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On June 10, 2019 the "Asteroid Terrestrial-Impact Last Alert System" (ATLAS)-Team discovered an asteroidal object of magnitude 19 near the border of the constellations Lacerta/Andromeda. Follow-up observations showed a strongly condensed 8" coma of magnitude 18.5, but no tail. Comet C/2019 L3 (ATLAS) will pass perihelion at the solar distance of 3.55 AU not before Jan. 9, 2022 (CBET 4644). Assuming an activity factor of n=4 it will peak at magnitude 12, assuming n=3 at magnitude 13. It should be brighter than 16 mag between fall 2020 and spring 2023. During this interval it will move through the constellations Cassiopeia, Perseus, Auriga, Gemini (perihelion), Canis Minor, Hydra, Antlia and Pyxis. It should be observable from mid-European locations until the opening of May 2022 during the whole night (at first) and the evening sky (at the end) and between the mid-October and the start of December 2022 in the morning sky.
Pre-perihelion the comet showed a very high level of activity, regarding the great solar distance of about 3.5 AU. It reappeared in the morning sky in mid-October 2022 as an object of about 11.0 mag – about 1.5 mag brighter compared to the pre-perihelion development. Thus the comet fades very slowly and will be much longer visible than expected, regarding the pre-perihelion development. Based on 1006 observations from 68 observers (until the start of July 2023) the brightness development can be represented quite well with the formulaepre-perihelion: m = -4.3 mag + 5×log D + 21.5×log r post-perihelion: m = +1.8 mag + 5×log D + 9.5×log r
In consequence the comet peaked at magnitude 9.1 in mid-January 2022. Until the opening of April 2023 it had only faded to 12.0 mag.Total Brightness and Coma Diameter
Until the start of 2021 the apparent coma diameter was constant at 0.5', thereafter increasing to short of 1.0' in July and to the maximum of 4.5' at the opening of November 2021. In November/December 2021 the coma diameter was constant, but began decreasing slowly at the turn of 2021/22, measuring 4' in February, 2.5' in May, 2.0' in October 2022, 2.0' at the start of 2023 and 1.7' in April 2023. The absolute coma diameter measured constant 125,000 until the start of 2021, increased to short of 175,000 km until July and reached the maximum of 675,000 km in mid-October 2021. Thereafter it shrunk slowly, measuring 470,000 km at the start of February 2022 and 425,000 km between the opening of May and the start of 2023. Thereafter it shrunk, measuring 350,000 km in April 2023.
The degree of condensation was constant at DC 4 until September 2021, followed by an increase to DC 5-6 until perihelion. Thereafter it decreased slowly, reaching DC 4 in May 2022. This value was hold for several months. At the start of 2023 DC 3 was reported.
CCD-observers could document a tail between September 2021 and December 2022 (visual observers between November 2021 and April 2022). Around perihelion the tail reached a maximum length of 15' (12 Mill. km). I fall 2022 a length of 6' (8 mio. km) was observed. Until January 2022 it pointed towards WNW, thereafter turning to North until May, to WNW in fall 2022 and to NW at the start of 2023.