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Comets are small solar system bodies that orbit the Sun. When they are close enough to the Sun a comet displays a visible coma and a tail. Both are the results of the acting of solar radiation upon the comets nucleus. Comet nuclei are themselves loose collections of ice, dust and small rocky particles, ranging from a few kilometers to tens of kilometers across. Aristotle first used the derivation komets to depict comets as “stars with hair.” The astronomical symbol for comets accordingly consists of a disc with a hairlike tail.
Comets have a variety of different orbital periods. Some are only a few years, others are hundreds of thousands of years, while some are believed to pass only once through the inner Solar System before being thrown out into interstellar space. Short-period comets are thought to originate in the Kuiper Belt or the associated scattered disc, both of which lie beyond the orbit of Neptune. Long-period comets are believed to originate in the Oort Cloud, consisting of debris left over from the condensing of the solar nebula. The cloud is located well-beyond the Kuiper Belt. Comets are thrown from these outer reaches of the Solar System towards the Sun by gravitational perturbations from the outer planets or nearby stars, or on occasion as a result of collisions 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 volatile materials may come to resemble asteroids because they lack that distinguishing tail. Asteroids are also believed to have a different origin from comets, having formed in the inner Solar System rather than the outer Solar System, but recent findings have somewhat blurred the distinction between asteroids and comets.
Comet nuclei are known to range from about 100 meters to more than 40 kilometers across. They are composed of rock, dust, ice, and frozen gases such as Carbon monoxide, carbon dioxide, methane, and ammonia. They are often popularly described as “dirty snowballs”, though recent observations have revealed dry dusty or rocky surfaces, suggesting that the ices are hidden beneath a crust. Comets also contain a variety of organic compounds in addition to the gases already mentioned, these may include methanol, hydrogen, hydrogen cyanide, formaldehyde, ethanol, and ethane. Perhaps they may contain more complex molecules such as long-chain hydrocarbons and amino acids. Because of their low mass, comets cannot become round under their own gravity and will have irregular shapes. Surprisingly, cometary nuclei are among the darkest objects known to exist in the solar system. The Giotto space probe found that Halley’s comet nucleus reflects approximately 4% of the light that falls on it and Deep Space 1 discovered that Borrelly’s Comet’s surface reflects 2.4–3.0% of the light that falls on it. In comparison, asphalt reflects 7% of the light that falls on it. It is thought that complex organic compounds 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 oil. The very darkness of cometary surfaces allows them to absorb the heat necessary to drive their outgassing.
As a comet approaches the inner solar system, radiation causes the volatile materials within the comet to vaporize and stream out of the nucleus, carrying dust away with them. The streams of dust and gas form a huge, 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.
Both the coma and tail are illuminated by the Sun and may become visible from Earth when a comet passes through the inner solar system, the dust reflecting sunlight directly and the gases glowing from ionization. The streams of dust and gas each form their own distinct tail, pointing in slightly different directions. The tail of dust is left behind in the comet’s orbit in such a manner that it often forms a curved tail called the anti-tail. At the same time, the ion tail, made of gases, always points directly away from the Sun, as this gas is more strongly affected by the solar wind than is dust, following magnetic field lines rather than an orbital trajectory. While the solid nucleus of comets is generally less than 50 km across, the coma may be larger than the Sun, and ion tails have been observed to extend 1 AU or more.
There is a lot more information to be had about comets. Here is a good article to start with. Universe Today has several great articles about comets. Here is one on the difference between comets and asteroids and here is another on smaller comets being released from Comet Holmes. Astronomy Cast offers a nice episode on the role of molecules in space.