The Hubble Space Telescope has captured images of a comet breaking apart into tiny fragments, 400 million km (250 million miles) from Earth, completely by accident.
The comet in question is C/2025 K1 (ATLAS) – also known as K1 – which, astronomers are keen to point out, is not to be confused with the interstellar comet 3I/ATLAS.
More on comets
What's surprising about Hubble observing the comet breaking up is that K1 wasn't even the original target of the space telescope's study.
Astronomers say the chances of spotting a comet breaking apart like this are incredibly slim.
A frozen relic from the dawn of the Solar System
Comets are essentially frozen lumps of ice and rock. The vast majority of the comets we know about originated within our Solar System and orbit our Sun.
These comets are formed from the ingredients out of which our Sun and all the planets – including Earth – formed.
That makes comets frozen time capsules, and studying them can reveal a lot about what our early Solar System was like.
As comets get closer to the Sun, they begin to heat up, causing their ancient ice to sublimate into gas, creating a fuzzy coma around the comet's head and a long tail streaking behind.
Scientists can study light from these gassy outflows to learn more about what the comet is made of.
But sometimes the heating caused by a close approach to the Sun is too much for a comet, and causes the comet to break apart.
The fate of C/2025 K1 (ATLAS)
"Sometimes the best science happens by accident," says John Noonan, a research professor in the Department of Physics at Auburn University in Alabama, USA and co-investigator on the study.
"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.
"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."
The team say they had tried before to use the Hubble Space Telescope to observe a comet break up before, but had never managed it.
"The irony is now we're just studying a regular comet and it crumbles in front of our eyes," says principal investigator Dennis Bodewits, 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," says 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."
How Hubble caught the break-up
The Hubble Space Telescope captured comet K1 breaking up into at least four pieces, the team say, and each of those pieces had its own coma.
Hubble’s images were taken just a month after K1’s closest approach to the Sun, when it got even closer to the Sun than the orbit of the planet Mercury.
At its closest, K1 was about one-third the distance between Earth and the Sun.
Because it got so close to the Sun, the comet would have undergone intense heating, scientists say, and this is the point at which many comets break up.
The team say K1 was probably about 8km (5 miles) wide, which is slightly larger than normal. They estimate the comet began to disintegrate eight days before Hubble saw it.
The space telescope captured three 20-second images, one on each day from 8 November to 10 November 2025. As it observed the comet, one of K1’s smaller pieces also broke up.
Working backwards
The team say they were able to use Hubble's observations to trace the history of the fragments back in time, to when they were one piece.
But they couldn't work out why there was a delay between when comet K1 broke up and when bright outbursts were seen from Earth.
Once the internal ice of the comet was exposed, it should have reflected light and made the comet brighten instantly.
The team say perhaps a layer of dust had formed over the ice, and then blew off. Or perhaps it took a while for internal heat to build up within the comet and blow of a shell of dust, to reveal the reflective ice underneath.
"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," says 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 team say the've still got a lot to learn about comet K1.
They say studies using ground-based telescopes show the comet is 'chemically strange', with a lot less carbon than is normally seen in other comets.
Spectroscopic studies using Hubble – i.e. analysing light from the comet – should reveal more about its chemical make-up, which in turn will give scientists more insight into the origins of our Solar System.
