Hubble accidentally captured a rare comet breakup: C/2025 K1 (ATLAS) disintegrated before astronomers’ eyes
NASA’s and ESA’s Hubble Space Telescope recorded one of those scenes that occur exceptionally rarely in astronomy: the breakup of comet C/2025 K1 (ATLAS) at the very moment the telescope was already observing its nucleus. It is a long-period comet that has just passed perihelion, that is, the point closest to the Sun, and then, on its way out of the inner part of the Solar System, began to break into several pieces. Instead of a single object, at least four appeared in Hubble’s images, and detailed analysis indicates that the fragment system was even more complex. For scientists, such a scene is valuable precisely because the breakup happened almost in real time, only a few days after the start of the disintegration, which, compared with earlier observations, is almost an exception.
An observation that was not planned for this comet
Particular weight to the entire discovery is given by the fact that comet K1 was not the original target of that Hubble program at all. After technical constraints, the researchers had to replace the observation target, and the replacement choice turned out to be extraordinarily fortunate. It was precisely during those three days, from 8 to 10 November 2025, that Hubble imaged the comet at the moment when its nucleus was already breaking apart. The scientific paper on this case was published on 18 March 2026 in the journal
Icarus, and the authors point out that they had tried to organize similar observations earlier as well, but without success, because it is almost impossible to predict in advance the exact moment when a nucleus will begin to disintegrate.
For comet astronomy, this is especially important because during a breakup, parts of the interior are revealed that had long been protected from solar heating and the effects of cosmic radiation. Such processes can open a view into ancient material from the early stages of the Solar System’s formation. That is precisely why comets remain among the most interesting bodies for research: they are not merely picturesque objects with tails, but rather a kind of icy-dusty time capsules that carry traces of conditions from the era of planet formation.
What Hubble actually saw
In Hubble’s images, each of the observed pieces has its own coma, a cloud of gas and dust surrounding the icy nucleus. Because of its very high angular resolution, Hubble managed to separate the fragments much more cleanly than telescopes on Earth, which, in the same period, mostly showed them as difficult-to-distinguish bright clumps. It was precisely this sharpness that enabled the team to reconstruct the motion of the fragments backward and estimate when they still formed a single body.
Official NASA and ESA announcements speak of at least four clearly distinguishable parts, while the scientific paper’s abstract states that Hubble’s images resolved five fragments and showed a hierarchical sequence of breakup, including additional cracking of one smaller piece. That difference does not change the basic conclusion: the comet did not break up in a single outburst, but through a series of successive fractures. It is precisely this sequence of events that makes K1 especially interesting, because it suggests that the nucleus is not a homogeneous sphere of ice and dust, but a fragile structure composed of material with different strength, porosity, and thermal history.
The team estimates that the main breakup began about eight days before Hubble imaged the comet. More importantly, during Hubble’s tracking itself, one smaller fragment continued to break apart. Thus, the scientists did not receive just a “photograph after the event,” but a time series showing that the disintegration process continued right in front of the telescope. In that sense, this is one of Hubble’s earliest and most detailed views of a comet immediately after nucleus breakup.
Why the comet broke up after passing by the Sun
Comet C/2025 K1 (ATLAS) reached perihelion on 8 October 2025 at a distance of about 0.33 astronomical units from the Sun, therefore inside Mercury’s orbit and at roughly one third of Earth’s distance from the Sun. In that phase, the comet goes through the strongest thermal load and the greatest mechanical stress. Ices and volatile substances in its interior begin to heat up rapidly, evaporate, and expand, and such changes can weaken an already porous nucleus. Astronomers have long known that it is precisely long-period comets, especially those entering deep into the inner Solar System for the first time, that often disintegrate not long after perihelion.
According to researchers’ estimates, before the breakup K1’s nucleus was probably about eight kilometers in diameter, meaning it was not an entirely small object. This further emphasizes how structurally weak such comets can be. Long-period comets come from very distant parts of the Solar System, most often from the Oort Cloud, and are considered among the oldest accessible bodies in our planetary neighborhood in terms of composition. But “old” does not mean “untouched”: during the long journey toward the Sun and through repeated exposure to radiation and heat, their outer layers change, crack, and lose volatile components. That is precisely why a breakup is an opportunity to briefly see fresher interior material.
The puzzle that surprised the researchers
One of the key questions that opened up after the data analysis is: why did a time delay occur between the breakup itself and the stronger optical brightening of the comet recorded from Earth? If the nucleus fracture had immediately released a large quantity of reflective dust, the comet should have brightened visibly almost instantly. But observations point to a different sequence, that is, to a delayed brightness response.
For now, the researchers offer several possible explanations. One is that after fresh icy material is exposed, a dry surface dust layer must first form, which gas then ejects into the surrounding space. Another is that heat must penetrate below the surface, increase gas pressure there, and only then cause the ejection of a broader shell of dust. If that is confirmed, Hubble’s images do not show only the picturesque breakup of one comet, but also the timescale of the processes governing the transition from a crack in the nucleus to a visible outburst of activity.
Such a result is important also because it helps explain how comets behave at the surface and just below it. A comet’s brightness largely does not come directly from ice, but from sunlight reflected off tiny dust grains. In other words, the very moment of nucleus cracking and the moment when the comet “flares up” in the sky do not have to be the same event. It is precisely that delay that could be one of the most valuable physical clues revealed by this observation.
A chemically unusual visitor from afar
The story of K1 does not end with the mechanics of breakup. According to available ground-based analyses cited by the researchers, the comet is chemically unusual because it shows a strong depletion of carbon compared with most other comets. Abstracts of the paper report very low ratios of carbon-bearing compounds, which places it among highly atypical objects and makes it an important candidate for detailed spectroscopic research.
This is precisely where Hubble still has to make its full scientific contribution. The STIS and COS instruments, intended for spectroscopy, can reveal more about the gases released from individual fragments and perhaps show whether differences exist between their compositions. If different pieces of the same original nucleus show chemical differences, that would indicate that the comet’s interior is not uniform, but layered or composed of material that formed under different conditions. Such a finding would be important for comet formation models, but also for a broader understanding of the chemical diversity of the early Solar System.
Carbon depletion is additionally interesting because such comets are not encountered often. Each new specimen helps compare how similar or different the reservoirs of material from which comets originated are. Translated into simpler language: K1 is interesting not only because it broke up, but also because what may have broken up was an object that was already unusual in its chemical signature even before that.
Where the comet is located and whether it will return
According to official data published alongside the observation results, the remains of the comet in March 2026 are located about 400 million kilometers from Earth, in the direction of the constellation Pisces, and continue traveling toward the outer parts of the Solar System. Everything indicates that this is not a comet that will return in the foreseeable future. Such long-period objects pass through the inner Solar System very rarely, and some, after passing once, are never again gravitationally returned toward the Sun in a way that would be relevant for human observations.
That is why such observations cannot easily be repeated. Once a comet is gone, the opportunity is over. It is precisely this combination of rarity, unexpectedness, and scientific value that explains why the breakup of K1 attracted so much interest among specialists in small Solar System bodies. In practice, astronomers received an almost laboratory-like case that allows them to study nucleus breakup immediately after perihelion, while also having both Hubble data and ground-based measurements of brightness changes.
What this case means for future missions
The scientific importance of observing K1 goes beyond a single comet. ESA’s Comet Interceptor mission, whose launch is planned for late 2028 or early 2029, is intended to be the first mission designed to visit a “primitive” comet coming from the outer edges of the Solar System, that is, a body carrying less processed material from the time of planet formation. When it receives a target, such a spacecraft will try to provide a much more detailed view of a similar type of object than telescopes from Earth or orbit can provide today.
In that context, the case of K1 serves as a valuable template. It shows how quickly a long-period comet can transition from the phase of a relatively orderly object into a chain of fragments, but also how much important data can be lost if observation does not happen in time. ESA therefore emphasizes that the Hubble results will help astronomers both in selecting a future target for Comet Interceptor and in understanding the risks and behavior of such bodies when they approach the Sun.
Also interesting is the comparison with ESA’s Rosetta mission, which investigated the short-period comet 67P/Churyumov-Gerasimenko in detail. Rosetta was the first in history to follow a comet from close range over a longer period and to land a lander on its surface. But short-period comets like 67P often behave more stably than long-period visitors such as K1. That is precisely why K1 is an important reminder that “comet” is not a single category of objects, but a broad range of bodies with different histories, structures, and resilience.
More than a spectacle in the sky
At first glance, the story of a comet that broke apart may seem like just another attractive space scene, but the scientific value of this event is much deeper. In the breaking of the nucleus, researchers see an opportunity to study more unprocessed material, the time lag between fracture and dust outburst, and the way heat and gases force their way through the fragile interior. All of that helps build a more precise picture of how the small bodies of the Solar System formed and why some of them fracture almost immediately after encountering the Sun, while others remain stable.
K1 is also a good reminder of the limits of planning in astronomy. The most valuable data sometimes do not come from observations that were ideally conceived, but from unexpected circumstances in which the telescope catches nature at a moment of change. That is exactly what happened with Hubble and comet C/2025 K1 (ATLAS): instead of a routine observation, one of the most interesting fresh studies of comet breakup and a rare view into the interior of a body that, on its brief journey through the vicinity of the Sun, brought material from the oldest layers of our cosmic neighborhood came into being.
Sources:- ESA – official announcement about Hubble’s observation of the breakup of comet C/2025 K1 (ATLAS) and the basic research results (link)- NASA Science – official NASA announcement about Hubble’s images, observation dates, and the scientific significance of the event (link)- Icarus / ScienceDirect – abstract of the paper “Sequential fragmentation of C/2025 K1 (ATLAS) after its near-sun passage” with emphasis on the sequence of breakup and the photometric context (link)- arXiv – available abstract of the same research with an additional description of five fragments, the time lag of the outburst, and the comet’s chemical unusualness (link)- Minor Planet Center – official announcement about comet C/2025 K1 (ATLAS), its discovery, and orbital data (link)- ESA Comet Interceptor – official description of the future mission and the planned launch period in late 2028 or early 2029 (link)- ESA Rosetta – official overview of the mission to comet 67P/Churyumov-Gerasimenko and its scientific legacy (link)
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