Comets have been observed since ancient times and have historically been considered bad omens, similar to eclipses. A comet is an small icy loosely bound collection of dust and ice and small rocky particles, ranging from a few hundred meters to tens of kilometres across that orbit the Sun in an ellipse and when close enough, displays a visible coma (a thin, fuzzy, temporary atmosphere), and sometimes also a tail. These phenomena are both due to the effects of solar radiation, 'boiling' away the ice of the comet, and the solar wind, which streams away from the Sun and makes the comet's tail point away from the Sun too. and small rocky particles,.
Comets have orbital periods ranging from a few years to hundreds of thousands of years. Short-period comets originate in the Kuiper Belt, Longer-period comets are thought to originate in the Oort Cloud, a cloud of icy bodies in the outer Solar System that were left behind during the condensation of the solar nebula. Long-period comets plunge towards the Sun from the Oort Cloud because of gravitational perturbations caused by either the massive outer planets of the Solar System or passing stars. Rare hyperbolic comets pass once through the inner Solar System before being thrown out into interstellar space never to be seen again.
As of May 2010 there are a reported 3,976 known comets. This number is steadily increasing. However, this represents only a tiny fraction of the total potential comet population: the reservoir of comet-like bodies in the outer solar system may number one trillion. The number visible to the naked eye averages roughly one per year, though many of these are faint and unspectacular.
Comets are useful because they contain the unprocessed material present at the birth of the Solar System. By examining it we gain some insight into the processes taking place during the birth of the Solar System and before.