A comet is a small body in the solar system that orbits the
Sun and (at least occasionally) exhibits a
coma (or atmosphere) and/or a tail — both primarily from the effects of solar radiation upon the comet's
nucleus, which itself is a minor body composed of rock, dust, and ice. Comets' orbits are constantly changing: their origins are in the outer solar system, and they have a propensity to be highly affected (or perturbed) by relatively close approaches to the major planets. Some are moved into Sun-grazing orbits that destroy the comets when they near the Sun, while others are thrown out of the solar system forever.
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[Picture] Any icy object that exists within the solar system. They are pieces of the primitive, unprocessed matter that formed the solar system 4.6 x 109 years ago. They are typically a few kilometres across and consist mainly of dust grains, frozen water, carbon monoxide and carbon dioxide; they contain many simple organic molecules.
Most comets are thought to lie in peripheral regions of the solar system known as the
Copier belt and
Oort cloud. The Kuiper belt is theorised to begin shortly outside the orbit of Pluto. It is shaped like a flared disc and gradually extends upwards and downwards to enclose the solar system within a spherical shell of comets known as the Oort cloud. Comets are very difficult to observe that far from the Sun because they are so small. They can fall inwards towards the Sun, because of gravitational instabilities in their orbits, and it is then that comets can be more easily studied. As a comet approaches the inner solar system, it receives increased amounts of solar radiation which sublimate the ices on its surface leading to the creation of a tail. There are two types of tail that a comet will develop: an ion tail (often referred to as a type I tail) and a dust (or type II) tail.
[Picture] The tails are formed because the sublimating ices are throwing off jets of dust and gas into space. The gas is ionized and is swept away by the
solar wind of particles boiling out of the sun . This means that the ion tail always points directly away from the Sun and thus appears straight. The dust tail is produced by the pressure of solar radiation (since
photons carry linear momentum). This causes the dust tail to be a sweeping fan-shaped object. Some comets, such as the 76 year Halley's comet, are short period comets. This means they have highly elliptical orbits which bring them into the inner solar system on timescales of up to one hundred years or so. With each passage they lose a little more of their mass until, eventually, they will break up altogether. Streams of dust particles in comet orbits are responsible for meteor showers. Long period comets are thought to fall inwards from the Oort cloud. They will often suffer gravitational perturbations from the giant planets, particularly Jupiter, which will alter their orbits. In some cases, they can be converted to short period comets, whilst in other cases their orbits are changed to be hyperbolic in shape. These will then escape the Sun's gravitational influence and wander the depths of interstellar space. See comet
Hyakutake and, the brightest seen with the naked eye,
Hale-Bopp (1995).
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