Don McDonouah, age 11, of Wichita, Kansas., or his question:

What keeps a planet in orbit?

There are two forces which work to keep a planet in orbit. One is gravity which pulls the planet towards our huge sun, dust as the earths gravity pulls down a falling stone. The other is centrifugal force. This force builds up as the planet swings around in its orbit. It operates to pull the planet away from the sun.

These two forces play a heavenly tug of war. Gravity pulls the planet towards the sun and centrifugal force pulls it away. The planet falls towards the sun, but because of the centrifugal force, it falls around in an orbit. In each of our nine planets, the two opposing forces are exactly balanced. Neither can win. Hence, each planet keeps falling around and around its orbit   and will continue t o do so.