Comet

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 due primarily to the effects of solar radiation upon the comet's Nucleus , which itself is a minor planet composed of rock, dust, and ices. Due to their origins in the outer solar system and their propensity to be highly affected (or ''perturbed'') by relatively close approaches to the major planets, comets' orbits are constantly changing. Some are moved into sungrazing orbits that destroy the comets when they near the Sun, while others are thrown out of the solar system forever.

Most comets are believed to originate in a cloud (the Oort Cloud ) at large distances from the Sun consisting of debris left over from the Condensation of the Solar Nebula ; the outer edges of such nebulae are Cool enough that Water exists in a Solid (rather than Gas eous) State . Asteroid s originate via a different process, but very old comets which have lost all their Volatile materials may come to resemble asteroids.

The word ''comet'' came to the English Language through 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."

PHYSICAL CHARACTERISTICS

Long-period comets are believed to originate in a distant cloud known as the Oort Cloud (after the astronomer Jan Hendrik Oort who hypothesised its existence). They are sometimes perturbed from their distant orbits by gravitational interactions, falling into extremely elliptical orbits that can bring them very close to the Sun . One theory says that as a comet approaches the Inner Solar System , Solar Radiation causes part of its outer layers, composed of ice and other materials, to melt and evaporate, but this has not been proven.
The streams of Dust and gas this releases form a very large, extremely tenuous atmosphere around the comet called the '' Coma '', and the force exerted on the Coma by the Sun's Radiation Pressure and Solar Wind cause an enormous ''tail'' to form, which points away from the sun. The dust and gas each form their own distinct tail, each pointed in slightly different directions. The tail made of dust is left behind in the comet's orbit in such a manner that it often forms a curved tail. At the same time, the ion tail, made of gases, always pointing directly away from the Sun, as this gas is more strongly affected by the solar wind than dust is, following magnetic field lines rather than an orbital trajectory. While the solid body of comets (called the '' Nucleus '') is generally less than 50km across, the coma may be larger than the Sun, and the ion tails have been observed to extend over 150 million km (1 Astronomical Unit ) or more.

Both Coma and tail are illuminated by the Sun, and may become visible from the Earth when a comet passes through the inner solar system, the dust reflecting sunlight directly and the gases glowing due to Ion ization. Most comets are too faint to be visible without the aid of a Telescope , but a few each decade become bright enough to be visible with the naked eye. Before the invention of the telescope, comets seemed to appear out of nowhere in the sky and gradually vanish out of sight. They were usually considered bad omens of deaths of kings or noble men, or coming catastrophes. From ancient sources, such as Chinese oracle bones, it is known that their appearance have been noticed by humans for millennia. One very famous old recording of a comet is the appearance of Halley's Comet on the Bayeux Tapestry , which records the Norman Conquest of England in 1066.¹

Surprisingly, cometary nuclei are among the Black est objects known to exist in the solar system. The Giotto probe found that Comet Halley 's nucleus reflects approximately 4% of the light that falls on it, and Deep Space 1 discovered that Comet Borrelly 's surface reflects only 2.4% to 3% of the light that falls on it; by comparison, Asphalt reflects 7% of the light that falls on it. It is thought that complex Organic Compound s are the dark surface material. Solar heating drives off volatile compounds leaving behind heavy long-chain organics that tend to be very dark, like Tar or crude Oil . The very darkness of cometary surfaces allows them to absorb the heat necessary to drive their outgassing.

In 1996, comets were found to emit fly through a cometary atmosphere, they collide with cometary atoms and molecules. In these collisions, the ions will capture one or more electrons leading to emission of X-rays and far ultraviolet photons.³

ORBITAL CHARACTERISTICS

and Earth , illustrating the high Eccentricity of the orbit and more rapid motion when closer to the Sun .]]
Comets are classified according to their orbital periods. ''Short period comets'' have orbits of less than 200 years, while ''Long period comets'' have longer orbits but remain gravitationally bound to the Sun, and Main-belt Comets orbit within the Asteroid Belt . ''Single-apparition comets'' have Parabolic or Hyperbolic orbits which will cause them to permanently exit the solar system after one pass by the Sun.

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Comet