Electronic Telegram No. 5597 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) On Aug. 13.47 UT, T. Prystavski (Lviv, Ukraine) obtained unfiltered CCD images of this comet (cf. CBET 5557) with a 0.51-m reflector at Siding Spring, NSW, Australia, that showed a coma diameter of 1'.5, total mag 12.1, and a tail about 7'.5 long in p.a. 107 degrees. On July 6.41, he obtained CMOS images with a 0.51-m reflector located at the Utah Desert Remote Observatory (near Beryl Junction, UT, USA) from which he measured a coma size of 0'.5 with total mag 14.7 and a 3'.5 tail in p.a. 227 degrees. CCD images taken on Aug. 18.87 UT with a 40-cm f/3 reflector (+ Bessel R filter) by E. Bryssinck at Kruibeke, Belgium, show a coma of size 1'.5 and total mag 12.5 with a 4'.2 tail in p.a. 100 degrees. Visual total-magnitude and coma-diameter estimates: July 16.95 UT, 13.9, 0'.5 (J. J. Gonzalez, Leon, Spain, 0.20-m reflector); 20.92, 13.3, 1'.1 (Gonzalez); 25.15, 13.0, 1' (J. G. de S. Aguiar, Campinas, Brazil, 0.30-m reflector); 26.14, 12.8, 1' (Aguiar); Aug. 14.88, 11.7, 2'.5 (Gonzalez); 20.90, 11.4, 2'.5 (Gonzalez). The following updated orbital elements by S. Nakano (Central Bureau) are from 1404 observations spanning 2025 Apr. 8-Aug. 14 (mean residual 0".4), with corresponding "original" and "future" values of 1/a being -0.000064 and -0.000714 (+/- 0.000020) AU**-1, respectively. The comet will pass 0.58 and 0.40 AU from the earth on 2025 Aug. 12 and 2025 Nov. 24 UT, respectively. Epoch = 2025 Oct. 12.0 TT T = 2025 Oct. 8.43903 TT Peri. = 271.02175 e = 1.0002880 Node = 97.55644 2000.0 q = 0.3342181 AU Incl. = 147.86492 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 (unchanged from those on CBET 5557). Date TT R. A. (2000) Decl. Delta r Elong. Phase Mag. 2025 08 13 17 36.53 +14 56.5 0.578 1.363 115.0 42.3 12.4 2025 08 18 16 44.34 +09 12.2 0.598 1.271 101.1 51.4 12.2 2025 08 23 16 00.53 +03 37.4 0.648 1.178 87.6 59.1 12.1 2025 08 28 15 25.47 -01 09.3 0.719 1.081 75.3 64.6 12.1 2025 09 02 14 57.54 -05 00.1 0.803 0.983 64.5 67.9 12.0 2025 09 07 14 34.73 -08 02.0 0.894 0.882 54.9 69.2 11.8 2025 09 12 14 15.23 -10 23.9 0.986 0.779 46.0 68.4 11.6 2025 09 17 13 57.55 -12 12.3 1.076 0.674 37.6 65.5 11.3 2025 09 22 13 40.39 -13 29.6 1.160 0.569 29.4 59.9 10.9 2025 10 22 11 54.71 -04 04.9 1.067 0.510 28.4 67.9 10.3 2025 11 01 11 38.34 +04 52.1 0.825 0.718 45.4 79.7 10.9 2025 11 11 11 20.78 +19 47.4 0.587 0.925 66.2 78.3 11.1 2025 11 21 10 32.88 +49 21.9 0.418 1.122 97.5 60.8 11.0 2025 12 01 04 27.51 +74 07.4 0.450 1.310 127.6 36.6 11.7 2025 12 11 01 25.19 +55 12.5 0.668 1.490 127.6 31.6 13.0 2025 12 21 01 01.32 +44 10.4 0.959 1.662 117.6 31.6 14.2 2025 12 31 00 57.11 +38 27.7 1.275 1.827 107.3 30.9 15.1 2026 01 10 00 59.02 +35 16.8 1.600 1.987 97.7 29.4 15.9 2026 01 20 01 03.73 +33 27.5 1.925 2.143 88.7 27.3 16.6 2026 01 30 01 09.99 +32 27.0 2.246 2.293 80.1 25.0 17.1 2026 02 09 01 17.16 +31 57.9 2.559 2.440 71.9 22.6 17.6 2026 02 19 01 24.93 +31 50.5 2.860 2.583 63.9 20.1 18.1 2026 03 01 01 33.07 +31 58.6 3.145 2.722 56.2 17.6 18.5 2026 03 11 01 41.42 +32 18.1 3.413 2.859 48.8 15.2 18.8 2026 03 21 01 49.86 +32 46.3 3.662 2.993 41.7 12.8 19.1 NOTE: These 'Central Bureau Electronic Telegrams' are sometimes superseded by text appearing later in the printed IAU Circulars. (C) Copyright 2025 CBAT 2025 August 21 (CBET 5597) Daniel W. E. Green