Colleen Bohan, age 11, of Staten Island, New York, for her question:

Exactly what are the asteroids?

Some weeks ago, these little bodies were mentioned in the science news. In fact, some people kept their fingers crossed, hoping that our long distance space probe would not collide with one of them on its fabulous voyage toward Jupiter. Astronomers have studied them for decades, counted thousands of them and charted many of their orbits in fine detail. But so far, the asteroids have kept their most interesting secrets to themselves.

The word "asteroids" means "little stars", which they are not. Astronomers, who like to be very precise, usually call them "planetoids", which means "little planets". This is because these small solid bodies travel in planet type orbits around the sun. Unlike a moon, an asteroid does not orbit a parent planet while the two of them circle the sun together. 'Each asteroid planetoid is on its own, orbiting its own path at its own speed.

We talk about the asteroid belt because most of the planetoids occupy a region between the orbits of Mars and Jupiter. This is what caused some to worry that our spacecraft on the way to Jupiter might collide with one of them. After all, we know that there are at least thousands of them and nobody can figure where ail of them are at the moment a spacecraft happens to pass through their neighborhood. Even a tiny one could be ruinous, but luckily nothing happened.

The asteroid territory is shaped somewhat like a celestial doughnut about 966 million miles wide. The hole in the center spans about 282 million miles, which is plenty wide enough for the orbits of Tsars and Earth, Venus and Mercury. Within this spaceous doughnut, nobody knows how many separate asteroids zoom around the sun. They are chunks of solid minerals and they come in assorted sizes. The smallest ones have not been measured but a few big ones are known to be several hundred miles wide.

Those in the asteroid belt are too small and too remote to be visible from the earth. But a few odd balls do not stay inside their spaceous playpen. Some of them have orbits that take them across the orbits of the planets. If this happens when the earth happens to be near the intersection at the right time, an asteroid may become visible, though not very bright, in our skies. There is not much chance of a collision, at least not more than once in perhaps 15 million years.

Until a few years a~     it was thought that there was     ~ly one asteroid belt in our Solar System. Now we know about another one, nay out there at the outer edge of the Solar System. They teem along the orbit of Pluto and far out beyond it. At present, nobody can say for certain why our planets share their territory with these swarms of midgets. One theory suggests that they may be building blocks for a couple of extra planets. Another suggests that they might be fragments of old planets that broke apart.