Study ‘Sequential fragmentation of C/2025 K1 (ATLAS) after its near-sun passage‘: The Hubble Space Telescope caught a rare moment on camera, getting a chance to watch a comet break up as it hurtled through space.

By sheer coincidence, Hubble was able to witness comet C/2025 K1 (ATLAS) fragmenting into at least four pieces after it had passed the sun last November. Now, a sequence of extraordinary images depicting this event is shedding light on what happens to comets as they split apart. Comet K1 reached perihelion at 0.33 au on 2025 October 8. Daily monitoring by the LCO Outbursting Objects Key Project revealed a major activity increase between November 2 and 4, accompanied by rapid changes in coma morphology. Serendipitous HST/STIS acquisition images obtained on November 8-10 captured the comet only days after this event and resolved five fragments, providing an early high-resolution view of a nucleus in the process of disruption. Fragment motions and morphologies indicate a hierarchical fragmentation sequence, including a slow secondary split of fragment II. Back extrapolation shows that both the primary and secondary breakups preceded their associated photometric outbursts by roughly one to three days. This consistent lag, together with the appearance of thin, short-lived arclets around fragment I in the first HST epoch, suggests that freshly exposed interior material warms rapidly but requires time before dust can be released efficiently. Given the comet’s close perihelion passage, rotational instability driven by enhanced outgassing torques is a plausible contributor to nucleus disintegration and dust release, and may represent the primary source of the observed brightening. These combined ground- and space-based observations provide rare, time-resolved constraints on the thermal and structural evolution of a fragmented comet near perihelion and highlight the scientific value of capturing a nucleus within days of disruption, when thermal adjustment, dust mantle re-formation, and outgassing-driven torques jointly govern the onset of activity.

Links:
Hubble Caught an Accidental Glimpse of a Comet Breakup, Snapping Pictures of Bright Blue Fragments

Icarus (https://doi.org/10.1016/j.icarus.2026.116996): Sequential fragmentation of C/2025 K1 (ATLAS) after its near-sun passage