Erin Merritt, age 10, of Sioux City, Iowa, for her question:

What causes a satellite to stay in orbit?

All through the universe there are mighty, invisible forces that keep the heavenly bodies in their place. Scientists know these laws, and when they launch a satellite, they can direct it exactly to the orbit they want it to take. When the satellite goes up, gravity tries hard to pull it back down    just as it pulls down a falling stone. But the satellite is programmed to turn in a curved path high above the globe. Gravity does not let go. But because the satellite is moving at orbital speed, about 18 miles per second, along a curved path, it keeps going in an orbit around the globe.

If the satellite is shot up too fast, it might escape the pull of gravity and go out into space. But traveling at the right speed, gravity keeps a hold on it. Actually, the orbiting satellite is really falling in a curved pattern around the globe. Little by little the pull of gravity tries to win this tug of war. The man made satellite may stay in orbit for a few weeks, ten years, or more than a hundred years if it has a small propulsion system that maintains its orbital velocity.