Guy Slafter, age 11, of Houston, Texas, for his question:

Are planetoids really moons?

A planet travels in an orbit around the sun. A moon travels in an orbit around a planet. The moon whirls around the planet as the two together whirl around the sun. A planet may have one moon or a dozen moons. A moon may be large, like our moon, or only a few miles across, like the little moons of Mars. But every moon must have a planet around which to orbit.

The planetoids, also called asteroids, have no planet around which to orbit. Each travels its own orbit around the sun. The name planetoid means a little planet. In the Solar System there are nine big planets and a whole host of these junior planets, each chugging along his own path around the sun.

So far, we have named and charted more than 1500 of these small travelers. It is estimated that there are 30,000 of them. They range in size from pebbles to sizeable bodies almost 500 miles in diameter. The largest planetoids are larger than say, the small moons of Mars. Most of them are far smaller than the smallest moons in the Solar System.

Whether a heavenly body is a moon or a planet does not depend upon size. The smallest heavenly body rates in the planet class if it has its own orbit around the sun. Most of the planetoids circle the sun between the orbits of Mars and giant Jupiter.

Mars is an average distance from the sun of 141 million miles, Jupiter an average distance of 488 million miles. This leaves a vast belt of empty space between their orbits ‑ plenty of room for the 30,000 or so little planetoids to play. Plenty of room for each to travel a safe orbit of his own without crashing into a neighbor.

Many of the planetoid orbits are almost circles, others are ovals. The long ovals of a few planetoids bring them at times quite close to the earth. A few swing way out close to Saturn. Once in countless egos planetoid may collide with a planet. The big meteor crater near Winslow, Arizona may have been made by a colliding planetoid.

A visiting planetoid might and in another type of accident with a planet. Suppose a little planetoid crossed earth's orbit when our planet was only a few hundred miles or a few thousand miles away. The gravity of the earth might pull the little lone traveler into orbit. The earth would grain a new moon and one little planetoid would lose forever its status in the planet class. Henceforth it would circle the earth as it vent around the sun.

Each little planetoid not only travels its own circular or oval orbit, it chooses the angle at which it travels around the sun. Though most of the orbits are more or less on a plane with the sun's equator, a few are tipped, up one side, down another. What's more, each planetoid travels as fast or as slowly as he chooses. Some make the round trap in less than two earth years, others take over 12 years to travel once around the sun.