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A comet is a small, icy celestial body that orbits around the Sun. It is made up of a nucleus (solid, frozen ice, gas and dust), a gaseous coma (water vapor, carbon dioxide, and other gases) and a tail (dust and ionized gases). Its long tail of gas and dust always points away from the sun, because of the force of the solar wind. The tail can be up to 250 million km long, and is most of what we see. Comets are only visible when they're near the sun in their highly eccentric orbits.