WHAT IS A COMET?
Imagine a snowball several miles wide, loosely packed with rocky or muddy material. Far from the Sun, a comet consists of little else. Within the snowball are ices of water, cyanogen, and other materials, as well as what scientists call CHON particles—organic material containing carbon, hydrogen, oxygen, and nitrogen.
As the snowball, or nucleus, near the Sun, its ices begin to sublimate, releasing gas and dust. This material forms a large head, or coma, around the nucleus, and then streams behind the coma to form a tail. The coma of a large comet might be thousands of miles across, its tail tens of millions of miles long. An active comet might bleed away only the topmost few feet of material during its months near the Sun. Even though the comet appears to be burning away, it has enought stored ice and dust to last for hundreds of revolutions about the Sun.