Gravity Launch

If you throw a ball into the air, it falls back down to the ground....Why? Because the gravity of the earth pulls it in. All objects (even you!) have gravity, but only really big objects, like planets and stars, have so much gravity that we can see the effect. The more massive the object, the stronger the gravity. When something like a rocket ship passes by the Moon or the Earth, it's pulled toward that object by gravity. The closer you are to an object, the more its gravity pulls on you. Rockets that are launched from the earth have to work against gravity - otherwise they would never get off the ground! So they have powerful engines that push them forward. The force the engines produce is called thrust. When you launched your rocket, the path it took depended on how much thrust you put behind it, the angle at which it was launched, and the gravity of the Moon and the Earth. When a rocket passes close enough to a planet or a moon, the combination of gravity and thrust will be just enough to lock it in an orbit.

http://www.tutorialsto.com/flash/games006/gravitylaunch.swf