Follow up monitoring of the fragmentation of comet C/2025 K1 (ATLAS)
ATel #17495; A. Kostov (IA and NAO, BAS, Bulgaria), M. Minev (IA and NAO, BAS, Bulgaria), G. Borisov (IA and NAO, BAS, Bulgaria), T. Bonev (IA and NAO, BAS, Bulgaria)
on 16 Nov 2025; 16:22 UT
Credential Certification: Andon Kostov (akostov@astro.bas.bg)
Following the current alerts from ATel #17482, ATel #17487 and ATel #17488 we observed the comet C/2025 K1 (ATLAS) in four consecutive nights - from 13 to 16 November. The observations were performed with the 1.5m AZ1500 telescope of the National Astronomical Observatory - Rozhen, Bulgaria. We clearly detect two fragments - first one, which is fainter and near to the core of the comet (named A), and the second one (named B), which is brighter and is farther. The distances between the core and the fragments, projected on the sky at the corresponding comet geocentric distance, are as follows:
| Date |
|
|
distance to A (km) |
distance to B (km) |
| 2025 Nov 13 |
|
|
1614 |
3597 |
| 2025 Nov 14 |
|
|
--- |
3922 |
| 2025 Nov 15 |
|
|
1608 |
4132 |
| 2025 Nov 16 |
|
|
1865 |
4374 |
We did not detect the fragment A on the observations from 14-November. This is probably due to the extended and bright coma of the comet at that date. Being fainter, and closer to the core, this fragment is probably overwhelmed by the coma. Assuming constant speed of dispersion of the fragments, and that no other forces affecting their speed, we can speculate that the fragmentation of the comet begin around the 2025-Nov-04 18:00 UT (calculated for both, A and B, separately), which is consistent with, and probably is a consequence of the observed outburst, reported by ATel #
17482. The estimated constant velocity is 6.5 metre per second.
In addition we observe rapid changes in the morphology of the comet in BVRI broadband filters, similar to that reported in ATel #
17482. For all the four nights, isophote contour maps of median images in each filter were prepared. The following links represent these maps based on the images obtained with filter R:
2025-Nov-13 Map
2025-Nov-14 Map
2025-Nov-15 Map
2025-Nov-16 Map
All maps are centered at the photocenter of the comet, orientation of the fields is standard astronomical (North at the top, East at left). The outermost contour is above 3 sigma of the night sky, and the step of the contours is 0.25mag. For each of the maps the corresponding heliocentric (r) and geocentric (Δ) distances are shown, along with the anti-solar direction (PsAng), the negative velocity vector (PsAMV), and the angle between Sun-Comet-Observer (Phase angle). The vector rose shows these angles, along with the orientation of the images. The dashed lines trough the photocenter of the comet represent the anti-solar direction and the negative velocity vector direction.
Our monitoring of C/2025 K1 (ATLAS) continue, more detailed analysis will be published.
The research that led to these results was carried out with the help of infrastructure purchased under the National Roadmap for Scientific Infrastructure, financially coordinated by the Ministry of Education and Science of the Republic of Bulgaria. It is also partially supported by Bulgarian Scientific Research Fund of the Ministry of Education and Science under the grant KΠ-06-H88/5 "Physical properties and chemical composition of asteroids and comets - a key to increasing our knowledge of the Solar System origin and evolution."
