Comet Hale Bopp (NASA)

Comets

The name comet comes from the greek word kometes, meaning "head of hair". Comets have been known for thousand of years. Some of them appeared so bright, that they could be seen during the day. Others, had beautiful tails that could stretch halfway across the sky. In the antiquity, comets were seen as bad signs that seemed to announce future disasters. In particular, Halley’s comet has been associated with the destruction of Jerusalem and the Norman Invasion of England in 1066 (click here to know more about the history of comets' science).


What is a comet?
Nowadays, comets are seen as dirty snowballs (or icy mudballs). In fact, comets are thought to be made of a mixture of ices and dust that wasn't incorporated into the planets when the solar system was formed. This is the main reason why they are believed to share the same composition of the primordial solar system, having formed essentially form the same material. (click here to know more about the primordial solar system).
But what is the mechanism that gives birth to the tail, normally associated with comets? When the solid snowball (called nucleus) enters the inner Solar System and comes near the Sun, it heats up and becomes active, causing volatile gas to sublime. The released gas and dust form a cloud, or coma, and the dust element of the tail (click here to know more about comets' composition).
Today comets are studied and observed and, as well as asteroids and the other celestial bodies, their physical characteristics are defined by some specific physical quantities. Anyway, in spite of the improvements in comets' sciences some very interesting questions are still unanswered. Among them, the most exciting are about the birth of a comet, the duration of its' life, and whether comets, in the last phases of their life, can become asteroids (click here to know more about a comet's life).


Where can comets be found in the Solar System?

Another interesting question about comets is about where they come from. The most qualified theory affirms that a cloud of comets, named Oort cloud, exists as a diffuse spherical shell at about 50,000 AU surrounding the entire solar system. This huge sphere is thought to contain perhaps up to objects. Another reservoir where comets are found is the Kuiper belt, just beyond Neptune's orbit, much nearer from the Earth than the Oort one. By now, only up to 31 bodies of this belt have been found. 

Short and long period comets: a first classification
Comets are usually divided into short and long period objects, taking in consideration where they form and their orbits: short period comets have predictable orbits and are thought to generate in the Kuiper Belt, while long period comets are thought to be generated in the Oort cloud,  so that only a very few number approaches the sun, with highly unpredictable orbits.
The composition corresponding to these two classes is slightly different, but this classification cannot be considered static, since a comet with a short period can easily leave its orbit for the gravitational influence of the planets it comes near to and become suddenly a long period comet (click here to know more about long and short period comets).