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On Sep. 24, 2021 the PANSTARRS team discovered a 19.5 mag comet in the constellation Lepus, which showed a 2" coma. Additional observations of comet C/2021 S3 (PANSTARRS) showed that this comet was still at a distance of 9 AU from the Sun, displaying a compact, 6" coma. It will pass perihelion on Feb. 18, 2024 at the solar distance of 1.33 AU, predicted to become as bright as 9 mag (CBET 5051 / MPECs). It should be brighter than 16 mag between February 2023 and January 2025. During this period it will move through the constellations Columba, Lepus, Canis Major, Puppis, Circinus, Antlia, Centaurus, Lupus, Scorpius, Ophiuchus/Serpens (perihelion), Aquila, Sagitta, Vulpecula towards the border of Cygnus/Draco, where it will display an opposition loop. It should be brighter than 12 mag between November 2023 and July 2024, when it will move from Centaurus to Cygnus/Draco. At mid-European locations it will appear above the southern morning horizon in February 2024, will then climb up and culminate at altitudes of 75° in June/July 2024. Thereafter it changes to the whole-night and evening sky, at which it will sink slowly down.
At the end of October 2023 the database is still too small for a meaningful prediction. Based on 41 observations from 8 observers, which can be used for a rough evaluation, the following brightness parameters result:
m0 = 7.7 mag / n = 2.5Indicating a maximum brightness of 9.0 mag at the beginning of March 2024.
Total Brightness and Coma DiameterDuring the covered period November 2022 to mid-October 2023 the comet brightened from 16.0 mag to 12.0 mag, while the coma diameter increased from 0.3' (75,000 km) to 1.0' (125,000 km). In contrast, the degree of condensation decreased noticeably in the same period, from DC 5-6 to DC 3.
Starting at the end of 2022 CCD observers detected a tail up to 2' (1.5 mio km) long, whose orientation changed from NW to North by the end of March 2023, then to WSW by mid-June, thereafter remaining constant until at least end of October.
Andreas Kammerer