Astronomers had been trying for years to catch a comet in the act of breaking apart. They submitted Hubble proposals hoping to be in the right place at the right time, but scheduling the telescope around an event this unpredictable rarely works out. None of those attempts succeeded.

Instead, it happened by accident — with a comet nobody had planned to study in the first place. The findings were published on 18 March 2026 in the journal Icarus. The comet is C/2025 K1 (ATLAS), not to be confused with the interstellar comet 3I/ATLAS.

A backup target that became the discovery

The team originally planned to observe a different comet. New technical constraints made it unviewable, forcing researchers to scramble for a replacement. They picked K1.

That was exactly when the comet started falling apart.

Sometimes the best science happens by accident. This comet got observed because our original comet was not viewable due to some new technical constraints after we won our proposal. We had to find a new target — and right when we observed it, it happened to break apart, which is the slimmest of slim chances.John Noonan, Auburn University

Noonan didn't realize it right away. He found out the next day, while reviewing the images and noticing four comets where only one had been planned.

A breakup inside Mercury's orbit

Hubble's images were taken about a month after K1's perihelion, its closest approach to the Sun. The comet passed within roughly one-third of the Earth-Sun distance — inside Mercury's orbit, the point where comets experience maximum heating and structural stress. It's also the point after which some long-period comets tend to break apart.

Before fragmenting, K1's nucleus measured roughly 8 kilometres across, somewhat larger than an average comet.

Hubble captured three 20-second exposures, one each day from 8 to 10 November 2025. Its sharp resolution let the team trace the fragments' history backward and pinpoint the onset of disintegration to eight days before the observations began. While Hubble watched, one of the smaller fragments split again.

Why the outburst came late

The data raised an unexpected question. When a comet cracks open, it exposes fresh ice that should reflect sunlight almost instantly. Yet ground-based telescopes recorded the brightness outburst several days after the actual breakup.

The team has two working theories. A layer of dry dust may need to form over the exposed ice before it can be blown off. Alternatively, heat may need time to penetrate beneath the surface, building pressure before ejecting an expanding shell of dust.

Noonan notes that Hubble has never before caught a fragmenting comet this close to the moment it actually broke apart — usually such observations come weeks or a month later, this time just days. That timing offers new constraints on the physics happening at a comet's surface.

A chemical anomaly and the road ahead

Beyond the mechanics of the breakup, K1 turned out to be chemically unusual. Ground-based analysis found it significantly depleted in carbon compared with other comets. Full spectroscopic analysis from Hubble's STIS and COS instruments is still underway and may reveal more about the primordial material of the early Solar System.

Comets are leftovers from the era of Solar System formation, made of ancient, largely unprocessed material. But they aren't pristine — solar heating and cosmic-ray exposure alter them over time. Cracking one open offers a rare look at material that hasn't been through that processing.

Astronomers have observed that long-period comets like K1 are more prone to fragmentation than short-period ones such as 67P/Churyumov–Gerasimenko, visited by ESA's Rosetta mission. Why remains unknown.

Toward the end of the decade, ESA plans to launch Comet Interceptor, the first mission to visit a long-period comet.

Hubble's chance observation of K1 will help us understand why some long-period comets split apart and give us a first view of their interiors. These new results will complement the detailed view of a long-period comet that we will obtain from Comet Interceptor, as well as helping astronomers to select the mission's target.Colin Snodgrass, University of Edinburgh

K1 is now a collection of fragments roughly 400 million kilometres from Earth, in the constellation Pisces. It is heading out of the Solar System and is unlikely ever to return.