Comets

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.