Electronic Telegram No. 5606 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) On Sept. 10, Vladimir Bezugly (Dnipro, Ukraine) informed the Central Bureau that he had found a moving object, possibly a comet, on low-resolution public website hydrogen Lyman-alpha images obtained during Sept. 5-9 with the Solar Wind Anisotropies (SWAN) camera on the Solar and Heliospheric Observer (SOHO) spacecraft (see CBETs 4068, 4136, 4939, 8619, 8587, 8344, 8346; and website URL http://swan.projet.latmos.ipsl.fr/). SWAN creates daily full-sky scans at the wavelength of hydrogen Lyman_alpha, but its images are at a very poor resolution (not better than about 1 degree). Bezugly provided the following approximate positions for the object, and it was circulated online including at the comets-ml discussion group to get additional observations. Bezugly's magnitude estimate is for a guessed optical brightness based on previous correlated SWAN vs. optical magnitudes of comets. 2025 UT R.A. (2000) Decl. Mag. Sept. 5.5 12h44m - 6d29' 8.5 8.5 12 54 - 8 04 9.5 12 58 - 9 07 10.5 13 03 - 9 39 Bezugly contacted M. Masek (Institute of Physics, Czech Academy of Sciences) to ask if he could obtain ground-based images of the object. Masek was able to obtain CCD images on Sept. 12.0 UT with a 135-mm f/2 lens at the Cherenkov Telescope array at Cerro Paranal, Chile, that show a comet of magnitude V = 7.4 and coma diameter 3'.7 with a long tail that was located close to Bezugly's positions (astrometry provided below). Masek adds that eleven 60-s stacked exposures show a 2.8-degree tail in p.a. 114 degrees. 2025 UT R.A. (2000) Decl. Mag. Observer Sept.11.98143 13 06 19.92 - 9 09 48.3 7.5 Masek 11.99373 13 06 23.32 - 9 09 49.5 7.4 " R. D. Matson (Irvine, CA, USA) subsequently reported on the comets-ml discussion forum the following approximate positions that he measured for identified images of this comet from SWAN images: 2025 UT R.A. (2000) Decl. Aug. 9.5 11 02 42 + 4 06 11.5 11 10 30 + 3 16 14.5 11 20 54 + 2 11 16.5 11 28 36 + 1 21 17.5 11 31 12 + 1 04 20.5 11 42 48 - 0 11 22.5 11 50 36 - 1 01 25.5 12 00 54 - 2 08 Sept. 8.5 12 52 54 - 8 34 9.5 12 58 06 - 9 07 The comet was then posted on the Minor Planet Center's PCCP webpage. Following are reports sent directly to the Central Bureau. Fourteen stacked 20-s CCD exposures taken remotely by H. Sato (Bunkyo-ku, Tokyo, Japan) using a 0.51-m f/6.8 astrograph located at Siding Spring, NSW, Australia, on Sept. 12.4 UT show a strongly condensed coma 2'.1 in diameter with a tail longer than 20' toward p.a. 115 deg; the magnitude was 8.1 as measured within a circular aperture of radius 64".8. E. Guido, Castellammare di Stabia, Italy, writes that five stacked 30-s CMOS exposures taken by M. Rocchetto, J. Ferguson, and himself with a 0.35-m f/3 reflector located at Siding Spring on Sept. 12.4 UT show a coma about 4' across with a tail at least 1 degree long in p.a. 116 degrees. Four stacked 15-s CMOS exposures taken remotely by K. Yoshimoto (Kumage, Yamaguchi, Japan) with a 0.25-m f/3.8 Newtonian astrograph located at Rio Hurtado, Chile, on Sept. 12.99 UT show a coma 2'.7 in diameter of total mag 8.1 and a straight tail over 45' long in p.a. 115 deg. Nine stacked 15-s exposures taken by Yoshimoto (same instrumentation) on Sept. 14.0 show a 3'.0 coma of total mag 7.1 (V) with a straight tail longer than 45' in p.a. 114 degrees. F. D. Romanov (Yuzhno-Morskoy, Nakhodka, Russia), obtained a single 60-s CCD exposure on Sept. 13.36 UT with an iTelescope 0.51-m f/6.8 reflector (+ Luminance filter) located Siding Spring that shows a 2' coma of mag 8.9 with a straight tail at least 18' long (to the edge of the frame) in p.a. 112 degrees. One-hundred 10-s CMOS exposures taken by A. Aletti, F. Bellini, L. Buzzi, and G. Galli with a 0.36-m f/8.6 reflector (+ R_c filter) located at Hakos, Namibia, on Sept. 13.7 UT show a condensed coma of size 50" with a straight tail at least 25' long in p.a. 114 degrees. S. J. O'Meara reports the comet's head to be of mag 7.2 on CMOS images obtained with a Canon 90D camera (+ 200-mm f/3.5 lens, ISO 1250) at Maun, Botswana, on Sept. 13.72 and 14.72 UT. Visual total-magnitude and coma-diameter estimates: Sept. 12.40 UT, 7.3, -- M. Mattiazzo (Strathalbyn, South Australia, 15x70 binoculars); 12.90, 7.5, 2' (J. G. de S. Aguiar, Campinas, Brazil, 25x100 binoculars); 12.92, 7.2, 2' (A. Amorim, Florianopolis, Brazil, 20x50 binoculars); 13.91, 7.0, 2' (Aguiar; greenish coma noted visually); 13.94, 6.9, 2' (M. Goiato, Aracatuba, Brazil, 20x80 binoculars; tail 0.25 deg long in p.a. 120 degrees; comet altitude 12 deg); 14.90, 6.9, 2' (Aguiar, 11x80 binoculars; 0.4-deg tail); 14.94, 7.1, 2' (Goiato); The available precise astrometry appears on MPEC 2025-R102. The following parabolic orbital elements by S. Nakano (Central Bureau) are from 60 observations spanning Sept. 12-14 (mean residual 0".6). The comet will pass 0.27 AU from the earth on 2025 Oct. 21 UT. T = 2025 Sept.12.36877 TT Peri. = 307.16312 Node = 336.00754 2000.0 q = 0.5028255 AU Incl. = 4.46794 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. Date TT R. A. (2000) Decl. Delta r Elong. Phase Mag. 2025 05 05 09 00.84 +16 52.0 2.235 2.426 88.4 24.6 13.8 2025 05 15 09 02.98 +16 34.5 2.246 2.285 79.3 25.8 13.6 2025 05 25 09 07.72 +16 05.6 2.247 2.141 70.9 26.5 13.4 2025 06 04 09 14.95 +15 24.9 2.235 1.994 63.1 27.0 13.1 2025 06 14 09 24.51 +14 31.7 2.206 1.843 56.1 27.2 12.8 2025 06 24 09 36.37 +13 24.9 2.159 1.688 49.6 27.3 12.5 2025 07 04 09 50.55 +12 02.5 2.092 1.529 43.8 27.4 12.1 2025 07 14 10 07.18 +10 22.1 2.003 1.366 38.7 27.7 11.6 2025 07 24 10 26.53 +08 20.1 1.892 1.198 34.3 28.5 11.0 2025 08 03 10 49.08 +05 51.8 1.757 1.028 30.9 30.4 10.3 2025 08 13 11 15.57 +02 52.0 1.597 0.857 28.6 34.4 9.5 2025 08 23 11 46.99 -00 44.6 1.409 0.694 27.6 42.4 8.5 2025 09 02 12 24.22 -04 55.0 1.187 0.561 28.1 58.0 7.4 2025 09 07 12 44.86 -07 05.2 1.065 0.519 28.9 69.5 6.9 2025 09 12 13 06.41 -09 10.1 0.936 0.503 29.7 82.9 6.5 2025 09 17 13 28.67 -11 02.9 0.808 0.515 30.6 96.4 6.2 2025 09 22 13 52.11 -12 40.8 0.684 0.554 31.7 107.9 6.1 2025 09 27 14 18.47 -14 05.7 0.570 0.612 33.2 116.1 6.1 2025 10 02 14 51.00 -15 20.1 0.466 0.683 36.2 120.0 6.0 2025 10 07 15 35.02 -16 19.7 0.377 0.761 41.8 118.9 5.9 2025 10 12 16 37.95 -16 36.6 0.306 0.844 51.9 111.5 5.8 2025 10 17 18 04.19 -15 00.6 0.265 0.929 67.8 96.9 5.9 2025 10 22 19 40.46 -10 46.0 0.265 1.015 86.7 78.2 6.2 2025 10 27 21 01.29 -05 37.3 0.305 1.101 102.4 61.9 6.8 2025 11 01 21 58.25 -01 28.8 0.373 1.186 112.3 50.8 7.5 2025 11 06 22 37.19 +01 25.6 0.458 1.270 117.5 43.8 8.1 2025 11 11 23 04.79 +03 28.1 0.554 1.354 119.8 39.4 8.8 2025 11 16 23 25.44 +04 58.2 0.656 1.436 120.2 36.5 9.3 2025 11 21 23 41.72 +06 08.1 0.764 1.517 119.4 34.6 9.9 2025 12 01 00 06.65 +07 54.6 0.993 1.676 115.8 32.0 10.8 2025 12 11 00 26.01 +09 18.2 1.235 1.832 110.7 30.2 11.6 2025 12 21 00 42.53 +10 31.2 1.488 1.983 104.8 28.7 12.2 2025 12 31 00 57.50 +11 39.2 1.750 2.130 98.5 27.2 12.8 NOTE: These 'Central Bureau Electronic Telegrams' are sometimes superseded by text appearing later in the printed IAU Circulars. (C) Copyright 2025 CBAT 2025 September 15 (CBET 5606) Daniel W. E. Green