What Keeps a Satellite Up?

What Keeps a Satellite Up?

Before we answer this question, we should first make sure we know which way is up.  Since the planet Earth is round, up means any direction that is away from the center of the Earth. And if we are talking about the planets, which are satellites of the Sun, up means any direction that is away from the Sun. Whatever keeps a man-made satellite up is the same thing that keeps the Moon from falling on the Earth and the Earth from falling into the Sun.

Now, if you had a ladder about 200 miles high, and if you climbed up to the top carrying a satellite under your arm (some satellites are small enough to carry under your arm), and let go of it, it would fall down exactly as you had dropped it off the top of a building. It wouldn’t stay up at all. So it isn’t just a matter of getting a satellite up there.

What does keep a satellite up is its velocity – the right amount, and in the right direction. And that leads us to Newton’s law stating that every action has an equal and opposite reaction. Newton has another law that states that anything that is moving will keep on moving at the same speed in a straight line forever, unless some force makes it do something else. This means that if we used a rocket to launch a satellite from the earth’s surface, would like to keep going in the same direction it had when the rocket burned out, and at the same speed. And in fact, if the earth were not there, that is just what it would do. But the Earth is there, and the Earth’s gravity is the outside force that makes it do something else. We have a tug-of-war, where the satellite is trying to sail off in a straight line into space and the Earth is trying to pull it back down.

Photo: Elena

Now if the satellite is going fast enough it will break loose from the pull of the Earth and sail off into space (but not in a straight line, because then the Sun’s gravity will start trying to pull it in toward the Sun). But if it is going just fast enough to balance the pull of the Earth it will keep going around the planet as though it were on a leash; it keeps trying to go straight and the leash (gravity) keeps pulling it in. That is what the Moon – our biggest and oldest satellite – has been doing for millions of years.

The force of gravity gets smaller and smaller as you get farther and farther from the Earth (that’s another of Newton’s laws). This means that for any distance from the Earth there is one particular speed that just balances the pull of gravity, and the higher a satellite goes (that is, the farther from the Earth), the less speed it needs to stay up. On the other hand, you need a more powerful rocket to get it there, because the rocket has to push it farther against the pull of gravity. And if a satellite is at a lower orbit, it has to be going faster just to hold its own against the pull of gravity. That is why lower satellite orbits have shorter periods of revolution – time it takes to go around the Earth – than higher ones.

Jupiter and its satellites. Photo in public domain

The first astronauts in their Mercury capsules went around the Earth in about an hour and a half. They were only a hundred miles up. And they were going over 17,000 miles an hour. The Moon, though, is 240,000 miles up and takes 28 days to go around the Earth. That means it goes only about 2200 miles an hour. For any altitude in between, there is a particular speed that you must go to stay in orbit at that altitude.

Of course, if the satellite or even the Moon were to stop in their orbits they would fall straight down to Earth. And if anything slows a satellite down it will fall a little bit because now the force of gravity is stronger than the force trying to keep it going in a straight line. Like anything else that falls, it will speed up as it falls until once again it is going fast enough to maintain a new orbit at the lower altitude. That is how the Gemini astronauts changed from one orbit to another. If the wanted to go to a lower orbit they fired their retro motors to slow them a little bit.

The process is just the reverse if you want to change from a low orbit to a high orbit.  You give a spacecraft a short push with your rocket motors, and that starts it moving up to the higher altitude. As soon as the motors stop thrusting, however, the spacecraft just coasts the rest of the way until it gets as high as it can go with the amount of thrust. While it is coasting, of course, it slows down, like a car that is coasting uphill after you shut off the engine. And if you’ve given it the right amount of thrust, when it gets to the higher orbit it will be going at the slower speed that corresponds to that orbit.

So although you speed a satellite up to get it to a higher orbit, by the time it gets there it is actually going more slowly than before. And although you slow a satellite down to get it to a lower orbit, by the time it gets there, it is actually going faster than before.

You already know that a satellite has to be lifted above the Earth’s atmosphere to stay in orbit at all, because it had to push its way through the air while it was in orbit, it would slow down so soon that it could stay up for only a very short time. Illustration: Megan Jorgensen.