Hubble Space Telescope accidentally witnesses comet C/2025 K1 (ATLAS) breaking apart

NASA's Hubble Space Telescope has captured a rare cosmic moment: a comet breaking apart in real time.

During its routine imaging of the universe, the space telescope spotted an unexpected object called C/2025 K1 (ATLAS), or comet K1 for short. Remarkably, Hubble's observations revealed that the comet's nucleus was actively fragmenting, according to a statement from NASA.

"Sometimes the best science happens by accident," John Noonan, co-author of the study and physics professor at Auburn University, said in the statement. "This comet [was] 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."

Comets are icy, rocky objects from the outer solar system that heat up as they approach the sun, releasing gas and dust to form a glowing coma and tail. They can also break apart when solar heating and gravitational forces overwhelm their fragile structure.

Astronomers observed K1 over three consecutive days, from Nov. 8 through Nov. 10, 2025, and found that it had already begun breaking apart before Hubble turned its gaze toward it. The telescope caught the comet splitting into at least four pieces, each surrounded by its own coma. One fragment appeared to split again during the observations, suggesting the process was still unfolding.

Researchers estimate the breakup began roughly a week earlier, likely triggered by the comet's close approach to the sun, known as perihelion, when heating and stress are at their peak.

K1 was discovered in May 2025 by the ATLAS (Asteroid Terrestrial-impact Last Alert System) survey and is a long-period comet from the distant Oort Cloud, a vast reservoir of icy bodies at the edge of the solar system. These objects are considered relics of the early solar system, preserving material largely unchanged for billions of years. Before fragmenting, K1 was likely a bit larger than an average comet, measuring about 5 miles (8 kilometers) across, according to the statement.

Comets like K1 are often described as "dirty snowballs," made of ice, dust and rock loosely bound together. As they near the sun, ices sublimate into gas, creating jets that can push outward on the nucleus. Combined with internal weaknesses and gravitational stress, these forces can cause the comet to fracture.

But K1's breakup revealed an additional mystery. Ground-based observers did not see the comet brighten immediately after it fragmented, as might be expected when fresh ice is exposed. Instead, there was a delay between the breakup and visible outbursts.

Scientists think this may be because a comet's brightness comes mainly from sunlight reflecting off dust, not ice. Newly exposed ice may first need to develop a thin dust layer that can then be blown off, or heat may need to build beneath the surface before releasing an expanding cloud of dust, according to the statement.

"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," Noonan said in the statement.

"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."

Watching a comet disintegrate in real time is extremely rare, as these events are unpredictable and often too faint to observe in detail. Hubble's high-resolution images allowed scientists to track the fragments as they drifted apart, offering an unusually clear view of the process. As those fragments continue to separate and fade, K1 is offering a rare and fleeting window into how some of the solar system's oldest objects evolve.

Their findings were published Feb. 6 in the journal Icarus.