Home=Current Comets: C/2025 R2
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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 an orbital period of 680 years (CBET 5618).
During the second half of October 2025 this comet was easily visible in binoculars, although the weather was not very cooperative. Based on 586 observations by 70 observers the brightness development can be best represented with the time-dependent formula
m = 6.8 mag + 5·log D + 0.057·|t – T|resulting in a peak brightness of 5.8 mag on Oct. 15, 2025. Alternatively the brightness development can be described with the classical parameters m0=9.3 mag / n=4. However, these parameters do not well suit the first and final weeks of the observations. Because the absolute magnitude was only slightly above the Bortle limit it came as no great surprise when images taken with the TTT3 telescope on Nov. 2, 2025 showed a fragment next to the main component (ATel 17469). After the comet had passed perigee (on Oct. 19) it faded rapidly, reaching 16 mag by mid-January 2026.
Total Brightness and Coma Diameter
The apparent coma diameter increased from 2.5' at the beginning of the apparition to 8' around Oct. 10, reaching a maximum of 15' on Oct. 22, 2025. Thereafter it rapidly decreased to 3.5' by the end of November, followed by a slower decrease to 2' until early January 2026. The absolute coma diameter expanded from 115,000 km at the beginning of the apparition to a maximum of 215,000 km around Nov. 10, 2025. Thereafter it shrunk, measuring only 100,000 km by the beginning of January 2026. While the coma was highly condensed at the beginning of the apparition (DC 7), it became very steadily more diffuse until around Nov. 20 (DC 2). By the beginning of January 2026 the degree of condensation had decreased to DC 0-1.During the first weeks the tail length increased rapidly from 0.4° (1.0 mio. km) to 1.0° (1.5 mio. km) at the end of September. Thereafter it steadily decreased. After Nov. 20 there were hardly any reports of tail sightings. The tail direction changed only from ENE to ESE.
Andreas Kammerer
