Electronic Telegram No. 5623 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 Q3 (ATLAS) An apparently asteroidal object discovered on CCD images taken on Aug. 20 UT with a 0.5-m f/2 Schmidt reflector at Haleakala, 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 it was posted to the Minor Planet Center's PCCP webpage. The discovery observations are tabulated below. 2025 UT R.A. (2000) Decl. Mag. Aug. 20.56601 4 34 29.80 +49 18 29.1 19.5 20.57433 4 34 30.12 +49 18 27.0 19.9 20.58265 4 34 30.48 +49 18 25.3 19.9 20.59097 4 34 30.82 +49 18 23.7 19.6 M. Jaeger, Vienna, reports that the object appeared somewhat more diffuse than stars of similar brightness on exposures taken on Aug. 25.98 UT with a 0.3-m f/4 reflector at Stixendorf, Austria; forty-three 80-s CMOS exposures showed a possible coma of size 7"-8" of total mag 19.3 (his photo was posted at the "ICQ Comet Observations" Facebook forum. R. Weryk, Department of Physics and Astronomy, University of Western Ontario, reports an accidental re-discovery of this comet in images obtained with the Pan-STARRS2 1.8-m Ritchey-Chretien reflector at Haleakala on Oct. 15.54-15.56 UT, in which three stacked 45-s w-band survey images taken in 1".2 seeing show a head of size 1".4 (full-width-at-half-maximum) but no apparent coma and a straight tail about 4" long toward p.a. 250 degrees. Twelve stacked 60-s CCD exposures taken remotely by H. Sato (Bunkyo-ku, Tokyo, Japan) with a 0.61-m f/6.5 astrograph located at the Sierra Remote Observatory near Auberry, CA, USA, on Aug. 23.5 UT showed only a stellar appearance; the magnitude was 19.6 as measured within a circular aperture of radius 1".9. The available astrometry appears on MPEC 2025-T165. The following orbital elements by S. Nakano (Central Bureau) are from 409 observations spanning 2025 Aug. 20-Oct. 15 (mean residual 0".4), with corresponding "original" and "future" values of 1/a being +0.001933 and +0.002175 (+/- 0.000040) AU**-1. There are no close approaches to major planets. Epoch = 2026 Feb. 9.0 TT T = 2026 Feb. 25.64355 TT Peri. = 218.45452 e = 0.9971972 Node = 233.14399 2000.0 q = 2.1125681 AU Incl. = 90.18407 The following ephemeris by the undersigned from the above orbital elements uses photometric power-law parameters H = 13.0 and 2.5n = 8 for the magnitudes. Date TT R. A. (2000) Decl. Delta r Elong. Phase Mag. 2025 10 12 04 24.53 +42 00.0 1.941 2.639 124.6 18.1 17.8 2025 10 22 04 09.07 +38 36.6 1.748 2.573 137.8 15.1 17.5 2025 11 01 03 49.32 +33 55.6 1.588 2.511 152.5 10.5 17.2 2025 11 11 03 26.91 +27 47.8 1.474 2.451 168.3 4.7 17.0 2025 11 21 03 04.18 +20 28.1 1.416 2.396 170.3 4.0 16.8 2025 12 01 02 43.56 +12 40.8 1.419 2.344 153.6 10.8 16.7 2025 12 11 02 26.79 +05 20.5 1.479 2.296 136.6 17.1 16.7 2025 12 21 02 14.56 -00 55.7 1.582 2.253 121.1 22.0 16.8 2025 12 31 02 06.85 -05 57.1 1.714 2.215 107.3 25.1 16.9 2026 01 10 02 03.17 -09 51.1 1.861 2.183 95.3 26.7 17.1 2026 01 20 02 02.93 -12 51.4 2.012 2.156 84.7 27.0 17.2 2026 01 30 02 05.56 -15 12.0 2.160 2.136 75.4 26.5 17.3 2026 02 09 02 10.59 -17 04.8 2.297 2.122 67.3 25.4 17.4 2026 02 19 02 17.64 -18 38.8 2.421 2.114 60.4 24.0 17.5 2026 03 01 02 26.44 -20 01.6 2.527 2.113 54.6 22.5 17.6 2026 03 11 02 36.77 -21 18.7 2.616 2.118 50.0 21.0 17.7 2026 03 21 02 48.53 -22 34.6 2.685 2.130 46.8 19.9 17.8 2026 03 31 03 01.62 -23 53.3 2.736 2.149 45.0 19.2 17.8 2026 04 10 03 16.01 -25 17.8 2.770 2.173 44.6 18.9 17.9 2026 04 20 03 31.73 -26 50.9 2.789 2.204 45.5 19.0 18.0 2026 04 30 03 48.81 -28 35.0 2.795 2.240 47.4 19.3 18.0 2026 05 10 04 07.34 -30 31.6 2.792 2.281 50.2 19.9 18.1 2026 05 20 04 27.42 -32 41.7 2.783 2.327 53.5 20.5 18.2 2026 05 30 04 49.17 -35 05.7 2.772 2.378 57.0 21.0 18.2 2026 06 09 05 12.72 -37 42.2 2.764 2.432 60.6 21.3 18.3 2026 06 19 05 38.21 -40 29.0 2.764 2.491 63.9 21.5 18.4 2026 06 29 06 05.73 -43 22.3 2.774 2.552 66.9 21.5 18.5 2026 07 09 06 35.33 -46 17.1 2.799 2.617 69.2 21.3 18.6 2026 07 19 07 06.99 -49 07.7 2.841 2.684 70.8 20.9 18.7 NOTE: These 'Central Bureau Electronic Telegrams' are sometimes superseded by text appearing later in the printed IAU Circulars. (C) Copyright 2025 CBAT 2025 October 16 (CBET 5623) Daniel W. E. Green