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COMET
(Lat. stella cometa, “hairy star”),
nebulous celestial body revolving around the sun. A comet is characterized
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 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, the British
astronomer Edmund Halley showed the comet of 1682 to be identical
with the two that had appeared in 1607 and 1531, and in 1705 he
successfully predicted the return of the comet in 1758. The earlier appearances
of Halley’s comet have now been identified from records
dating from as early as 240 bc, and it is probable that
the bright comet observed in 466 bc was also an apparition
of this famous comet. Halley’s comet most recently passed
around the sun again early in 1986. As it once more headed outward,
it was visited in March of that year by two Soviet probes and by
another instrumented package launched by the European Space Agency.
Two Japanese craft observed the comet at a great distance as it
passed.
The comet Hyakutake, which was discovered less than two months
before it passed close to earth in March 1996, was found to emit
X rays, confirming similar observations made during an all-sky survey
in 1990. Comet Hale-Bopp, which may be the brightest comet in 400
years, was discovered in 1995 and passed closest to earth in March
1997.
For many years astronomers have supposed comets to consist
of a small nucleus embedded in a nebulous disk called a coma. American
astronomer Fred L. Whipple (1906– ) proposed
in 1949 that the nucleus, containing practically all the comet’s
mass, is a “dirty snowball” consisting of ices
and dust. This view was confirmed by the 1986 Halley studies.
The gases and particles ejected from comets providing their
comae and tails consist mostly of molecules or radicals of elements
common in space: hydrogen, carbon, nitrogen, and oxygen. The radicals
of CH, NH, and OH that are observed may be broken away from the
stable molecules CH4 (methane), NH3 (ammonia), and
H2O (water), which exist as ices in the nucleus. The
escaping gases produce jet actions that propel the nucleus slightly
away from its otherwise predictable path. Short-period comets (see
below) observed over many revolutions, tend to fade slowly with
time, as would be expected of such objects. Even Halley’s
comet, although not a short-period comet, is losing about 100 million
metric tons with each passage around the sun. Thus, it will also
fade after several hundred more passes by the earth—a matter
of several hundred more centuries.
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.
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 kilometers 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 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.
As a comet recedes from the sun, the loss of gas and accompanying
dust decreases in quantity, and the tails disappear. Some of the
comets with small orbits have tails so short that they are practically
invisible. On the other hand, the tail of at least one comet has
exceeded 320 million km (200 million mi) in length. The variation
in length of the tail, together with the closeness of approach to
the sun and the earth, accounts for the variation in the visibility
of comets. Of the many comets on record, fewer than half the tails
were visible to the naked eye, and fewer than 10 percent were conspicuous.
Comets have elliptical orbits, and the periods of about 200 comets—the
time they take to orbit the sun once—have been calculated.
They range from 3.3 years for Encke’s comet to 2000 years
for Donati’s Comet of 1858; Halley’s comet has
a period of 75–76 years. The orbits of most comets are
vast and are probably ellipses of great eccentricity, with periods
as long as 40,000 years or possibly much longer.
No comets have been known to approach the earth on a hyperbolic
orbit; this would have meant an origin in outer space. Some comets,
however, may never return to the solar system because of extreme alteration
of their original orbits by the gravitational action of the planets.
Such action has been observed on a smaller scale. About 60 short-period
comets have orbits that have been influenced by the planet Jupiter,
and are said to belong to the family of Jupiter. Their periods range
from 3.3 to 9 years.
When several comets with different periods travel in nearly
the same orbit, they are said to be members of a comet group. The
most famous group includes the spectacular sun-grazing comet, Ikeya-Seki,
of 1965, and seven others having periods of nearly a thousand years.
The American astronomer Brian G. Marsden (1937– )
has concluded that the 1965 comet and the even brighter comet of
1882 split from a parent comet, possibly the one of 1106. This comet
and others of the group probably split away from a truly giant comet
thousands of years ago.
A close relationship also exists between the orbits of comets
and the orbits of meteor showers. The Italian astronomer Giovanni
Virginio Schiaparelli proved that the Perseid meteors, which appear
in August, move in the same orbit as Comet 1862 III. Similarly,
the Leonid meteors, which appear in November, were found to follow
the same orbit as Comet 1866 I. Several other showers have been
related with known cometary orbits, and are explained by the earth-type
debris scattered by a comet along its orbit.
Although no detailed theory of origin is generally accepted,
many astronomers now believe that comets originate in the outer,
colder edges of the solar system, where matter remains as it was
at the time when the solar system formed. A Dutch astronomer, Jan
Hendrik Oort, proposed in 1950 the existence of a “storage
cloud” of such material surrounding the solar system at
a great distance (as much as 50,000 to 150,000 times more than the
distance from the sun to earth), while the Dutch-American astronomer
Gerard Peter Kuiper proposed in 1951 a denser, nearer cloud, just
beyond Neptune’s orbit but lying more in the plane of the
system. Passing galactic visitors, such as molecular clouds or brown
dwarfs, may then disturb these clouds and send comets in toward
the realm of the planets. The first such comets were discovered
in the Kuiper cloud in 1992; by 1996, 31 had been found. The Oort
cloud accounts for long-period comets, such as Halley’s
and the Swift-Tuttle (with a period of 130 years), and the Kuiper
cloud for short-period comets (on the order of 10 years).
Comets have long been regarded by the superstitious as portents
of calamity or important events. The appearance of a comet has also
given rise to the fear of collision between the comet and the earth.
The earth, in fact, has passed through the tails of occasional comets
without measurable effect. Some scientists suggest, however, that
collisions have taken place in the astronomical past. A somewhat related
theory that became controversial during the 1980s is that the earth
is constantly being bombarded by tiny, hundred-ton ice comets that
were the source of the earth’s oceans over geological time.
Most scientists, however, have discounted this notion.