comet

A comet is a small solar system body

bigger than a meteoroid that, when close

enough to the Sun, exhibits a visible coma

(fuzzy “atmosphere”), and sometimes a tail,

both because of the effects of solar radiation

upon the comet’s nucleus. Comet nuclei are

themselves loose collections of ice, dust and

small rocky particles, ranging from a few

hundred meters to tens of kilometers 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 only once through

the inner Solar System before being thrown

out into interstellar space. Short-period

comets are thought to originate in the Kuiper

Belt, or associated scattered disc, which lie

beyond the orbit of Neptune. Long-period

comets are believed to originate in the Oort

cloud, consisting of debris left over from the

condensation of the solar nebula, located well

beyond the Kuiper Belt. Comets are thrown

from these outer reaches of the Solar System

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

between objects within these regions.