Comet
Comet Hale-Bopp, as seen on 29 March 1997
in Pazin, Croatia.
A comet is a Small Solar System Body that
orbits the Sun. When close enough to the
Sun, a comet exhibits a visible coma (fuzzy
"atmosphere"), and sometimes a tail, both be-
cause of the effects of solar radiation upon
the comet’s nucleus. Comet nuclei are them-
selves loose collections of ice, dust and small
rocky particles, ranging from a few kilomet-
ers to tens of kilometers across.
Background
Name and symbol
The word comet came to the English lan-
guage through the Latin cometes from the
Greek word komē, meaning "hair of the
head"; Aristotle first used the derivation
komētēs to depict comets as "stars with
hair." The astronomical symbol for comets (?)
accordingly consists of a disc with a hairlike
tail.
Orbits and origin
Comets have a variety of different orbital
periods, ranging from a few years, to hun-
dreds of thousands of years, while some are
believed to pass only once through the inner
Solar System before being thrown out into in-
terstellar space. Short-period comets are
thought to originate in the Kuiper Belt, or as-
sociated scattered disc,[1] which lie beyond
the orbit of Neptune. Long-period comets are
believed to originate in the Oort cloud, con-
sisting of debris left over from the condensa-
tion of the solar nebula, located well-beyond
the Kuiper Belt. Comets are thrown from
these outer reaches of the Solar System to-
wards the Sun by gravitational perturbations
from the outer planets (in the case of Kuiper
Belt objects) or nearby stars (in the case of
Oort Cloud objects), or as a result of colli-
sions between objects within these regions.
Comets are distinguished from asteroids
by the presence of a coma or tail, though
very old comets that have lost all their volat-
ile materials may come to resemble asteroids
(see extinct comets).[2] Asteroids are also be-
lieved to have a different origin from comets,
having formed in the inner Solar System
rather than the outer Solar System,[3] but re-
cent findings[4] have somewhat blurred