C/2025 K1 (ATLAS)
C/2025 K1 is a non-periodic comet first seen in May 2025. It is one of many comets discovered by the Asteroid Terrestrial-impact Last Alert System. The comet is dynamically new, having come directly from the Oort cloud. With perihelion only from the Sun, the comet was not expected to survive perihelion passage, but it did and was recovered on 18 October 2025. As of mid-December 2025, the comet is around apparent magnitude 14, and can be seen in the constellation of Camelopardalis. The comet has broken into multiple fragments and fragments A+B+C+D may be ejected from the Solar System.
Observational history
The comet was first discovered on 24 May 2025 as an apparently asteroid-like object from the ATLAS facility at Rio Hurtado, Chile. At the time, it was a 19th-magnitude object with a slightly diffuse coma within the constellation Pegasus. Observations from the Siding Spring Observatory noted a tail about 7.5 arcseconds long and a coma about 1.5 arcseconds across, brightening up to apparent magnitude 12.1 by 13 August 2025.Between 24 and 29 September, the comet was spotted in the same area of the sky from the Earth's perspective alongside C/2025 R2 (SWAN) as it gradually moved within the constellation Virgo. In early November 2025 the comet experienced two apparent outbursts, with the comet brightening by 0.9 magnitude during the largest of which, on 4 November. On the same date, images by ZTF revealed a parabola of a material up to 10 arcseconds from the nucleus that the next day was not present while the tail had become brighter, which is consistent with the material moving tailwards. This outburst later resulted with the comet experiencing a fragmentation event with two visible components. The fragmentation was first reported on the social media groups comets-ml.groups.io and Facebook by F. Kugel, Dauban Observatory MPC station A77 on October 10, 2025. and reported by ATel from the Teide Observatory on 10 November 2025. By 13 November 2025, three fragments were visible. By 25 November 2025 a fourth fragment had appeared and fragment-A had brightened significantly, which indicated other fragment could become visible.
On 24 November 2025 the comet passed from Earth, and about 12 degrees from Dubhe in the Big Dipper. Additional observations from the Gemini North telescope in December 2025, and the James Webb Space Telescope in January 2026, had detected the second fragmentation event that occurred on the comet's Fragment C.
Images captured by Gemini North on 11 November and 6 December 2025 showed three suspected fragments of the comet's nucleus, each fluctuating in brightness, density, and position from night to night.
| Fragment | Time | Earth distance |
| C/2025 K1-B | 2025-Nov-24 17:05:03 | |
| C/2025 K1-C | 2025-Nov-24 17:05:36 | |
| C/2025 K1-A | 2025-Nov-24 17:06:41 |
Physical characteristics
Spectroscopic observations of the comet conducted from the Asiago Astrophysical Observatory between 3 and 15 August 2025 had revealed that its coma is significantly depleted in carbon-chain molecules, especially diatomic carbon. Additional data obtained from the Lowell Observatory on 19 August 2025 indicated it also has very low CN-to-OH and dust-to-gas ratios compared to other comets. So far, only two other comets were known to have such carbon-poor composition, these were C/1988 Y1 (Yanaka) and 96P/Machholz. On 13 November 2025, as the comet was fragmenting, the optical spectrum was dominated by [Amino radical|] while it was depleted in carbon species.Fragmentation images
- at Groups.io
- at the Virtual Telescope Project