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.

Comet C/2023 A3 (Tsuchinshan-ATLAS) could be seen with the unaided eye from the southern hemisphere at the end of September/start of October and from the northern hemisphere from Oct. 11 onwards for about two weeks. It thus became the brightest comet of 2024, which also had a beautiful tail. The only major downer for Central European observers was the weather, which was not particularly cooperative in the first few days after the comet emerged from the twilight. It took a bit of luck to catch the comet in a cloud gap at just the right time (with twilight as deep as possible and the comet still at a sufficient altitude). And thereafter the waxing moon increasingly brightened the sky background, so that the contrast between the comet and the background was often too low to see the comet clearly with the unaided eye. Eventually, after Oct. 20 the conditions developed in a favorable manner.

The comet developed very steadily in a positive manner until the end of April 2024 and was observed intensively from spring onwards. No wonder, as it was already easily recognizable even in medium-sized instruments at around 10 mag, showing a very small coma of high surface brightness. But then the brightness development stagnated, which led to extensive discussions in the comet forums in early summer, regarding the comet's fate. Opinions ranged from the view that the comet is already in the process of disintegration to the conviction that it is continuing to develop normally and that the standstill could be caused by a phase effect (due to the opposition), which could have led to a temporary brightening of the comet before opposition.

Based on 1195 observations from 102 observers (until the start of November 2024) the brightness development has to be divided into five periods (three pre- and two post-perihelion). Until Apr. 25, 2024 (r = 2.85 AU) the brightness increased slightly above-average. The second period (Apr. 25 to June 20) of 55 days length, was characterized by a constant heliocentric magnitude. After June 20, 2024 (r = 2.05 AU) the comet resumed its increase in heliocentric magnitude, however with a lower activity parameter compared to the first period and with an absolute magnitude 2.0 mag fainter).

During the first 20 days after perihelion (until Oct. 17, r = 0.64 AU) the predicted forward-scattering resulted in a significant brightening, with the magnitude increasingly deviating from the standard formula, which predicted a maximum brightness of 2.4 mag on Oct. 6. Actually the comet peaked at magnitude -3.2 on Oct. 9.4, according to CBET 5468! Thus the forward-scattering caused an additional brightening of 5.6 mag! However, the period of maximum forward-scattering and of maximum brightness could not be followed by terrestrial observers, but instead only via the SOHO probe. Terrestrial observers could observe the comet until Oct. 5, 18UT (0.5 mag) and again starting on Oct. 11, 18UT (-1.0 mag). Since Oct. 17 the comet shows a steady decrease of activity. Below are the appropriate formulae:

The pre-perihelion development is very similar to the behavior of comet Hale-Bopp in autumn 1996, which showed almost a standstill in activity for a similarly long period (60 days) - albeit at a greater solar distance (r = 3.65 AU to r = 3.00 AU) - and thereafter an almost identical activity parameter compared to the first phase, but with a fainter absolute magnitude. It was therefore unlikely that the temporary standstill of comet C/2023 A3 (Tsuchinshan-ATLAS) hinted towards a disintegration. Thus I predicted a maximum brightness of 2.4 mag ± 0.3 mag on Oct. 6, 2024 (neglecting forward-scattering!).

Development of the heliocentric magnitude (pre-perihelion)

J. N. Marcus made the following prediction regarding a probable significant forward-scattering for this dust-rich comet in the days around the smallest phase angle (Oct. 9.4 UT), assuming a gas-to-dust ratio of 10. He predicted the associated increase in brightness as follows: 2024 Sep. 29.0 UT: -0.3 mag, Oct. 2.0 UT: -1.1 mag, 5.0 UT: -2.5 mag, 6.0 UT: -3.2 mag; 7.0 UT: -4.1 mag, 8.0 UT: -5.3 mag, 9.0 UT: -6.7 mag, 9.4 UT: -6.9 mag, 10.0 UT: -6.5 mag, 11.0 UT: -5.1 mag, 12.0 UT: -3.9 mag, 13.0 UT: -3.0 mag, 14.0 UT: -2.3 mag, 15.0 UT: -1.7 mag, 18.0 UT: -0.7 mag, 21.0 UT: -0.2 mag. Assuming that the comet continues to brighten beyond perihelion according to the brightness parameters m₀=6.0 mag / n=3.2 it should be brighter than 0 mag between Oct. 5 and 13, brighter than -2 mag between Oct. 7 and 11, brighter than -3 mag between Oct. 8 and 10 and brighter than -4 mag on Oct. 9.4 UT (CBET 5445). Actually the brightness increase associated with forward-scattering reached a maximum of 5.6 mag! In the diagram "Total Brightness and Coma Diameter", the development of the brightness according to the fourth formula for the period t=0d to t=+20d is shown as a dashed line to visualize the influence of the forward scattering.

Brightness variations

Concerning the maximum brightness, M. Paradowski determined a maximum brightness of -4.9 mag on Oct. 9.75 UT from the SOHO/LASCO C3 images (in which the comet made an impressive appearance), although he used an aperture of 37'. The CBAT then asked the SOHO-satellite comet specialists for their assessment of the comet's peak brightness in the LASCO C3 coronagraph images. Q.-c. Zhang (Lowell Observatory) derived a maximum brightness of -2.9 mag for an aperture of 5' and of -3.8 mag for an aperture of 15' on Oct. 9.40 UT. A deeper analysis showed that the aperture may only be 7-10' to prevent that parts of the tail are taken into account. This results in an actual maximum brightness of -3.2 mag. This is confirmed by the fact that (with the exception of a small number of unverified sightings) it was not possible to find the comet in the daytime sky. C. Schur (Payson, AZ, USA) was only able to document the comet on Oct. 10.71 UT (with the comet 6.5° from the sun) on images taken with a 25.4cm telescope from an observation site 1,680m above sea level, applying extensive image processing. Claimed sightings with the unaided eye or binoculars are therefore not plausible (CBET 5468). The diagrams "Brightness variations" and "Total Brightness and Coma Diameter" contain the SOHO/LASCO estimates, although the published brightnesses have been corrected according to the analysis above.

Total Brightness and Coma Diameter (standard time scale)

Total Brightness and Coma Diameter (compressed time scale)

Development of the Dust Tail

CCD observers were able to detect the dust tail already in summer 2023, while visual observers had to wait until spring 2024 to detect the tail, which then exhibited a surprisingly high surface brightness for several weeks. In May 2024 the visual tail length reached 10' and by early August it had grown to 20'. After the solar conjunction in mid-September a visual tail length of 20' was initially recorded, although this may have been influenced by the unfavorable observing circumstances during those days. Thereafter the visual estimates increased very quickly: Sep. 26: 5°, Sep. 29: 10°, Oct. 1: 15°, Oct. 2: 20°. The maximum of 25° was reached on Oct. 5. On Oct. 10 Michael Mattiazzo (Australia) was able to record the extensions of the dust tail on a deep image up to a distance of 45° from the nucleus! After reappearing in the evening sky only a significantly shorter tail could be detected at first - partly due to the significant twilight during those days - but quickly the reported visual tail lengths increased: Oct. 12: 5°, Oct. 13: 10°, Oct. 15: 15°, Oct. 16: 20°. Thereafter the tail began to shorten: Oct. 20: 15°, Oct. 22: 10°, Oct. 27: 5° and Nov. 5: 1.5°. To derive the absolute lengths of the dust tail is only approximately possible due to the fact that it was significantly curved over a long time. Assuming a linear, anti-solar tail the following absolute lengths result (but the true tail length was actually longer): until March 2024: 1 mio. km, first maximum in mid-April: 5 mio. km, thereafter the tail shortened to 2.5 mio. km by mid-June, early August: 6 mio. km, mid-September: 2 mio. km (probably partly influenced by the poor conditions), Sep. 26: 10 mio. km, Sep. 29: 20 mio. km, Oct. 1: 25 mio. km, Oct. 2: 35 mio. km, Oct. 5: 45 mio. km. After the reappearance on the evening sky: Oct. 12: 10 mio. km, Oct. 13: 15 mio. km, Oct. 15: 30 mio. km, Oct. 16: 30 mio. km, Oct. 20: 25 mio. km, Oct. 22: 15 mio. km, Oct. 27: 10 mio. km, Nov. 5: 6 mio. km.

Between the beginning of July and the beginning of November 2024 a temporarily highly dynamic gas tail appeared, which overlapped with the dust tail for most of the time and was therefore not particularly noticeable in many photos and practically indistinguishable visually. While the gas tail was significantly longer than the dust tail before the solar conjunction (at most twice as long), it was always shorter than the dust tail after the solar conjunction.

On Oct. 15, 2024 Earth crossed the comet's orbital plane. The appearance of a possibly very bright anti-tail was predicted around this date. An analysis of the images in the archive of the German Comet Section shows the anti-tail between Oct. 13 (when twilight first permitted to document it) and Oct. 28. Between Oct. 13 and 21 it was very narrow (narrowest between Oct. 13 and 16), directed nearly perfectly towards Sun. Thereafter it became broader, more diffuse and deviated more and more from the direction to the Sun. Although the anti-tail is conspicuous in the photos, it does not appear to have been observed visually, i.e. it was noticeably fainter than the anti-tail of comet Arend-Roland. It is difficult to determine its maximum length because it stretches beyond the edge of numerous images (although sometimes just). According to my measurements it was longest on Oct. 15 at 4°, whereby this length did not deviate by more than 0.5° between Oct. 14 and 19.

In summer 2023 the tail was directed towards East (p.a. ≈ 100°). Between December 2023 and the end of March 2024 it pointed towards Northwest (p.a. ≈ 310°), but quickly turned over North towards East (p.a. ≈ 100°) by the beginning of May. This orientation remained constant until the beginning of August. Between mid-September and early October the orientation changed only from p.a. = 235° to p.a. = 260° (approximately Southwest). Since mid-October the tail is directed rather constantly towards p.a. = 75°.

Andreas Kammerer

FGK observations