Electronic Telegram No. 5354 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 C4 (ATLAS) An apparently asteroidal object discovered on CCD images taken on Feb. 14 UT UT with a 0.5-m f/2 Schmidt reflector at Sutherland, South Africa, in the course of the "Asteroid Terrestrial-Impact Last Alert System" (ATLAS) search program has been found to show cometary appearance by CCD and CMOS astrometrists elsewhere. The discovery observations are tabulated below: 2024 UT R.A. (2000) Decl. Mag. Feb. 14.08138 16 55 01.64 -48 34 21.2 17.4 14.08457 16 55 01.82 -48 34 16.2 17.5 14.10474 16 55 03.01 -48 33 43.9 16.5 14.10753 16 55 03.22 -48 33 39.6 17.3 While the object was posted on the Minor Planet Center's NEOCP webpage, S. Deen (Simi Valley, CA, USA) obtained three 300-s CMOS images remotely on Feb. 14.2 UT with a "Deep Sky Chile" 0.43-m f/6.8 reflector (+ luminance filter) located at Rio Hurtado, Chile, which show a diffuse coma of size about 3" (full-width-at-half-maximum) and a broad tail around 9" long spanning p.a. 180-250 degrees; the seeing was estimated as around 1".0. Twenty stacked 30-s CMOS exposures taken on Feb. 14.3 UT by K. Yoshimoto (Kumage, Yamaguchi, Japan) with a "Deep Sky Chile" 0.36-m f/7.2 Corrected Dall-Kirkham reflector located at Rio Hurtado show this to be definitely a comet, with a condensed coma 13" in diameter and total mag 16.9, and a broad tail 13" long in p.a. 225 deg. Ten stacked 30-s CCD exposures taken remotely by H. Sato (Tokyo, Japan) on Feb. 14.37 UT with a "Deep Sky Chile" 0.51-m f/6.8 astrograph at Rio Hurtado show a strongly condensed comet with an outer coma 17" in diameter and no tail; the magnitude was 16.4 as measured within a circular aperture of radius 8".6. Seven additional stacked CCD exposures taken on Feb. 14.77 by Sato using a 0.51-m f/6.8 astrograph located at Siding Spring, NSW, Australia, show a strongly condensed coma 15" in diameter and a broad tail 10" long spanning p.a. 230-260 degrees; the magnitude was 16.0 as measured within a circular aperture of radius 7".6. Eight 60-s stacked exposures taken remotely by M. Mattiazzo (Swan Hill, Victoria, Australia) on Feb. 15.26 UT with a Planewave CDK14 reflector (+ luminance filter) located at Rio Hurtado Valley, Chile, show a moderately condensed coma 20" wide with total broadband G magnitude 15.8 (Gaia reference stars) and a hint of a tail 10" long in p.a. 230 degrees. Twenty 30-s follow-up exposures taken by Mattiazzo on Feb. 16.77 with a Celestron 11 RASA f/2.2 reflector (+ Canon 6D camera) at Swan Hill show a diffuse coma 2' wide with total V magnitude 14.2 (UCAC4 reference stars). T. Prystavski (Lviv, Ukraine) writes that CCD exposures taken remotely on Feb. 14.74-14.75 UT with a 0.32-m f/9.0 Ritchey-Chretien reflector at Siding Spring and on Feb. 16.36 with a "Deep Sky Chile" 0.51-m f/6.8 Corrected Dall-Kirkham telescope at Rio Hurtado show a diffuse comet with a moderately condensed coma 0'.4 in diameter and a broad tail 0'.5 long in p.a. 227 deg; the total magnitude was given as 15.9 and 15.7 for the two nights. The available astrometry appears on MPEC 2024-D98. The following preliminary parabolic orbital elements by S. Nakano (Central Bureau) are from 69 observations spanning 2024 Feb. 14-20 (mean residual 0".4). The comet is may have a medium-short-period orbit, but the orbital period is highly uncertain. T = 2024 Jan. 30.86304 TT Peri. = 322.05035 Node = 220.80431 2000.0 q = 1.4784287 AU Incl. = 79.45612 The following ephemeris by the undersigned from the above orbital elements uses photometric power-law parameters H = 13.5 and 2.5n = 8 for the magnitudes. Date TT R. A. (2000) Decl. Delta r Elong. Phase Mag. 2024 01 11 15 49.03 -60 43.5 1.860 1.505 53.8 31.8 16.3 2024 01 16 16 02.98 -59 16.1 1.824 1.493 54.9 32.6 16.2 2024 01 21 16 15.28 -57 42.5 1.783 1.485 56.4 33.5 16.1 2024 01 26 16 26.06 -56 02.7 1.738 1.480 58.3 34.5 16.1 2024 01 31 16 35.44 -54 16.3 1.689 1.478 60.5 35.5 16.0 2024 02 05 16 43.52 -52 22.7 1.635 1.480 63.2 36.5 15.9 2024 02 10 16 50.35 -50 20.9 1.579 1.485 66.2 37.4 15.9 2024 02 15 16 55.96 -48 09.4 1.519 1.494 69.5 38.3 15.8 2024 02 20 17 00.39 -45 46.8 1.456 1.506 73.2 39.0 15.7 2024 02 25 17 03.63 -43 10.8 1.392 1.520 77.3 39.4 15.7 2024 03 01 17 05.69 -40 19.1 1.327 1.538 81.8 39.6 15.6 2024 03 06 17 06.53 -37 08.8 1.262 1.559 86.6 39.4 15.5 2024 03 11 17 06.11 -33 36.9 1.199 1.582 91.9 38.9 15.5 2024 03 16 17 04.37 -29 40.2 1.139 1.609 97.6 37.8 15.4 2024 03 21 17 01.28 -25 15.9 1.083 1.637 103.8 36.2 15.4 2024 03 26 16 56.82 -20 22.7 1.035 1.668 110.3 34.1 15.4 2024 03 31 16 50.98 -15 01.1 0.996 1.700 117.0 31.6 15.3 2024 04 05 16 43.79 -09 15.4 0.968 1.735 123.7 28.7 15.3 2024 04 10 16 35.31 -03 13.8 0.953 1.771 129.9 25.7 15.4 2024 04 15 16 25.70 +02 51.4 0.953 1.809 135.3 23.0 15.5 2024 04 20 16 15.17 +08 45.7 0.969 1.849 139.0 20.9 15.6 2024 04 25 16 04.00 +14 15.4 1.000 1.889 140.8 19.7 15.7 2024 04 30 15 52.50 +19 10.4 1.045 1.931 140.4 19.4 15.9 2024 05 05 15 41.00 +23 24.6 1.103 1.974 138.4 19.8 16.1 2024 05 10 15 29.84 +26 56.7 1.172 2.018 135.1 20.7 16.3 2024 05 15 15 19.30 +29 48.5 1.251 2.063 131.3 21.6 16.5 2024 05 20 15 09.63 +32 04.2 1.336 2.108 127.2 22.5 16.7 2024 05 25 15 00.98 +33 48.6 1.428 2.154 123.0 23.2 16.9 2024 05 30 14 53.45 +35 06.8 1.525 2.201 118.9 23.8 17.2 2024 06 04 14 47.05 +36 03.2 1.625 2.248 114.8 24.2 17.4 2024 06 09 14 41.80 +36 41.9 1.727 2.296 111.0 24.4 17.6 2024 06 14 14 37.66 +37 06.2 1.832 2.344 107.3 24.4 17.8 2024 06 19 14 34.55 +37 19.1 1.938 2.392 103.7 24.4 18.0 2024 06 24 14 32.40 +37 22.9 2.045 2.441 100.3 24.2 18.2 2024 06 29 14 31.13 +37 19.5 2.151 2.490 97.1 23.9 18.3 2024 07 04 14 30.65 +37 10.4 2.258 2.539 93.9 23.5 18.5 2024 07 09 14 30.90 +36 56.8 2.364 2.589 90.9 23.1 18.7 2024 07 14 14 31.82 +36 39.8 2.470 2.638 88.0 22.6 18.8 2024 07 19 14 33.32 +36 20.3 2.574 2.688 85.3 22.1 19.0 2024 07 24 14 35.35 +35 58.8 2.677 2.738 82.6 21.6 19.1 2024 07 29 14 37.86 +35 36.1 2.778 2.788 80.0 21.0 19.3 2024 08 03 14 40.80 +35 12.4 2.877 2.838 77.6 20.4 19.4 2024 08 08 14 44.13 +34 48.3 2.975 2.888 75.2 19.8 19.6 2024 08 13 14 47.82 +34 24.1 3.071 2.938 72.9 19.3 19.7 2024 08 18 14 51.83 +34 00.2 3.164 2.988 70.8 18.7 19.8 2024 08 23 14 56.13 +33 36.7 3.255 3.038 68.7 18.1 19.9 2024 08 28 15 00.69 +33 14.0 3.344 3.088 66.7 17.5 20.0 2024 09 02 15 05.49 +32 52.1 3.430 3.138 64.9 16.9 20.1 2024 09 07 15 10.51 +32 31.3 3.514 3.187 63.1 16.4 20.3 NOTE: These 'Central Bureau Electronic Telegrams' are sometimes superseded by text appearing later in the printed IAU Circulars. (C) Copyright 2024 CBAT 2024 February 21 (CBET 5354) Daniel W. E. Green