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C/2017 K2 (PanSTARRS)
	Time-lapse with an 8" Celestron telescope on July 14, 2022 - the night of its closest approach to Earth. Below the comet is the Globular Cluster Messier 10.
Discovery
Discovery date	21 May 2017
Orbital characteristics
Epoch	2022-12-07 (2459920.5)
Observation arc	9.46 years
Number of; observations	477
Orbit type	Oort cloud
Aphelion	~50000 AU (inbound); ~1400 AU (outbound)
Perihelion	1.7969 AU
Eccentricity	~0.99992 (inbound); ~0.9975 (outbound)
Orbital period	Millions of years inbound; ~19,000 years (outbound)
Inclination	87.563°
Longitude of; ascending node	88.26730
Last perihelion	19 December 2022
Earth MOID	1.10 AU (165 million km)
Jupiter MOID	1.29 AU
Physical characteristics
Dimensions	Rn between 14 km (8.7 mi) and 80 km (50 mi); maximum Dn ~18 km (11 mi)

C/2017 K2 (PanSTARRS) is an Oort cloud comet with an inbound hyperbolic orbit, discovered in May 2017 at a distance beyond the orbit of Saturn when it was 16 AU (2.4 billion km) from the Sun. Precovery images from 2013 were located by July.^[7] It had been in the constellation of Draco from July 2007 until August 2020. As of June 2022^[update], the 3-sigma uncertainty in the current distance of the comet from the Sun is ±6000 km.^[8]

The comet is record breaking because it was already becoming active at such a distance. Only Comet Hale–Bopp produced such a show from that distance with a similar nucleus. However, this comet will not be as visible as Hale–Bopp was in 1997 in part because it does not come nearly as close to the Sun.^[b] Astronomers had never seen an active inbound comet this far out, where sunlight is 1/225th its brightness as seen from Earth. Temperatures, correspondingly, are at -440 °F (-262 °C) in the Oort cloud. However, as it was approaching the Sun at a distance of 16 AU at discovery, a mix of ancient ices on the surface containing oxygen, nitrogen, carbon dioxide and carbon monoxide began to sublimate and shed the dust frozen into it. This material expands into a vast 130,000 km (81,000 mi) wide halo of dust, called a coma, enveloping the solid nucleus.^[9] Outgasing of carbon monoxide was detected when the comet was 6.72 AU from the Sun.^[10]

Research with the Canada–France–Hawaii Telescope (CFHT) infers the comet nucleus to have a radius between 14–80 km (9–50 mi), so there is a chance the nucleus could be as large as C/1995 O1 (Hale-Bopp).^[5] However, research with the Hubble Space Telescope (HST) estimates the nucleus to have a circular equivalent diameter of less than 18 km (11 mi).^[6] On 17 September 2020, morphological studies of the inner coma, observed on 12 September 2020, were reported, noting that two jet-streamed structure were emitted from the nucleus and, as well, that the length of the tail was about 800,000 km (500,000 mi) long.^[11]

The comet was within 5 AU (750 million km) of Earth by 11 January 2022.^[12] Around 6 July 2022, the comet crossed the celestial equator, and then on 14 July 2022, it passed 1.8 AU (270 million km) from Earth^[13] and shone around 9.0 magnitude making it a decent binoculars object.^[14]^[4] It reached perihelion on 19 December 2022, close to the orbit of Mars, and was not visible to naked eye at 8.0 magnitude.^[3]^[4]^[a]

On 27 July 2021, further detailed observations of the comet were reported on The Astronomer's Telegram.^[15]^[16]

JPL Horizons models that C/2017 K2 took millions of years to come from the Oort cloud at a distance of roughly 50,000 AU (0.8 ly).^[2] The heliocentric orbital eccentricity drops below 1 in December 2023.^[17] The outbound orbital period will be around 19000 years with aphelion being around 1400 AU.^[2] There was a dispute whether that was the first time the comet entered the inner solar system, but its orbit suggests that the comet isn't dynamically new and there is a 29% chance that the comet is of interstellar origin and was captured by the solar system within the last 3 million years.^[18]

Gallery

June 2017 by Hubble's Wide Field Camera 3^[19]
11" fast Astrograph on 2022-07-16 while near Messier 10
Path of C/2017 K2 in the sky

References

^ "MPEC 2017-K35 : COMET C/2017 K2 (PANSTARRS)". IAU Minor Planet Center. 2017-05-24. Retrieved 2017-10-21. (CK17K020) (2.6 day observation arc with perihelion estimated to be in 2027)
^ ^a ^b ^c ^d ^e ^f ^g Horizons output. "Barycentric Osculating Orbital Elements for Comet C/2017 K2 (PANSTARRS)". Solution using the Solar System Barycenter. Ephemeris Type:Elements and Center:@0 (To be outside planetary region, inbound epoch 1950 and outbound epoch 2050. For epoch 2050-Jan-01 orbit period is "PR= 6.58E+06 / 365.25 days" = ~18,000 years)
^ ^a ^b ^c "C/2017 K2 (PANSTARRS) Orbit". Minor Planet Center. Archived from the original on 2017-11-13. Retrieved 2017-11-13.
^ ^a ^b ^c Seiichi Yoshida. "C/2017 K2 ( PanSTARRS )". Seiichi Yoshida's Comet Catalog. Retrieved 2020-06-02.
^ ^a ^b Meech, Karen (2017). "CO-Driven Activity in Comet C/2017 K2 (PANSTARRS)". The Astrophysical Journal Letters. 849 (1): L8. arXiv:1710.03876. Bibcode:2017ApJ...849L...8M. doi:10.3847/2041-8213/aa921f. S2CID 119214098.
^ ^a ^b Jewitt, David (2017). "A Comet Active Beyond the Crystallization Zone". The Astrophysical Journal. 847 (2): L19. arXiv:1709.10079. Bibcode:2017ApJ...847L..19J. doi:10.3847/2041-8213/aa88b4. S2CID 119347880.
^ "MPEC 2017-N26 : COMET C/2017 K2 (PANSTARRS)". IAU Minor Planet Center. 2017-07-01. Retrieved 2017-10-21.
^ Horizons output. "Current distance from the Sun with 3-sigma uncertainty in km".
^ Byrd, Deborah (2 October 2017). "Farthest active inbound comet yet". Earth & Sky. Retrieved 19 September 2020.
^ Yang, Bin; Jewitt, David; Zhao, Yuhui; Jiang, Xuejian; Ye, Quanzhi; Chen, Ying-Tung (10 June 2021). "Discovery of Carbon Monoxide in Distant Comet C/2017 K2 (PANSTARRS)". The Astrophysical Journal Letters. 914 (1): L17. arXiv:2105.10986. Bibcode:2021ApJ...914L..17Y. doi:10.3847/2041-8213/ac03b7. S2CID 235166808.
^ Manzini, Federico; et al. (17 September 2020). "ATel #14026: Morphological structures in the inner coma of comet C/2017 K2 (PanStarrs)". The Astronomer's Telegram. Retrieved 19 September 2020.
^ "5au from Earth". JPL Horizons. Retrieved 2022-01-07.
^ "Closest Approach to Earth in July 2022 (3 hour interval)" (Closest Earth approach occurs when deldot flips from negative to positive). JPL Horizons. Archived from the original on 2022-06-22. Retrieved 2022-06-21.
^ Hubble Spots Farthest-Ever Incoming Active Comet September 29, 2017
^ Manzini, Federico; et al. (27 July 2021). "ATel #14813: Comet C/2017 K2 (PanStarrs) shows jet-shaped dust emissions modulated by extremely slow rotation at greater-than-usual distance from the Sun". The Astronomer's Telegram. Retrieved 28 July 2021.
^ Manzini, Federico; et al. (27 July 2021). "ATel #14814: Erratum to ATEL #14813: Comet C/2017 K2 (PanStarrs) shows jet-shaped dust emissions modulated by extremely slow rotation at greater-than-usual distance from the Sun". The Astronomer's Telegram. Retrieved 28 July 2021.
^ Horizons output. "Heliocentric Osculating Orbital Elements for Comet C/2017 K2 (PANSTARRS) dropping below 1.0".
^ Marcos, Raúl de la Fuente; Marcos, Carlos de la Fuente (24 April 2018). "Comet C/2017 K2 (PANSTARRS): Dynamically Old or New?". Research Notes of the AAS. 2 (2): 10. arXiv:1804.07292. Bibcode:2018RNAAS...2...10D. doi:10.3847/2515-5172/aabf8f. S2CID 118938922.
^ "Distant active comet C/2017 K2". www.spacetelescope.org. Retrieved 3 October 2017.

Notes

^ ^a ^b Perihelion passage: Upon discovery perihelion was still 5 years away. An epoch of 2022 gives a more accurate perihelion date that properly accounts for planetary perturbations. An epoch 2017 unperturbed two-body solution (Sun+comet) gives a date of 2022-Dec-21. Integrating the orbit with JPL Horizons which accounts for planetary perturbations gives a date of 2022-Dec-19. The Minor Planet Center's Epoch Dec 2022 solution also gives a date of 2022 Dec. 19
^ Comet Hale–Bopp came to perihelion (inside the orbit of Earth) at 0.9 AU whereas C/2017 K2 only comes to perihelion (outside the orbit of Mars) at 1.79 AU.