A comet is generally considered to consist of a small, sharp nucleus
embedded in a nebulous disk called the coma. American astronomer Fred L. Whipple
proposed in 1949 that the nucleus, containing practically all the mass of the
comet, is a “dirty snowball” conglomerate of ices and dust. Major proofs of the
snowball theory rest on various data. For one, of the observed gases and
meteoric particles that are ejected to provide the coma and tails of comets,
most of the gases are fragmentary molecules, or radicals, of the most common
elements in space: hydrogen, carbon, nitrogen, and oxygen. The radicals, for
example, of CH, NH, and OH may be broken away from the stable molecules CH4
(methane), NH3 (ammonia), and H2O (water), which may exist as ices or more
complex, very cold compounds in the nucleus. Another fact in support of the
snowball theory is that the best-observed comets move in orbits that deviate
significantly from Newtonian gravitational motion. This provides clear evidence
that the escaping gases produce a jet action, propelling the nucleus of a comet
slightly away from its otherwise predictable path. In addition, short-period
comets, observed over many revolutions, tend to fade very slowly with time, as
would be expected of the kind of structure proposed by Whipple. Finally, the
existence of comet groups shows that cometary nuclei are fairly solid units. The
head of a comet, including the hazy coma, may exceed the planet Jupiter in size.
The solid portion of most comets, however, is equivalent to only a few cubic
kilometers. The dust-blackened nucleus of Halley\'s comet, for example, is about
15 by 4 km (about 9 by 2.5 mi) in size.

Category: Science