Electronic Telegram No. 5488 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/2024 G3 (ATLAS) This comet (cf. CBET 5384) has apparently brightened more rapidly than expected. Included here are reports sent directly to the Central Bureau and the ICQ, or taken from the "ICQ Comet Observations" Facebook forum (where numerous photos are posted). A. Pearce (Nedlands, W. Australia) analyzed a photo of comet C/2024 G3 taken by R. Kaufman at Mt. Porepunkah, Victoria, Australia, on Jan. 3.74 UT, yielding total mag 2.4 with coma diameter 2' and a 6' tail in p.a. 247 degrees; Kaufman notes that the comet was much brighter than on Jan. 1.73, and that he was able to see in with 8x42 binoculars though not via naked eye. Pearce measured total mag 5.9 from five 15-s stacked CCD images taken by M. Mattiazzo with a 0.25-m reflector located at the Rio Hurtado Valley in Chile on 2024 Dec. 27.36, with a 1'.5 coma and a tail 7'.5 long in p.a. 227 deg. Pearce measured total mag 6.9 with a 1'.2 coma and a 7' tail in p.a. 213 degrees from five stacked 15-s CCD images taken by T. Lovejoy on 2024 Dec. 23.36 also with a 0.25-m reflector located at Rio Hurtado. T. Lovejoy notes that exposures taken with a Sony A7iii camera (+ 135-mm-f.l. f/1.8 lens) on 2025 Jan. 2.76 UT at Cleveland Point, Qld., Australia, indicate total mag 3.7; the comet was clearly visible in Nikon 10-22x50mm binoculars, at total mag approximately 3.2 with ICQ extinction correction applied for 45 Oph (8.3 degree higher and similar apparent brightness). Approximate total-magnitude and coma-diameter measurements for C/2024 G3 made by T. Prystavski (Lviv, Ukraine) via remote CMOS imaging with an iTelescope 0.25-m f/3.8 Newtonian reflector located at Rio Hurtado Valley in Chile: 2024 Dec. 21.35 UT, 6.9, 1'.7; Dec. 22.35, 6.7, 1'.7; Dec. 23.35, 6.7, 1'.6; 2025 Jan. 2.36, 4.7, 2'.0; tail about 19'.3 long in p.a. 242 deg. M. Masek (Institute of Physics, Czech Academy of Sciences) reported total magnitudes R = 6.1 and V = 6.5 with a 1'.5 coma from eight stacked CCD images obtained on 2024 Dec. 26.33 UT with the FRAM 0.30-m telescope near Malargue, Argentina. Total-magnitude and coma-diameter measurements from "Deep-Sky Chile" CMOS images taken remotely by K. Yoshimoto, Yamaguchi, Japan (extinction corrections applied for total magnitudes): 2024 Dec. 20.36 UT, 7.5*, 1'.3 (135-mm f/3.5 lens; 8' tail in p.a. 215 deg); Dec. 30.37, 5.5*, 1'.9 (tail 7' long in p.a. 240 deg); 2025 Jan. 3.37, 2.8:*, 2'.0 (tail 10' long in p.a. 248 deg). On Dec. 20.36, CMOS images taken by Yoshimoto with a 0.25-m f/3.8 Newtonian Astrograph at the same site show a coma 1'.3 in diameter and a tail 6' long in p.a. 215 deg. CCD images taken by Yoshimoto using the same 0.25-m telescope on Dec. 25.37 show a coma of diameter 1'.6 and a tail 8' long in p.a. 220 deg (total mag 6.5). Eight stacked 10-s CMOS exposures taken by H. Sato (Bunkyo-ku, Tokyo, Japan) with a "Deep Sky Chile" 0.25-m f/3.8 astrograph on 2024 Dec. 22.36 UT show a strongly condensed coma 1'.1 in diameter with an 8'.5 tail in p.a. 220 degrees; the magnitude was 7.1 as measured within a circular aperture of radius 34".8. Selected visual total-magnitude and coma-diameter estimates sent to the Central Bureau and the ICQ, all made with the comet at low altitude (generally 4-6 degrees above the horizon; an asterisk indicates that the observer stated use of the ICQ extinction tables): 2024 Dec. 23.72 UT, 6.5, 2'.2 (C. Wyatt, Walcha, NSW, Australia, 15x70 binoculars; faint tail visible in p.a. about 232 deg); Dec. 24.30, 6.2, 2' (J. G. de S. Aguiar, Campinas, Brazil, 25x100 binoculars); Dec. 26.72, 6.0, 2'.3 (Wyatt; faint tail visible in p.a. 234 deg); Dec. 29.32, 5.5, 1' (W. Souza, Atibaia, Brazil, 7x50 binoculars); Dec. 30.32, 5.1*, 2' (M. Goiato, Aracatuba, Brazil, 20x100 binoculars; 0.2-deg tail in p.a. 235 deg); Dec. 31.32, 4.9, 2' (Goiato; 0.2-deg tail in p.a. 240 deg); 2025 Jan. 1.30, 4.8*, 2' (Aguiar; 0.2-deg tail); Jan. 3.31, 2.0*, 3' (A. Amorim, Florianopolis, Brazil, 10x50 binoculars); Jan. 3.76, 2.0*, 3' (M. Mattiazzo, Swan Hill, Victoria, Australia, 8x40 binoculars; 15' tail in p.a. 240 deg); Jan. 4.33, 2.1*, 3' (Goiato, 7x50 binoculars; 0.5-deg tail in p.a. 250 deg; clouds). Mattiazzo noted that in 15x70 binoculars a prominent dark lane or nuclear shadow was visible on Jan. 3.76 (comet altitude 6 degrees; barely detectable with the naked eye under clear conditions); the comet was still visible when the sun was 8 degrees below the horizon. The following orbital elements by S. Nakano (Central Bureau) are from 285 observations spanning 2024 Apr. 5-Dec. 22 (mean residual 0".5), with corresponding "original" and "future" values of 1/a being +0.000301 and +0.000129 (+/- 0.000002) AU**-1. Epoch = 2025 Jan. 5.0 TT T = 2025 Jan. 13.42888 TT Peri. = 108.12332 e = 1.0000092 Node = 220.33941 2000.0 q = 0.0935414 AU Incl. = 116.84559 The following ephemeris by the undersigned from the above orbital elements uses photometric power-law parameters H = 5.0 and 2.5n = 8 Date TT R. A. (2000) Decl. Delta r Elong. Phase Mag. 2024 12 16 16 33.59 -37 35.1 1.811 0.941 19.4 20.3 6.1 2024 12 18 16 39.75 -36 59.5 1.760 0.892 19.3 21.4 5.8 2024 12 20 16 46.19 -36 20.3 1.707 0.843 19.2 22.6 5.6 2024 12 22 16 52.96 -35 36.8 1.653 0.792 19.2 24.0 5.3 2024 12 24 17 00.11 -34 48.4 1.597 0.740 19.0 25.6 5.0 2024 12 26 17 07.71 -33 54.0 1.540 0.687 18.8 27.5 4.6 2024 12 28 17 15.86 -32 52.5 1.481 0.631 18.5 29.7 4.3 2024 12 30 17 24.65 -31 42.5 1.420 0.573 18.1 32.2 3.8 2025 01 01 17 34.28 -30 21.9 1.357 0.513 17.5 35.2 3.3 2025 01 03 17 44.97 -28 48.2 1.293 0.450 16.7 38.8 2.8 2025 01 05 17 57.10 -26 58.0 1.226 0.384 15.6 43.5 2.1 2025 01 06 18 03.90 -25 55.4 1.192 0.349 14.9 46.3 1.7 2025 01 07 18 11.33 -24 46.8 1.157 0.314 14.1 49.7 1.3 2025 01 08 18 19.58 -23 31.6 1.121 0.277 13.1 53.8 0.8 2025 01 09 18 28.90 -22 09.1 1.085 0.238 12.0 59.0 0.2 2025 01 10 18 39.72 -20 39.2 1.048 0.199 10.6 65.8 -0.5 2025 01 11 18 52.72 -19 03.8 1.011 0.159 9.0 75.5 -1.4 2025 01 12 19 09.08 -17 31.7 0.976 0.122 7.1 90.0 -2.4 2025 01 13 19 30.12 -16 32.5 0.947 0.097 5.3 109.3 -3.2 2025 01 14 19 54.08 -16 56.9 0.937 0.099 5.2 115.4 -3.2 2025 01 15 20 15.23 -18 33.0 0.948 0.127 7.2 102.8 -2.3 2025 01 16 20 32.04 -20 26.7 0.969 0.164 9.6 90.2 -1.3 2025 01 17 20 45.67 -22 14.5 0.996 0.204 11.8 80.8 -0.5 2025 01 18 20 57.17 -23 51.5 1.024 0.243 13.7 73.7 0.1 2025 01 19 21 07.16 -25 17.6 1.053 0.281 15.4 68.1 0.7 2025 01 20 21 16.03 -26 34.1 1.083 0.318 16.8 63.6 1.2 2025 01 21 21 24.05 -27 42.1 1.113 0.354 18.1 59.9 1.6 2025 01 22 21 31.38 -28 42.9 1.142 0.388 19.3 56.7 2.0 2025 01 23 21 38.15 -29 37.6 1.172 0.422 20.3 53.9 2.3 2025 01 24 21 44.45 -30 26.8 1.201 0.455 21.2 51.5 2.7 2025 01 25 21 50.36 -31 11.4 1.230 0.486 22.0 49.3 2.9 2025 01 30 22 15.56 -34 02.5 1.368 0.635 25.2 41.3 4.1 2025 02 04 22 35.95 -35 56.8 1.498 0.770 27.4 36.1 5.0 2025 02 09 22 53.36 -37 18.0 1.618 0.895 29.1 32.4 5.7 2025 02 14 23 08.75 -38 18.6 1.729 1.014 30.7 29.8 6.2 2025 02 19 23 22.70 -39 06.1 1.832 1.126 32.2 27.9 6.7 2025 02 24 23 35.58 -39 45.0 1.928 1.233 33.7 26.4 7.2 2025 03 01 23 47.65 -40 18.6 2.017 1.337 35.3 25.3 7.5 2025 03 06 23 59.08 -40 48.9 2.099 1.436 37.0 24.6 7.9 2025 03 11 00 09.99 -41 17.7 2.175 1.533 38.8 24.0 8.2 2025 03 16 00 20.49 -41 45.9 2.245 1.627 40.8 23.5 8.4 2025 03 21 00 30.65 -42 14.5 2.309 1.718 42.9 23.2 8.7 2025 03 26 00 40.54 -42 44.2 2.368 1.808 45.1 23.0 8.9 2025 03 31 00 50.20 -43 15.7 2.423 1.895 47.4 22.8 9.1 2025 04 05 00 59.66 -43 49.4 2.472 1.980 49.8 22.7 9.3 2025 04 10 01 08.95 -44 25.7 2.518 2.063 52.3 22.6 9.5 2025 04 15 01 18.10 -45 05.0 2.560 2.145 54.8 22.5 9.7 2025 04 20 01 27.12 -45 47.4 2.599 2.226 57.4 22.4 9.9 2025 04 25 01 36.05 -46 33.3 2.635 2.305 60.1 22.2 10.0 2025 04 30 01 44.88 -47 22.9 2.668 2.383 62.8 22.1 10.1 2025 05 05 01 53.64 -48 16.4 2.699 2.460 65.6 21.9 10.3 2025 05 10 02 02.31 -49 13.7 2.728 2.535 68.3 21.7 10.4 2025 05 15 02 10.91 -50 15.0 2.756 2.609 71.0 21.5 10.5 2025 05 20 02 19.45 -51 20.2 2.784 2.683 73.8 21.2 10.7 2025 05 25 02 27.93 -52 29.3 2.810 2.755 76.5 20.9 10.8 2025 05 30 02 36.35 -53 42.3 2.837 2.827 79.1 20.6 10.9 2025 06 04 02 44.69 -54 59.1 2.864 2.898 81.7 20.3 11.0 2025 06 09 02 52.96 -56 19.4 2.893 2.968 84.2 19.9 11.1 2025 06 14 03 01.16 -57 43.0 2.922 3.037 86.6 19.5 11.2 2025 06 24 03 17.27 -60 38.9 2.986 3.173 91.1 18.7 11.4 2025 07 04 03 32.90 -63 44.3 3.060 3.306 94.9 17.8 11.6 2025 07 14 03 47.82 -66 55.7 3.145 3.437 97.9 17.0 11.8 2025 07 24 04 01.70 -70 09.8 3.244 3.565 100.1 16.3 12.0 2025 08 03 04 13.93 -73 23.1 3.357 3.691 101.2 15.6 12.2 2025 08 13 04 23.43 -76 32.0 3.485 3.815 101.3 15.1 12.4 2025 08 23 04 28.06 -79 33.5 3.626 3.937 100.4 14.6 12.6 2025 09 02 04 22.84 -82 23.7 3.781 4.057 98.7 14.2 12.8 2025 09 12 03 54.49 -84 55.7 3.947 4.176 96.1 13.9 12.9 2025 09 22 02 23.76 -86 48.7 4.122 4.293 93.0 13.5 13.1 2025 10 02 23 29.37 -87 03.9 4.304 4.408 89.4 13.1 13.3 2025 10 12 21 43.82 -85 41.1 4.490 4.522 85.4 12.7 13.5 2025 10 22 21 09.28 -83 50.4 4.679 4.634 81.3 12.3 13.7 2025 11 01 21 01.34 -81 57.6 4.867 4.746 77.1 11.8 13.8 2025 11 11 21 04.25 -80 09.1 5.053 4.856 72.9 11.2 14.0 2025 11 21 21 12.36 -78 26.7 5.233 4.964 68.9 10.7 14.2 NOTE: These 'Central Bureau Electronic Telegrams' are sometimes superseded by text appearing later in the printed IAU Circulars. (C) Copyright 2025 CBAT 2025 January 4 (CBET 5488) Daniel W. E. Green