Electronic Telegram No. 5555 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 J1 (BORISOV) Gennady Borisov, Nauchnij, Crimea, reports his discovery of another comet on CCD images obtained on May 2.98-3.01 UT with a 0.50-m f/1.9 astrograph. The object showed a diffuse coma of diameter about 1".5 and a broad tail 7" long spanning p.a. 250-260 degrees; the total green magnitude was 19.3 as measured in a circular aperture of size 0'.25. Additional images yielded a 9" tail in p.a. 210-240 degrees. The discovery observations are tabulated below. 2025 UT R.A. (2000) Decl. Mag. Observer May 2.97693 19 39 00.47 +84 32 25.8 19.3 Borisov 2.99154 19 38 59.84 +84 32 40.3 19.4 " 3.00620 19 38 59.22 +84 32 54.3 19.2 " After the comet was posted on the Minor Planet Center's PCCP webpage, other observers also commented on the cometary appearance. D. Buczynski (Portmahomack, Scotland) writes that images obtained by N. James and himself on May 5.0 UT with a 0.35-m f/6 reflector show a slight elongation to the west. One-hundred-sixty 12-s exposures taken with a 0.41-m reflector by P. Birtwhistle (Great Shefford, Berkshire, England) on May 5.9 show a very condensed head of size 4" with a 20" tail in p.a. 230 degrees. Twenty-two stacked 60-s CCD exposures taken remotely by H. Sato (Bunkyo-ku, Tokyo, Japan) on May 7.34-7.35 with a 0.61-m f/6.5 astrograph located at the Sierra Remote Observatory near Auberry, CA, USA, show only a stellar appearance; the magnitude was 19.9 as measured within a circular aperture of radius 2".9. The available astrometry (including pre-discovery observations obtained on Apr. 29 and May 2 at mag 19.6-20.1 with the 1.2-m Schmidt telescope at Palomar and reported post-discovery by the "Zwicky Transient Facility" survey) appears on MPEC 2025-K5. The following parabolic orbital elements by S. Nakano (Central Bureau) are from 134 observations spanning Apr. 29-May 16 (mean residual 0".3). There have been no close approaches to major planets. T = 2026 June 11.59267 TT Peri. = 166.69620 Node = 274.00429 2000.0 q = 3.5757985 AU Incl. = 95.43800 The following ephemeris by the undersigned from the above orbital elements uses photometric power-law parameters H = 10.0 and 2.5n = 8 for the magnitudes. Date TT R. A. (2000) Decl. Delta r Elong. Phase Mag. 2025 03 26 19 12.71 +74 05.2 5.271 5.267 84.4 10.9 19.4 2025 04 05 19 23.27 +76 49.4 5.240 5.207 82.6 11.0 19.3 2025 04 15 19 32.42 +79 35.5 5.217 5.148 80.5 11.1 19.3 2025 04 25 19 38.74 +82 21.5 5.201 5.089 78.1 11.2 19.2 2025 05 05 19 37.68 +85 05.2 5.189 5.030 75.4 11.2 19.2 2025 05 15 19 05.01 +87 42.0 5.181 4.972 72.6 11.2 19.1 2025 05 25 12 41.76 +89 03.1 5.173 4.915 69.7 11.1 19.1 2025 06 04 09 55.17 +86 53.6 5.165 4.858 66.9 11.1 19.1 2025 06 14 09 40.51 +84 32.2 5.153 4.801 64.4 11.0 19.0 2025 06 24 09 42.26 +82 16.7 5.136 4.746 62.1 10.9 19.0 2025 07 04 09 48.71 +80 09.3 5.111 4.690 60.2 10.8 18.9 2025 07 14 09 56.97 +78 11.1 5.078 4.636 58.9 10.8 18.9 2025 07 24 10 06.05 +76 22.8 5.035 4.582 58.2 10.9 18.8 2025 08 03 10 15.47 +74 45.2 4.980 4.530 58.3 11.0 18.7 2025 08 13 10 24.94 +73 19.2 4.913 4.478 59.1 11.2 18.7 2025 08 23 10 34.30 +72 05.6 4.833 4.426 60.7 11.5 18.6 2025 09 02 10 43.38 +71 05.7 4.740 4.376 63.1 11.9 18.5 2025 09 12 10 52.01 +70 20.5 4.634 4.327 66.2 12.3 18.4 2025 09 22 11 00.03 +69 51.5 4.516 4.279 70.0 12.7 18.3 2025 10 02 11 07.17 +69 40.2 4.387 4.232 74.5 13.2 18.2 2025 10 12 11 13.10 +69 48.2 4.250 4.186 79.6 13.6 18.1 2025 10 22 11 17.32 +70 17.3 4.105 4.141 85.1 13.9 18.0 2025 11 01 11 19.02 +71 08.7 3.958 4.097 91.0 14.0 17.9 2025 11 11 11 16.95 +72 23.0 3.810 4.055 97.2 14.0 17.8 2025 11 21 11 08.99 +73 58.9 3.667 4.014 103.5 13.8 17.7 2025 12 01 10 51.36 +75 51.5 3.533 3.975 109.6 13.5 17.5 2025 12 11 10 17.83 +77 47.2 3.415 3.937 115.2 13.1 17.4 2025 12 21 09 20.41 +79 18.0 3.317 3.901 119.8 12.6 17.3 2025 12 31 08 00.10 +79 37.3 3.244 3.866 122.8 12.3 17.3 2026 01 10 06 41.22 +78 12.2 3.200 3.833 123.6 12.3 17.2 2026 01 20 05 46.23 +75 18.5 3.188 3.802 121.9 12.7 17.2 2026 01 30 05 14.66 +71 34.8 3.208 3.772 118.0 13.3 17.1 2026 02 09 04 58.43 +67 31.3 3.259 3.744 112.2 14.1 17.2 2026 02 19 04 51.49 +63 26.6 3.337 3.719 105.1 14.9 17.2 2026 03 01 04 50.29 +59 31.9 3.437 3.695 97.2 15.4 17.2 2026 03 11 04 52.74 +55 53.1 3.554 3.673 89.0 15.7 17.3 2026 03 21 04 57.62 +52 32.3 3.682 3.654 80.6 15.6 17.3 NOTE: These 'Central Bureau Electronic Telegrams' are sometimes superseded by text appearing later in the printed IAU Circulars. (C) Copyright 2025 CBAT 2025 May 17 (CBET 5555) Daniel W. E. Green