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C/2023 H2 (Lemmon)
	The comet on 13 November 2023
Discovery
Discovered by	Mount Lemmon Survey
Discovery date	23 April 2023
Orbital characteristics
Epoch	7 October 2023
Observation arc	292 days
Earliest precovery date	26 March 2023
Aphelion	480.2 AU
Perihelion	0.894 AU
Semi-major axis	240.6 AU
Eccentricity	0.9963
Orbital period	3,731 years
Inclination	113.75°
Longitude of; ascending node	217.04°
Argument of; periapsis	150.65°
Last perihelion	29 October 2023
Earth MOID	0.036 AU
Comet total; magnitude (M1)	16.1

C/2023 H2 (Lemmon) is a non-periodic comet discovered by the Mount Lemmon Survey in images obtained on 23 April 2023.^[1] It passed perihelion on 29 October 2023 and on 10 November 2023 approached Earth to a distance of 0.19 AU (28 million km) brightening to an apparent magnitude of 6.

The comet was discovered as an asteroidal object with an apparent magnitude of about 21 by the Mount Lemmon Survey in images obtained on 23 April 2023. Consequently, precovery images by PanSTARRS dating from 26 March 2023. Cometary activity was observed on 14 May, when the comet had a coma about 12 arcseconds across.^[1]^[3] The comet reached perihelion on 29 October 2023 and on 10 November 2023 approached Earth to a distance of 0.19 AU (28 million km). On that day the comet moved nine degrees across the sky.^[4] The comet brightened far above the expectations up to an apparent magnitude of 6, showing also a weak ion tail.^[5]

The close encounter with Earth provided the rare opportunity to measure the composition of an otherwise faint comet. The water production rate was estimated to be (59±7)×10²⁶ mol/s, which is unusually low compared to that measured in other Oort Cloud comets at similar heliocentric distances. The comet had also low production rate for hydroxyl radical and low dust signal.^[5] However, C/2023 H2 displays very high mixing ratio with respect to water of carbon monoxide (16.3 ± 0.6%), as well as high abundances of ethane, methane, hydrogen cyanide, and methanol. The hyper-volatile species CO, C2H6, and CH4 probably play an important role in the activity of this comet.^[5]

References

^ ^a ^b ^c Chastel, S.; Fairlamb, J.; Huber, M.; Ramanjooloo, Y.; Wainscoat, R.; Weryk, R.; Chastel, S.; Fairlamb, J.; Huber, M.; Ramanjooloo, Y.; Wainscoat, R.; Weryk, R.; Christensen, E. J.; Fay, D.; Fazekas, J. B.; Fuls, D. C.; Gibbs, A. R.; Grauer, A. D.; Groeller, H.; Hogan, J. K.; Kowalski, R. A.; Larson, S. M.; Leonard, G. J.; Rankin, D.; Seaman, R. L.; Serrano, A.; Shelly, F. C.; Wierzchos, K. W.; Christensen, E. J.; Fay, D.; Fazekas, J. B.; Fuls, D. C.; Gibbs, A. R.; Grauer, A. D.; Groeller, H.; Hogan, J. K.; Kowalski, R. A.; Larson, S. M.; Leonard, G. J.; Rankin, D.; Seaman, R. L.; Serrano, A.; Shelly, F. C.; Wierzchos, K. W.; Linder, T.; Holmes, R.; Horn, L.; Maikner, J.; Balam, D. D.; Green, D. W. E.; Sato, H.; Pittichova, J.; Korlevic, K.; Lejoly, C.; Jaeger, M.; Masek, M.; Kugel, F.; Aletti, A.; Bellini, F.; Hug, G.; Koch, B.; Sicoli, P. (1 May 2023). "COMET C/2023 H2 (Lemmon)". Minor Planet Electronic Circulars. 2023-K122. doi:10.48377/MPEC/2023-K122.
^ "Small-Body Database Lookup:C/2023 H2 (Lemmon)". ssd.jpl.nasa.gov. Retrieved 29 July 2023.
^ Green, Daniel (24 May 2023). "COMET C/2023 H2 (Lemmon)". Central Bureau for Astronomical Telegrams. 5264: 1. Retrieved 7 September 2024.
^ Dickinson, David (2 November 2023). "Comet H2 Lemmon Brightens in Early November Ahead of Expectations". Universe Today. Retrieved 7 September 2024.
^ ^a ^b ^c Lippi, M.; Ferellec, L.; Opitom, C.; Faggi, S.; Mumma, M. J.; Villanueva, G. L. (September 2024). "The chemical composition of CO-rich comet C/2023 H2 (Lemmon)". Astronomy & Astrophysics. 689: A77. doi:10.1051/0004-6361/202450634.