How Orbits Work

What Is an Orbit?

The drawings below 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 (drawing no.1).

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 (drawing no.2).

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 (drawing no.3).

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.

Getting Into Orbit

The cannonball provides us with a pretty good analogy. It makes it clear that to get a spacecraft into orbit you need to

• Raise It Up (the mountain) to a high enough altitude so that Earth's atmosphere isn't going to slow it down too much. In practical terms you don't generally want to be less than about 100 miles above the surface of the Earth. At that altitude, the atmosphere is so thin that it doesn't present much frictional drag to slow you down.
• Accelerate It until it is going so fast that as it falls, it just falls completely around the planet.