A
comet is an icy
small Solar System body that, when passing close to the
Sun, heats up and begins to
outgas, displaying a visible atmosphere or
coma, and sometimes also a
tail. These phenomena are due to the effects of
solar radiation and the
solar wind upon the nucleus of the comet.
Comet nuclei range from a few hundred metres to tens of kilometres across and are composed of loose collections of ice, dust, and small rocky particles. The coma and tail are much larger and, if sufficiently bright, may be seen from the Earth without the aid of a telescope. Comets have been observed and recorded since ancient times by many cultures.
