Gemini multi-band follow-up of fragmenting Oort cloud comet C/2025 K1 (ATLAS)

ATel #17570; Bryce Bolin (Eureka Scientific), Marco Micheli (ESA NEO Coordination Centre), Davide Farnocchia (Jet Propulsion Laboratory, California Institute of Technology), Eunchong Kim (Gemini Observatory), Hyewon Suh (Gemini Observatory), Jennie Berghuis (Gemini Observatory), Brian Lemaux (Gemini Observatory), Laura-May Abron (Griffith Observatory), Matthew Belyakov (Caltech), Christine Chen (STScI), Marco Delbo (OCA), Yanga Fernandez (UCF), Christoffer Fremling (Caltech), Dean Hines (STScI), Carl Ingebretsen (JHU), Oleksandra Ivanova (Slovak Academy of Sciences, Astronomical Institute), Ludmilla Kolokolova (University of Maryland), Meredith MacGregor (JHU), Keith Noll (PSI), David Tholen (University of Hawaii), Ian Wong (STScI)
on 1 Jan 2026; 02:45 UT
Credential Certification: Bryce Bolin (bolin.astro@gmail.com)

Tweet

We present deep multi-band (g = 465 nm, r = 622 nm, i = 771 nm) follow-up imaging of the fragmenting Oort cloud comet C/2025 K1 (hereafter K1) taken from the summit of Maunakea using the Gemini North 8.1-m telescope (observatory code T15) on 2025 December 24 06:17:27 UTC (GN-2025B-DD-115/PI: Bolin). The telescope was tracked at the comet's on-sky rate of motion, and the comet was imaged several times with exposures ranging from 3 s to 30 s using the Gemini Multi-Object Spectrograph (GMOS). The seeing was measured to be 0.6 arcsec at the time of image acquisition. The g-, r-, and i- images (see link to figure below) show the presence of three separate components: the parent body, and two fragments reported as fragment D and fragment C in previous reports (e.g., Noonan et al. 2025, ATel #17488, Koston et al. 2025, ATel #17495, Guzik & Drahus 2025, ATel #17501, Mugrauer et al. 2025, ATel #17501, #17510, Bolin et al. 2025a, ATel #17529, Romanov et al. 2025, MPEC X108). However, fragment E, previously reported in observations taken on December 6 by ATel #17529, appears to have dissipated. The angular distance between the parent body and the D and C fragments is ~3 arcsec and ~13 arcsec (~2,300 km and ~10,000 km at the 1.05 au geocentric distance of the comet). A dust tail, mostly emanating from the C fragment, has a position angle of ~170 degrees. The photometry of the parent body, D and C fragments were measured using a 0.7 arcsec aperture and using the cometary sky-background removal technique demonstrated in Bolin et al. 2021, AJ, 161, 3, id.116, 15 pp, and Bolin et al. 2025, MNRAS:L, 542, 1, pp. L139-L143. The brightness in r-band is 20.19 +/- 0.02, 21.02 +/- 0.04, and 18.72 +/- 0.02, corresponding to a phase angle-correct A(0)f(rho) of 20.00 +/- 0.43 cm, 9.31 +/- 0.36 cm, and 77.44 +/- 1.10 cm (heliocentric distance 1.712 au and phase angle 31.7 degrees). We determined the g-, r-, and i- color indices of the parent body: g-r = 0.38 +/- 0.03, r-i = 0.19 +/- 0.03, fragment D: g-r = 0.47 +/- 0.06, r-i = 0.13 +/- 0.05, and fragment C: g-r = 0.41 +/- 0.02, r-i = 0.17 +/- 0.02 (Solar g-r = 0.44, r-i = 0.11). We measured the astrometry of the parent body and fragments in our images and submitted the measurements to the Minor Planet Center. We then used the astrometric data to update the orbits of the comet and its fragments in the Jet Propulsion Laboratory Horizons System. This study is based on observations obtained at the international Gemini Observatory, a program of NSF's NOIRLab, which is managed by the Association of Universities for Research in Astronomy (AURA) under a cooperative agreement with the National Science Foundation on behalf of the Gemini Observatory partnership. This work was enabled by observations made from the Gemini North telescope, located within the Maunakea Science Reserve and adjacent to the summit of Maunakea. We are grateful for the privilege of observing the Universe from a place unique in both its astronomical quality and cultural significance.

g-, r-, and i-band images of C/2025 K1 (ATLAS) taken on 2025 December 24 UTC