Electronic Telegram No. 5565 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 K4 (SIVERD) Robert Siverd, Institute for Astronomy, University of Hawaii, reports the discovery of a diffuse comet on CCD images taken on Mar. 30 UT with a 0.5-m f/2 Schmidt reflector at Sutherland, South Africa, in the course of the "Asteroid Terrestrial-Impact Last Alert System" (ATLAS) search program. The coma showed a size of 14".4 (full-width-at-half-maximum) in 6" seeing, with no apparent tail. The discovery observations are tabulated below: 2025 UT R.A. (2000) Decl. Mag. May 30.95302 15 52 45.21 -67 26 34.0 19.1 30.95667 15 52 44.85 -67 26 32.1 19.2 30.96257 15 52 44.92 -67 26 27.5 19.3 30.97268 15 52 44.19 -67 26 21.1 19.0 After the comet was posted on the Minor Planet Center's PCCP webpage, H. Sato (Bunkyo-ku, Tokyo, Japan) reported that he obtained twenty stacked 60-s CCD exposures remotely on May 31.7 UT using a 0.32-m f/9.0 Ritchey-Chretien telescope at Siding Spring, NSW, Australia, which show a strongly condensed coma 7" in diameter and no tail; the magnitude was 19.3 as measured within a circular aperture of radius 3".8. The available astrometry appears on MPEC 2025-L55. The following parabolic orbital elements by S. Nakano (Central Bureau) are from 23 observations spanning May 30-June 6 (mean residual 0".6). There are no close approaches to major planets. T = 2025 Aug. 6.39183 TT Peri. = 330.73822 Node = 296.04565 2000.0 q = 2.5420753 AU Incl. = 39.33625 The following ephemeris by the undersigned from the above orbital elements uses photometric power-law parameters H = 14.0 and 2.5n = 8 for the magnitudes. Date TT R. A. (2000) Decl. Delta r Elong. Phase Mag. 2025 05 05 16 11.72 -69 47.0 2.028 2.732 124.8 17.6 19.0 2025 05 15 16 05.29 -69 26.1 1.942 2.695 129.0 16.9 18.9 2025 05 25 15 57.31 -68 24.2 1.871 2.662 132.5 16.3 18.8 2025 06 04 15 50.06 -66 38.6 1.817 2.632 134.8 15.9 18.7 2025 06 14 15 45.48 -64 11.8 1.781 2.606 135.7 15.8 18.6 2025 06 24 15 44.49 -61 10.5 1.766 2.585 134.7 16.2 18.5 2025 07 04 15 47.23 -57 44.4 1.773 2.567 132.0 17.1 18.5 2025 07 14 15 53.31 -54 04.9 1.802 2.555 127.8 18.3 18.5 2025 07 24 16 02.11 -50 22.0 1.852 2.546 122.5 19.7 18.6 2025 08 03 16 13.09 -46 44.5 1.922 2.542 116.5 20.9 18.7 2025 08 13 16 25.71 -43 18.2 2.011 2.543 110.1 22.0 18.8 2025 08 23 16 39.57 -40 06.3 2.115 2.548 103.5 22.7 18.9 2025 09 02 16 54.35 -37 10.0 2.233 2.558 96.9 23.1 19.0 2025 09 12 17 09.81 -34 28.6 2.362 2.572 90.3 23.0 19.1 2025 09 22 17 25.74 -32 00.5 2.499 2.591 83.8 22.7 19.3 2025 10 02 17 42.00 -29 43.7 2.642 2.614 77.4 21.9 19.4 2025 10 12 17 58.44 -27 35.9 2.789 2.641 71.1 21.0 19.6 2025 10 22 18 14.98 -25 34.8 2.937 2.672 64.9 19.7 19.8 2025 11 01 18 31.53 -23 38.4 3.085 2.707 58.8 18.3 19.9 2025 11 11 18 48.00 -21 45.0 3.230 2.745 52.7 16.7 20.1 2025 11 21 19 04.32 -19 53.2 3.371 2.786 46.6 14.9 20.2 NOTE: These 'Central Bureau Electronic Telegrams' are sometimes superseded by text appearing later in the printed IAU Circulars. (C) Copyright 2025 CBAT 2025 June 9 (CBET 5565) Daniel W. E. Green