The drawings at the right simplify the physics of orbiting Earth. We see Earth with a huge, tall mountain rising from it. The mountain, as Isaac Newton first envisioned, has a cannon at the top. When the cannon is fired, the cannonball follows its ballistic arc, falling as a result of Earth's gravity, and it hits Earth some distance away from the mountain. If we put more gunpowder in the cannon, the next time it's fired, the cannonball goes halfway around the planet before it hits the ground. With still more gunpowder, the cannonball goes so far that it never touches down at all. It falls completely around Earth. It has achieved orbit.
If you were riding along with the cannonball, you would feel as if you were falling. The condition is called free fall. You'd find yourself falling at the same rate as the cannonball, which would appear to be floating there (falling) beside you. You'd never hit the ground. Notice that the cannonball has not escaped Earth's gravity, which is very much present -- it is causing the mass to fall. It just happens to be balanced out by the speed provided by the cannon.
The cannonball provides us with a pretty good analogy. It makes it clear that to get a spacecraft into orbit you need to
The required speed for a particular altitude A can be found from the formula
where A is in miles and v comes out in miles per hour. So for example the shuttle, orbiting at 200 miles up travels at
At that speed, it takes about 90 minutes to complete one orbit (an hour and a half to go all the way around the Earth!).
If we place a satellite way up - at an altitude of 22,284 miles, then to stay in orbit, the satellite should travel at
At that speed, you can show that it takes 24 hours to orbit the Earth. But since the Earth is rotating once every 24 hours, the satellite is going around the Earth at the same exact rate that the Earth is turning. The satellite stays above the same point on the Earth, or looking at it from the Earth's surface, the satellite stays in the same place in the sky. This is called a "geostationary" orbit, since the satellite seems to be stationary - it looks like it doesn't move! This is great if you have to point your satellite dish to pick up a signal from this satellite. Point it once and you're done.
How does a satellite get from low earth orbit (where the shuttle lets go of it) to geosynchronous orbit?
Orbital Mechanics Web Page an outstanding reference!
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Updated 8 September 2008