What is geostationary orbit?

When you point your dish at the 101 satellite, do you ever stop to think about what that means?

Communications satellites have only been around for 50 years, but we take them for granted. For many of us, our television comes from a satellite. For some, phone and internet service do, too. It’s hard to wrap your head around how this is even possible, but it is, and the key part of it geostationary orbit. When you talk about the 101 satellite, you mean a satellite hanging in space approximately 35,780km from the surface of the planet, and staying precisely at 101 degrees west longitude.

It’s impossible, right? How can something hover in space in the region of the equator, south of Lubbock, Texas, and not move? It’s obviously possible because it does happen. The math is a little hard to understand but really, the concepts aren’t.

Why does orbit work anyway?

Pretty much everything orbits something. “To orbit” just means to go around over and over, like if your kid or pet runs around the couch until they get tired. An orbit is a very delicate dance that happens when two forces balance each other out. On the one hand you have gravity.

The first rule of gravity, as any child can tell you, is Things fall. There’s a lot of math behind that statement but that’s not important right now. If you have a big ball of rock in the middle of space and you are relatively near to it, you will fall towards it. If you doubt me, try jumping off a chair (carefully, don’t hurt yourself.) You will not fly endlessly into space, I promise.

Then, there’s another force, also known to children. This is called centrifugal force and it means that if you’re spinning fast, you want to fly away. Spin something on your fingers, do it fast enough and it will get away from you. (There are a million ways to demonstrate it and most of them involve damage to another person or property, so just trust me.)

So… here’s the trick. Start something spinning really fast, so it wants to fly away from the earth. Then, of course, the earth will want to pull it back, because of gravity. If you get something spinning at just… the… right… speed, its tendency to fly away will EXACTLY match the earth’s tendency to pull it back. It will be stuck going around and around until it loses enough speed and gravity eventually wins.

You might have seen this before. You start a coin spinning and it spins and spins around until it falls. This is basically like orbiting. Of course, in the spiral wishing well, air and friction slow down the coin and the orbit decays quickly. In space things can orbit for a much longer time.

Although it seems like orbiting might be pretty hard to do, if you have millions of years it must be quite easy, since just about everything orbits something. The moon orbits the earth, the earth orbits the sun, and on and on.

Geostationary orbit — the hard part

None of that explains how a communications satellite can hang in space over Lubbock. In fact, even though the basic principles of orbiting have been known for hundreds of years, the idea of a geostationary orbit is less than 70 years old. It takes a little bit of math, and a lot of vision. Luckily, a fellow named Arthur C. Clarke had both mathematical vision and talent.

Here’s the basics… the further away an object is, the slower it needs to go in order to orbit. The moon orbits the earth once a month. The earth orbits the sun once a year. The first person in space orbited the earth in a couple of hours, because he was pretty close (in comparison.)

So… get out your calculators… there must be a point where you can orbit, where it takes exactly one day to go all the way around the earth. I mean, there has to be a point like that. You just have to figure out where it is.

That’s what Dr. Clarke did. He proposed the idea of that magic distance — 35,786km. Put something up there in a stable orbit, right at the equator, and it will orbit the earth in exactly 24 hours. So why is that important?

Imagine you’re in a moving car on the highway. Another car pulls up next to you at exactly the same speed. It seems like you’re both standing still, even though you’re both moving. It’s the same with that orbit. The earth is spinning around once every 24 hours. The satellite is spinning around once every 24 hours. So… the satellite sits over the same point on the earth, like neither one is moving.

Either you think that’s extremely cool, or it blew your mind, or you just clicked off and went to Starbucks.

Geostationary orbits mean you can have GPS satellites, satellite TV, telephone conversations beamed off satellites, all sorts of stuff because you know where the satellite is. You don’t have to constantly re-aim the dish. Your dish sits happily on the roof not moving.

The first applications of geostationary orbit were in the 1960s but since then, thousands upon thousands of satellites have been launched into geostationary orbit. It’s a much higher orbit than things like the International Space Station, by the way, so that whole scene in Gravity where the space junk comes rushing toward them really wouldn’t happen.

That’s not to say there isn’t a whole lot of space junk out there, but it’s not a big problem right now because there’s a lot of space out there in geostationary orbit. When a satellite reaches the end of its useful life, it’s moved out of geostationary orbit to an even higher orbit where it will presumably spend the rest of time, at least until we have space garbage trucks that can go up there and get it.

About the Author

Stuart Sweet