Comet K1, whose full name is Comet C/2025 K1 (ATLAS) – not to be confused with interstellar comet 3I/ATLAS – was not the original target of a recent Hubble study. The findings were published today in the journal Icarus.

“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 in the United States. “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.”

John 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 John. “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,” explained Dennis. “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 that 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 they time they only appeared as barely distinguishable 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 the 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 8 km 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 8 November through 10 November 2025. As it watched the comet, one of K1’s smaller pieces also broke up.