Electronic Telegram No. 5657 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 Y3 (PANSTARRS) R. Weryk, Department of Physics and Astronomy, University of Western Ontario, reports the discovery of another comet in images obtained on 2025 Dec. 27 UT with the Pan-STARRS1 1.8-m Ritchey-Chretien reflector at Haleakala, when four 45-s w-band survey images taken in 1".2-1".6 seeing showed only marginal cometary appearance. The discovery observations are tabulated below, together with pre-discovery observations from Dec. 13 with the Pan-STARRS2 1.8-m Ritchey-Chretien reflector at Haleakala that Weryk was able to identify later. 2025 UT R.A. (2000) Decl. Mag. Dec. 13.45300 6 43 32.65 -20 02 37.4 21.1 13.46487 6 43 30.70 -20 03 08.3 21.1 27.38819 6 02 15.69 -29 05 37.3 21.6 27.39986 6 02 13.47 -29 06 00.0 21.0 27.41155 6 02 11.28 -29 06 23.1 20.9 27.42322 6 02 09.05 -29 06 46.0 21.6 27.52063 6 01 50.62 -29 09 56.5 20.9 27.52127 6 01 50.52 -29 09 57.4 21.1 Three stacked 60-s gri-band follow-up images obtained by Weryk and R. Wainscoat with the 3.6-m Canada-France-Hawaii Telescope on 2026 Jan. 11.3 UT (queue observer C. Wipper; queue coordinator H. Flewelling) in 1".7-1".8 seeing show a diffuse head of size 1".9 (full-width-at-half-maximum) with a broad 12" tail spanning p.a. 15-45 degrees. Three additional 60-s CFHT exposures obtained on Jan. 14.3 (queue observer L. Wells; queue coordinator V. Khatu) in 1".4-1".6 seeing show a diffuse head of size 1".6 (FWHM) and a broad tail 4" long in p.a. 10-40 degrees. After the comet was posted on the Minor Planet Center's PCCP webpage, H. Sato (Bunkyo-ku, Tokyo, Japan) obtained twenty-two stacked 60-s CCD exposures on Jan. 15.4 UT with a 0.51-m f/6.8 astrograph located at Siding Spring, NSW, Australia, that show a starlike head with a possible broad tail 25" long toward p.a. 40 degrees; the magnitude was 21.9 as measured within a circular aperture of radius 3".2. S. Deen (Simi Valley, CA, USA) identified the following archival observations of this comet obtained with the Mt. Lemmon 1.5-m reflector, when it was too faint to see any definitive cometary activity. 2025 UT R.A. (2000) Decl. Dec. 21.35084 6 20 48.51 -25 29 54.7 21.36625 6 20 45.73 -25 30 29.7 Deen also observed the comet remotely with a 0.43-m reflector at Rio Hurtado, Chile, on 2026 Jan. 8.05 UT, but it appears only stellar in 1" seeing. The available astrometry appears on MPEC 2026-B128. The following parabolic orbital elements by S. Nakano (Central Bureau) are from 29 observations spanning 2025 Dec. 13-2026 Jan. 15 (mean residual 0".4). The comet passed 2.57 AU from Jupiter on 2025 Aug. 17 UT. T = 2026 Jan. 13.72680 TT Peri. = 219.39110 Node = 272.96215 2000.0 q = 2.2270809 AU Incl. = 89.54101 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 12 31 05 51.02 -30 57.1 1.522 2.233 124.7 21.2 20.7 2026 01 10 05 20.93 -34 55.0 1.613 2.227 116.0 23.4 20.8 2026 01 20 04 54.67 -37 18.3 1.735 2.228 106.8 25.0 21.0 2026 01 30 04 34.04 -38 31.8 1.873 2.235 98.0 25.9 21.2 2026 02 09 04 19.35 -39 02.1 2.018 2.248 90.1 26.0 21.3 2026 02 19 04 10.09 -39 09.1 2.161 2.266 83.1 25.7 21.5 2026 03 01 04 05.45 -39 07.2 2.296 2.290 77.2 25.0 21.7 2026 03 11 04 04.58 -39 05.5 2.420 2.319 72.3 24.1 21.8 2026 03 21 04 06.82 -39 09.6 2.529 2.353 68.4 23.2 22.0 NOTE: These 'Central Bureau Electronic Telegrams' are sometimes superseded by text appearing later in the printed IAU Circulars. (C) Copyright 2026 CBAT 2026 January 20 (CBET 5657) Daniel W. E. Green