Fragmentation of C/2025 K1 (ATLAS)
ATel #17488; John Noonan (Auburn University), Dennis Bodewits (Auburn University), Colin Snodgrass (University of Edinburgh), Kacper Wierzchos (Lunar and Planetary Laboratory), Michael Combi (University of Michigan)
on 10 Nov 2025; 18:31 UT
Distributed as an Instant Email Notice Comets
Credential Certification: Dennis Bodewits (dennis@auburn.edu)
We report the possible fragmentation of comet C/2025 K1 (ATLAS) observed in HST/STIS images obtained at 13:17 and 13:52 UTC on 2025 November 8 as part of program GO-18135. At the time of observation, the comet was 0.88 au from the Sun, 0.65 au from Earth, and at a phase angle of ~80 degrees.
Following reports of two apparent outbursts between October 31 and November 4 2025 (ATel #17482) that included a smaller event of ~ -0.3 mag around Oct 31-Nov 1 and a larger event of ~ -0.9 mag detected by Nov 4 relative to Nov 2, we detect four fragments in the HST data obtained with the MIRVIS Clear filter (exposure times of 10 s and 42 s). The fragments span ~5.2 arcsec along a position angle of 197 degrees relative to north. The two leading fragments, separated by 3.6 arcsec, each display parabolic comae, with the second fragment appearing noticeably brighter than the first. Two additional fainter fragments are visible farther along the same orientation; these are diffuse and lack sharply defined comae.
We encourage continued monitoring of C/2025 K1 (ATLAS) to track the evolution of its morphology and to assess whether additional fragments or renewed activity emerge in the coming weeks.
