Elaine Watson, age 12, of Dally s, Texas, for her question:

What is a comet made of?

The heart or nucleus of a comet is thought to be a loose bundle of boulders, pebbles and gravel. This is the only solid material in the comet. And this nucleus ray be from a few miles up to 10,000 miles in diameter.

Around the nucleus is a huge, glowing halo called the coma. It may be 10,000 to several hundred thousand miles in diameter. It is made of gases thinner than the air around us. The tail of the comet, many millions of miles long, is made of material so fine that the earth can pass right through it without noticing part from the sun, a big showy comet is brighter than any object in the sky. It is bright enough to be seen at high noon. Yet its bulk is less than any planet, less even than the moon. It would take 100,000 big comets to equal the weight of the earth.  Comets have been described as an icy snowball.