Electronic Telegram No. 5453 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 S1 (ATLAS) L. Denneau reports the discovery of a comet by Robert Siverd (who noted the object to be extended) on CCD images taken on Sept. 27.6 UT with a 0.5-m f/2 Schmidt reflector at Haleakala, Hawaii, in the course of the "Asteroid Terrestrial-Impact Last Alert System" (ATLAS) search program. Siverd measured a size of 6".3 (full-width-at-half-maximum) for the nuclear condensation (in 4".9 seeing) with an extended coma of size 30" and no tail. The discovery observations are tabulated below, 2024 UT R.A. (2000) Decl. Mag. Sept.27.61651 9 05 19.74 -11 39 21.5 15.3 27.62061 9 05 20.74 -11 39 26.1 15.2 27.62977 9 05 22.96 -11 39 36.1 15.3 After the comet was posted on the Minor Planet Center's PCCP webpage, many other observers elsewhere reported on the cometary appearance. S. Deen (Simi Valley, CA, USA) reports that three 300-s images taken remotely on Sept. 28.3 UT with a 0.43-m f/6.8 CDK reflector located at Rio Hurtado, Chile, show a large, somewhat-diffuse coma of size about 5" (FWHM) with a tail about 15" long in p.a. 257 degrees. Eight stacked 60-s CMOS exposures taken remotely on Sept. 28.4 by Hidetaka Sato (Bunkyo-ku, Tokyo, Japan) using a "Deep Sky Chile" 0.25-m f/3.8 astrograph located at Rio Hurtado show a strongly condensed object with an outer coma 2'.0 in diameter and a 45" tail toward p.a. 260 degrees; the magnitude was 13.5 as measured within a circular aperture of radius 60".2. J. Drummond (Gisborne, New Zealand) writes that thirty stacked 60-s CCD exposures taken on Sept. 28.7 with a 0.35-m f/10 Schmidt-Cassegrain reflector show a coma about 1' in diameter with an apparent faint 30" tail in p.a. 255 degrees. Stacked CCD images obtained remotely on Sept. 28.77 by F. D. Romanov (Yuzhno-Morskoy, Nakhodka, Russia) with an "iTelescope" 0.43-m f/6.8 reflector at Siding Spring Observatory, NSW, Australia (+ Luminance filter) show a moderately condensed coma up to 30" in diameter. Fifteen stacked 30-s unfiltered CMO images taken remotely by E. Guido (Castellammare di Stabia, Italy), M. Rocchetto, and A. Valvasori on Sept. 28.78 with a 0.35-m f/3 telescope of the SPACEFLUX network (Australian station) show a coma about 30" arcsec in diameter that is elongated toward p.a. 263 deg. Thirty stacked 30-s CCD exposures taken by A. Aletti, F. Bellini, L. Buzzi (Varese, Italy), and G. Galli with a 0.36-m f/8.4 reflector at the Schiaparelli Southern Observatory (Hakos, Namibia) on Sept. 29.1 show a condensed 12" coma with a faint straight tail at least 20" long in p.a. 265 deg. A. Pearce (Nedlands, Western Australia) writes that images obtained on Sept 29.14 using an 0.1-m f/5 refractor (+ clear filter) at the Skygem Observatory in Namibia show a moderately condensed comet with a faint outer coma 1'.7 in diameter with total mag 13.0. Twenty stacked 15-s CCD exposures taken on Sept. 29.36 by K. Yoshimoto (Kumage, Yamaguchi, Japan) with a 0.51-m f/6.8 corrected Dall-Kirkham telescope at Rio Hurtado show a diffuse coma 1'.6 in diameter (total mag 13.4) and a faint tail 30" long in p.a. 265 deg. CCD images taken remotely by T. Prystavski (Lviv, Ukraine) on Sept. 29.4 with a 0.25-m f/3.8 Newtonian reflector at Rio Hurtado show a 2'.9 diffuse coma of total red mag 12.6 with a 2'.6 tail in p.a. 265 deg. Fourteen stacked 60-s CCD exposures taken on Sept. 30.8 by Hirohisa Sato (Sukagawa, Japan) with a 0.25-m f/4 reflector show a moderately condensed coma about 0'.9 in diameter (total mag 13.6) with no tail. Additional CCD total-magnitude and coma-diameter measurements by Pearce with a 0.05-m f/5 refractor at Nedlands: Sept. 29.88 UT, 13.1, 3'.2; 30.87, 12.9, 4'. J. G. de S. Aguiar (Campinas, Brazil) reports total visual mag 11.7 and coma diameter 1' on Oct. 1.32 with a 0.30-m reflector (Tycho-2 Catalogue comparison stars). A post by Michael Jaeger (Vienna) on the "ICQ Comet Observations" Facebook forum with a photograph taken by D. Moeller with a 0.30-m f/2.8 astrograph on Sept. 29.11 UT clearly shows the strong greenish color that is presumed due to C_2, with a 2' coma, a tail, and total magnitude estimated to be about 11.5. The strong C_2 color suggests that the total magnitude is indeed closer to 11.5 than to 13-14 (the latter coming from digital astrometrists; some have suggested a rapid rise in brightness based on poor astrometric magnitudes, but such a rise is doubtful since Sept. 27). The available astrometry appears on MPEC 2024-T22. The following parabolic orbital elements by S. Nakano (Central Bureau) are from 90 observations spanning 2024 Sept. 27-30 (mean residual 0".7). These are the elements of a Kreutz sungrazing comet. T = 2024 Oct. 28.50321 TT Peri. = 68.08324 Node = 345.56952 2000.0 q = 0.0074991 AU Incl. = 141.67800 The comet has been at solar elongations under 50 degrees since April (when it would naturally have been much fainter), and under 40 degrees for most of that time -- explaining why it was missed by professional surveys. The following ephemeris by the undersigned from the above orbital elements uses photometric power-law parameters H = 11.0 and 2.5n = 8 for the magnitudes. Date TT R. A. (2000) Decl. Delta r Elong. Phase Mag. 2024 09 07 08 04.24 -07 05.6 2.021 1.516 46.5 28.8 14.0 2024 09 09 08 08.79 -07 25.3 1.966 1.476 47.0 29.9 13.8 2024 09 11 08 13.54 -07 46.3 1.909 1.436 47.5 31.1 13.7 2024 09 13 08 18.53 -08 08.6 1.853 1.395 47.9 32.4 13.5 2024 09 15 08 23.77 -08 32.3 1.796 1.354 48.3 33.7 13.3 2024 09 17 08 29.30 -08 57.4 1.739 1.312 48.6 35.1 13.1 2024 09 19 08 35.16 -09 24.1 1.681 1.269 48.8 36.5 13.0 2024 09 21 08 41.39 -09 52.4 1.624 1.225 48.9 38.1 12.8 2024 09 23 08 48.04 -10 22.6 1.566 1.181 48.9 39.8 12.6 2024 09 25 08 55.16 -10 54.5 1.509 1.136 48.8 41.6 12.3 2024 09 27 09 02.83 -11 28.4 1.452 1.090 48.6 43.6 12.1 2024 09 29 09 11.14 -12 04.4 1.395 1.043 48.2 45.8 11.9 2024 10 01 09 20.17 -12 42.3 1.339 0.995 47.7 48.1 11.6 2024 10 03 09 30.04 -13 22.3 1.283 0.946 47.0 50.6 11.3 2024 10 05 09 40.89 -14 04.1 1.228 0.895 46.0 53.4 11.1 2024 10 07 09 52.88 -14 47.6 1.175 0.843 44.7 56.5 10.8 2024 10 09 10 06.19 -15 32.2 1.123 0.790 43.2 60.0 10.4 2024 10 11 10 21.05 -16 17.1 1.074 0.734 41.3 63.8 10.1 2024 10 13 10 37.68 -17 01.0 1.028 0.677 39.0 68.1 9.7 2024 10 15 10 56.35 -17 42.1 0.986 0.617 36.2 72.9 9.3 2024 10 17 11 17.31 -18 17.4 0.948 0.553 32.9 78.3 8.8 2024 10 18 11 28.73 -18 31.7 0.932 0.520 31.1 81.3 8.6 2024 10 19 11 40.81 -18 43.0 0.917 0.486 29.1 84.5 8.3 2024 10 20 11 53.56 -18 50.5 0.904 0.451 26.9 87.9 8.0 2024 10 21 12 07.00 -18 53.4 0.894 0.414 24.6 91.5 7.7 2024 10 22 12 21.13 -18 50.9 0.887 0.376 22.1 95.4 7.3 2024 10 23 12 35.96 -18 41.7 0.882 0.336 19.4 99.6 6.9 2024 10 24 12 51.49 -18 24.6 0.881 0.293 16.6 104.2 6.5 2024 10 25 13 07.74 -17 57.9 0.885 0.247 13.5 109.4 5.9 2024 10 26 13 24.79 -17 19.0 0.894 0.196 10.3 115.4 5.1 2024 10 27 13 42.87 -16 23.1 0.912 0.138 6.7 123.2 3.9 2024 10 28 14 02.96 -14 53.7 0.946 0.063 2.4 138.0 1.3 2024 10 29 13 59.85 -13 17.3 1.013 0.062 3.4 69.8 1.4 2024 10 30 13 45.75 -14 17.0 1.012 0.136 7.7 78.1 4.1 2024 10 31 13 35.63 -15 13.2 1.010 0.195 11.1 79.5 5.3 2024 11 01 13 27.27 -16 06.1 1.007 0.246 14.1 79.6 6.1 2024 11 02 13 19.94 -16 56.9 1.004 0.292 16.8 79.2 6.7 2024 11 03 13 13.31 -17 45.9 1.002 0.335 19.3 78.6 7.2 2024 11 04 13 07.20 -18 33.7 1.000 0.375 21.7 77.9 7.6 2024 11 05 13 01.48 -19 20.4 0.997 0.414 24.0 77.2 7.9 2024 11 06 12 56.06 -20 06.2 0.995 0.451 26.2 76.4 8.2 2024 11 08 12 45.89 -21 35.6 0.991 0.520 30.4 74.8 8.7 2024 11 10 12 36.34 -23 02.8 0.987 0.585 34.4 73.1 9.1 2024 11 12 12 27.17 -24 28.1 0.982 0.647 38.3 71.5 9.4 2024 11 14 12 18.23 -25 51.8 0.978 0.705 42.0 69.9 9.7 2024 11 16 12 09.37 -27 14.2 0.974 0.762 45.7 68.2 10.0 2024 11 18 12 00.49 -28 35.2 0.970 0.816 49.3 66.6 10.2 2024 11 20 11 51.51 -29 54.7 0.966 0.869 52.8 64.9 10.4 2024 11 22 11 42.34 -31 12.6 0.962 0.920 56.3 63.3 10.6 2024 11 24 11 32.91 -32 28.7 0.958 0.970 59.8 61.6 10.8 2024 11 26 11 23.18 -33 42.6 0.956 1.019 63.2 59.9 11.0 2024 11 28 11 13.08 -34 53.8 0.953 1.066 66.7 58.2 11.1 NOTE: These 'Central Bureau Electronic Telegrams' are sometimes superseded by text appearing later in the printed IAU Circulars. (C) Copyright 2024 CBAT 2024 October 1 (CBET 5453) Daniel W. E. Green