In a remarkable stroke of luck, NASA’s Hubble Space Telescope observed a comet in the middle of breaking apart. The odds of witnessing such an event at exactly the right moment are extremely low. The findings were published in the journal Icarus.
The comet K1, formally known as C/2025 K1 (ATLAS) — not to be confused with interstellar comet 3I/ATLAS — was not the intended focus of the Hubble observations.
“Sometimes the best science happens by accident,” said co-investigator John Noonan, a research professor in the Department of Physics at Auburn University in Alabama. “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.”
A Surprise Discovery in Hubble Data
Noonan did not realize the comet was breaking up until he reviewed the images the following day. “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. “So we knew this was something really, really special.”
Capturing a comet in the act of fragmenting is something the team had long hoped to achieve. They had submitted multiple proposals to observe such an event with Hubble, but timing these observations has proven extremely difficult, and previous attempts had not succeeded.
“The irony is now we’re just studying a regular comet and it crumbles in front of our eyes,” said principal investigator Dennis Bodewits, also a professor in Auburn University’s Department of Physics.
“Comets are leftovers of the era of solar system formation, so they’re made of ‘old stuff’ — the primordial materials that made our solar system,” said Bodewits. “But they are not pristine — they’ve been heated; they’ve been irradiated by the Sun and by cosmic rays. So, when looking at a comet’s composition, the question we always have is, ‘Is this a primitive property or is this due to evolution?’ By cracking open a comet, you can see the ancient material that has not been processed.”
Hubble Reveals Comet K1 Splitting Into Pieces
Hubble observed K1 breaking into at least four separate fragments, each surrounded by its own coma, the cloud of gas and dust that forms around a comet’s icy core. While Hubble clearly resolved these pieces, ground-based telescopes could only detect them as faint, barely separated points of light.
The images were taken about a month after the comet’s closest approach to the Sun, known as perihelion. At that point, K1 had traveled inside Mercury’s orbit, roughly one-third of the distance between Earth and the Sun. This is when comets experience the most intense heat and stress. Many long-period comets, including K1, tend to begin breaking apart shortly after this phase.
Timing the Breakup and Tracking the Fragments
Before it began to disintegrate, K1 was likely slightly larger than a typical comet, measuring about 5 miles across. Researchers estimate the breakup started around eight days before Hubble captured it. The telescope recorded three 20-second images taken on consecutive days from Nov. 8 through Nov. 10, 2025. During that short window, one of the smaller fragments also split further.
Thanks to Hubble’s high resolution, scientists were able to trace the fragments backward to their original state as a single object. This allowed them to reconstruct the sequence of events. However, their analysis revealed an unexpected puzzle. Why was there a delay between the breakup and the bright outbursts later seen from Earth? If fresh ice was exposed, why did the comet not brighten right away?
A New Mystery About Comet Brightness
The team has proposed several possible explanations. A comet’s brightness is largely 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 dry dust layer must first form and then be blown off. Another idea is that heat penetrates beneath the surface, builds pressure, and eventually releases a shell of dust into space.
“Never before has Hubble caught a fragmenting comet this close to when it actually fell apart. Most of the time, it’s a few weeks to a month later. And in this case, we were able to see it just days after,” said Noonan. “This is telling us something very important about the physics of what’s happening at the comet’s surface. We may be seeing the timescale it takes to form a substantial dust layer that can then be ejected by the gas.”
Strange Chemistry and Future Insights
The research team plans to continue analyzing gases released by the comet. Early observations from ground-based telescopes suggest that K1 has an unusual chemical makeup, showing significantly lower levels of carbon 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 it can reveal about the origins of the solar system.
A Comet That Will Not Return
K1 is now a cluster of fragments located about 250 million miles from Earth. It can be found in the constellation Pisces and is moving away from the Sun, likely never to return to the inner solar system.
The Hubble Space Telescope has been in operation for more than 30 years and continues to deliver important discoveries that expand our understanding of the universe. It is a joint project between NASA and ESA (European Space Agency). NASA’s Goddard Space Flight Center in Greenbelt, Maryland, oversees the mission, with support from Lockheed Martin Space in Denver. Scientific operations are conducted by the Space Telescope Science Institute in Baltimore, which is run by the Association of Universities for Research in Astronomy.