Why doesn't a satellite fall down? Orbit is about going sideways fast
The International Space Station is only about 400 km up, where gravity is almost as strong as on the ground. It stays up because it is moving sideways at 7.67 km/s.
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A satellite does fall. It is falling toward the Earth all the time, but it is also moving sideways so fast that the ground curves away beneath it as quickly as it drops, so it never lands. Orbit is not about being high up. It is about going sideways fast: a stable needs a speed of about 7.8 kilometres per second, or 28,000 km/h.
Isaac Newton explained it with a cannon on a very high mountain, with no air to slow the shot. Fire the ball slowly and it drops back to the ground. Fire it faster and it lands farther away. Fire it at orbital speed and it keeps falling around the curved Earth and comes back to the mountain it started from. Try the throw below: a bigger speed means a longer flight before the ground gets in the way.
Try it
Throw it faster, it lands farther
- Lands at
- 3.49 m
- Highest point
- 0.154 m
Challenge · Keep the angle low and raise the speed from 10 to 30 m/s. How much farther does the ball land?
Gravity has not gone away up there. At low orbit, its pull is only slightly weaker than on the Earth's surface, because even a few hundred kilometres is small next to the Earth's radius. Astronauts float because they and their spacecraft are falling together, in what is called , so nothing presses them against the floor.
The International Space Station shows the numbers. It circles about 400 kilometres up at 7.67 km/s, one lap roughly every 93 minutes, more than 15 times a day. Rockets are the only way to get there, because the speed is the hard part, not the height.
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Recap
Orbit is falling sideways so fast that the ground keeps curving away: about 7.8 km/s in low Earth orbit.
💡 A trick to remember it · Throw it so fast the Earth runs out of ground to hit: a satellite is a cannonball that keeps missing.
Surprising fact · The ISS orbits where gravity is only slightly weaker than on the ground; it stays up thanks to its 7.67 km/s sideways speed.
Connects to
- 🚀 How does a rocket work in empty space, with nothing to push against?
- 🍎 Did Newton's laws of the heavens make people hope for laws of society?
- 🛰️ Why did a beeping metal ball shock America into the space race?
- 🚀 Do rockets really need to hit a magic speed to escape Earth?
- 🛰️ Why do spacecraft have to slow down to catch up?
Sources (6)
No source, no claim. Every fact in this lesson (15 claims) cites at least one of these.