Electronic Telegram No. 5051 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 S3 (PANSTARRS) R. Weryk, Department of Physics and Astronomy, University of Western Ontario, reports the discovery of another comet in images obtained with the Pan-STARRS2 1.8-m Ritchey-Chretien reflector at Haleakala on Sept. 24 UT (discovery observations tabulated below). The 45-s i-band survey images show a diffuse coma of size 2".2 (full-width-at-half-maximum) in 1".9 seeing, with no obvious tail. 2021 UT R.A. (2000) Decl. Mag. Sept.24.56091 5 23 02.09 -20 37 12.2 20.1 24.57108 5 23 02.10 -20 37 15.7 19.8 24.59140 5 23 02.10 -20 37 22.0 19.6 Weryk adds that two stacked 60-s w-band follow-up exposures taken with the Pan-STARRS1 1.8-m reflector at Haleakala on Sept. 28.6 UT in 1".8 seeing show a diffuse coma of size 2".25 (FWHM), again with no obvious tail. Four additional 45-s w-band Pan-STARRS2 survey exposures from Oct. 4.6 show a diffuse coma of size 2".2 in 1".7-1".8 seeing with no clear tail (but point-spread-function asymmetry towards p.a. 280 degrees). After the comet was posted on the Minor Planet Center's PCCP webpage, other CCD astrometrists have sent reports on the object's appearance to the Central Bureau. E. Bryssinck, Kruibeke, Belgium, writes that 180-s stacked luminance-filtered images taken remotely by a large group of observers on Oct. 4.3 UT with a "Telescope Live" 0.5-m f/3.6 reflector located in the Rio Hurtado Valley of Chile show a near-stellar appearance with a strongly condensed coma 6" in diameter and no tail; the magnitude was 19.6 as measured within a circular aperture of radius 5".2. Twelve stacked 60-s CCD exposures taken remotely by H. Sato, Tokyo, Japan, with a 0.51-m f/6.8 astrograph on Oct. 4.6 show a stellar appearance; the magnitude was 19.5 as measured within a circular aperture of radius 4".9. The available astrometry appears on MPEC 2021-T166. The following parabolic orbital elements by S. Nakano (CBAT) are from 27 observations spanning 2021 Sept. 24-Oct. 5 (mean residual 0".5). T = 2024 Feb. 11.72413 TT Peri. = 7.21645 Node = 215.42290 2000.0 q = 1.3049617 AU Incl. = 58.40185 The following ephemeris by the undersigned from the above orbital elements uses photometric power-law parameters H = 7.0 and 2.5n = 8 for the magnitudes. Date TT R. A. (2000) Decl. Delta r Elong. Phase Mag. 2021 09 23 05 23.02 -20 28.8 8.714 8.919 98.6 6.4 19.3 2021 10 03 05 22.79 -21 23.1 8.524 8.844 105.4 6.3 19.2 2021 10 13 05 21.71 -22 17.6 8.345 8.768 112.0 6.1 19.2 2021 10 23 05 19.79 -23 10.6 8.178 8.692 118.1 5.8 19.1 2021 11 02 05 17.04 -24 00.3 8.028 8.616 123.5 5.5 19.0 2021 11 12 05 13.53 -24 44.7 7.897 8.539 127.8 5.3 18.9 2021 11 22 05 09.39 -25 22.0 7.786 8.462 130.6 5.1 18.9 2021 12 02 05 04.79 -25 50.5 7.698 8.385 131.6 5.0 18.8 2021 12 12 04 59.93 -26 08.8 7.633 8.308 130.7 5.2 18.8 2021 12 22 04 55.05 -26 16.5 7.590 8.230 127.9 5.4 18.7 2022 01 01 04 50.38 -26 13.2 7.568 8.152 123.5 5.8 18.7 2022 01 11 04 46.17 -25 59.7 7.565 8.074 118.1 6.2 18.7 2022 01 21 04 42.59 -25 36.8 7.578 7.996 111.8 6.6 18.6 2022 01 31 04 39.82 -25 06.2 7.603 7.917 105.1 6.9 18.6 2022 02 10 04 37.94 -24 29.4 7.637 7.838 98.1 7.2 18.6 2022 02 20 04 37.02 -23 48.3 7.676 7.758 91.1 7.3 18.5 2022 03 02 04 37.06 -23 04.7 7.717 7.678 84.1 7.4 18.5 2022 03 12 04 38.05 -22 20.2 7.755 7.598 77.3 7.3 18.5 2022 03 22 04 39.95 -21 36.4 7.787 7.518 70.7 7.2 18.5 2022 04 01 04 42.68 -20 54.6 7.812 7.437 64.5 7.0 18.4 2022 04 11 04 46.18 -20 16.0 7.825 7.356 58.8 6.7 18.4 NOTE: These 'Central Bureau Electronic Telegrams' are sometimes superseded by text appearing later in the printed IAU Circulars. (C) Copyright 2021 CBAT 2021 October 8 (CBET 5051) Daniel W. E. Green