Lisa Gallagher, age 11, of Philadelphia, Pa., for her question:

WHAT KEEPS SATELLITES IN THE SKY?

The earth's gravity, as we know, pulls down falling stones and hugs everything else to the surface. A satellite is way up there above the ground, so we would expect it to come plummeting down with everything else. But there is a reason why this does not happen. The force of gravity plays tug of war with objects that spin around in orbits.

A satellite is pulled down down by the earth's gravity. But so long as it keeps spinning around in an orbit, it counteracts this pull. In order to stay above the earth, it must orbit at exactly the right speed and at exactly the right distance above the surface.  Then the opposite forces of the tug of war are evenly balanced. And instead of plummeting down to the ground, the satellite falls around and around the curved path of its orbit.