21P/Giacobini–Zinner

Comet Giacobini–Zinner (officially designated as 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 20 December 1900. It was recovered two orbits later by Ernst Zinner, while he was observing variable stars near Beta Scuti on 23 October 1913.

21P/Giacobini–Zinner

Comet Giacobini–Zinner photographed from Moscow, Russia on 9 September 2018.
Discovery
Discovered by	Michel Giacobini Ernst Zinner
Discovery date	20 December 1900 23 October 1913
Designations
MPC designation	P/1900 Y1 P/1913 U1
Alternative designations	1900 III; 1913 V; 1926 VI; 1933 III; 1940 I; 1946 V; 1959 VIII; 1966 I; 1972 VI; 1979 III; 1985 XIII; 1992 IX
Orbital characteristics[1][2]
Epoch	13 November 2017 (JD 2458070.5)
Observation arc	4,922 days (13.48 years)
Number of observations	1,993
Aphelion	5.987 AU
Perihelion	1.013 AU/2192480km (2031 aparition)
Semi-major axis	3.492 AU
Eccentricity	0.71047
Orbital period	6.549 years
Inclination	32.002°
Longitude of ascending node	195.40°
Argument of periapsis	172.81°
Last perihelion	25 March 2025[3]
Next perihelion	8 October 2031
TJupiter	2.465
Earth MOID	0.018 AU
Jupiter MOID	0.248 AU
Physical characteristics[2][4]
Dimensions	2.0 km (1.2 mi)
Synodic rotation period	7.39±0.01 hours
Comet total magnitude (M1)	13.2

Physical properties

The comet nucleus is estimated to be 2.0 km (1.2 mi) in diameter.^[2] During its apparitions, Giacobini–Zinner can reach about the 7-8th magnitude,^[5] 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 0.035 AU (5.2 million km).^[2]

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]

Exploration

See also: List of minor planets and comets visited by spacecraft

Giacobini–Zinner was the target of the International Cometary Explorer spacecraft, which passed through its plasma tail at a distance of 7,800 km (4,800 mi) on 11 September 1985, becoming the first comet ever visited in space exploration.^[7] Earlier in the same month the comet was observed by the Pioneer Venus Orbiter.^[8] 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.

2025 Perihelion

21P/Giacobini-Zinner most recently passed perihelion on 25 March 2025, making its closest approach to Earth 4 days before, on 21 March. It brightened to magnitude ~11.^[9]