Electronic Telegram No. 5566 Central Bureau for Astronomical Telegrams Mailing address: Hoffman Lab 209; Harvard University; 20 Oxford St.; Cambridge, MA 02138; U.S.A. e-mail: cbatiau@eps.harvard.edu (alternate cbat@iau.org) URL http://www.cbat.eps.harvard.edu/index.html Prepared using the Tamkin Foundation Computer Network COMET C/2025 L1 (ATLAS) A. Fitzsimmons, Queen's University, Belfast, reported finding a possibly cometary object in four 30-s CCD images taken on June 6.4 UT with a 0.5-m f/2 Schmidt reflector at Rio Hurtado, Chile, in the course of the "Asteroid Terrestrial-Impact Last Alert System" (ATLAS) search program; he noted "a marginally extended" object with size 6".1 (full-width-at-half-maximum) in 5".1 seeing. The discovery observations are tabulated below: 2025 UT R.A. (2000) Decl. Mag. June 6.38630 2 42 12.00 -49 21 50.8 18.2 6.40411 2 42 14.05 -49 22 05.0 18.4 6.40684 2 42 14.30 -49 22 07.1 18.8 6.41003 2 42 14.69 -49 22 10.0 18.4 After the object was posted on the Minor Planet Center's PCCP webpage, S. Deen (Simi Valley, CA, USA) obtained CMOS images of this comet (observers A. Schnabel and D. Ruhela) with a 0.43-m reflector at Rio Hurtado, Chile, on June 7.4 UT that he then measured, finding a very condensed, near-stellar coma of V magnitude 18.6 and size 1".3 (FWHM) in 1".2 seeing, with a a much-fainter tail about 3" long towards p.a. 295-300 degrees. With the help of his new astrometry, Deen then was able to find and measure pre-discovery images of the comet in ATLAS archival imagery taken at Rio Hurtado on 2025 May 11.4 and 17.4 (at mag 18.6-18.9 and 18.3-18.6, respectively) and at Sutherland, South Africa, on May 13.2 (at mag 18.6-18.8); Deen notes that the comet was too faint in all these May imags to be able to make any certain comments about cometary activity. Also, eight stacked 60-s CMOS exposures taken remotly by H. Sato (Bunkyo-ku, Tokyo, Japan) on June 9.42 with a 0.25-m f/3.8 astrograph located at Rio Hurtado show only a stellar appearance; the magnitude was 18.1 as measured within a circular aperture of radius 5".2. R. Weryk, Department of Physics and Astronomy, University of Western Ontario, reports that he has identified apparent images of this object from 2024 Oct. 1 (astrometry tabulated below). with the Pan-STARRS1 1.8-m Ritchey-Chretien reflector at Haleakala, adding that "the image quality was not sufficient to determine if it showed faint activity". 2024 UT R.A. (2000) Decl. Mag. Oct. 1.44152 1 50 17.22 -45 27 45.7 21.8 1.44999 1 50 16.69 -45 27 51.6 22.0 1.45845 1 50 16.18 -45 27 57.2 22.1 The available astrometry appears on MPEC 2025-L56. The following orbital elements by S. Nakano (Central Bureau) are from 30 observations spanning 2024 Oct. 1-2025 June 9 (mean residual 0".5), with corresponding "original" and "future" values of 1/a being -0.000293 and -0.000896 (+/- 0.000589) AU**-1, respectively. The residuals for the observations on 2024 Oct. 1 were -39" in R.A. and +40" in Decl. from an orbit based on the 2025 observations alone. The comet passed 4.79 AU from Uranus on 2017 Aug. 30 UT. Epoch = 2025 Dec. 31.0 TT T = 2026 Jan. 12.67484 TT Peri. = 341.24449 e = 1.0019710 Node = 208.11092 2000.0 q = 1.6794947 AU Incl. = 114.13659 The following ephemeris by the undersigned from the above orbital elements uses photometric power-law parameters H = 12.0 and 2.5n = 8 for the magnitudes. Date TT R. A. (2000) Decl. Delta r Elong. Phase Mag. 2025 05 25 02 20.62 -46 49.6 3.479 3.306 71.8 16.9 18.9 2025 06 04 02 37.79 -48 49.1 3.287 3.212 76.9 17.9 18.6 2025 06 14 02 57.23 -51 17.6 3.100 3.118 81.6 18.8 18.4 2025 06 24 03 19.82 -54 16.3 2.923 3.025 85.9 19.6 18.2 2025 07 04 03 46.87 -57 43.9 2.761 2.932 89.4 20.3 17.9 2025 07 14 04 20.55 -61 34.1 2.621 2.839 91.7 21.0 17.7 2025 07 24 05 04.36 -65 32.7 2.506 2.747 92.6 21.7 17.5 2025 08 03 06 03.19 -69 13.0 2.422 2.656 91.8 22.4 17.3 2025 08 13 07 21.18 -71 52.5 2.370 2.566 89.3 23.3 17.1 2025 08 23 08 53.47 -72 44.6 2.348 2.477 85.2 24.0 17.0 2025 09 02 10 22.12 -71 35.5 2.355 2.390 79.7 24.5 16.9 2025 09 12 11 33.03 -68 59.3 2.383 2.305 73.3 24.7 16.8 2025 09 22 12 25.82 -65 42.1 2.427 2.223 66.4 24.4 16.7 2025 10 02 13 05.48 -62 15.8 2.478 2.144 59.1 23.6 16.6 2025 10 12 13 36.42 -58 55.9 2.529 2.069 51.9 22.3 16.5 2025 10 22 14 01.59 -55 48.9 2.573 1.997 45.0 20.6 16.5 2025 11 01 14 22.75 -52 55.7 2.605 1.931 38.6 18.7 16.4 2025 12 31 15 43.91 -37 43.6 2.314 1.688 40.2 22.1 15.6 2026 01 10 15 51.64 -34 47.2 2.174 1.680 47.8 25.7 15.5 2026 01 20 15 57.47 -31 23.0 2.011 1.682 56.5 29.2 15.3 2026 01 30 16 01.01 -27 16.7 1.832 1.695 66.2 32.1 15.1 2026 02 09 16 01.67 -22 08.0 1.644 1.718 77.1 34.0 15.0 2026 02 19 15 58.61 -15 28.9 1.458 1.751 89.2 34.4 14.8 2026 03 01 15 50.63 -06 45.5 1.291 1.792 102.7 32.6 14.6 2026 03 11 15 36.22 +04 24.8 1.164 1.842 117.0 28.7 14.5 2026 03 21 15 13.77 +17 31.1 1.104 1.898 129.3 24.0 14.4 2026 03 31 14 42.42 +30 32.4 1.129 1.961 134.3 21.4 14.6 2026 04 10 14 03.71 +41 00.2 1.236 2.030 129.9 22.2 14.9 2026 04 20 13 22.41 +47 49.2 1.406 2.103 120.5 24.3 15.3 2026 04 30 12 44.89 +51 28.9 1.616 2.180 110.2 25.7 15.8 2026 05 10 12 15.32 +53 03.4 1.848 2.261 100.4 26.0 16.2 2026 05 20 11 54.56 +53 26.6 2.089 2.344 91.5 25.6 16.6 2026 05 30 11 41.52 +53 13.2 2.329 2.430 83.3 24.5 16.9 2026 06 09 11 34.53 +52 43.0 2.564 2.518 76.0 23.0 17.3 2026 06 19 11 32.12 +52 06.2 2.788 2.607 69.3 21.4 17.6 2026 06 29 11 33.14 +51 28.8 2.998 2.698 63.3 19.7 17.8 2026 07 09 11 36.74 +50 54.0 3.192 2.789 58.0 18.0 18.1 2026 07 19 11 42.33 +50 23.5 3.368 2.882 53.5 16.5 18.3 2026 07 29 11 49.48 +49 58.9 3.524 2.975 50.0 15.1 18.5 2026 08 08 11 57.87 +49 41.1 3.661 3.068 47.4 14.1 18.7 2026 08 18 12 07.28 +49 31.2 3.778 3.162 46.1 13.3 18.9 2026 08 28 12 17.55 +49 30.2 3.875 3.256 46.0 12.9 19.0 2026 09 07 12 28.55 +49 38.9 3.953 3.349 47.1 12.7 19.2 2026 09 17 12 40.22 +49 58.7 4.013 3.443 49.4 12.8 19.3 2026 09 27 12 52.48 +50 30.5 4.055 3.537 52.6 13.0 19.4 2026 10 07 13 05.30 +51 15.5 4.084 3.631 56.6 13.3 19.5 2026 10 17 13 18.67 +52 14.8 4.099 3.725 61.3 13.6 19.6 2026 10 27 13 32.56 +53 29.3 4.106 3.818 66.4 13.8 19.7 2026 11 06 13 46.98 +54 59.7 4.106 3.911 71.8 13.9 19.8 2026 11 16 14 01.95 +56 46.4 4.104 4.004 77.3 13.9 19.9 NOTE: These 'Central Bureau Electronic Telegrams' are sometimes superseded by text appearing later in the printed IAU Circulars. (C) Copyright 2025 CBAT 2025 June 9 (CBET 5566) Daniel W. E. Green