Electronic Telegram No. 5640 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 3I/ATLAS M. Jaeger, Vienna, writes that comet 3I/ATLAS has shown a complex tail structure since it has emerged from conjunction with the sun. Stacked CMOS images obtained on Nov. 8.2 UT with a 0.28-m f/2.2 RASA reflector at by Jaeger, G. Rhemann, and E. Prosperi show a 5' coma of total mag 9.1 and tails or jets as follows: 400" long in p.a. 0.5 deg; 500" long in p.a. 316 deg; 900" long in p.a. 295 deg; 430" long in p.a. 278 deg; 200" long in p.a. 109 deg. The comet was at altitude 7-10 degrees above the horizon. Numerous of their images of comet 3I, as well as many other images of this comet by other observers, have been posted online at the "ICQ Comet Observations" forum on Facebook. An anti-sunward ion tail stretches more than a degree from the coma on numerous photos; there is also visible a short sunward tail presumably composed of dust trailing the comet, typical of comets when seen near the plane of the ecliptic. Visual total-magnitude and coma-diameter estimates: Nov. 9.22 UT, 9.8, 3' (V. Gonano, Udine, Italy, 20x80 binoculars); 10.31, 10.0, 2' (J. G. de S. Aguiar, Campinas, Brazil, 0.27-m reflector); 11.23, 9.9, 1'.5 (Gonano); 11.31, 10.0, 1' (Aguiar); 12.31, 10.1, 1' (Aguiar); 18.24, 9.3, 4' (J. J. Gonzalez, Cueto, Rosales, Leon, Spain, 0.20-m reflector); 19.20, 9.8, 2' (Gonano); 19.31, 9.7, 2' (M. Goiato, Aracatuba, Brazil, 0.22-m reflector); 20.31, 9.8, 2' (Goiato); 21.29, 9.7, 3' (W. Souza, Atibaia, Brazil, 20x80 binoculars); 22.29, 9.8, 3' (Souza); 22.30, 9.7, 3' (Goiato, 20x100 binoculars). Total digital magnitudes (unfiltered unless otherwise noted) and coma- diameter measurements: Nov. 3.84 UT, 9.6, -- (K. Kadota, Ageo, Japan, 0.25-m f/5 reflector + CCD); 4.19, 9.1, -- (Jaeger); 5.85, 9.8, 2'.3 (K. Yoshimoto, Yamaguchi, Japan, 0.3-m f/4 reflector + CMOS); 10.84, 9.7, 3'.5 (Yoshimoto; V filter; faint tail 18' long in p.a. 296 deg and anti-tail 2' long in p.a. 110 deg); 13.84, 9.7, 3'.6 (Yoshimoto; V filter; 20' tail in p.a. 295 deg and anti-tail 3' long in p.a. 110 deg); 16.11, 11.1, 0'.67 (S. E. Shurpakov, Baran, Belarus, 0.41-m reflector + CCD). Revised orbital elements by S. Nakano (Central Bureau) from 4405 observations spanning 2025 May 15-Nov. 21 (mean residual 0".4), with corresponding "original" and "future" values of 1/a being -3.787577 and -3.789458 (+/- 0.000028) AU**-1, respectively. There are no obvious non- gravitational forces manifested yet from the available astrometry. The comet will pass 0.36 AU from Jupiter on 2026 Mar. 16 UT. Epoch = 2025 Oct. 12.0 TT T = 2025 Oct. 29.48339 TT Peri. = 128.00820 e = 6.1390461 Node = 322.15453 2000.0 q = 1.3563954 AU Incl. = 175.11295 The comet has been some 3 magnitudes brighter in recent weeks, as seen both in spacecraft imaging and from the ground this month, than would have been expected by the ephemeris on CBET 5596. The following ephemeris by the undersigned from the above orbital elements uses photometric power-law parameters H = 6.5, 2.5n = 8 for the magnitudes to reflect that brightening. Date TT R. A. (2000) Decl. Delta r Elong. Phase Mag. 2025 11 21 12 34.14 -02 22.9 2.033 1.586 49.8 28.4 9.6 2025 11 26 12 18.73 -01 02.3 1.970 1.687 58.9 30.1 9.8 2025 12 01 12 01.78 +00 27.6 1.912 1.801 68.5 30.6 10.0 2025 12 06 11 43.18 +02 06.9 1.862 1.925 78.4 30.1 10.1 2025 12 11 11 22.88 +03 54.8 1.824 2.056 88.9 28.6 10.3 2025 12 16 11 01.02 +05 49.4 1.802 2.195 99.7 26.2 10.5 2025 12 21 10 37.87 +07 47.3 1.799 2.338 110.9 23.2 10.7 2025 12 26 10 13.92 +09 44.1 1.818 2.486 122.2 19.6 11.0 2025 12 31 09 49.81 +11 34.8 1.862 2.637 133.5 15.7 11.2 2026 01 05 09 26.24 +13 15.3 1.931 2.791 144.5 11.8 11.5 2026 01 10 09 03.83 +14 42.7 2.026 2.948 155.2 8.0 11.8 2026 01 15 08 43.08 +15 56.2 2.144 3.106 165.4 4.6 12.1 2026 01 20 08 24.30 +16 56.1 2.285 3.266 174.7 1.6 12.4 2026 01 25 08 07.63 +17 43.9 2.446 3.427 175.0 1.4 12.7 2026 01 30 07 53.06 +18 21.5 2.624 3.590 166.8 3.6 13.0 2026 02 04 07 40.49 +18 50.7 2.818 3.753 158.8 5.5 13.3 2026 02 09 07 29.74 +19 13.3 3.024 3.918 151.2 7.0 13.6 2026 02 14 07 20.63 +19 30.9 3.243 4.083 144.0 8.2 13.9 2026 02 19 07 12.98 +19 44.5 3.471 4.248 137.1 9.1 14.2 2026 02 24 07 06.60 +19 55.1 3.707 4.415 130.6 9.8 14.5 2026 03 01 07 01.33 +20 03.4 3.949 4.581 124.3 10.3 14.8 2026 03 06 06 57.03 +20 09.9 4.197 4.748 118.3 10.6 15.0 2026 03 11 06 53.57 +20 15.0 4.450 4.916 112.5 10.8 15.3 2026 03 16 06 50.83 +20 19.0 4.705 5.084 106.9 10.8 15.5 2026 03 21 06 48.72 +20 22.1 4.963 5.252 101.4 10.7 15.7 2026 03 26 06 47.17 +20 24.5 5.223 5.420 96.1 10.5 16.0 2026 03 31 06 46.09 +20 26.3 5.483 5.588 90.9 10.3 16.2 2026 04 05 06 45.42 +20 27.6 5.743 5.757 85.8 10.0 16.4 2026 04 10 06 45.10 +20 28.4 6.003 5.926 80.8 9.6 16.6 2026 04 15 06 45.09 +20 28.8 6.261 6.095 75.9 9.2 16.8 2026 04 20 06 45.35 +20 28.9 6.517 6.263 71.1 8.7 16.9 2026 04 25 06 45.84 +20 28.6 6.770 6.433 66.3 8.2 17.1 2026 04 30 06 46.53 +20 28.0 7.021 6.602 61.6 7.7 17.3 2026 05 05 06 47.38 +20 27.1 7.268 6.771 57.0 7.2 17.5 2026 05 10 06 48.37 +20 26.0 7.510 6.940 52.4 6.6 17.6 2026 05 15 06 49.48 +20 24.6 7.749 7.110 47.8 6.0 17.8 2026 05 20 06 50.69 +20 22.9 7.982 7.279 43.3 5.5 17.9 2026 05 25 06 51.97 +20 21.1 8.211 7.449 38.8 4.9 18.0 2026 05 30 06 53.31 +20 19.0 8.434 7.618 34.3 4.3 18.2 2026 06 04 06 54.70 +20 16.8 8.651 7.788 29.9 3.7 18.3 2026 08 28 07 13.82 +19 28.2 11.335 10.672 47.0 4.0 20.0 2026 09 02 07 14.18 +19 25.9 11.437 10.842 51.8 4.2 20.1 2026 09 07 07 14.42 +19 23.8 11.535 11.011 56.5 4.4 20.1 2026 09 12 07 14.53 +19 21.9 11.628 11.181 61.4 4.5 20.2 2026 09 17 07 14.50 +19 20.1 11.718 11.351 66.2 4.6 20.3 2026 09 22 07 14.34 +19 18.7 11.805 11.520 71.1 4.7 20.4 2026 09 27 07 14.03 +19 17.4 11.890 11.690 76.1 4.8 20.4 2026 10 02 07 13.59 +19 16.4 11.973 11.859 81.1 4.8 20.5 2026 10 07 07 13.01 +19 15.7 12.055 12.029 86.2 4.8 20.5 NOTE: These 'Central Bureau Electronic Telegrams' are sometimes superseded by text appearing later in the printed IAU Circulars. (C) Copyright 2025 CBAT 2025 November 23 (CBET 5640) Daniel W. E. Green