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Comets are celestial bodies of small mass that travel around
the Sun (see SOLAR SYSTEM), usually in elongated orbits.  They
become visible as they near the Sun, and sometimes they form a
For centuries, comets have been considered harbingers of
catastrophe.  Their appearances, and sometimes their motions,
were accurately chronicled.  Babylonian and Chinese astronomers 
believed that comets were celestial bodies that moved through
space just like the planets.  The Greeks considered comets a
phenomenon in the Earth's atmosphere--a kind of vapor or
exhalation from the Earth.  This view was later generally
accepted.  Only in the 16th century did Tycho BRAHE establish
that comets are not atmospheric phenomena but are considerably
farther away from the Earth than the Moon is.
A century later Sir Isaac NEWTON discovered a method of
deriving the true orbit of a comet from its observed trajectory 
in the sky, and he determined that the comet of December 1680
followed a very elongated, parabolic orbit.  Edmond HALLEY, a
contemporary of Newton's, found that the orbits of the comets
of 1531, 1607, and 1682 were almost identical, a discovery that 
led him to conclude that they were in reality a single comet,
whose return he predicted in 1758.  The return actually
occurred, and since then the comet has been known as HALLEY'S
COMET.  It has been observed 20 times since 239 BC, its most
recent appearance occurring in 1985-86.
A newly discovered comet is provisionally designated by the
year of discovery and a letter indicating its order in comet
sightings that year.  Once the date at which the comet reaches
perihelion (closest approach to the Sun) is determined, the
comet is officially designated by the calendar year followed by a 
Roman numeral specifying the chronological order of the
perihelion passages in that year (for example, 1882 II).  Some
comets are named for their discoverers.  Thus the comet
IRAS-Araki-Alcock, which was discovered in 1983 and came within 
4.7 million km (2.9 million mi) of Earth--the closest known
comet approach in more than 200 years--was named for the
Infrared Astronomy Satellite and the two amateur astronomers
who first detected the comet.
At present comets are sought with the help of large reflecting
telescopes, which can photograph extensive parts of the sky
simultaneously.  This technique is used by many amateur
astronomers.  There are about ten comet discoveries a year.  On 
the average, one comet every three years is visible without a
telescope.  In 1985 the U.  S.  Interplanetary Sun Earth
Explorer (ISEE) 3 was maneuvered into an encounter with comet
P/Giacobini-Zinner, and in 1986 the European Space Agency,
Japan, and the Soviet Union all obtained closer views of
Halley's comet with their separate spacecraft.
All comet orbits that have been established are elliptical.
Short-period comets have periods of less than 200 years, and
their orbits are mostly inclined at a small angle to the
orbital plane of the Earth (the ecliptic).  The comet with the
shortest known period is Encke's comet (3.3 years).
Long-period comets have periods of more than 200 years--some
may reach several thousand years--and their orbital planes may
lie at various angles to the ecliptic.  About 100 of each group 
of periodic comets are known, and 800 nonperiodic comets have
been observed.  Some comets observed only once appear to have
parabolic or hyperbolic orbits that would bring them near the
Sun only once, suggesting a possible origin beyond the solar
system, but lack of data may account for such seeming orbits.
Almost all known comets approach to between 0.005 and 2.5
astronomical units of the Sun at perihelion (1 AU = mean
Earth-Sun distance).  If a comet's perihelion is farther from
the Sun than 2.5 AU, it is usually not observable.  Many comets 
have their aphelia (points of greatest distance from the Sun)
in the region of the outer planets.  A group of about 75 comets 
known as the Jupiter family have their aphelia near the orbit
of Jupiter.  There are other comet groups consisting of comets
that move in strikingly similar orbits around the Sun.  The
members of such a group are the remnants of a larger comet that 
broke up because of tidal forces exerted by the Sun or a

Nucleus and Coma
Almost the entire mass of a comet is concentrated in its
nucleus.  The diameter of the nucleus is on the order of a few
kilometers.  The density, between 0.1 and 1 g/cu cm (6 and 60
lb/cu ft), indicates that the nucleus is very tenuous.
According to Fred L.  WHIPPLE's "dirty snowball"
model--confirmed by recent comet observations--the nucleus
consists of a conglomerate of such compounds as water, carbon
dioxide, ammonia, and methane, all frozen and mixed with grit
and dust.  When the comet approaches the Sun, this frozen
matter sublimes and forms a cloud of gas and grit--called the
coma--around the nucleus.  Closer to the Sun, the production of 
gases increases.  The gas and dust particles are repelled from
the nucleus by the solar RADIATION PRESSURE and the SOLAR WIND
(a stream of charged particles), forming the tail.
The average diameter of the coma is about 100,000 km (62,000
mi), but its mass and density are small.  Some molecules are
decomposed and ionized by ultraviolet light from the Sun on
their way from the nucleus to the tail.  The chemical
composition of the nucleus can be discovered indirectly by
spectral analysis of this released gas.  The main products
observed are hydrogen and oxygen atoms, water, and hydroxyl
(OH) radicals.  The coma of a comet generally becomes smaller
as the distance from the Sun decreases.  Near the Sun, the
molecules of the coma are decomposed more rapidly by the solar
wind and pulled into the tail.
The apparent brightness of a comet depends on its distance from 
the Sun and from the Earth:  the brightness depends on about
the fourth power of the distance from the Sun, which indicates
that a comet not only reflects light, but also absorbs and then 
itself emits light.  Consequently the brightness may increase
quite rapidly upon approach to the Sun.  Some comets show
abrupt, striking increases in brightness in the vicinity of the 
Sun, the result of temporary increases in solar activity.
Other comets fade away upon approaching the Sun, probably
because the nucleus disintegrates (for example, Comet Ensor,
1926 III).  In some short-period comets the brightness
decreases slightly with each revolution, probably owing to loss 
of matter.
When a bright comet becomes visible, the most noticeable
feature is the tail.  At the appearance of Halley's comet in
1910, the tail stretched for more than 90 deg over the
celestial sphere.  During the comet's most recent appearance,
however, this elongation of the tail took place while the comet 
was on the far side of the Sun from the Earth, so that the show 
was far less dramatic.  In the majority of faint comets, no
tail is ever observed.
The length of the tail ranges from 1 to 100 million km (0.62 to 
62.14 million mi).  Usually it first appears at a distance of
about 1.5 AU from the Sun.  Despite the enormous size of a
comet's tail, it usually contains less material in 1 cu km than 
in 1 cu mm of ordinary air.
The tail is formed of gas from the coma and always points away
from the Sun.  It was initially thought that the solar
radiation pressure alone was responsible for driving the tail
away;  it is now clear that the solar wind has a far greater
effect.  The solar wind consists of charged particles ejected
from the Sun.  The force exerted by these particles on the gas
molecules of the coma is about 100 times stronger than the
gravitational force of the Sun, so that the molecules are
pulled along by the wind.  The solar wind is not constant, and
its variations are responsible for the threadlike structure of
the tail.  Solar flares or other perturbations on the Sun can
sometimes be seen to affect the tail:  it becomes turbulent and 
is sometimes bent.
A comet may have one of two types of tail, and many comets have 
both types--a double tail.  One is elongated and almost
straight, has a fibrous structure, and consists of ionized
gases.  This type of tail is called a Type I tail, a gas tail,
or a plasma tail.  Type II tails, or dust tails, are more
strongly curved and hazier;  they consist of dust repelled by
sunlight.  A comet may possess several dust tails in addition
to a gas tail.  Some comets have an anomalous tail, or
antitail, which points toward the Sun (for example, the
Arend-Roland comet, 1957 III).  The antitail is actually a very 
thin layer of dust lying along the orbit of the comet, having
been expelled at an earlier stage.
Many comets, especially short-period ones, slowly disintegrate, 
mainly under the influence of the Sun's gravitational force.
Recently, several collisions of comets with the Sun have been
observed with coronographs aboard satellites.  A regular
decrease in the brightness of short-period comets is often
observed.  Comets also leave waste products behind in their
orbits, in the form of millions of meteoroids.  When the Earth
crosses such an orbit, METEOR showers are frequently observed
Scientists speculate that collisions of comets--or cometary
fragments--with the Earth may occasionally occur, with
devastating results.  In one hypothetical scenario, past
cometary collisions are thought to have thrown enough dust into 
the Earth's atmosphere to have caused the extinctions of some
species of plants and animals (see EXTINCTION).  Also, the
impact of a comet, or piece of a comet, remains one of the more 
plausible explanations for a tremendous blast that occurred in
the Tunguska region of Russia in June 1908.
Various theories have been developed in recent centuries to
account for comets, but the one most widely accepted at present 
is that comets were formed at the same time as the rest of the
solar system.  In 1950 the Dutch astronomer Jan OORT proposed
that the Sun is surrounded by an enormous "cloud" of comet
material at a distance about 1,000 times that of the radius of
the known solar system.  This theory was followed in 1951 by
Dutch-American astronomer Gerard KUIPER's proposal that a ring
of cometary material lies in the plane of the solar system,
several hundred times as far from the Sun as the Earth is.
Both proposals have been widely accepted, with some astronomers 
suggesting that the Oort and Kuiper clouds actually merge along  
the Oort cloud's much denser inner boundary.  Perturbations by
interstellar clouds or passing stars would cause some of the
comet materials to dislodge and enter the inner solar system in 
the form of comets, with short-period comets more likely
arising from the Kuiper belt.
Steven J. Dick
Bibliography:  Brandt, John C., and Chapman, Robert D.,
Introduction to Comets (1981);  Brandt, John C., and Niedner,
Malcolm B., Jr., "The Structure of Comet Tails," Scientific
American (January 1986);  Delsemme, A.  H., "Whence Come
Comets?" Sky & Telescope, March 1989;  Sagan, Carl, and Druyan, 
Ann, Comet (1985);  Whipple, F.  L., The Mystery of Comets
(1985);  Yeomans, Donald K., Comets:  A Chronological History
of Observation, Science, Myth, and Folklore (1991).

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