Electronic Telegram No. 5483 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 X2 (ATLAS) A. Fitzsimmons reported to the Minor Planet Center on Dec. 12 the discovery of an object with "a soft cometary appearance" on CCD images taken 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, noting a coma approximately 11" in diameter in individual 30-s exposures. The discovery observations are tabulated below: 2024 UT R.A. (2000) Decl. Mag. Dec. 12.30201 11 00 36.30 -67 22 37.7 18.8 12.30870 11 00 35.99 -67 22 40.9 18.5 12.31150 11 00 35.98 -67 22 40.7 18.5 12.31747 11 00 35.30 -67 22 43.5 18.8 12.32391 11 00 35.01 -67 22 46.7 18.7 12.32832 11 00 34.50 -67 22 49.4 18.5 12.33775 11 00 34.12 -67 22 51.9 18.8 After the comet was posted on the MPC's NEOCP and PCCP webpages, other CCD astrometrists have reported on the cometary appearance. Ten stacked 60-s exposures taken remotely by H. Sato (Bunkyo-ku, Tokyo, Japan) using a 0.51-m f/6.8 astrograph located at Rio Hurtado, Chile, on Dec. 13.21 UT show a strongly condensed coma 5" in diameter with a 10"-long tail in p.a. 270 deg; the magnitude was 19.0 as measured within a circular aperture of radius 2".9. A. Hale (Cloudcroft, NM, USA) found a condensed 4" coma of mag 18.3 (noting the appearance only slightly non-stellar) on images taken remotely on Dec. 14.28 with a 0.35-m f/3 Cassegrain reflector located at Cerro Tololo. Additional images taken by Hale on Dec. 20.0 with a 0.35-m f/3 Cassegrain reflector located at Sutherland, South Africa, show a slightly diffuse appearance (condensed 4" coma of mag 19.2-19.4) with no tail. The available astrometry appears on MPEc 2024-Y71. The following parabolic orbital elements by S. Nakano (Central Bureau) are from 50 observations spanning 2024 Dec. 12-21 (mean residual 0".5). It appears that the comet passed 5.18 AU from Saturn in 2020 November. T = 2025 July 15.07353 TT Peri. = 317.87986 Node = 121.95525 2000.0 q = 3.6256588 AU Incl. = 109.33896 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. 2024 12 16 10 56.30 -67 45.0 4.282 4.087 72.0 13.2 18.0 2024 12 26 10 40.72 -68 39.2 4.159 4.047 76.7 13.7 18.0 2025 01 05 10 18.96 -69 15.9 4.032 4.009 81.6 14.0 17.9 2025 01 15 09 51.49 -69 22.5 3.905 3.972 86.7 14.3 17.8 2025 01 25 09 20.41 -68 46.4 3.783 3.937 91.7 14.5 17.7 2025 02 04 08 49.19 -67 18.1 3.670 3.903 96.3 14.5 17.6 2025 02 14 08 21.39 -64 54.9 3.573 3.871 100.2 14.5 17.5 2025 02 24 07 59.16 -61 41.5 3.495 3.841 103.0 14.5 17.4 2025 03 06 07 43.02 -57 47.6 3.441 3.813 104.5 14.6 17.3 2025 03 16 07 32.45 -53 25.4 3.414 3.786 104.4 14.7 17.3 2025 03 26 07 26.47 -48 47.5 3.416 3.761 102.6 15.0 17.3 2025 04 05 07 24.14 -44 05.8 3.445 3.738 99.2 15.3 17.3 2025 04 15 07 24.62 -39 30.5 3.499 3.717 94.6 15.6 17.3 2025 04 25 07 27.24 -35 09.3 3.576 3.699 89.1 15.8 17.3 2025 05 05 07 31.50 -31 07.5 3.669 3.682 82.8 15.8 17.4 2025 05 15 07 36.98 -27 28.2 3.775 3.667 76.2 15.5 17.4 2025 05 25 07 43.37 -24 12.5 3.887 3.655 69.3 15.0 17.5 2025 06 04 07 50.43 -21 20.2 4.001 3.645 62.4 14.3 17.5 2025 06 14 07 57.95 -18 49.9 4.112 3.637 55.6 13.3 17.6 NOTE: These 'Central Bureau Electronic Telegrams' are sometimes superseded by text appearing later in the printed IAU Circulars. (C) Copyright 2024 CBAT 2024 December 24 (CBET 5483) Daniel W. E. Green