Hubble observes comet split

Telescope unexpectedly captures comet splitting into four.

Comet K1 (whose full name is Comet C/2025 K1 (ATLAS)) had just passed its closest approach to the Sun and was heading out of the Solar System when the NASA/ESA Hubble Space Telescope managed to capture K1 as it fragmented into at least four pieces, each with a distinct coma, the fuzzy envelope of gas and dust that surrounds a comet’s icy nucleus.

The odds of that happening while Hubble viewed the comet are extraordinarily small: researchers had proposed many Hubble observations to catch a comet breaking up, but these are very difficult to schedule, and previous attempts were unsuccessful.

Before it fragmented, K1 was likely a bit larger than an average comet, probably around 8 kilometres 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 to 10 November 2025. As it watched the comet, one of K1’s smaller pieces also broke up.

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 of the distance from the Earth to 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.

Because Hubble’s sharp vision can distinguish extremely fine details, the team could trace the history of the fragments back to when they were one piece. That allowed them to reconstruct the timeline. But in doing so, they uncovered a mystery: Why was there a delay between the comet breaking up and the bright outbursts seen from the ground? When the comet fragmented and exposed fresh ice, why didn’t it brighten almost instantaneously?

The team has some theories. Most of a comet’s brightness is sunlight reflected from dust grains. But when a comet cracks open, it reveals pure ice. Perhaps a layer of dry dust needs to form over the pure ice and then blow off. Or maybe heat needs to get below the surface, build up pressure, and then eject an expanding shell of dust.

The team is looking forward to finishing the analysis of the gases that come from the comet. Already, ground-based analysis shows that K1 is chemically very strange — it is significantly depleted in carbon, compared with other comets. Spectroscopic analysis from Hubble’s instruments is likely to reveal much more about the composition of K1 and the very origins of our Solar System.

Astronomers are aware that long-period comets such as K1 are more likely to fragment than their short-period cousins, but it is not known why. Launching towards the end of the decade, ESA’s (European Space Agency) Comet Interceptor will be the first mission to visit a long-period comet.

Professor Colin Snodgrass of the Institute for Astronomy, and an Interdisciplinary Scientist for the Comet Interceptor mission, said:

Hubble’s chance observation of K1 will help us understand why some long-period comets split apart and give us a first view of their interiors. These new results will complement the detailed view of a long-period comet that we will obtain from ESA’s Comet Interceptor, as well as helping astronomers to select the mission’s target.

At present, the comet K1 is now a collection of fragments about 400 million kilometers from Earth. Located in the constellation Pisces, it is heading out of the Solar System, and is not likely to ever return.