NASA's Hubble unexpectedly catches comet breaking up
by NASALisa Lock
scientific editor
Meet our editorial team
Behind our editorial process
Robert Egan
associate editor
Meet our editorial team
Behind our editorial process
Editors' notes
This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility:
fact-checked
peer-reviewed publication
trusted source
proofread
The GIST
Add as preferred source
In a happy twist of fate, NASA's Hubble Space Telescope witnessed a comet in the act of breaking apart. The chance of that happening while Hubble watched is extraordinarily minuscule. The findings are published in the journal Icarus.
The comet K1, whose full name is C/2025 K1 (ATLAS)—not to be confused with interstellar comet 3I/ATLAS—was not the original target of the Hubble study.
"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."
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. "So we knew this was something really, really special."
This is an experiment the researchers always wanted to do with Hubble. They had proposed many Hubble observations to catch a comet breaking up. Unfortunately, these are very difficult to schedule, and they were never successful.
"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 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. Hubble cleanly resolved the fragments, but to ground-based telescopes, at the time they only appeared as barely distinguishable, bright blobs.
Hubble's images were taken just a month after K1's closest approach to the sun, called perihelion. The comet's perihelion was inside Mercury's orbit, about one-third the distance of Earth from the sun. During perihelion, a comet experiences its most intense heating and maximum stress. Just past perihelion is when some long-period comets like K1 tend to fall apart.
Before it fragmented, K1 was likely a bit larger than an average comet, probably around 5 miles across. The team estimates the comet began to disintegrate eight days before Hubble viewed it. Hubble took three 20-second images, one on each day from Nov. 8 through Nov. 10, 2025. As it watched the comet, one of K1's smaller pieces also broke up.
Because Hubble's sharp vision can distinguish extremely fine details, the team could trace the history of the fragments back to when they were one piece. That allowed them to reconstruct the timeline. But in doing so, they uncovered a mystery: Why was there a delay between when the comet broke up and when bright outbursts were seen from the ground? When the comet fragmented and exposed fresh ice, why didn't it brighten almost instantaneously?
Discover the latest in science, tech, and space with over 100,000 subscribers who rely on Phys.org for daily insights. Sign up for our free newsletter and get updates on breakthroughs, innovations, and research that matter—daily or weekly.
Subscribe
The team has some theories. Most of a comet's brightness is sunlight reflected off of dust grains. But when a comet cracks open, it reveals pure ice. Maybe a layer of dry dust needs to form over the pure ice and then blow off. Or maybe heat needs to get below the surface, build up pressure, and then eject an expanding shell of dust.
"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."
The research team is looking forward to finishing the analysis of the gases to come from the comet. Already, ground-based analysis shows that K1 is chemically very strange—it is significantly depleted in carbon, compared with other comets. Spectroscopic analysis from Hubble's STIS (Space Telescope Imaging Spectrograph) and COS (Cosmic Origins Spectrograph) instruments is likely to reveal much more about the composition of K1 and the very origins of our solar system, as NASA's space telescopes continue to contribute to our understanding of planetary science.
The comet K1 is now a collection of fragments about 250 million miles from Earth. Located in the constellation Pisces, it is heading out of the solar system, not likely to ever return.
Publication details
D. Bodewits et al, Sequential fragmentation of C/2025 K1 (ATLAS) after its near-sun passage, Icarus (2026). DOI: 10.1016/j.icarus.2026.116996
Journal information: Icarus
Key concepts
Provided by NASA