| 21P/Giacobini–Zinner | |
| Discoverer: | Michel Giacobini, Ernst Zinner |
| Discovery Date: | December 20, 1900 |
| Designations: | 1900 III; P/1900 Y1; 1913 V; P/1913 U1; 1926 VI; 1933 III; 1940 I; 1946 V; 1959 VIII; 1966 I; 1972 VI; 1979 III; 1985 XIII; 1992 IX |
| Epoch: | 2025-03-26 |
| Earth Moid: | 0.018AU |
| Semimajor: | 3.492 AU |
| Perihelion: | 1.009 AU |
| Aphelion: | 5.97 AU |
| Eccentricity: | 0.7110 |
| Period: | 6.52 yr |
| Inclination: | 32.050° |
| Last P: | 2018-Sep-10[1] February 11, 2012 July 2, 2005 |
| Next P: | 2025-Mar-25[2] |
| Dimensions: | 2 km |
Comet Giacobini–Zinner (officially designated 21P/Giacobini–Zinner) is a periodic comet in the Solar System. It was discovered by Michel Giacobini, who observed it in the constellation of Aquarius on December 20, 1900. It was recovered two orbits later by Ernst Zinner, while he was observing variable stars near Beta Scuti on October 23, 1913.
The comet nucleus is estimated to be 2.0 kilometers in diameter.[3] During its apparitions, Giacobini–Zinner can reach about the 7-8th magnitude,[4] but in 1946 it underwent a series of outbursts that made it as bright as 5th magnitude. It is the parent body of the Giacobinids meteor shower (also known as the Draconids). The comet currently has a minimum orbit intersection distance to Earth of 0.035abbr=onNaNabbr=on.[3]
Giacobini–Zinner was the target of the International Cometary Explorer spacecraft, which passed through its plasma tail on September 11, 1985. Earlier in the same month the comet was observed by the Pioneer Venus Orbiter[5] In addition, Japanese space officials considered redirecting the Sakigake interplanetary probe toward a 1998 encounter with Giacobini–Zinner, but that probe lacked the propellant for the necessary maneuvers and the project was abandoned.
During the apparition of 2018, the optical spectra have revealed the comet is depleted in carbon-chain molecules and carbon dioxide, likely indicating its origin in relatively warm portion of the Solar system.[6]