Electronic Telegram No. 5624 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 T1 (ATLAS) An apparently asteroidal object discovered on CCD images taken on Oct. 11 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 the object was posted to the Minor Planet Center's PCCP webpage. The discovery observations are tabulated below. 2025 UT R.A. (2000) Decl. Mag. Oct. 11.62114 11 22 00.39 +48 13 52.0 17.5 11.62482 11 22 01.52 +48 13 59.2 16.3 11.63180 11 22 03.61 +48 14 10.0 17.2 11.63877 11 22 05.65 +48 14 20.7 17.6 S. Deen (Simi Valley, CA, USA) writes that he identified archival observations of this object taken with the discovery telescope on Oct. 3, with his measures tabulated below. The comet appeared clearly diffuse with a coma of size 6"-8" (full-width-at-half-maximum) in 3".0 seeing, with a possible slight elongation westwards (noting that the anti-solar angle then was p.a. 328 deg). 2025 UT R.A. (2000) Decl. Mag. Oct. 3.61781 10 48 11.20 +44 55 58.2 18.2 3.62333 10 48 12.30 +44 56 07.4 18.1 3.63024 10 48 13.83 +44 56 16.6 18.4 P. Birtwhistle (Great Shefford, Berkshire, England) writes that one- hundred-nineteen 10-s CCD images were obtained on Oct. 11.8 UT with a 0.41-m Schmidt-Cassegrain reflector in difficult conditions at low altitude (12-13 degrees), revealing a condensed coma of size 11" and a possible tail 30" long in p.a. 330 degrees. Ten stacked 30-s CMOS images obtained by K. Yoshimoto (Yamaguchi, Japan) with a 0.3-m f/4 reflector on Oct. 11.85 UT show this object to be definitely a comet, with a diffuse coma 18" in diameter and a broad tail 10" long in p.a. 330 deg; the total magnitude was 16.5 (Tycho2-catalogue V-magnitudes). Thirty-four stacked 30-s exposures taken by Yoshimoto on Oct. 16.82-16.84 show an apparent notable increase in brightness to total mag 13.8 (comparison V magnitudes from APASS catalogue), with a diffuse coma 2'.3 in diameter and no tail. M. Jaeger, Vienna, reports that fourteen 80-s images obtained on Oct. 13.8 UT with a 0.28-m f/2.2 Celestron RASA Schmidt telescope at Martinsberg, Austria, show a 110' coma of mag 14.5-15.0 with a possible short tail toward p.a. 160 degrees. Twelve stacked 60-s CCD exposures taken remotely on Oct. 16.5 UT by H. Sato (Bunkyo-ku, Tokyo, Japan) with a 0.25-m f/3.4 astrograph located at the Utah Desert Remote Observatory (near Beryl Junction, UT, USA) show a strongly condensed coma 1'.6 in diameter with no tail; the magnitude was 14.0 as measured within a circular aperture of radius 50".0. The available astrometry appears on MPEC 2025-Uxxx. The following parabolic orbital elements by S. Nakano (Central Bureau) are from 89 observations spanning 2025 Oct. 3-17 (mean residual 0".5). There are no close approaches to major planets. T = 2025 Dec. 2.37048 TT Peri. = 114.14642 Node = 140.50140 2000.0 q = 1.1087527 AU Incl. = 123.43703 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 10 12 11 23.91 +48 23.9 1.570 1.380 60.2 38.9 14.1 2025 10 17 11 52.66 +50 39.3 1.446 1.336 63.2 41.8 13.8 2025 10 22 12 29.59 +52 51.1 1.329 1.294 65.8 44.6 13.5 2025 10 27 13 17.07 +54 36.2 1.223 1.255 68.0 47.3 13.2 2025 11 01 14 15.88 +55 14.9 1.135 1.220 69.6 49.7 13.0 2025 11 06 15 22.30 +53 58.9 1.069 1.189 70.3 51.8 12.7 2025 11 11 16 27.70 +50 20.3 1.032 1.162 70.1 53.3 12.6 2025 11 16 17 24.15 +44 38.1 1.026 1.140 68.9 54.0 12.5 2025 11 21 18 09.04 +37 46.9 1.052 1.124 66.8 53.9 12.5 2025 11 26 18 43.71 +30 44.2 1.107 1.114 63.9 52.8 12.6 2025 12 01 19 10.61 +24 08.6 1.186 1.109 60.6 50.8 12.7 2025 12 06 19 31.88 +18 17.8 1.281 1.110 56.9 48.0 12.9 2025 12 11 19 49.14 +13 15.1 1.388 1.118 52.9 44.7 13.1 2025 12 16 20 03.50 +08 56.6 1.502 1.131 48.8 40.9 13.3 2025 12 21 20 15.75 +05 16.0 1.619 1.149 44.6 37.0 13.5 2025 12 26 20 26.44 +02 07.1 1.735 1.173 40.4 32.9 13.8 2025 12 31 20 35.94 -00 36.1 1.849 1.202 36.0 28.8 14.0 2026 01 10 20 52.45 -05 03.4 2.063 1.272 27.3 20.8 14.4 2026 01 20 21 06.75 -08 34.2 2.251 1.355 18.6 13.4 14.8 2026 01 30 21 19.63 -11 27.5 2.410 1.449 9.8 6.7 15.2 2026 02 09 21 31.52 -13 56.4 2.537 1.550 1.1 0.7 15.5 2026 02 19 21 42.65 -16 10.4 2.631 1.657 7.8 4.6 15.9 2026 03 01 21 53.14 -18 17.1 2.694 1.767 16.6 9.2 16.1 2026 03 11 22 02.99 -20 22.8 2.727 1.880 25.6 13.2 16.4 2026 03 21 22 12.19 -22 33.2 2.732 1.994 34.6 16.5 16.6 2026 03 31 22 20.64 -24 53.8 2.714 2.109 43.8 19.1 16.8 2026 04 10 22 28.20 -27 29.6 2.676 2.225 53.2 21.1 16.9 2026 04 20 22 34.67 -30 25.8 2.622 2.340 62.8 22.4 17.0 2026 04 30 22 39.73 -33 47.0 2.560 2.456 72.7 23.0 17.2 2026 05 10 22 42.98 -37 36.7 2.495 2.570 82.7 22.9 17.3 2026 05 20 22 43.81 -41 56.3 2.435 2.684 92.9 22.1 17.4 2026 05 30 22 41.32 -46 43.7 2.388 2.798 103.1 20.7 17.5 2026 06 09 22 34.25 -51 50.8 2.363 2.910 112.8 18.8 17.6 NOTE: These 'Central Bureau Electronic Telegrams' are sometimes superseded by text appearing later in the printed IAU Circulars. (C) Copyright 2025 CBAT 2025 October 19 (CBET 5624) Daniel W. E. Green