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Analysis of Comet Apparitions

C/2025 R2 (SWAN)

On Sep. 10, 2025, Vladimir Bezugly (Ukraine) discovered an object in the publicly available SWAN images which he assumed could be a comet. He informed the comet community, and finally, on Sep. 12, M. Masek managed to capture an image of the comet with a 135mm telephoto lens from Cerro Paranal. The comet, located in the constellation Virgo, was of magnitude 7.5, showing a coma short of 4' and a 2.8° tail in p.a. 114°. During the following days other observers confirmed these values. Comet C/2025 R2 (SWAN) passed perihelion at a solar distance of 0.50 AU on Sep. 12. The comet will approach Earth to within 0.26 AU (Oct. 19), expected to reach magnitude 6.0 at those days (CBET 5606). If it experiences an average brightness development the comet should be brighter than 16.0 mag until the end of March 2026. During this period it will move through the constellations Libra, Scorpius, Ophiuchus, Serpens, Scutum (perigee), Aquila, Aquarius, Pisces and Aries. From mid-European locations it will appear above the southwestern evening horizon around Oct. 10, then quickly gaining altitude until the first week of November. It will reach greatest altitudes (around 50°) in mid-January 2026, thereafter descending towards the horizon.

The comet has only been observed for 10 days, so only a very rough analysis can be made. In addition, I can only use 32 observations from 13 observers, done at low altitudes. Therefore, I can only assume that the comet will develop at an average rate. This results in brightness parameters of

m₀ = 9.5 mag / n = 3,

with the absolute magnitude close to the Bortle-limit. This means that the comet is likely to reach magnitude 6.4 mag during the days of perigee. During those days it should show a visual coma diameter of about 15'. On Sep. 22 the comet was of magnitude 6.7, showing a strongly (DC 7) condensed coma of 4' diameter. A tail of about 0.8° (1.3 mio. km), pointing toward ESE, was also observed.

Andreas Kammerer

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