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Periodic comets usually have elongated elliptical orbits, and usually return to the vicinity of the Sun after a number of decades.
The official names of non-periodic comets begin with a "C"; the names of periodic comets begin with "P" or a number followed by "P". Comets that have been lost or disappeared have names with a "D". Comets whose orbit has not been determined are designated with a "X" prefix.