Electronic Telegram No. 5557 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 K1 (ATLAS) An apparently asteroidal object discovered on CCD images taken on May 24 UT with a 0.5-m f/2 Schmidt reflector at Rio Hurtado, Chile, in the course of the "Asteroid Terrestrial-Impact Last Alert System" (ATLAS) search program has been found to show cometary appearance by CCD astrometrists elsewhere following posting to the Minor Planet Center's NEOCP and PCCP webpages. The discovery observations are tabulated below: 2025 UT R.A. (2000) Decl. Mag. May 24.35354 22 25 46.46 +15 07 01.6 18.6 24.35670 22 25 46.47 +15 07 04.0 18.5 24.36215 22 25 46.51 +15 07 07.0 18.3 24.37938 22 25 46.65 +15 07 14.8 18.3 A. Hale (Cloudcroft, NM, USA) writes that single 180-s CCD exposures taken remotely on May 25.13 UT with a 0.35-m f/3 Cassegrain reflector at Sutherland, South Africa, show a condensed head that is slightly diffuse (mag 17.9-18.3) when compared to stars of similar brightness, with no tail; he adds that individual 180-s CCD exposures taken on May 26.15 with a 0.35-m f/3 Cassegrain reflector at Tenerife show a condensed and diffuse coma of size 5" with no tail. M. Mattiazzo, Swan Hill, Victoria, Australia, writes that eight 60-s stacked exposures taken remotely on May 25.45 with a 0.61-m f/6.5 reflector located at the Sierra Remote Observatory near Auberry, CA, USA, show a diffuse coma 8" wide with V magnitude 18.1. Eight stacked 60-s CCD exposures taken remotely by H. Sato (Bunkyo-ku, Tokyo, Japan) with a 0.51-m f/6.8 astrograph located at the Utah Desert Remote Observatory (near Beryl Junction, UT, USA) on May 26.43 show only a stellar appearance; the magnitude was 17.7 as measured within a circular aperture of radius 4".9. R. Weryk, Department of Physics and Astronomy, University of Western Ontario, reports that three 60-s gri-band images taken with the 3.6-m Canada-France-Hawaii Telescope on May 26.62 (queue observer J. Silva; queue coordinator H. Flewelling in 0".6 seeing show a very condensed head of size 1".3 (full-width-at-half-maximum) with an obvious 11"-long tail in p.a. 255 degrees. Twenty stacked 60-s CCD exposures taken remotely by K. Yoshimoto (Kumage, Yamaguchi, Japan) with a 0.51-m f/6.8 reflector at the Utah Desert Remote Observatory on May 27.4 show a condensed coma 15" in diameter with total mag 17.8; it appears to have a faint tail in p.a. 260 deg. The available astrometry appears on MPEC 2025-K110 and includes pre- discovery observations taken with the 1.2-m Schmidt telescope at Palomar on May 19.4 (mag 18.5-19.0) and 22.43-22.47 (mag 18.2-18.9) that were reported after the discovery ATLAS observations were reported to the MPC. The following parabolic orbital elements by S. Nakano (Central Bureau) are from 85 observations spanning May 19-27 (mean residual 0".4); there are no close approaches to major planets. T = 2025 Oct. 8.41891 TT Peri. = 271.05754 Node = 97.57478 2000.0 q = 0.3342494 AU Incl. = 147.85198 The following ephemeris by the undersigned from the above orbital elements uses photometric power-law parameters H = 12.5 and 2.5n = 8 for the magnitudes. The extended ephemeris is only provided for an idea of the comet's projected path and brightness, though its small absolute magnitude does not hold promise for survival past perihelion. Date TT R. A. (2000) Decl. Delta r Elong. Phase Mag. 2025 05 05 22 19.87 +12 46.3 3.182 2.897 64.7 18.3 18.7 2025 05 15 22 23.58 +13 56.0 2.894 2.761 72.4 20.4 18.3 2025 05 25 22 25.87 +15 12.1 2.596 2.623 80.3 22.4 17.9 2025 06 04 22 26.16 +16 35.0 2.290 2.481 88.5 24.1 17.5 2025 06 14 22 23.55 +18 05.4 1.981 2.335 97.1 25.6 16.9 2025 06 24 22 16.55 +19 44.2 1.673 2.186 106.1 26.5 16.3 2025 07 04 22 02.49 +21 30.3 1.372 2.032 115.8 26.8 15.7 2025 07 14 21 36.38 +23 16.2 1.087 1.873 125.9 26.1 14.9 2025 07 24 20 48.81 +24 25.9 0.834 1.710 134.8 24.9 14.0 2025 08 03 19 26.75 +22 49.0 0.647 1.539 134.5 28.0 13.1 2025 08 13 17 36.24 +14 54.8 0.578 1.362 115.0 42.4 12.4 2025 08 18 16 44.04 +09 10.2 0.598 1.271 101.1 51.4 12.2 2025 08 23 16 00.25 +03 35.5 0.648 1.177 87.5 59.1 12.1 2025 08 28 15 25.23 -01 10.9 0.719 1.081 75.3 64.7 12.1 2025 09 02 14 57.33 -05 01.5 0.803 0.982 64.5 68.0 12.0 2025 09 07 14 34.54 -08 03.1 0.894 0.882 54.8 69.2 11.8 2025 09 12 14 15.05 -10 24.8 0.986 0.778 46.0 68.4 11.6 2025 09 17 13 57.38 -12 13.0 1.076 0.674 37.6 65.5 11.3 2025 09 22 13 40.22 -13 30.0 1.161 0.569 29.3 59.9 10.9 2025 11 01 11 38.29 +04 54.6 0.825 0.719 45.4 79.7 10.9 2025 11 06 11 30.57 +11 12.7 0.702 0.823 55.0 80.6 11.1 2025 11 11 11 20.79 +19 51.3 0.586 0.925 66.2 78.3 11.1 2025 11 16 11 05.21 +32 14.0 0.486 1.025 80.2 71.9 11.0 2025 11 21 10 32.98 +49 28.2 0.418 1.123 97.6 60.7 11.0 2025 11 26 08 57.06 +68 35.6 0.404 1.218 115.7 46.9 11.2 2025 12 01 04 26.34 +74 10.4 0.450 1.310 127.6 36.6 11.7 2025 12 06 02 05.65 +64 16.9 0.545 1.401 130.3 32.4 12.4 2025 12 11 01 24.71 +55 13.1 0.669 1.490 127.5 31.6 13.0 2025 12 16 01 08.42 +48 45.2 0.809 1.577 122.8 31.7 13.6 2025 12 21 01 01.02 +44 11.1 0.960 1.662 117.5 31.7 14.2 2025 12 26 00 57.79 +40 53.6 1.116 1.745 112.3 31.4 14.7 2025 12 31 00 56.86 +38 28.3 1.275 1.827 107.3 30.9 15.1 NOTE: These 'Central Bureau Electronic Telegrams' are sometimes superseded by text appearing later in the printed IAU Circulars. (C) Copyright 2025 CBAT 2025 May 27 (CBET 5557) Daniel W. E. Green