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On May 21, 2017 the PanSTARRS team discovered a comet of magnitude 21.0 with a miniscule coma in the constellation Draco. Follow-up observations of comet C/2017 K2 (PANSTARRS) showed a moderately condensed 12" coma of total magnitude 19.0 (CBET 4393 / MPEC 2017-N26). The comet was detected in the record distance of 16.1 AU, indicating an absolute brightness of 1 mag and an absolute coma diameter of 140.000 km. The comet will reach perihelion at a distance of 1.80 AU not prior to Dec. 19, 2022. It should be brighter than 16.0 mag from early 2020 to the end of 2025. During this interval it will move from the northern parts of Hercules through Ophiuchus and Scorpius, diving far south between October 2022 and March 2023, then reappearing for mid-European observers in Eridanus, thereafter moving through Lepus, Orion and Taurus into Auriga. Its maximum brightness of about 5.5 mag should be reached around the turn of the years 2022/23. During the most interesting months it will be observable from mid-European locations in the evening sky until the end of August 2022 (then of magnitude 6.0), then from early September 2023 (then of magnitude 9.5) onwards in the morning sky.
At the start of the apparition (spring 2019) the comet was of magnitude 17.0. On Oct. 20, 2020 the comet was at a solar distance of 8 AU, but already of magnitude 15.0. It brightened to magnitude 12.5 until the opening of 2022 and to magnitude 10.5 in early May 2022. In summer 2022 the brightening of the comet slowed – the brightness increased from magnitude 10.0 to 8.6.
At the end of October 2023 1124 observations from 67 observers can be used for the analysis. These show a significantly smaller activity parameter pre-perihelion than post-perihelion. This is surprising for a long-period comet, probably indicating a comet which had passed perihelion for several times, thereby exhausting its reservoir of lighter gases. The appropriate formulae arepre-perihelion: m = 4.8 mag + 5×log D + 6.2×log r post-perihelion: m = 4.0 mag + 5×log D + 9.6×log r
Between August and October 2022 the comet showed a constant brightness around magnitude 8.6 (maximum), because the comet receded temporarily from Earth. Its brightness peaked at magnitude 8.3 during the first half of January 2023. In mid-April 2023 it had already faded to 10.0 mag, at the start of June to 11.0 mag and at the end of October to 12.5 mag.
Total Brightness and Coma DiameterAt the start of the apparition the comet showed an apparent coma diameter of 0.5', in October 2020 of 0.7'. Until the opening of 2022 it had increrased to 1.5'. During the next months it increased further, peaking at 6.5' at the end of June 2022 (holding this value for one month). Thereafter it began to shrink, measuring 4' in mid-August, then holding this value until mid-November. In mid-January 2023 the maximum apparent coma diameter of 6.5' was reached, decreasing to 2.5' until the start of June and to 1.1' at the end of October. The absolute coma diameter measured 275,000 km until April 2021, thereafter expanded, measuring 350,000 km in August 2021. This value was hold until April 2022. Thereafter the coma diameter began to increase again, measuring 500,000 km in July 2022. During the following weeks it decreased to 400,000 km until mid-August. In January 2023 the absolute coma diameter peaked at 625,000 km, thereafter decreasing to 350,000 km until the start of June and to 200,000 km until the end of October.
Until the turn of 2021/22 the coma was medium-condensed with the degree of condensation constant at DC 3-4. During 2022 it increased to the maximum of DC 5 in August/September. Thereafter it decreased slowly to DC 4 until mid-April 2023 and to DC 3 until the end of October.
Since start of 2021 a tail is observed, which reached an apparent length of 5' (6 Mio. km) in winter 2021/22 and of constantly 20' (8 Mio. km) in summer 2022. In October 2022 the observers measured a length of only 12' (3.5 Mio. km). However, at this time the comet approached the horizon even for observers in the Southern hemisphere. Thus it is possible that they could not discern the whole length. During the first four months of 2023 the number of published tail estimates were small, indicating a length of about 25' (7-8 Mio. km). In October a length of 8' (3 Mio. km) was reported.
During summer 2022 the tail was oriented towards NNW at first, slowly rotating towards NNE. The anti-solar direction changed from SE towards NE during the same period. Thus the dust tail was inclined towards the anti-solar direction by more than 45° before November 2021 and again starting in March 2022, and by more than 90° between May and July 2022, hinting towards massive dust particles in the tail. During the first four months of 2023 the tail's orientation turned from North over West to SSW, whereas the anti-solar direction changed from South towards Southwest.