A
comet is a small body in the
Solar System that orbits the
Sun and, when close enough to the Sun, exhibits a visible
coma (or atmosphere) and/or a tail — both primarily from the effects of solar radiation upon the comet's
nucleus. The nucleus itself measures a few kilometres or tens of kilometres across, and is composed mostly of rock, dust and ice.
Comets originate in the outer solar system; they are thrown inwards towards the Sun by gravitational perturbations from planets or nearby stars. They have a variety of different orbital periods, ranging from a few years, to 50 or 100 years, to 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.
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.
(From Wikipedia)
Imaging comets can be both rewarding and challenging, as they are sometimes fast moving objects that may be difficult to track during an exposure.
