How fast is orbit speed?
Why high satellites go slower than low ones, what an oval orbit is, and how fast you need to go to leave Earth for good.
Every planet and moon has its own orbit speed. Go slower and you fall. Go faster and your path stretches out. In this article, we find out how fast is just right, and what happens when you go even faster.
Higher means slower
You might think that a satellite far away needs to go faster. It is the other way around.
The farther you are from a planet, the weaker its pull. A weaker pull means you need less sideways speed to stay in a circle.
Here are real satellites around Earth:
| Satellite | Height | Speed | One lap takes |
|---|---|---|---|
| Space Station | 400 km | 7.7 km/s | 1 h 32 min |
| A high satellite | 2,000 km | 6.9 km/s | 2 h 7 min |
| GPS satellites | 20,200 km | 3.9 km/s | 12 hours |
| Weather satellites | 35,786 km | 3.1 km/s | 24 hours |
| The Moon | 384,400 km | 1.0 km/s | 27 days |
Look at the last two rows. A weather satellite takes exactly one day to go around, and Earth spins once in one day too. So the satellite stays above the same spot on the ground. It is like a lamp on a very tall pole. That is why one satellite dish can point at the same satellite all the time.
In Orbit Keeper, Mission 3 is exactly this: a weather satellite. Does it feel slow?
Other worlds, other speeds
The Moon is much lighter than Earth, so its pull is weaker. A satellite 100 km above the Moon needs only about 1.6 km/s. Mars is in between: about 3.4 km/s at 300 km up.
Space agencies work out the orbit speed for each world before a mission leaves home.
What if you go a bit faster?
Say you launch faster than the circle speed. The satellite starts to move away from the planet. As it climbs, gravity tugs it back and it slows down. Then it turns around, falls back in, and speeds up again.
The path is an oval. Scientists call it an ellipse.
On an oval orbit:
- The satellite goes fastest at the closest point.
- It goes slowest at the farthest point.
Think of a roller coaster. When you roll down a hill you speed up, and when you climb you slow down. A satellite on an oval is doing exactly that, rolling downhill toward the planet and climbing away again.
Comets follow very long ovals. And so do the planets going around the Sun, although most of them are almost circles.
How fast to escape?
Keep making the launch speed bigger and the oval gets longer and longer. At some point, it never comes back at all.
That special speed is called escape speed. From the Space Station's height, it is about 10.9 km/s.
There is a neat pattern: escape speed is always the circle speed multiplied by about 1.41 (that is the square root of 2). At 400 km: 7.7 × 1.41 is about 10.9 km/s.
Space probes that visit other planets must reach escape speed to leave Earth behind. Rockets are big because they must carry a lot of fuel to build up all that speed.
Scientists write the circle speed like this: speed = √(GM ÷ r). Here r is the distance from the centre of the planet, and GM is a number that says how strong the planet's pull is. A bigger r makes the answer smaller. That is why higher satellites go slower. Escape speed is √(2GM ÷ r), which is √2 times bigger.
Try it
Play Orbit Keeper and try these:
- Mission 5 asks you to make an oval that climbs about 20,000 km high.
- Mission 6 asks you to escape Earth completely.
- In Free Play, pick the Moon and a high orbit. Compare the speed with Earth.