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ORBIT

Why do things ORBIT in space?

Orbiting explained.


Orbit. Just about everyone knows that things in space do this. They ORBIT. What's that? Again, ask anyone and they'll most likely know that ORBITING is "going round and round". Also, things that are IN ORBIT generally keep on going round and round and somehow just stay up there and don't come down.

If you've ever wondered WHY things can be "in orbit" and how they can just stay up there and keep on going round and round, here's one of these science-made-easy explanations.

Well, you might say, there's no gravity "up there" in space, so stuff in space will just hang around and not come down anyway. But then again, if that's the case, you might wonder why they go round and round, when instead they could just save themselves the trouble and just hover there in a kind of weightless limbo.

The first thing is that there IS gravity in space, or at least the earth's gravity extends well beyond the height where there's no air worth arguing about. Objects such as satellites, spacecraft, the moon, and space-junk are all going around in various orbits in part of space which has some gravity. The further into deep space you go, the less gravity there is. But in the realm of low earth orbit satellites there is almost as much gravity as there is on the earth! So, why do those things stay up there?

This is where the idea of ORBIT comes in. It is the business of going round and round which keeps the stuff there. It's like the forces in a spin-dryer, known as either centrifugal force or centripetal force depending on who you'd like to argue with. These forces balance the force of gravity. But there is a better way of explaining orbiting...

Imagine travelling at high speed horizontally along the flat top of a high plateau, and after going off the edge of the cliff, you land some distance away. The faster you travel off the edge, the further away from the cliff you come down. But, suppose you could travel extremely fast, then you land even further away than expected, because the surface of the earth is not flat, and to some extent you are travelling over the horizon! But now make that speed several miles per second, and as you travel off the edge, you are travelling along so fast that for every metre you fall, you've gone so far over the horizon that the ground has fallen a metre. At this rate of travel, "orbital velocity", you are falling but not getting any closer to the ground, because the ground is falling away beneath you as you continue to go over the horizon.

Extraordinary as it may seem, at this speed you continue to travel on and on right around the earth, always falling but never getting any closer to the ground! Incidentally, "down" is always towards the piece of ground you happen to be over, so you continue on right around the earth. After about 90 minutes you pass over the spot you started and keep on going. Well, when I say "the same spot" it might be the same spot in your own reference frame in space, but the Earth will have rotated under you, so you might have a continuously changing view of the surface. Global Positioning System (GPS) satellites are in such orbits, which is good news if you're lost somewhere on the ground, because there's nearly always a few GPS satellites flying over near where you are.

Because of the high speeds involved, orbiting is usually done above the layer of air around the earth, to avoid the problems of the misbehaviour of air at high speed which would class as "hypersonic travel". Many tens of miles up, with no air friction, there is very little to stop something continuing going round and round indefinitely. This is the state known as being "in orbit". Space stations and satellites can remain in orbit for many years without much adjustment. The moon has been in orbit for longer than that, and shows no signs of stopping.

Another thing about orbiting is that in a spacecraft in orbit, you are "weightless" and feel no gravity. That's not because you are without gravity, but because you and the spacecraft are travelling together and are in the same situation of being in orbit and are in effect continuously travelling and falling without getting any lower. In the same sort of way, cups of tea on high-speed trains remain with the tea in the cup without it all being swooshed away at 200MPH in the wind. Also, if you are "weightless" in space, you don't have gravity pulling you down, but you still have mass. See the difference between weight and mass


Special kinds of orbit, in case you're interested, include: polar, equatorial, geostationary, and retrograde.

Loads of other curious stuff at this website at the science page and the site index

There's more stuff about gravity at the page of Gravity on Different Planets

Various things in orbit around the Earth: Satellite Navigation / Global Positioning System satellites, weather satellites, Satellite TV satellites, spy satellites, and The Moon.

Earth / Moon / Sun - to scale - on a golf course!

If you found this page about HOW ORBITING WORKS some help, you may be interested in the What Next?... [response]