Hubble Was Not Even Watching the Right Comet. Then It Saw Four.
Hubble Was Not Even Watching the Right Comet. Then It Saw Four.
Comet C/2025 K1 (ATLAS) was supposed to be a fallback target. Dennis Bodewits and John Noonan at Auburn University had proposed to study a different comet; that target became unviewable due to technical constraints after their Hubble proposal was approved. They needed a replacement, and K1 — a routine long-period comet that had just passed the Sun and was heading back out of the Solar System — fit the window.
The timing turned out to be extraordinary. K1 fragmented into at least four pieces while Hubble was watching. The researchers did not know it had happened until they viewed the images the next 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," Noonan said. "So we knew this was something really, really special."
Catching a comet fragmenting during an observation is extraordinarily unlikely. The observations were taken November 8 through 10, 2025, just a month after K1 passed perihelion — inside Mercury's orbit, about one-third the Earth-Sun distance, where comets experience their maximum thermal stress. K1 had likely begun breaking apart eight days before Hubble viewed it. As Hubble watched, one of the smaller pieces continued to fragment.
Hubble's sharp resolution cleanly separated the fragments; ground-based telescopes at the time saw only barely distinguishable blobs. The combination of resolution and near-simultaneity is what makes this observation unusual.
"Never before has Hubble caught a fragmenting comet this close to when it actually fell apart," Noonan said. "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. This is telling us something very important about the physics of what's happening at the comet's surface."
The team found a gap in the timeline that is not yet explained. When a comet cracks open and exposes fresh ice, why did the expected brightening take time to appear? The leading theory involves dust layer formation: heat builds below the surface, pressure builds, and an expanding shell of dust ejects rather than an immediate flare. The timing of that process is what the K1 observation is helping to constrain.
Ground-based spectroscopy shows K1 is "significantly depleted in carbon compared with other comets" — an anomalous compositional flag that Hubble's STIS and COS instruments are now analyzing in more detail. That data will take time.
The observation has a downstream use. ESA's Comet Interceptor, launching toward the end of the decade, will be the first mission to visit a long-period comet. Colin Snodgrass, an interdisciplinary scientist for the Comet Interceptor mission at the University of Edinburgh and a co-author on the study, said Hubble's K1 data will help select the mission's target and "give us a first view of their interiors." Long-period comets like K1 are more likely to fragment than short-period comets — Rosetta's target 67P is short-period — but the reason is not well understood. The K1 fragmentation timeline is data that did not exist before November.
K1 is now a collection of fragments about 400 million kilometers from Earth, heading out of the Solar System and not expected to return.