Electronic Telegram No. 5398 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 J3 (ATLAS) A comet discovered on CCD images taken on May 6.3 UT with a 0.5-m f/2 Schmidt reflector at Rio Hurtado, Chile, in the course of the "Asteroid Terrestrial-Impact Last Alert System" (ATLAS) search program has been reported to the Minor Planet Center (cf. MPEC 2024-K118). James Robinson reported that the four stacked 30-s discovery images taken in 5".7 seeing show a head of size 9".0 (full-width-at-half-maximum). The discovery observations are tabulated below: 2024 UT R.A. (2000) Decl. Mag. May 6.26497 19 28 46.96 -42 55 22.4 17.6 6.26678 19 28 47.03 -42 55 23.3 18.6 6.26964 19 28 46.87 -42 55 23.8 17.1 6.27477 19 28 46.78 -42 55 23.9 18.0 After the comet was posted on the MPC's PCCP webpage, other CCD astrometrists also commented on the cometary appearance. Six stacked 120-s exposures taken remotely by H. Sato (Bunkyo-ku, Tokyo, Japan) with a 0.51-m f/6.8 astrograph at Siding Spring, NSW, Australia, on May 12.8 UT show a strongly condensed coma 8" in diameter with no tail; the magnitude was 17.8 as measured within a circular aperture of radius 7".6. Seventy-five stacked 60-s exposures taken by A. Aletti, F. Bellini, L. Buzzi, G. Galli, and M. Auteri using a 0.36-m f/7.5 reflector at Hakos, Namibia, on May 19.0 show a condensed 6" coma with a faint, straight tail at least 12" long in p.a. 220 degrees. The following parabolic orbital elements by S. Nakano (Central Bureau) are from 70 observations spanning May 6-28 (mean residual 0".5). There are no close approaches to major planets. T = 2026 Nov. 24.92260 TT Peri. = 74.11797 Node = 285.98343 2000.0 q = 3.8620938 AU Incl. = 75.56342 The following ephemeris by the undersigned from the above orbital elements uses photometric power-law parameters H = 6.0 and 2.5n = 8 for the magnitudes. Date TT R. A. (2000) Decl. Delta r Elong. Phase Mag. 2024 05 10 19 27.81 -42 56.8 7.771 8.324 120.1 6.0 17.8 2024 05 20 19 24.43 -42 59.8 7.583 8.262 129.4 5.4 17.7 2024 05 30 19 19.96 -43 00.7 7.413 8.200 138.6 4.7 17.7 2024 06 09 19 14.51 -42 58.3 7.266 8.139 147.3 3.9 17.6 2024 06 19 19 08.27 -42 51.2 7.146 8.077 154.8 3.1 17.5 2024 06 29 19 01.50 -42 38.3 7.054 8.015 159.7 2.5 17.5 2024 07 09 18 54.49 -42 19.0 6.992 7.953 159.7 2.5 17.4 2024 07 19 18 47.56 -41 53.1 6.960 7.891 154.7 3.2 17.4 2024 07 29 18 41.04 -41 20.9 6.958 7.829 147.1 4.0 17.4 2024 08 08 18 35.21 -40 43.1 6.982 7.767 138.2 5.0 17.3 2024 08 18 18 30.30 -40 01.0 7.031 7.706 128.8 5.9 17.3 2024 08 28 18 26.46 -39 15.8 7.100 7.644 119.2 6.6 17.3 2024 09 07 18 23.79 -38 28.7 7.185 7.582 109.6 7.2 17.3 2024 09 17 18 22.32 -37 40.9 7.280 7.520 100.0 7.6 17.3 2024 09 27 18 22.03 -36 53.1 7.382 7.458 90.5 7.7 17.3 2024 10 07 18 22.86 -36 06.1 7.485 7.396 81.1 7.7 17.3 2024 10 17 18 24.74 -35 20.2 7.584 7.334 71.8 7.4 17.3 2024 10 27 18 27.55 -34 35.6 7.676 7.273 62.6 7.0 17.3 2024 11 06 18 31.20 -33 52.3 7.755 7.211 53.6 6.4 17.3 NOTE: These 'Central Bureau Electronic Telegrams' are sometimes superseded by text appearing later in the printed IAU Circulars. (C) Copyright 2024 CBAT 2024 May 29 (CBET 5398) Daniel W. E. Green