C/2025 K1 (ATLAS) 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 (ATLAS). 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. The comet has broken into multiple fragments and fragments A+B+D should be ejected from the Solar System. Fragment-C may remain bound to the Solar System.

Observational history The comet was first discovered on 24 May 2025 as an apparently asteroid-like object (temporarily designated as A11nyuL) 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. (Image linked externally) 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(s) 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 (JWST) 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.

+Earth approaches on 24 November 2025 Fragment Time Earth distanceC/2025 K1-B 2025-Nov-24 17:05:03 C/2025 K1-C 2025-Nov-24 17:05:34 C/2025 K1-A 2025-Nov-24 17:06:43

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 while it was depleted in carbon species. Multiband optical observations from the Gemini Observatory in December 2025 show that the comet fragments have a predominantly blue (g–r) color of ~0.40, which are likely explained by the dust particle size and light-scattering effects.

See also C/2025 A6 (Lemmon) C/2025 R2 (SWAN)

Notes

References

External links

at the Minor Planet Center database C/2025 K1 at COBS (Comet Observation database) C/2025 K1 at Gideon van Buitenen's website C/2025 K1 at Seiichi Yoshida's website

Fragmentation images Comet C/2025 K1 (ATLAS) fragmentation at Groups.io (10 Nov 2025) Comet C/2025 K1 (ATLAS) fragmentation at the Virtual Telescope Project (13 Nov 2025)

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