In a surprising stroke of luck, NASA’s Hubble Space Telescope observed a comet in the process of breaking up. The probability of witnessing such an event at exactly the right moment is very low. The research results were published in a magazine Icarus.
Comet K1, officially known as C/2025 K1 (ATLAS) (not to be confused with interstellar comet 3I/ATLAS), was not the intended focus of Hubble’s observations.
“Sometimes the best science happens by chance,” says study co-author John Noonan, a research professor in the physics department at Auburn University in Alabama. “This comet was observed because after we won the proposal, new technical constraints made it impossible to observe the original comet. We needed to find a new target, and the moment we observed it, the comet happened to fall apart. This is the slimmest of the slimmest of possibilities.”
Surprising discoveries in Hubble data
Noonan didn’t realize the comet was breaking up until he looked at the images the next day. “When we were initially looking at the data, we found that those images contained four comets, even though we had suggested looking at only one,” Noonan said. “So we knew this was something really, really special.”
Capturing the fragmentation of comets was something the team had long wanted to accomplish. They had submitted multiple proposals to observe such phenomena with Hubble, but timing these observations proved extremely difficult and previous attempts had been unsuccessful.
“The irony is that right now we’re just studying an ordinary comet, and it’s collapsing right before our eyes,” said lead researcher Dennis Bodewicz, a professor in Auburn University’s Department of Physics.
“Comets are remnants of the solar system’s formation, so they’re made of ‘old stuff’ — the primordial material that created the solar system,” Bordowitz said. “But they’re not in their original state. They’re heated. They’re irradiated by the sun and cosmic rays. So when we look at the composition of comets, the question we always have is, ‘Is this primitive in nature or is it due to evolution?'” When you crack open a comet, you see ancient material that hasn’t been processed. ”
Comet Hubble reveals split comet K1
Hubble observed K1 split into at least four separate pieces, each surrounded by its own coma, a cloud of gas and dust that forms around the comet’s icy core. Hubble clearly resolved these debris, but ground-based telescopes could detect them only as faint, barely separated points of light.
These images were taken about a month after the comet’s closest approach to the Sun, known as perihelion. At that point, K1 had moved inside Mercury’s orbit, about one-third of the distance between Earth and the sun. At this time, the comet experiences its most intense heat and stress. Many long-period comets, including K1, tend to start breaking up shortly after this stage.
Timing of splits and tracking of fragments
Before it began to disintegrate, K1 was probably slightly larger than a typical comet, about 8 miles in diameter. Researchers estimate that the breakup began about eight days before Hubble detected it. The telescope recorded three 20-second images taken consecutively from November 8 to November 10, 2025. During that short time frame, one of the smaller pieces also split further apart.
Thanks to Hubble’s high resolution, scientists were able to trace the debris back to its original state as a single object. This allowed us to reconstruct the sequence of events. However, their analysis revealed an unexpected mystery. Why was there a delay between the breakup and the subsequent bright explosion observed from Earth? If fresh ice was exposed, why didn’t the comet brighten immediately?
A new mystery about the brightness of comets
The research team has proposed several possible explanations. A comet’s brightness is primarily due to sunlight reflecting off dust particles. When a comet first breaks open, it exposes clean ice rather than dust. One possibility is that a layer of dry dust forms first and then needs to be blown away. Another idea is that heat seeps below the Earth’s surface, increasing pressure and eventually ejecting a shell of dust into space.
“Hubble has never before captured a comet fragmenting so close to the moment it actually disintegrates. In most cases, it’s a few weeks to a month later. And in this case, we were able to see it just a few days later,” Noonan said. “This tells us something very important about the physics of what’s happening on the comet’s surface. We may be able to tell the timescale it takes for a significant dust layer to form, which can then be ejected by the gas.”
Strange chemistry and future insights
The research team plans to continue analyzing the gas emitted by the comet. Early observations with ground-based telescopes suggest that K1 has an unusual chemical composition, with significantly lower carbon levels compared to most comets. Additional data from Hubble’s STIS (Space Telescope Imaging Spectrograph) and COS (Cosmic Origins Spectrograph) instruments are expected to provide deeper insight into its composition and what can be revealed about the origins of the solar system.
A comet that never returns
K1 is a collection of debris currently located about 250 million miles from Earth. It can be found in the constellation Pisces, where it is moving away from the Sun and will probably never return to the inner solar system.
The Hubble Space Telescope has been in operation for more than 30 years and continues to make important discoveries that expand our understanding of the universe. This is a joint project between NASA and ESA (European Space Agency). NASA’s Goddard Space Flight Center in Greenbelt, Maryland, is overseeing the mission, with support from Lockheed Martin Space in Denver. Scientific operations are conducted by the Space Telescope Science Institute in Baltimore, operated by the Association of Universities for Research in Astronomy.
