Electronic Telegram No. 5467 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) As noted on CBET 5459, this comet is unlikely to survive perihelion due to its small apparent size (absolute magnitude). Observers have been reporting some sporadic morphological behavior in this comet, including multiple outbursts in brightness with rapidly changing coma appearance. M. Masek posted photos of C/2024 S1 on the "ICQ Comet Observations" Facebook forum that were taken with a 0.30-m reflector in Argentina, showing the comet fading from total V mag 8.2 on Oct. 20.36 UT (at altitude 10.4 degrees) to mag 10.1 on Oct. 22.36 (at altitude 7.4 degrees), with the coma diameter shrinking from 2'.2 to 0'.9 and the tail shrinking from length 8'.6 to 3'.2; sky conditions on the two nights were not specified. A series of images have been analyzed by H. Sato (Bunkyo-ku, Tokyo, Japan). CCD images taken with a 0.51-m f/6.8 astrograph located at Siding Spring, NSW, Australia, on Oct. 2.8 UT show a strongly condensed coma 2'.2 in diameter with a tail 1'.2 long toward p.a. 265 degrees; the magnitude was 13.6 as measured within a circular aperture of radius 66".0. Images taken with the same telescope on Oct. 6.76 show a strongly condensed coma 2'.2 in diameter with a tail 3'.3 long toward p.a. 265 deg; the magnitude was 13.0 as measured within a circular aperture of radius 66".0. Eight stacked 15-s CMOS exposures taken with a "Deep Sky Chile" 0.25-m f/3.8 astrograph at Rio Hurtado, Chile, on Oct. 17.37 show a strongly condensed coma 1'.5 in diameter with a 2'.8 tail toward p.a. 255 deg; the magnitude was 12.0 as measured within a circular aperture of radius 45".2. Eight stacked 15-s exposures taken with the same telescope on Oct. 19.38 show a strongly condensed triangular-shaped coma 2'.6 in diameter with a tail 24' long toward p.a. 255 deg; the magnitude was 8.4 as measured within a circular aperture of radius 78".3, indicating a large outburst in brightness during the intervening two days. S. Deen (Simi Valley, CA, USA) writes that monitoring of C/2024 S1 with a 0.43-m reflector at Rio Hurtado on Oct. 18 showed a total magnitude of V = 10.0. By Oct. 19, the coma and tail had noticeably brightened in an apparently outburst, with total mag estimated as V = 9.0-9.5. The comet was brighter yet on Oct. 20 (V = 8.4). Deen notes that the C_2 coma had virtually vanished by Oct. 21, leaving only its tail with very little coma. A. Pearce (Nedlands, W. Australia) has posted some photos of C/2024 S1 on the "ICQ Comet Observations" Facebook forum that are indicative of the appearance of Kreutz sungrazing comets close to the sun in satellite coronagraph imagery. A photo taken by M. Mattiazzo with an iTelescope 0.25-m telescope in Chile on Oct. 23.37 UT shows a 1'.0 coma of total mag 10.8 with weak condensation and a 17' tail in p.a. 251 degrees, but the comet was only 4.5 degrees above the horizon. Similar imaging by Mattiazzo on Oct. 22.37 yielded total mag 10.7 and coma diametr 1'.2; Pearce notes that the coma had become more elongated compared to that on previous days. The following updated orbital elements by S. Nakano (Central Bureau) are from 302 observations spanning 2024 Sept. 27-Oct. 23 (mean residual 1".0). Many of the observations had to be rejected due to high residuals, including thirteen of the last fourteen observations (after Oct. 20; i.e., from Oct. 21-23). These elements indicate an orbital period of 847 years. Epoch = 2024 Oct. 17.0 TT T = 2024 Oct. 28.48605 TT Peri. = 69.19696 e = 0.9999111 Node = 347.19053 2000.0 q = 0.0079594 AU Incl. = 141.90710 NOTE: These 'Central Bureau Electronic Telegrams' are sometimes superseded by text appearing later in the printed IAU Circulars. (C) Copyright 2024 CBAT 2024 October 23 (CBET 5467) Daniel W. E. Green