NASA's Hubble Space Telescope witnessed comet K1, whose full name is C/2025 K1 (ATLAS), breaking apart in November 2025, with findings published in the journal Icarus. The observation itself was almost incidental. K1 was not the original target of the Hubble study, as the original comet became unviewable due to new technical constraints. Researchers had to find a new target, and right when they observed it, it happened to break apart, which is described as the slimmest of slim chances.
Noonan didn't know K1 was fragmenting until he viewed the images the day after Hubble took them. "While I was taking an initial look at the data, I saw that there were four comets in those images when we only proposed to look at one," said Noonan. This was the first time Hubble has witnessed a comet so early in the process of breaking up.
The scientific value of this accidental discovery lies in what it reveals about comet structure and behaviour. Hubble caught K1 fragmenting into at least four pieces, each with a distinct coma, the fuzzy envelope of gas and dust that surrounds a comet's icy nucleus. Before it fragmented, K1 was likely a bit larger than an average comet, probably around 8 kilometres across. Hubble's images were taken just a month after K1's closest approach to the Sun, called perihelion, which was inside Mercury's orbit. During perihelion, a comet experiences its most intense heating and maximum stress, and just past perihelion is when some long-period comets like K1 tend to fall apart.
What makes these images particularly valuable is a puzzle they revealed. By tracking the fragments' motions backward in time, the researchers reconstructed when the comet had been intact. The pieces had separated several days before any outburst was reported from Earth, which challenges a long-standing assumption that fresh ice exposed by fragmentation immediately drives the brightening seen from the ground. The team proposes two leading explanations. Freshly exposed ice may initially release mostly gas, which does not reflect sunlight efficiently. Only after a layer of dust accumulates and is lifted away does the comet brighten visibly.
This discovery has implications that extend beyond a single comet. Astronomers see that long-period comets such as K1 are more likely to fragment than their short-period cousins, such as 67P/Churyumov-Gerasimenko that was visited by ESA's Rosetta mission, but it is not known why. Principal investigator Dennis Bodewits noted that the irony is they're studying a regular comet and it crumbles in front of their eyes. Comets are leftovers of the era of Solar System formation, made of primordial materials that made our Solar System.
The findings are already shaping future research directions. Launching towards the end of the decade, ESA's Comet Interceptor will be the first mission to visit a long-period comet. Hubble's chance observation of K1 will help scientists understand why some long-period comets split apart and give a first view of their interiors. These new results will complement the detailed view of a long-period comet that will be obtained from Comet Interceptor, as well as helping astronomers to select the mission's target.
For cometary science, the real lesson is methodological. A research team that had already secured telescope time was positioned to capture this breakup as it unfolded. That preparedness, combined with an unexpected target, produced insights that would have taken deliberate planning to achieve. The research team is looking forward to finishing the analysis of the gases that came from the comet. Ground-based analysis shows that K1 is chemically very strange, significantly depleted in carbon compared with other comets. Those chemical details may yet reveal more about the solar system's origins.