JWST NIRCam detection of secondary fragmentation in C/2025 K1-C (ATLAS)

ATel #17598; Bryce Bolin (Eureka Scientific), Marco Micheli (ESA NEO Coordination Centre), Ian Wong (STScI), Blair Porterfield (STScI), Davide Farnocchia (JPL), John Stansberry (STScI), Laura-May Abron (Griffith Observatory), Matthew Belyakov (Caltech), Christine Chen (STScI), Marco Delbo (OCA), Dean Hines (STScI), Carl Ingebretsen (JHU), Oleksandra Ivanova (AISAS), Ludmilla Kolokolova (University of Maryland), Meredith MacGregor (JHU), Katherine Murray (STScI), Keith Noll (PSI), David Tholen (University of Hawaii)
on 15 Jan 2026; 05:06 UT
Credential Certification: Bryce Bolin (bolin.astro@gmail.com)

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We present follow-up images of fragmenting Oort cloud comet C/2025 K1 taken with JWST/NIRCam on 2026 January 10 05:09 (JWST-DD-12488/PI: Bolin), showing secondary fragmentation of C/2025 K1-C, a fragment of the Oort cloud comet C/2025 K1. Images obtained with the F150W2 filter (pivot wavelength = 1.672 microns) using a combined integration time of 537 s reveal an additional fragment. The new fragment, designated F in the image linked below, is located approximately 1 arcsec southwest of the C fragment (James et al. 2025 in MPEC W56), ~1200 km at the 1.595 au distance of the comet. The heliocentric distance and phase angle of the comet at the time of the observations are 1.987 au and 29.723 degrees. The orientation of the F fragment, C fragment, and the tail of the C fragment is approximately 158 degrees, similar to the projected anti-solar direction as viewed from JWST as seen in high-resolution imaging of other interstellar and solar system comets (Bolin & Lisse 2020, MNRAS, 497, 4, pp. 4031-4041, Bolin et al. 2021, AJ, 161, 3, id.116, 15 pp.). The linked image shows a 12 arcsec wide image cutout centered on fragments F, C, and fragment C's tail. North is up, and east is to the left. The projected solar and negative heliocentric orbital velocity directions are shown with orange and blue arrows. The effective PSF FWHM of an unresolved solar-type source in the F150W2 images is 0.045 arcsec (1.452 pixels at the short NIRCam channels pixel scale of 0.031 arcsec/pixel). The linked image is a composite stack of five dithered 107.4-s exposures. Using the photometry technique demonstrated in Bolin et al. 2025, MNRAS, 542, 1, L139-L143, 5 pp., the flux measured within a 0.23 arcsec radius for the F and C fragments is ~0.33 muJy and ~0.22 muJy. This work is based on observations made with the NASA/ESA/CSA James Webb Space Telescope. The data were obtained from the Mikulski Archive for Space Telescopes at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-03127 for JWST. These observations are associated with the program JWST-DD-12488/PI: Bolin.

JWST/NIRCam F150W2 images of C/2025 K1 (ATLAS) taken on 2026 January 10 UTC