Electronic Telegram No. 5643 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 W2 (ATLAS) An apparently asteroidal object discovered on CCD images taken on Nov. 28.6 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 has been found to show cometary appearance by observers elsewhere after posting on the Minor Planet Center's PCCP webpage. The discovery observations are tabulated below. 2025 UT R.A. (2000) Decl. Mag. Nov. 28.61032 12 24 35.00 +67 57 56.4 19.1 28.61625 12 24 41.07 +67 58 19.4 18.9 28.61991 12 24 44.45 +67 58 32.5 18.9 28.63604 12 25 00.33 +67 59 31.6 18.6 R. Weryk, Department of Physics and Astronomy, University of Western Ontario, reports that four 45-s i-band survey pre-discovery images obtained with the Pan-STARRS1 1.8-m Ritchey-Chretien reflector at Haleakala on Nov. 8.6 UT in 1".5-1".8 seeing show a diffuse head. Twenty stacked 30-s CCD exposures taken remotely by H. Sato (Bunkyo-ku, Tokyo, Japan) on Dec. 1.55 with a 0.43-m f/6.8 astrograph located at the Utah Desert Remote Observatory (near Beryl Junction, UT, USA) show a moderately condensed coma 18" in diameter with a hint of a tail 15" long toward p.a. 310 degrees; the magnitude was 17.8 as measured within a circular aperture of radius 9".6. Additional pre-discovery astrometric observations were reported to the MPC after the ATLAS discovery observations were reported, including Mt. Lemmon 1.5-m reflector observations from Oct. 26.5 at mag 20.1-20.4, Catalina 0.68-m Schmidt telescope observations from Nov. 14.5 (mag 18.6-19.3), and Palomar 1.2-m f/2.4 Schmidt telescope observations from Nov. 27.5 (mag 19.2-19.3) and 28.4-28.5 (mag 18.7-19.1). The available astrometry appears on MPEC 2025-X47. The following 2-body orbital elements by S. Nakano (Central Bureau) are from 38 observations spanning 2025 Oct. 26-Dec. 2 (mean residual 0".5). The formal uncertainty in the orbital period is +/- 2.7 yr. The comet passed 3.43 AU from Jupiter on 2025 Aug. 31 UT. T = 2025 Dec. 7.84030 TT Peri. = 41.03532 e = 0.9452639 Node = 90.83971 2000.0 q = 1.4531836 AU Incl. = 94.70666 a = 26.5488973 AU n = 0.00720501 P = 136.8 years The following ephemeris by the undersigned from the above orbital elements uses photometric power-law parameters H = 17.0 and 2.5n = 8 for the magnitudes. Date TT R. A. (2000) Decl. Delta r Elong. Phase Mag. 2025 11 21 10 51.60 +58 24.1 0.954 1.472 98.5 41.6 18.2 2025 11 26 11 45.63 +65 02.6 0.935 1.462 99.1 41.8 18.2 2025 12 01 13 07.78 +70 03.4 0.938 1.456 98.3 42.1 18.2 2025 12 06 14 54.81 +71 56.0 0.964 1.453 96.4 42.4 18.2 2025 12 11 16 33.91 +70 24.7 1.009 1.454 93.6 42.5 18.3 2025 12 16 17 43.13 +66 58.3 1.070 1.458 90.3 42.5 18.5 2025 12 21 18 28.03 +63 02.0 1.143 1.465 86.7 42.1 18.6 2025 12 26 18 58.24 +59 16.7 1.224 1.475 83.1 41.5 18.8 2025 12 31 19 19.81 +55 56.6 1.310 1.488 79.5 40.5 19.0 2026 01 05 19 36.07 +53 04.6 1.399 1.504 76.1 39.4 19.1 2026 01 10 19 48.90 +50 39.1 1.488 1.524 72.9 38.1 19.3 2026 01 15 19 59.37 +48 37.6 1.577 1.546 69.9 36.7 19.5 2026 01 20 20 08.14 +46 57.1 1.663 1.570 67.2 35.3 19.7 2026 01 25 20 15.63 +45 34.7 1.746 1.597 64.7 33.9 19.8 2026 01 30 20 22.10 +44 28.1 1.825 1.626 62.6 32.5 20.0 2026 02 04 20 27.73 +43 35.2 1.899 1.657 60.7 31.3 20.1 2026 02 09 20 32.66 +42 54.1 1.968 1.690 59.2 30.1 20.3 2026 02 14 20 36.96 +42 23.5 2.031 1.725 58.0 29.0 20.4 2026 02 19 20 40.69 +42 02.3 2.089 1.762 57.2 28.1 20.6 2026 02 24 20 43.86 +41 49.4 2.141 1.799 56.7 27.4 20.7 2026 03 01 20 46.49 +41 43.9 2.187 1.839 56.6 26.7 20.8 2026 03 06 20 48.58 +41 45.0 2.227 1.879 56.9 26.2 20.9 2026 03 11 20 50.13 +41 52.0 2.261 1.920 57.5 25.9 21.0 2026 03 16 20 51.10 +42 04.3 2.290 1.963 58.5 25.6 21.1 2026 03 21 20 51.47 +42 21.4 2.312 2.006 59.8 25.4 21.2 NOTE: These 'Central Bureau Electronic Telegrams' are sometimes superseded by text appearing later in the printed IAU Circulars. (C) Copyright 2025 CBAT 2025 December 6 (CBET 5643) Daniel W. E. Green