Electronic Telegram No. 5618 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 R2 (SWAN) New CCD observations from H. Abe (Yatsuka, Japan) obtained with a 0.26-m f/6 reflector: 2025 UT R.A. (2000) Decl. Mag. Observer Oct. 5.40571 15 17 10.47 -15 58 53.9 Abe 5.41081 15 17 13.18 -15 58 56.8 6.8 " Further to CBET 5606, S. Nakano (Central Bureau) has calculated the following improved orbital elements from 196 observations spanning 2025 Sept. 12-Oct. 5 (mean residual 0".5), with corresponding a = 77.33 AU and P = 680.0 years. He notes that Oct. 2 astrometry yield residuals of -0.31 deg in R.A. and +0.05 deg in Decl. from his orbit on CBET 5606. T = 2025 Sept.12.83280 TT Peri. = 308.32323 e = 0.9934817 Node = 335.34402 2000.0 q = 0.5040566 AU Incl. = 4.47366 Imaging posted at the "ICQ Comet Observations" forum on Facebook with the comet at low altitude shows an ion tail up to 4 degrees long with very little evident dust (a much-shorter dust tail). Visually, the tail is faint and difficult. Selected visual total-magnitude and coma-diameter estimates: Sept. 17.35 UT, 6.7, 1'.3 (C. Stoitsis, Caulfield, Vic., Australia, 20x80 binoculars); 21.37, 6.6, 4' (C. Wyatt, Walcha, NSW, Australia, 15x70 binoculars); 23.39, 6.6, 4'.5 (Wyatt); 24.94, 6.7, 4' (M. Goiato, Aracatuba, Brazil, 7x50 binoculars); 26.42, 6.0, -- (M. Mattiazzo, Strathalbyn, South Australia, 8x40 binoculars); 26.94, 6.2, 4' (Goiato; 0.5-deg tail); 27.41, 5.9, -- (Mattiazzo); 27.90, 6.1, 2' (J. G. de S. Aguiar, Campinas, Brazil, 11x80 binoculars); 29.73, 5.7, -- (S. J. O'Meara, Maun, Botswana, 8x42 binoculars); 30.73, 5.9, -- (O'Meara); Oct. 1.73, 6.1, -- (O'Meara); 2.73, 5.8, -- (O'Meara); 3.73, 5.7, -- (O'Meara); 5.92, 5.9, 5' (W. Souza, Sao Paulo, Brazil, 20x80 binoculars); 6.72, 5.7, -- (O'Meara). Total CCD magnitudes from K. Kadota, Ageo, Japan, with a 0.25-m f/5 reflector: Sept. 17.40 UT, 6.9 (3' coma; 10' tail in p.a. 120 deg; altitude 4 deg); Sept. 27.39, 6.6. Total magnitudes from K. Yoshimoto, Yamaguchi, Japan, using a 7.7-cm f/4.3 refractor (+ Canon EOS Kiss X7i digital camera G channel), corrected for atmospheric extinction: Oct. 1.41, 6.7 (coma diameter 4'.5; tail 20' in p.a. 103 deg); Oct. 5.41, 6.2 (coma diameter 7'.2). The following ephemeris by the undersigned from the above orbital elements uses photometric power-law parameters H = 9.0 and 2.5n = 8 for the magnitudes; note that this supercedes that on CBET 5606. Date TT R. A. (2000) Decl. Delta r Elong. Phase Mag. 2025 09 22 13 52.02 -12 40.6 0.696 0.550 31.6 106.8 6.1 2025 09 27 14 18.06 -14 04.5 0.581 0.606 33.1 115.2 6.1 2025 10 02 14 49.82 -15 17.0 0.477 0.675 35.9 119.6 6.0 2025 10 07 15 32.29 -16 14.5 0.386 0.753 41.2 119.1 5.9 2025 10 12 16 32.56 -16 32.4 0.314 0.835 50.6 112.5 5.9 2025 10 17 17 55.72 -15 06.3 0.269 0.919 65.7 98.8 5.9 2025 10 22 19 31.17 -11 03.7 0.264 1.004 84.4 80.5 6.1 2025 10 27 20 54.00 -05 53.6 0.300 1.090 100.6 63.7 6.7 2025 11 01 21 53.41 -01 36.7 0.365 1.174 111.1 52.1 7.4 2025 11 06 22 34.13 +01 24.9 0.447 1.258 116.8 44.7 8.1 2025 11 11 23 02.93 +03 32.0 0.541 1.341 119.4 40.0 8.7 2025 11 16 23 24.38 +05 04.9 0.642 1.423 120.0 37.0 9.3 2025 11 21 23 41.21 +06 16.6 0.749 1.504 119.3 34.9 9.8 2025 12 01 00 06.81 +08 04.8 0.975 1.662 115.9 32.2 10.7 2025 12 11 00 26.57 +09 29.0 1.215 1.817 110.9 30.4 11.5 2025 12 21 00 43.35 +10 42.2 1.467 1.967 105.1 28.9 12.2 2025 12 31 00 58.51 +11 50.2 1.726 2.114 98.8 27.4 12.8 2026 01 10 01 12.72 +12 55.2 1.992 2.257 92.3 25.8 13.3 2026 01 20 01 26.36 +13 58.4 2.262 2.397 85.6 24.2 13.8 2026 01 30 01 39.65 +15 00.1 2.533 2.533 78.8 22.4 14.2 2026 02 09 01 52.70 +16 00.3 2.802 2.667 72.0 20.6 14.6 2026 02 19 02 05.56 +16 58.8 3.067 2.799 65.2 18.7 15.0 2026 03 01 02 18.29 +17 55.4 3.325 2.928 58.3 16.7 15.3 2026 03 11 02 30.86 +18 49.8 3.574 3.055 51.4 14.7 15.6 2026 03 21 02 43.29 +19 41.8 3.812 3.179 44.5 12.7 15.9 NOTE: These 'Central Bureau Electronic Telegrams' are sometimes superseded by text appearing later in the printed IAU Circulars. (C) Copyright 2025 CBAT 2025 October 7 (CBET 5618) Daniel W. E. Green