Home=Current Comets: C/2024 G3
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An asteroidal object of magnitude 19, discovered by the ATLAS team on Apr. 5, 2024 near the borders of the constellations Apus/Pavo/Octans showed a cometary morphology in the course of detailed observations. Comet C/2024 G3 (ATLAS) showed a significantly condensed coma of diameter approximately 5" and of total magnitude 18.5 and a 7" tail in p.a. 300°. It will pass perihelion at the solar distance of only 0.09 AU on Jan. 13, 2025, when it should peak at magnitude 2 if it follows a standard development (CBET 5384). However, its absolute magnitude appears to be well below the Bortle limit, meaning that it could possibly disintegrate while approaching the Sun. In addition, the comet will stay north of the ecliptic plane for only seven days, as its parabola is oriented almost perpendicular to the ecliptic and it approaches the Sun from the South and will disappear towards the South thereafter. In the case it will not disintegrate the comet should be brighter than 16 mag between mid-August 2024 and mid-June 2025. During this period it will move through the constellations Centaurus, Lupus, Scorpius, Sagittarius (perihelion), Capricornus, Piscis Austrinus, Grus, Phoenix, Eridanus and Horologium. The comet cannot be observed from Central European locations during this period.
The comet reached perihelion and became a very bright comet! However, it only just managed to get extremely close to the sun. Then it rapidly disintegrated before the eyes of the observers after it emerged from the sun's rays. Apart from the very first days after perihelion passage the comet presented itself as another example of a headless comet with a conspicuous tail. On the basis of only 166 observations by 32 observers the brightness development can be well represented with the following formulae (whereas it is noteworthy that the brightness faded very steadily even after the disintegration of the nucleus according to the second formula):
pre-perihelion : m = 7.1 mag + 5×log D + 10.9×log r post-perihelion: m = 5.8 mag + 5×log D + 9.4×log rThe peak brightness, which was reached on perihelion date, can only be determined to be approximately -3.0 mag to -3.5 mag, which is equal to that of comet C/2023 A3 (Tsuchinshan-ATLAS). This estimate is primarily based on published magnitudes that were determined from SOHO images, resulting in a peak brightness of -3.8 mag. As with comet C/2023 A3, however, too large an aperture (38') was used to determine these SOHO-magnitudes. Thus it is probable, that the SOHO-magnitudes included parts of the bright tail (as for C/2023 A3). According to the analysis of comet C/2023 A3 (CBET 5468) this would make it necessary to reduce the determined SOHO-magnitudes by as much as 1-1.5 mag. However, the correct aperture and a possible forward-scattering for C/2024 G3 was not determined. In addition, the published SOHO-magnitudes "fit" rather well the brightness development determined above, both pre- and post-perihelion. I therefore did not make any correction to the published SOHO-magnitudes.
To estimate the effect of possible forward scattering I have plotted the difference between the published brightness estimates and the brightness derived via the formulae above over time. A significant forward scattering is not apparent in the diagram. However, it shows that the brightness increased above average during the first weeks pre-perihelion and average during the following weeks. Starting on Jan. 2 the comet brightens rapidly, reaching an amplitude of 1.5 mag on Jan. 4. This outburst was immediately followed by a dip, which reached an amplitude of 1.2 mag on Jan. 12. This short-term outburst could have been caused by the initial break-up of the nucleus, with its complete disintegration not taking place until shortly after perihelion passage. Post-perihelion the brightness estimates follow the post-perihelion formula. Again, it is surprising that the comet did not fade in a very rapid manner after disintegration, but instead the activity only gradually decreased (estimates during the last days of February show the comet still at magnitude 8.5). Due to this erratic behavior near perihelion, it is even more difficult to determine the maximum brightness with certainty. Photographically the comet could still be detected as a very elliptical cloud of very low surface brightness at the end of April.
Total Brightness and Coma Diameter
Until the start of December 2024 the coma diameter measured around 0.6' (75,000 km), reaching 2' (125,000 km) by the end of December. During the days around perihelion it shrunk rapidly (a consequence of the extremely poor observing conditions and the extremely strong solar wind). Thereafter it increased rapidly, reaching a maximum of short of 8' (450,000 km) at the end of January 2025. The degree of condensation increased from DC 4 in mid-October 2024 to DC 8-9 at perihelion, only to decrease to DC 1-2 by Feb. 10 (due to the disintegration of the nucleus).Starting in October 2024 the CCD observers could observe a tail. Visually it was first detected at the end of 2024. Pre-perihelion the visually discernible tail did not exceed 0.5°. Visually the maximum tail length of 10° was observed around Jan. 22, 2025. This corresponds to 45 mio. km (very similar to that of C/2023 A3). Thereafter the tail shortened rapidly, measuring only 1° on Feb. 10. Pre-perihelion its orientation changed from South to WNW, post-perihelion it turned very quickly from Northeast towards East.
Andreas Kammerer
