Susan Mann, age 12, of Lincolnwood, Ill., for her questions

Why don't the planets fall into the sun?

The sun contains more than 99 percent of all the mass, or material, in the :polar System. It is mass which gives a heavenly body its gravity. Surely then, the gravity of the mighty sun should be able to pull all its planets into its seething heart. But this does not happen far a very good reason. Each planet orbits the sun and the speed of its orbit builds up centrifugal fcrce,

Centrifugal force pulls a planet away from the center of its orbit, in the opposite direction from the sun's gravity. The two forces pull in an endless tug of war. Actually, the planet is always f alling towards the sun, but its orbital speed keeps it falling around and around in more or less a circle.