18D/Perrine–Mrkos is a periodic comet in the Solar System, originally discovered by the American-Argentine astronomer Charles Dillon Perrine (Lick Observatory, California, United States) on December 9, 1896. For some time it was thought to be a fragment of Biela's Comet.
It was considered lost after the 1909 appearance, but was rediscovered by the Czech astronomer Antonín Mrkos (Skalnate Pleso Observatory, Slovakia) on October 19, 1955, using ordinary binoculars, it was later confirmed as 18D by Leland E. Cunningham (Leuschner Observatory, University of California, Berkeley).
The comet was last observed during the 1968 perihelion passage when it passed 0.3144 AU (47,030,000 km; 29,230,000 mi) from the Earth. The comet has not been observed during the following perihelion passages:
- 1975 Aug. 2
- 1982 May 16
- 1989 Feb. 28
- 1995 Dec. 6
- 2002 Sept.10
- 2009 Apr. 17
The next predicted perihelion passage would be on 2017 Feb. 26, but the comet is currently considered lost.