By a stroke of luck, we have seen a comet just days after it cracked into four pieces. This could provide a crucial window into the history of the solar system.

John Noonan at Auburn University in Alabama and his colleagues had planned to observe a different comet with the Hubble Space Telescope, but limitations to the spacecraft’s ability to turn quickly made that impossible, so they found a new target: a comet called C/2025 K1 (ATLAS). When they pointed Hubble at K1, they saw not a single comet but four fragments.

“We have seen comets break up before – we’ve seen them break up from the ground all the time – but this one wasn’t known to have broken up when we looked at it,” says Noonan. “The amount of sheer luck that came into acquiring these images cannot be overstated.”

We have never taken such clear pictures of a comet that’s just broken up before, because it is hard to predict when one will start to crack and even harder to point a space telescope at one just in time. Thanks to the high resolution of the images, the researchers managed to calculate when K1 began to fragment, about a week before the pictures were taken.

Comets are made of pristine ice from the early days of solar system formation, but their exteriors are eroded over time by sunlight and other space radiation. To get at those pristine ices, which could tell us about the environment that formed the planets, we have to look under the surface, which is exactly what a fragmenting comet allows.

When a comet breaks, the ices inside it are expected to start sublimating, turning into gas and floating off. “These really cold ices that are being exposed to heat for the first time in billions of years, and they should start sublimating really fast,” says Noonan. But that doesn’t seem to be what happened in this case – it took about two days after K1 broke up for it to brighten, which is typically seen as a sign of sunshine lighting up sublimated gas and dust.

The cause of this delay is a mystery for now, but Noonan and his colleagues are currently working to analyse the rest of their data on K1, which should both explain the delay and reveal the makeup of the comet’s insides. “We’re about to get a really fascinating look into this comet and the early solar system,” he says.