• 7.7 km/s: just right. The ground curves away at exactly the same rate the satellite falls. It goes around in a circle.
• 8.8 km/s: a bit fast. The path stretches into an oval. The satellite swings out away from Earth, then falls back in.
• 11.2 km/s: very fast. The satellite is going so fast that gravity can never pull it back. It leaves for good.

Why do astronauts float?

Many people think there is no gravity in space. That is not true. Gravity is what keeps the Space Station in orbit.

Astronauts float because they are falling too, together with the station. Think about a lift that suddenly drops. For a moment, you would float inside it, because you and the lift fall at the same speed. The Space Station and its astronauts are doing exactly that, all the time. They are in free fall around the Earth.

Why doesn't it slow down?

Things on Earth slow down because of air and friction. A ball that rolls on the ground stops. But 400 km up there is almost no air. With nothing to slow it down, the Space Station just keeps going, lap after lap.

It is not totally empty up there, though. A tiny bit of thin air touches the station and slows it very slightly. Every few months, its rockets give it a small push to lift it back up.

The big idea

An orbit needs two things working together:

1. Gravity pulls the satellite toward the planet.
2. Sideways speed carries it past the planet.

Too little sideways speed and the satellite falls. Just the right amount and it circles. The next question is: how fast is just right? That is next.