A minor celestial icy body that orbits a star in a highly elliptical path. It is made up of a nucleus (solid, frozen ice, gas and dust) containing almost the entire cometary mass, a gaseous coma (water vapour, CO2, 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 kilometres long, and is most of the visible part of the comet. Comets are highly prized as a source of water and other useful habitat and terraforming materials.
Types of Comets
Short Period Comet: A comet with a revolution period less than about 100 standard years.
Long-period Comet: A comet moving on a nearly parabolic orbit and thus having an orbital period of hundreds of thousands of years. Some haloist and spacer clades build their habitats on such comets.