Michelle Shriber, age 7, of Farmington, Utah, for her question:

How do scientists know when to expect comets?

Most comets are too dim to be seen. But telescopes spot about half a dozen of them daily. Scientists expect tiny comets of this sort all the time. A few comets are big enough to show their long golden tails    and some of them arrive when expected. Scientists know from past records that Halley's famous comet pays us a visit every 76 years or so. The last time was in 1910. So its next visit is expected in 1986.

A few comets pay regular visits to our part of the sky. Scientists have names for them and know when to expect them to return. But most comets are strangers from afar. Some visit us only once. Some came thousands of years ago and nobody remembered them. Even the beat telescopes cannot spot them way out beyond the Solar System. But telescope pictures show them as they get closer, long before we can see them. Comet Kohoutek was expected because telescopes spotted it a year ago. Then it was way out near the orbit of the planet Jupiter.