To judge these fascinating possibilities, we need to know more about comets, and we need to understand how individual comets are alike or different. For the first time, we will assess how diverse these original building blocks of the solar system are. If a comet approaches the Sun, the surface of the nucleus heats up, ice evaporates, and the released gases and dust form the coma, or atmosphere. These conspicuous fuzzy parts of comets can extend for thousands of miles from the nucleus.
Sunlight sweeps the gases and dust back into a tail that can stretch millions of miles.
Comas and tails are big enough for easy study with Earth-based telescopes, but a comet's nucleus is so tiny that we can study it only by getting close. Comets are the most numerous sizeable bodies in the solar system. Current estimates place their total number at a million million. Although comets belong to the Sun's family, the vast majority remain where they formed, in the immense region that stretches beyond the orbits of the farthest planets, Neptune and Pluto, half way to the nearest star. On rare occasions, a comet is nudged in its orbit and approaches the Sun.