Analysis of Comet Apparitions

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C/2023 A3 (Tsuchinshan-ATLAS)

The "Asteroid Terrestrial-Impact Last Alert System" (ATLAS) team discovered an 18th magnitude asteroidal object on Feb. 22, 2023 near the border of the constellations Serpens/Libra/Virgo. Subsequently, Minor Planet Center staff identified three positions of a Jan. 9, 2023 object assumed lost, which had been transmitted from China's Purple Mountain Observatory, with the asteroidal object. Since comets discovered by this observatory are designated Tsuchinshan by Chinese astronomers, this comet was given the designation C/2023 A3 (Tsuchinshan-ATLAS). Additional observations of the comet in the last days of February showed a strongly condensed 6" coma of total magnitude 18.0, but no tail. The comet will come quite close to both the Sun and Earth in the fall of 2024. It was at the large solar distance of 7.3 AU at the end of February 2023. Thus, it is a very interesting comet, but its activity level is currently very uncertain. In the following a new comet is assumed, whichs brightness increases with n=3. The comet will pass perihelion on Sep. 28, 2024 at the distance of 0.39 AU and could then reach 3rd magnitude (CBET 5228). Thereafter it will pass Earth on Oct. 13 at a distance of 0.47 AU, but will be near the Sun-Earth line und thus close to the Sun on the sky. Between November 2023 and September 2025 the comet should be brighter than 16 mag. During this period it will move through the constellations Libra, Virgo, Leo, Sextant, Leo/Crater (perihelion), Virgo (perigee), Serpens Caput, Ophiuchus, Serpens Cauda, Aquila, Delphinus, Vulpecula, Lyra and Hercules. For central European locations the first observing window will last from early December 2023 (15-16 mag, morning sky) to mid-June 2024 (11 mag, evening sky). The second, very unfavorable observation window lasts from about Sep. 25 to Oct. 5, 2024 (3-4 mag), but during this time the comet can only be seen extremely low above the eastern morning horizon during twilight. The third observation window is the most interesting. The comet will emerge above the western evening horizon on Oct. 12 (3-4 mag), reaching altitudes near 40° by early November, but fading to 6-7 mag during these two weeks. According to my empirical formulae, it should show a coma diameter of 20' during the days of greatest approach to Earth and a tail up to 5° long, which probably will be rather faint, thus difficult to observe visually. In the mid-January (11-12 mag) the comet disappears again above the western evening horizon. Exactly at this time it appears in the morning sky and finally reaches maximum elevations of 70° in June (14-15 mag). During the next weeks it will change to the night and finally to the evening sky, where it should become fainter than 16 mag. Overall an interesting, but probably not a spectacular object.

Until end of April 2024 the brightness developed in a very positive manner, with the comet being quite intensively observed in spring 2024. No wonder, considering the current brightness of about 10.0 mag and the very small coma, showing a high surface brightness. Thus it can already be recognized easily even with medium-sized instruments. Of course the most exciting question about the comet is the one concerning its maximum brightness. And precisely this question is currently difficult to answer. Based on 384 observations by 64 observers the comet showed a very steady development until the last week of April, which could be described very well with the formula

If the comet would follow this development until perihelion, it could reach a maximum brightness of 0.0 ± 0.2 mag around Oct. 5, 2024!

However, the apparent brightness remained at 10.0-10.5 mag during the first half of May. The reason for this significant deviation from the previous steady development could be of temporary nature, such as with comet Hale-Bopp in the late summer of 1996. In this case the comet could resume activity as before after a certain time, with a fainter absolute magnitude, i.e. the comet would be (slightly) fainter than 0 mag at maximum. Another scenario would be a break in the brightness development. In this case the comet would subsequently show a significantly lower activity and the maximum brightness could be considerably fainter than the mentioned 0 mag (as was the case with comet Austin 1990, for example). It should be possible to determine the actual cause during the next about four weeks.

Until the end of 2023 the apparent coma diameter measured less than 0.5' (about 100,000 km), but thereafter it increased, reaching 2.2' (180,000 km) in mid-May 2024. The degree of condensation was estimated at DC 8 in spring/summer 2023, but has remained constant at DC 6 since the end of 2023, with the coma showing a dominant false nucleus in spring 2024.

First tail sightings were reported in summer 2023. Since the end of 2023 a dust tail has been observed continuously, which has a surprisingly high surface brightness and could easily be recognised visually since at least the second half of April 2024. In mid-May it reached a length of just less than 10' (3.5 mio. km). The tail was pointing towards East in summer 2023 and towards NNW at the end of 2023. Between Mar. 29 and Apr. 8 it turned rapidly towards NE, thereafter continued to turn more slowly towards ESE until mid-May.

Andreas Kammerer

FGK observations