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Analysis of Comet Apparitions


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C/2024 S1 (ATLAS)


On Sep. 27, 2024 the ATLAS project discovered a comet in the western part of the constellation Hydra, estimating a total magnitude of 15.5 for the 30" coma; a tail could not be detected. During the following days comet C/2024 S1 (ATLAS) was observed by numerous observers and it was identified as a Kreutz comet that would pass perihelion on Oct. 28, 2024 at the solar distance of just 0.0075 AU. The comet was not discovered earlier because of the small elongations, which had been less than 50° since May 2024. Follow-up observations showed a conspicuous central condensation and a moderately condensed, 3' coma of total magnitude 11.5-12.5, as well as a 150" tail in p.a. 265°. Assuming a brightness development according to n=3, the comet would peak at magnitude 1.5 on the day of perihelion (CBET 5453). However, the absolute magnitude of the comet is well below the Bortle limit for this small solar distance, so there is a high probability that it will disintegrate on its way towards the Sun. If this should not happen it would be brighter than 16 mag until February 2025. During this period it moves through the constellations Hydra, Sextans, Corvus, Virgo (perihelion), Corvus, Hydra, Antlia, Vela, Puppis, Canis Major and Lepus. From mid-European locations it can only be seen until around Oct. 15 and then again from early November to early December. Starting in the second week of January 2025 it will reappear above the midnight southern horizon, reaching altitudes of 25° in the evening sky by the end of February. Determined from astrometric data the comet orbits the Sun with a period of about 850 years (CBET 5467).

Total Brightness and Coma Diameter (standard time scale)

Total Brightness and Coma Diameter (stretched time scale)

During the first few weeks the comet caused some excitement in the comet's communities, because it brightened above average. There were even rumors that it could be a second Tsuchinshan-ATLAS - just two weeks after this one passed perihelion. However, it soon became apparent that this comet was considerably smaller than Tsuchinshan-ATLAS. Images taken on Oct. 8 with the 1m LCO telescope in South Africa showed no central condensation in the inner coma, which also appeared elliptical. The researchers interpreted this as the first signs of the expected fragmentation (ATel 16857). Observers reported an erratic morphological behavior of the comet, with several outbursts, accompanied with a rapidly changing coma. Between Oct. 17.37 UT and 19.38 UT it brightened from 12.0 mag to 8.4 mag, associated with an increase of the coma diameter from 1.5' to 2.6' and of the tail length from 2.8' to 24'. Between Oct. 20.36 UT and 22.36 the comet faded from 8.2 mag to 10.1 mag; in parallel to this the coma diameter decreased from 2.2' to 0.9' and the tail length from 8.6' to 3.2' (CBET 5467). In fact, these outbursts proved to be harbingers of the fragmentation of the nucleus. A few hours before the comet passed perihelion it could be detected for the last time in the SOHO/LASCO images, exhibiting a state of total disintegration.

Brightness Variations

A total of 96 observations from 15 observers could be used for the analysis, including a dozen magnitudes determined from the SOHO/LASCO images. However, some of the latter had to be corrected, because the applied aperture was too large. This does not apply to the maximum brightness (2.8 mag at Oct. 28.25 UT), because a correct aperture was used for this. After reaching maximum brightness the comet faded very quickly. In the diagram above (based on the brightness parameters m0=12.6 mag / 2.5n=6.6) the erratic behavior is clearly visible, with two large outbursts on Oct. 17-19 and Oct. 27/28. The coma diameter was fairly constant at 2' (90,000 km). I only have estimates of the degree of condensation up to Oct. 19. These show an increase from DC 3-4 to DC 8 between Sep. 30 and Oct. 19. Terrestrial observers were able to observe the tail up to a length of 0.4° (0.9 mio. km). The SOHO/LASCO images showed the tail with a maximum length of 1.8° (5.5 mio. km), whose orientation changed from West towards Southwest.

Andreas Kammerer


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