When a comet comes close enough that we might hope to see some detailed features through a telescope, it veils its head (nucleus) in a diffuse cloud (coma), so that all we see is a fuzzy ball of mist. The light from the coma, analyzed in a spectroscope, tells us some of the things it is made of: water vapor, ammonia, methane, cyanogen. Also, atoms of common metals are revealed: iron, nickel, manganese, calcium, magnesium, sodium, and so on. All of these are driven out of the coma by the sun’s radiation to form the tail. The tail shines, as does the coma, by fluorescence and reflection of sunlight.

The size of comets is tremendous. The coma is often larger than the planets, sometimes even as large as the sun. The tails of comets are tens of millions of miles long; some have been over a hundred million miles, long enough to reach from the earth to the sun. The solid nucleus, however, is tiny in comparison. It is probably no more than a few miles across.

In keeping with the small nucleus, the bulk of the entire comet is billions of times less massive than the earth. The tail of a comet so great that it seems to fill the sky has so little substance that stars shine right through it. It is more rarefied than the best vacuum man can make. Realizing this has allayed earlier fears that the earth’s passage through a comet’s tail might be disastrous. Such a scare spread when Halley’s comet was last here. People panicked at the thought that the gases in the tail would poison the atmosphere and sought to protect themselves before the fateful day of May 18, 1910. But the comet’s tail brushed right over the earth without the slightest discernible effect.

In the currently popular theory of comets, the nucleus is described as a “dirty snowball,” made up of ices of water, methane, and ammonia, mixed with solid grains containing metallic elements. As the comet approaches the sun, it sublimates, emitting vapors and spewing out dust particles, to form the cloudlike coma. Closer to the sun, the vapors and dust are blown away from the coma by a solar wind of particles and by solar radiation to form the tail.

Also, a periodic comet suffers from aging. Every time it passes the sun, it expends some of its substance to recreate its coma and tail. Some short-period comets have disappeared after repeated circuits, leaving nothing behind but a shower of meteors.