Detailed view of the fragmenting comet C/2025 K1 (ATLAS)
ATel #17501; Piotr Guzik, Michal Drahus, Mikolaj Sabat (Astronomical Observatory, Jagiellonian University in Krakow)
on 18 Nov 2025; 20:30 UT
Credential Certification: Piotr Guzik (piotr1.guzik@uj.edu.pl)
We report Gemini North/GMOS imaging observations of comet C/2025 K1 (ATLAS) obtained on 2025 Nov 11.609 UT (three 30-s r-band exposures, 2x2 binning) and 2025 Nov 16.605 UT (two 30-s r-band exposures, 1x1 binning), which show a detailed view of the comet's fragmenting nucleus and the surrounding coma. The nominal image quality at both epochs is approximately 0.8 arcsec in FWHM.
The 2025 Nov 11 images show three distinct fragments aligned approximately along PA ~ 290 deg (about 10 deg south of the antisolar direction). The central fragment is evidently brightest, but cannot be measured photometrically due to the image saturation. The westward component is located ~5 arcsec from the central fragment and is also saturated, but not as bright. The eastward fragment is significantly fainter and located ~2 arcsec from the brightest one. The two bright components display their own comae and anti-sunward tails, while the faintest one shows a fan-like sunward structure.
The 2025 Nov 16 images reveal four fragments aligned approximately along PA ~ 285 deg (about 15 deg south of the antisolar direction). Three of them are comparable in brightness, with the separations of 6.0 arcsec and 13.4 arcsec measured from the eastward fragment (supposedly the same as the brightest fragment detected on November 11). Furthermore, the eastward fragment is clearly elongated, which indicates further fragmentation. The faint component is visible close to the central fragment and separated from it by ~1 arcsec in the sunward direction. The bright fragments again display their own comae and anti-sunward tails.
The images also show a broad, parabolic dust envelope, which exhibits a rapid expansion between the two epochs. Additionally, a large, diffuse, approximately circular gas coma is present on both nights.
These observations independently confirm the presence of multiple discrete components of the nucleus of C/2025 K1 and show that the morphology of the fragmenting system is evolving on a timescale of days, consistent with earlier reports of activity changes and fragment detections (ATel #17482, #17487, #17488, #17495).
The Gemini North/GMOS images are available at: C/2025 K1 (ATLAS) - Gemini North images.
Based on observations obtained at the international Gemini Observatory, a program of NSF NOIRLab [ include additional acknowledgment here, see section below ], which is managed by the Association of Universities for Research in Astronomy (AURA) under a cooperative agreement with the U.S. National Science Foundation on behalf of the Gemini Observatory partnership: the U.S. National Science Foundation (United States), National Research Council (Canada), Agencia Nacional de Investigación y Desarrollo (Chile), Ministerio de Ciencia, Tecnología e Innovación (Argentina), Ministério da Ciência, Tecnologia, Inovações e Comunicações (Brazil), and Korea Astronomy and Space Science Institute (Republic of Korea).
