Electronic Telegram No. 5047 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/2021 S1 (ATLAS) A. Fitzsimmons, Astrophysics Research Centre, Queen's University, Belfast, reports the discovery of a comet on four 30-s 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; the comet appears non-stellar and "soft", with a size of 6".0 (full-width-at-half-maximum) in 4".4 seeing, with no apparent tail. The discovery observations are tabulated below, 2021 UT R.A. (2000) Decl. Mag. Sept.27.57014 6 54 12.03 +57 38 49.2 17.8 27.57425 6 54 12.11 +57 38 50.5 18.6 27.57978 6 54 12.31 +57 38 50.8 18.4 27.59031 6 54 12.77 +57 38 50.8 18.6 After the object was posted on the Minor Planet Center's PCCP webpage, other CCD astrometrists have noted cometary appearance. L. Buzzi, Varese, Italy, writes that one-hundred-fifty stacked 15-s exposures taken in strong moonlight with a 0.84-m f/3.5 reflector on Sept. 28.1 UT (measured by A. Aletti) show a moderately condensed 8" coma of mag 18.8 with a straight tail at least 30" long in p.a. 315 deg. E. Bryssinck, Kruibeke, Belgium, notes that fourteen stacked luminance-filtered 120-s exposures taken on Sept. 29.14 remotely by a large group of observers using a "Telescope Live" 0.7-m f/8 Ritchey-Chretien reflector at Oria, Spain, show a diffuse coma of size 5" x 8", elongated toward p.a. 211 degrees but with no obvious tail; the magnitude was 19.7 in an aperture of radius 5". A. Valvasori, Padulle, Italy, relates that thirty-eight stacked 120-s unfiltered exposures taken remotely on Oct. 1.0 by E. Guido and himself with a 0.30-m f/4 reflector show a compact coma about 10" in diameter and of mag 18.4-18.9. F. Kugel, Dauban, France, finds a tail 1'.8 long in p.a. 325 degrees and a magnitude of 18.4-18.9 in an aperture of radius 5".8 on exposures taken on Oct. 1.14-1.15 with a 0.4-m f/2.8 reflector. The available astrometry appears on MPEC 2021-T21. The following preliminary parabolic orbital elements by S. Nakano (CBAT) are from 72 observations spanning 2021 Sept. 27-Oct. 2 (mean residual 0".4). T = 2022 Mar. 12.18306 TT Peri. = 149.30071 Node = 299.79073 2000.0 q = 6.1011120 AU Incl. = 52.14792 The following ephemeris by the undersigned from the above orbital elements uses photometric power-law parameters H = 8.0 and 2.5n = 8 for the magnitudes. Date TT R. A. (2000) Decl. Delta r Elong. Phase Mag. 2021 09 23 06 51.55 +57 34.9 6.256 6.215 83.0 9.2 18.3 2021 10 03 06 56.88 +57 44.0 6.113 6.202 90.4 9.3 18.3 2021 10 13 07 00.41 +57 54.9 5.971 6.190 98.1 9.2 18.2 2021 10 23 07 01.98 +58 06.5 5.832 6.179 105.9 8.9 18.2 2021 11 02 07 01.46 +58 17.4 5.701 6.168 113.8 8.5 18.1 2021 11 12 06 58.81 +58 25.5 5.581 6.158 121.7 7.9 18.0 2021 11 22 06 54.10 +58 28.3 5.476 6.149 129.3 7.1 18.0 2021 12 02 06 47.58 +58 23.0 5.391 6.141 136.2 6.4 18.0 2021 12 12 06 39.67 +58 07.1 5.329 6.133 141.8 5.7 17.9 2021 12 22 06 30.98 +57 38.5 5.292 6.127 145.3 5.2 17.9 2022 01 01 06 22.18 +56 56.4 5.282 6.121 145.9 5.2 17.9 2022 01 11 06 13.98 +56 01.2 5.300 6.116 143.2 5.5 17.9 2022 01 21 06 06.97 +54 54.8 5.345 6.111 138.0 6.2 17.9 2022 01 31 06 01.54 +53 39.7 5.415 6.108 131.1 7.0 18.0 2022 02 10 05 57.93 +52 18.9 5.508 6.105 123.2 7.8 18.0 2022 02 20 05 56.18 +50 55.3 5.621 6.103 114.9 8.5 18.0 2022 03 02 05 56.23 +49 31.4 5.748 6.102 106.3 9.0 18.1 2022 03 12 05 57.94 +48 08.8 5.887 6.101 97.7 9.3 18.1 2022 03 22 06 01.12 +46 49.1 6.033 6.101 89.2 9.4 18.2 2022 04 01 06 05.57 +45 32.6 6.181 6.103 80.8 9.3 18.2 2022 04 11 06 11.10 +44 19.8 6.329 6.105 72.6 9.0 18.3 NOTE: These 'Central Bureau Electronic Telegrams' are sometimes superseded by text appearing later in the printed IAU Circulars. (C) Copyright 2021 CBAT 2021 October 3 (CBET 5047) Daniel W. E. Green