A 'Comet' (Latin name 'stella cometa', meaning hairy star) is a nebulous celestial body
revolving around the Sun. A comet is characterised by a long, luminous tail, but only
in the segment of the comet's orbit when it passes closest to the Sun.
Appearances of large comets were regarded as atmospheric phenomena until 1577,
when the Danish astronomer Tycho Brahe proved that they were, in fact, celestial
bodies. In the 17th century the British scientist Sir Isaac Newton demonstrated that
the movements of comets are subject to the same laws that control the planets in their
orbits.
By comparing the orbital elements of a number of earlier comets, British astronomer
Edmund Halley showed the comet of 1682 to be identical with the two that had
appeared in 1607 and 1531, and he successfully predicted the return of the comet in
1759. This comet is now known as Halley's comet and has been identified from
records dating from as early as 240 BC.
A comet is generally considered to consist of a small, sharp nucleus embedded in a
nebulous disk called the coma. American astronomer Fred 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 example, 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
kilometres. The dust-blackened nucleus of Halley's comet, for example, is about 15 by
4 km (about 9 by 2.5 mi) in size.
Solar Effects
As a comet approaches the Sun, the solar heat evaporates, or sublimates, the ices so
that the comet brightens enormously. It may develop a brilliant tail, sometimes
extending many millions of kilometres into space. The tail is generally directed away
from the Sun, even as the comet recedes again. The great tails of comets are
composed of simple ionized molecules, including carbon monoxide and dioxide. The
molecules are blown away from the comet by the action of the solar wind, a thin
stream of hot gases continuously ejected from the solar corona, the outermost
atmosphere of the Sun, at a speed of 400 km (250 mi) per sec. Comets frequently also
display smaller, curved tails composed of fine dust blown from the coma by the
pressure of solar radiation.