Electronic Telegram No. 5279 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/2023 F3 (ATLAS) An apparently asteroidal object discovered on CCD images taken on Mar. 28 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 found to show cometary appearance by CCD astrometrists elsewhere after it was posted on the Minor Planet Center's PCCP webpage. The discovery observations are tabulated below: 2023 UT R.A. (2000) Decl. Mag. Mar. 28.35310 18 43 22.97 -35 18 21.2 18.8 28.37010 18 43 22.82 -35 18 25.5 18.8 28.37285 18 43 22.76 -35 18 25.9 18.6 28.38519 18 43 22.65 -35 18 29.2 18.8 Six stacked 60-s exposures taken remotely on Mar. 29.4 UT by H. Sato (Tokyo, Japan) with a "Deep Sky Chile" 0.51-m f/6.8 astrograph located at Rio Hurtado, Chile, show a strongly condensed coma 5" in diameter with no tail; the magnitude was 18.8 as measured within a circular aperture of radius 2".9. Six additional stacked 120-s exposures taken on Mar. 31.79 by Sato with a 0.43-m f/6.8 astrograph located at Siding Spring, NSW, Australia, show only a stellar appearance; the magnitude was 19.2 as measured within a circular aperture of radius 3".8. The object looked slightly elongated on single images taken remotely by T. Prystavski (Lviv, Ukraine) on May 2.4 with an iTelescope 0.51-m f/6.8 Corrected Dall-Kirkham telescope located in the Rio Hurtado Valley, Chile; stacked images show a tail about 5" long in p.a. 345 degrees, and the total magnitude was measured as 18.7. Additional exposures obtained on June 1.4 by Prystavski with the same telescope showed the comet at total mag 18.1 in a very dense star field with a very condensed head (but no obvious coma) and a possible tiny tail in p.a. 351 degrees. Eight stacked images obtained by Prystavski with a 0.32-m f/9.0 Ritchey-Chretien reflector on July 16.5 show a fuzzy head of size 0'.2 and total mag 17.6 (with nearby stars showing sharp points), appearing slightly elongated toward p.a. 24 degrees. Fifty 60-s exposures obtaned by A. Aletti, F. Bellini, L. Buzzi, and G. Galli on July 14.74-14.77 with a 0.36-m f/8.4 reflector located at Hakos, Namibia, show a condensed 5" coma that is softer with respect to nearby stars (4".0 full-width-at-half-maximum, vs. 3".2) and a hint of a broad tail toward p.a. 35 deg. The available astrometry appears on MPEC 2023-O43. The following orbital elements by S. Nakano (Central Bureau) are from 108 observations spanning Mar. 28-July 17 (mean residual 0".4), with corresponding "original" and "future" values of 1/a being +0.000082 and -0.000097 (+/- 0.000059) AU**-1, respectively. The comet passed 2.13 AU from Saturn on 2021 Jan. 3 and will pass 2.62 AU from Jupiter on 2026 Nov. 21 UT. Epoch = 2025 Feb. 14.0 TT T = 2025 Feb. 2.71195 TT Peri. = 265.53547 e = 1.0034168 Node = 109.46237 2000.0 q = 5.1906360 AU Incl. = 145.96184 The following ephemeris by the undersigned from the above orbital elements uses photometric power-law parameters H = 7.0 and 2.5n = 8 for the magnitudes. Date TT R. A. (2000) Decl. Delta r Elong. Phase Mag. 2023 06 25 17 32.04 -41 05.0 5.801 6.767 160.4 2.9 17.5 2023 07 05 17 19.56 -41 14.2 5.801 6.722 153.0 3.9 17.4 2023 07 15 17 07.66 -41 14.2 5.834 6.678 143.4 5.2 17.4 2023 07 25 16 56.81 -41 06.4 5.898 6.633 133.1 6.4 17.4 2023 08 04 16 47.32 -40 53.1 5.987 6.589 122.6 7.5 17.4 2023 08 14 16 39.40 -40 36.7 6.097 6.546 112.1 8.2 17.5 2023 08 24 16 33.13 -40 19.4 6.220 6.503 101.8 8.8 17.5 2023 09 03 16 28.47 -40 03.2 6.350 6.460 91.7 9.0 17.5 2023 09 13 16 25.35 -39 49.5 6.482 6.417 81.9 8.9 17.5 2023 09 23 16 23.64 -39 39.4 6.609 6.376 72.3 8.6 17.5 2023 10 03 16 23.18 -39 33.4 6.727 6.334 62.9 8.1 17.6 2023 10 13 16 23.80 -39 31.9 6.831 6.293 53.8 7.4 17.6 2023 10 23 16 25.36 -39 35.2 6.917 6.253 45.0 6.5 17.6 2024 01 11 16 51.37 -42 46.9 6.690 5.949 38.3 5.9 17.3 2024 01 21 16 53.71 -43 31.9 6.544 5.913 46.8 7.0 17.3 2024 01 31 16 55.15 -44 21.8 6.377 5.878 55.7 8.0 17.2 2024 02 10 16 55.42 -45 16.7 6.195 5.844 64.9 8.8 17.1 2024 02 20 16 54.21 -46 16.2 6.000 5.811 74.3 9.4 17.0 2024 03 01 16 51.18 -47 19.5 5.799 5.778 83.9 9.8 16.9 2024 03 11 16 45.98 -48 25.4 5.596 5.746 93.7 9.9 16.8 2024 03 21 16 38.22 -49 31.4 5.399 5.715 103.6 9.8 16.7 2024 03 31 16 27.60 -50 34.2 5.213 5.685 113.4 9.3 16.6 2024 04 10 16 13.95 -51 29.1 5.045 5.655 123.1 8.5 16.5 2024 04 20 15 57.36 -52 10.4 4.902 5.626 132.1 7.6 16.5 2024 04 30 15 38.30 -52 32.2 4.790 5.598 139.8 6.7 16.4 2024 05 10 15 17.66 -52 30.0 4.712 5.570 145.1 6.0 16.3 2024 05 20 14 56.64 -52 01.7 4.672 5.544 146.4 5.8 16.3 2024 05 30 14 36.54 -51 08.8 4.670 5.518 143.5 6.3 16.3 2024 06 09 14 18.39 -49 56.0 4.705 5.493 137.2 7.2 16.3 2024 06 19 14 02.88 -48 30.3 4.773 5.469 129.0 8.3 16.3 NOTE: These 'Central Bureau Electronic Telegrams' are sometimes superseded by text appearing later in the printed IAU Circulars. (C) Copyright 2023 CBAT 2023 July 21 (CBET 5279) Daniel W. E. Green