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
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.
Imaging comets can be both rewarding and challenging, as they are sometimes fast moving objects that may be difficult to track during an exposure.