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205P/Giacobini
	Comet 205P/Giacobini photographed from the Zwicky Transient Facility on 25 October 2021
Discovery
Discovered by	Michel Giacobini
Discovery site	Nice, France
Discovery date	4 September 1896
Designations
MPC designation	D/1896 R2; P/2008 R6
Alternative designations	1896d; 1896 V
Orbital characteristics
Epoch	19 February 2014 (JD 2456707.5)
Observation arc	112.05 years
Number of; observations	1,143
Aphelion	5.558 AU
Perihelion	1.536 AU
Semi-major axis	3.547 AU
Eccentricity	0.5669
Orbital period	6.681 years
Inclination	15.287°
Longitude of; ascending node	179.625°
Argument of; periapsis	154.257°
Last perihelion	13 January 2022
Next perihelion	12 September 2028
TJupiter	2.779
Earth MOID	0.543 AU
Jupiter MOID	0.373 AU
Physical characteristics
Comet total; magnitude (M1)	12.0–18.5
Comet nuclear; magnitude (M2)	15.7
Apparent magnitude	11.3; (1896 apparition)

205P/Giacobini is a periodic Jupiter-family comet with an orbital period of 6.68 years. It was discovered by Michel Giacobini on 4 September 1896 and then it was lost until it was recovered by Koichi Itagaki on 10 September 2008. The comet was then found to have fragmented into three pieces.^[1]

Observational history

The comet was discovered by Michel Giacobini on 4 September 1896. He described the comet as faint and with a coma about one arcminute across. Other observers confirmed the presence of the comet the next day, and estimated it to have an apparent magnitude of 11.3. The comet was then located in Serpens.^[1] The comet subsequently grew fainter and it was last observed on 5 January 1897 by William Hussey.^[1] The first elliptical orbit was calculated by Perrotin and Giacobini and indicated an orbital period of 6.65 years and perihelion date on October 28.80.^[1]

Starting from 26 September, Henri Perrotin observed an extremely faint companion, indicating that the comet had split. Other observers recorded a second condensation within the comet's coma in late September and early October using the Lick Observatory and an elongated nucleus on 10 October.^[1] Zdenek Sekanina estimated the fragment separated from the main nucleus in 24 April 1896, indicating a short-lived companion.^[8]

The comet was lost until Koichi Itagaki and Hiroshi Kaneda rediscovered it on 10 September 2008 during their patrol survey for supernovae. The comet then had an apparent magnitude of 13.5, a coma about 25 arcseconds across and a tail about 2 arcminutes long. The comet was initially linked with the comet 1896V by Maik Meyer and the link was confirmed by Nakano.^[9] A companion about 4 magnitudes fainter than the primary (nucleus B) was observed on 17 September and one more than 5 magnitudes fainter (nucleus C) was observed on 19 September, further away.^[10] Zdenek Sekanina estimated that nucleus B separated in 2006, about 700 days before perihelion, while nucleus C had separated in 1998, before the comet's previous perihelion.^[11] The two fragments weren't observed in the 2015 perihelion.^[12]

During the next perihelion a secondary nucleus was observed drifting away from the primary, starting 15 October 2021. It is unclear if the secondary is a new fragmentation event or the reactivation of a previously observed fragment.^[12]

References

^ ^a ^b ^c ^d ^e ^f G. W. Kronk. "D/1896 R2 (Giacobini)". Cometography.com. Retrieved 24 August 2024.
^ "Comet Names and Designations". International Comet Quarterly. Retrieved 29 December 2024.
^ ^a ^b S. Yoshida. "205P/Giacobini". www.aerith.net. Retrieved 17 August 2024.
^ "205P/Giacobini – JPL Small-Body Database Lookup". ssd.jpl.nasa.gov. Retrieved 20 July 2023.
^ "205P/Giacobini-A – JPL Small-Body Database Lookup". ssd.jpl.nasa.gov. Retrieved 8 January 2025.
^ "205P/Giacobini-B – JPL Small-Body Database Lookup". ssd.jpl.nasa.gov. Retrieved 8 January 2025.
^ "205P/Giacobini-C – JPL Small-Body Database Lookup". ssd.jpl.nasa.gov. Retrieved 8 January 2025.
^ Z. Sekanina (1979). "Relative motions of fragments of the split comets". Icarus. 38 (2): 300–316. doi:10.1016/0019-1035(79)90185-4.
^ D. W. Green (1 September 2008). "Comet P/2008 R6 (Giacobini)". International Astronomical Union Circular. 8975. ISSN 0081-0304.
^ D. W. Green (22 September 2008). "Comet 205P/Giacobini". International Astronomical Union Circular. 8978. ISSN 0081-0304.
^ D. W. Green (2 October 2008). "Comet 205P/Giacobini". International Astronomical Union Circular. 8987. ISSN 0081-0304.
^ ^a ^b Q. Ye; M. S. P. Kelley (2023). "Fragments of comet 205P/Giacobini in 2021". The Astronomer's Telegram. 16104: 1.