Home=Current Comets: C/2025 K1
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On May 24, 2025, the ATLAS team discovered an asteroidal object of magnitude 18.5 in the constellation Pegasus. Follow-up observations identified the object as cometary. Comet C/2025 K1 (ATLAS) showed a strongly condensed 15" coma and a faint, 11" tail in p.a. 255°. The comet will pass perihelion very close to the Sun (solar distance: 0.33 AU) on Oct. 8, 2025 and could then reach 9.5 mag. It will approach the Earth to within 0.58 AU on Aug. 13 and should then be of magnitude 12. However, its absolute brightness is well below the Bortle limit, so there is a high probability that the comet will disintegrate on its way towards perihelion (CBET 5557). If this should not happen the comet would be brighter than 16 mag between July 2025 and mid-January 2026, experiencing a second perigee of 0.40 AU on Nov. 25. During this period, it will move through the constellations Pegasus, Vulpecula, Hercules (1st perigee), Serpens, Libra, Virgo, Corvus (perihelion), Leo, Ursa Major (2nd perigee), Camelopardalis, Cassiopeia and Andromeda. From Central European locations it can be followed in the evening sky until the end of August (then of magnitude 12.0). If it survives the perihelion passage intact it will reappear in the morning sky around Oct. 20 (about 10.5 mag), quickly gaining altitude and finally moving towards the midnight sky.
This comet showed a very interesting development. Surprisingly it survived its close approach to the Sun, even though its absolute brightness is below the Bortle limit. At the beginning of June it was only of magnitude 17.0 but when it disappeared from the evening sky around Sep. 25 it showed as an object of magnitude 10.0. Would it survive perihelion? Starting around Oct. 25 the comet was recovered in the morning sky – showing a strongly condensed coma – at magnitude 9.5, making it a surprisingly easy object visually. By Nov. 10 the comet had faded to 10.0 mag. Based on 242 observations by 52 observers (until the start of November 2025) the brightness development can be reasonably well represented with the parameters
m0 = 11.8 mag / n = 3.7,with estimates between mid-August and the end of September lying about 0.5 mag below this development. This results in a (non-observable) maximum brightness of 8.0 mag on Oct. 9 and an observable maximum brightness of 9.5 mag around Oct. 25, 2025.
Total Brightness and Coma Diameter
Observations with the Hubble Space Telescope showed an increase in brightness of the near-nucleus region of 0.3 mag on the night of Oct. 31/Nov. 1 and of 0.9 mag on Nov. 4. The latter can just be discerned in the moving weighted 3-day means of the total magnitude (amplitude appx. 0.5 mag). As a result of this short-term outbursts an image taken on Nov. 8 showed four fragments in p.a. 197°, with the two brightest ones separated by 3.6", exhibiting parabolic comae. The other two fragments showed at a greater distance from the nucleus and looked quite diffuse (ATel 17482 and 17488). Three fragments could be tracked with Earth-based telescopes in mid-November, with the two brightest also visible visually.
The apparent coma diameter increased from 0.4' at the start of the apparition to the maximum of 1.6' in mid-August, which has remained constant since then. The absolute coma diameter measured rather constant 35,000 km until mid-August, thereafter expanded rapidly to 70,000 km by mid-September. Post-perihelion it measures about 60,000 km. The coma has been noticeably condensed so far. Pre-perihelion the degree of condensation was rather constant at DC 4, post-perihelion it is reported as DC 5-6. So far a tail could always be recognized on CCD images. It reached a length of 12' in mid-August. Post-perihelion the tail is easily discernible visually with a length of up to 12' (up to 35' on CCD images), which corresponds to an absolute length of 400,000 km. While the tail was directed toward WSW at the beginning of the apparition it rotated rapidly over South towards East between the end of July and mid-August. Post-perihelion it is oriented towards WNW.
The future development is uncertain. If fragmentation continues it may cause a quick fading. However, even if this does not happen the comet will fade rapidly after perigee (on Nov. 25 at a distance of 0.40 AU), probably reaching 16.0 mag by mid-January 2026. Until mid-January 2026 it will move from the constellation Ursa Major towards the border of the constellations Andromeda/Pisces. Thus it will be a morning object at first, switching to the evening sky in a few weeks. Earth will cross the comet's orbital plane on Dec. 29.
Andreas Kammerer
