Co to je kometa?

O tom, co to je kometa, z čeho se skládá, a jak je asi velká.

reference:
67P/Churyumov–Gerasimenko: ESA/Rosetta/NAVCAM – CC BY-SA IGO 3.0 http://creativecommons.org/licenses/by-sa/3.0/igo/ (no changes were made)