Non-periodiccomets are seen only once. They are usually on near-parabolic orbits that will not return to the vicinity of theSun for thousands of years, if ever.
Periodic comets usually have elongatedelliptical 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.
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