The name „comet" comes from the Latin word stella cometa or „hairy star". Comets orbit the sun and are loose accumulations, which mostly consist of cosmic dust, meteorites, ice particles and gases.
The name „comet" comes from the Latin word stella cometa or „hairy
star". Comets orbit the sun and are loose accumulations, which mostly consist of
cosmic dust, meteorites, ice particles and gases.
Composition of a
comet
A comet consists of a sharply defined nucleus, looking like a star to the eye, embedded in
a nebulous cloud called the coma. There are many different theories about the composition
of comets. One of the best known theories is that of the american astronomer Fred L.
Whipple who proposed in 1949 that the nucleus, containing practically all the mass of the
comet, is a "dirty snowball" consisting of ices and dust. Evidence for the
snowball theory rests on various data. For one thing, the best-observed comets move in
orbits that are not identical to the one before. 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
revolutions, tend to fade very slowly with time, as would be expected of the kind of
structure proposed by Whipple. The head of a comet, Including
the hazy coma, may exceed the planet Jupiter, or sometimes even the Sun, in size. The
solid portion of most comets, however, has a volume of only a few cubic kilometers. The
dust-blackened nucleus of Halley's comet, for example, is about 15 by 8km (9 by 5 mi) in
size.
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History of comets
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 English scientist 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, 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, which reappeared in 1758. The earlier appearances of Halley's
comet have now been identified from records dating from as early as 240 BC. Halley's comet
passed round the Sun most recently early in 1986. As it once
more headed outward, it was visited in March of that year by two Soviet-constructed
probes, Vega 1 and 2, and by another instrumented package called Giotto, launched by the
European Space Agency. Two Japanese craft observed it from a great distance as it passed.
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Solar Effects of comets
As a comet approaches the Sun, the solar heat evaporates, or sublimates, the ices so that
the comet brightens enormously. It may also develop one or more tails, sometimes extending
many millions of kilometers into space. The tail is generally directed away from the Sun,
even as the comet recedes again. The gaseous tails of comets are often straight and highly
structured, being composed of simple ionized molecules, including carbon monoxide and
carbon 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/s (250mi/s). Comets frequently also display
curved tails, much smoother in appearance, composed of fine dust blown from the coma by
the pressure of solar radiation. As a comet recedes from the Sun, less gas and dust are
lost, 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 miles) in length. The variation in length of the
tail, together with the closeness of approach to the Sun and Earth, accounts for the
variation in the visibility of comets. Of some 1,400 comets on record, fewer than half the
tails were visible to the naked eye, and fewer than 10 per
cent were conspicuous.
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Periods and Orbits
Comets have elliptical orbits, and the periods of about 200 comets-the time they take to
orbit the Sun once-have been calculated. Encke's comet has a period of only 3.3 years;
Donati's comet of 1858 has one of 2,000 years; But the
majority of comets have even longer periods. The orbits of most comets are so vast that
they are virtually indistinguishable from parabolas-open curves that would take the comets
out of the solar system-but from careful observations astronomers have found that they too
are ellipses, albeit of great eccentricity, and with periods measured in millions or tens
of millions of years. At the extremes of their motions, far from the Sun, the orbits of many comets are significantly affected by passing stars, and
even by the overall gravitational effect of the galactic disc. No comets have been known
to approach the Sun on a strongly hyperbolic orbit; this would have meant an origin beyond
the solar system, in interstellar space. Some comets, however, may leave the solar system,
never to return, because of small alterations of their original orbits by the
gravitational action of the planets. Such action has been
observed on a smaller scale. About 160 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 or more.
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Comet Groups
When several comets 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 astronomer Brian G. Marsden has concluded that the 1965 comet and the even brighter
comet of 1882 split from a parent comet, possibly that of 1106. This comet and others of
the group Probably split away from a truly gigantic comet
thousands of years ago. A close relationship also exists between the orbits of comets and
those of the particles (meteoroids) that produce meteor showers. The Italian astronomer
Giovanni Virginio Schiaparelli proved that the Perseid meteors, which appear annually
around August 12, are associated with Comet Swift -Tuttle. Similarly, the Leonid meteors,
which appear annually in November, are closely associated with Comet Tempel -Tuttle.
Several other showers have been related to known cometary orbits, and are explained as the
stream of debris scattered by a comet along its orbit. Comets were once believed to come
from interstellar space. Although no detailed theory of origin is generally accepted, many
astronomers now believe that comets originated in the outer, colder part of the solar
system from residual planetary matter in the early days of the solar system. The Dutch
astronomer Jan Hendrik Oort has proposed that a "storage cloud" of comet
material has accumulated far beyond the orbit of Pluto, and that the gravitational effects
of passing stars may send some of the material towards the Sun, where it becomes visible
as comets. 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 Earth. Earth has in fact passed through the tails of
occasional comets without measurable effect. The collision of the nucleus of a comet with
Earth would probably have catastrophic worldwide effects, but the probability of such an
event occurring is exceedingly small. Some scientists suggest, however, that collisions
have taken place in the astronomical past and may even, for example, have had a climatic
role in the extinction of the dinosaurs. In 1992 the progenitor of Comet Shoemaker -Levy 9
produced 21large fragments following its exceptionally close approach to the planet
Jupiter. During a week-long bombardment in July 1994, the fragments crashed into Jupiter's
dense atmosphere at speeds of about 210,000 km/h (130,000 mph). Upon impact, the
dissipation of their tremendous kinetic energy into heat led to massive explosions, some
resulting in fireballs larger than Earth.
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