Free Astronomy Magazine May-June 2026

50 MAY-JUNE 2026 ASTRO PUBLISHING weeks to a month later. And in this case, we were able to see it just days after,” said Noonan. “This is telling us something very important about the physics of what’s happening at the comet’s surface. We may be see- ing the timescale it takes to form a substantial dust layer that can then be ejected by the gas.” The team is looking forward to fin- ishing the analysis of the gases to 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. Spec- troscopic analysis from Hubble’s STIS (Space Telescope Imaging Spectro- graph) and COS (Cosmic Origins Spectrograph) instruments is likely to reveal much more about the com- position of K1 and the very origins of our Solar System. The comet K1 is now a collection of fragments about 400 million kilome- ters from Earth. Located in the con- stellation Pisces, it is heading out of the Solar System, not likely to ever return. Astronomers see that long- period comets such as K1 are more likely to fragment than their short- period cousins, such as 67P/Churyu- mov-Gerasimenko that was visited by ESA’s Rosetta mission, but it is not known why. Launching towards the end of the decade, ESA’s Comet In- terceptor will be the first mission to visit a long-period comet. “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 interi- ors,” said co-author Prof. Colin Snod- grass of the University of Edinburgh in Scotland and an Interdisciplinary Scientist for the Comet Interceptor mission. “These new results will com- plement the detailed view of a long- period comet that we will obtain from Comet Interceptor, as well as helping astronomers to select the mission’s target.” https://esahubble.org/videos/heic2606a/ T his animation steps through the three Hubble Space Telescope images of the fragmenting comet C/2025 K1 (ATLAS)], or K1 for short, taken consecutively on November 8, 9, and 10, 2025. Captured by Hubble’s STIS (Space Telescope Im- aging Spectrograph) instrument, the sequence shows the progressive disintegra- tion of the comet over the three-day period. 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 from the ground they only appeared at that time as barely distinguishable blobs. Hubble chronicled the sequence of events and showed exactly how the breakup happened. [NASA, ESA, D. Bodewits (Auburn), J. DePasquale (STScI)] able blobs. Hubble’s images were taken just a month after K1’s closest approach to the Sun, called perihe- lion. The comet's perihelion was in- side Mercury’s orbit, about one-third the distance of the Earth from the Sun. During perihelion, a comet ex- periences its most intense heating and maximum stress. Just past peri- helion 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 kilometres across. The team estimates the comet be- gan to disintegrate eight days be- fore 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 pie- ces also broke up. Because Hubble’s sharp vision can distinguish ex- tremely 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 when the comet broke up and when bright outbursts were seen from the ground? When the comet fragmented and exposed fresh ice, why didn’t it brighten al- most instantaneously? The team has some theories. Most of a comet’s brightness is sunlight re- flected off of dust grains. But when a comet cracks open, it reveals pure ice. Maybe 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 pres- sure, and then eject an expanding shell of dust. “Never before has Hubble caught a fragmenting comet this close to when it actually fell apart. Most of the time, it’s a few !