Billy Bayer, age 13, of Staten Island, N.Y., for his question:

Is it possible for planets to collide?

It is impossible, absolutely impossible, for two planets of our solar system to collide with each other. It was proved long ago that our solar system is a stable unit of heavenly bodies and a traffic accident between two of its planets can never happen. The pulling gravity of the sun is offset exactly by the planets' speed around their orbits.

Planets are small bodies in the vast spaces of the solar system. Jupiter is only 88,770 mils in diameter and it is bigger than all our other planets combined. The nine little orbiting bodies are separated by millions of miles. The closest are Mercury and Venus and they are 31 million miles apart. It is impossible for a planet to come off course and even more impossible for it to collide with another planet.