Electronic Telegram No. 5605 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 R1 (ATLAS) A. Fitzsimmons, Queen's University, Belfast, reports the discovery of a comet on CCD images taken on Sept. 3 UT with a 0.5-m f/2 Schmidt reflector at Mauna Loa, Hawaii, in the course of the "Asteroid Terrestrial-Impact Last Alert System" (ATLAS) search program. He notes that seven 30-s exposures show a very condensed head of size 9".0 (full-width-at-half maximum) in 6".3 seeing; stacked images show a compact coma approximately 10" in size with no tail. The discovery observations are tabulated below. 2025 UT R.A. (2000) Decl. Mag. Sept. 3.55226 4 45 53.33 +62 43 04.6 19.3 3.55505 4 45 53.44 +62 43 10.1 19.1 3.56154 4 45 53.28 +62 43 28.4 19.3 3.57357 4 45 53.10 +62 44 00.6 19.2 3.57639 4 45 52.97 +62 44 06.5 19.0 3.58426 4 45 53.11 +62 44 26.8 18.8 3.59444 4 45 52.94 +62 44 53.9 19.3 After the comet was posted on the Minor Planet Center's PCCP webpage, other astrometrists reported on the cometary appearance. M. Jaeger (Vienna) relates that nineteen 80-s stacked CMOS images taken with a 0.40-m f/3.2 reflector at Martinsberg, Oed, Austria, by G. Rhemann, E. Prosperi, and himself on Sept. 4.0 UT show a strongly condensed coma of size 10" with a very faint tail 25" long in p.a. 245 degrees. A. Hale (Cloudcroft, NM, USA) writes that 300-s CCD exposures taken remotely on Sept. 4.11 with a 0.35-m f/3 Cassegrain reflector located at Tenerife (Canary Islands) show the comet to slightly diffuse with a condensed head and no tail when compared to stars of similar brightness (magnitude given as 18.9-19.1). The available astrometry appears on MPEC 2025-R73. The following parabolic orbital elements by S. Nakano (Central Bureau) are from 113 observations spanning Sept. 3-9 (mean residual 0".5). The comet passed 1.68 AU from Jupiter on 2024 Nov. 30 UT. T = 2025 Nov. 22.73564 TT Peri. = 77.27952 Node = 60.40869 2000.0 q = 1.9796303 AU Incl. = 109.98056 The following ephemeris by the undersigned from the above orbital elements uses photometric power-law parameters H = 14.0 and 2.5n = 8 for the magnitudes. Date TT R. A. (2000) Decl. Delta r Elong. Phase Mag. 2025 09 02 04 45.99 +61 35.9 2.102 2.213 82.7 26.9 18.4 2025 09 07 04 44.99 +65 18.5 2.010 2.187 86.2 27.4 18.2 2025 09 12 04 41.34 +69 20.1 1.925 2.163 89.4 27.7 18.1 2025 09 17 04 32.62 +73 39.7 1.849 2.139 92.3 28.0 18.0 2025 09 22 04 12.35 +78 12.8 1.784 2.117 94.7 28.2 17.9 2025 09 27 03 17.66 +82 42.9 1.730 2.097 96.6 28.4 17.8 2025 10 02 00 15.56 +85 40.9 1.690 2.077 97.8 28.5 17.7 2025 10 07 20 33.56 +83 28.0 1.664 2.060 98.2 28.7 17.6 2025 10 12 19 22.53 +78 40.2 1.653 2.044 97.8 28.9 17.6 2025 10 17 18 57.58 +73 26.2 1.656 2.030 96.7 29.2 17.6 2025 10 22 18 46.94 +68 13.2 1.673 2.017 94.8 29.5 17.6 2025 10 27 18 42.27 +63 11.6 1.703 2.006 92.3 29.7 17.6 2025 11 01 18 40.60 +58 27.3 1.744 1.997 89.4 29.8 17.6 2025 11 06 18 40.62 +54 03.8 1.794 1.990 86.1 29.8 17.7 2025 11 11 18 41.69 +50 02.5 1.852 1.985 82.6 29.7 17.7 2025 11 16 18 43.42 +46 23.7 1.916 1.981 79.0 29.3 17.8 2025 11 21 18 45.61 +43 06.9 1.984 1.980 75.3 28.9 17.9 2025 11 26 18 48.10 +40 11.0 2.054 1.980 71.7 28.3 17.9 2025 12 01 18 50.79 +37 34.4 2.125 1.982 68.2 27.5 18.0 2025 12 06 18 53.60 +35 15.6 2.195 1.986 64.8 26.7 18.1 2025 12 11 18 56.49 +33 12.9 2.264 1.992 61.5 25.8 18.2 2025 12 16 18 59.40 +31 24.7 2.331 2.000 58.4 24.8 18.2 2025 12 21 19 02.32 +29 49.8 2.394 2.009 55.6 23.8 18.3 2025 12 26 19 05.19 +28 26.7 2.453 2.021 53.1 22.9 18.4 2025 12 31 19 08.00 +27 14.5 2.507 2.034 50.8 22.0 18.5 2026 01 10 19 13.30 +25 18.1 2.600 2.065 47.4 20.5 18.6 2026 01 20 19 18.05 +23 53.9 2.668 2.103 45.7 19.6 18.7 2026 01 30 19 22.02 +22 56.6 2.711 2.146 45.9 19.3 18.8 2026 02 09 19 24.97 +22 21.9 2.728 2.195 48.1 19.5 18.9 2026 02 19 19 26.66 +22 06.5 2.719 2.249 51.9 20.2 19.0 2026 03 01 19 26.75 +22 07.3 2.686 2.307 57.3 21.2 19.1 2026 03 11 19 24.86 +22 21.2 2.632 2.369 63.9 22.1 19.1 2026 03 21 19 20.57 +22 45.1 2.560 2.435 71.5 22.8 19.1 2026 03 31 19 13.34 +23 14.8 2.476 2.504 80.0 23.1 19.2 2026 04 10 19 02.67 +23 44.5 2.386 2.576 89.2 22.9 19.2 2026 04 20 18 48.09 +24 06.9 2.298 2.650 99.1 22.0 19.2 2026 04 30 18 29.39 +24 11.4 2.220 2.726 109.4 20.4 19.2 NOTE: These 'Central Bureau Electronic Telegrams' are sometimes superseded by text appearing later in the printed IAU Circulars. (C) Copyright 2025 CBAT 2025 September 10 (CBET 5605) Daniel W. E. Green