Comets are small Solar System bodies that orbit the Sun and, when close enough to the Sun, exhibit a visible coma (or atmosphere) and/or a tail — both primarily from the effects of solar radiation upon the comet's nucleus. Comet nuclei are themselves loose collections of ice, dust and small rocky particles, measuring a few kilometres or tens of kilometres across.
Comets have a variety of different orbital periods, ranging from a few years, to hundreds of thousands of years, while some are believed to pass through the inner Solar System only once before being thrown out into interstellar space. Short-period comets are thought to originate in the Kuiper Belt, which lies beyond the orbit of Neptune. Long-period comets are believed to originate at a very much greater distance from the Sun, in a cloud (the Oort cloud) consisting of debris left over from the condensation of the solar nebula. Comets are thrown from these outer reaches of the Solar System inwards towards 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 collisions. Asteroids originate via a different process, but very old comets which have lost all their volatile materials may come to resemble asteroids.
There are a reported 3,354 known comets as of November 2007, of which several hundred are short-period. This number is steadily increasing. However, this represents only a tiny fraction of the total comet population: the reservoir of comet-like bodies in the outer solar system may number one trillion. The number of naked-eye comets averages to roughly one per year, though many of these are faint and unspectacular. When a historically bright or notable naked-eye comet is witnessed by many people, it is often considered a Great comet.
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