Electronic Telegram No. 5217 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/2023 B2 (ATLAS) An apparently asteroidal object discovered on CCD images taken on Jan. 21 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 astrometrists elsewhere. The discovery observations are tabulated below: 2023 UT R.A. (2000) Decl. Mag. Jan. 21.96831 10 42 45.00 -53 03 56.4 18.8 21.97056 10 42 45.16 -53 03 58.5 18.8 21.97373 10 42 45.32 -53 04 01.2 18.7 21.98381 10 42 46.22 -53 04 09.7 18.8 After the comet was posted on the Minor Planet Center's PCCP webpage, R. Weryk (Department of Physics and Astronomy, University of Western Ontario) reported three pre-discovery observations obtained on 2022 Nov. 16.6 UT with the Pan-STARRS1 1.8-m Ritchey-Chretien reflector at Haleakala, showing the comet at mag 21.7; Weryk notes that "it's difficult to judge if it's a comet", but adds that it appears much fainter than one would expect based on a normal extrapolation for a minor-planet magnitude. Ten stacked 60-s exposures taken remotely by H. Sato (Bunkyo-ku, Tokyo, Japan) with a 0.51-m f/6.8 astrograph located at Siding Spring, NSW, Australia, on 2023 Jan. 23.61 UT show a moderately condensed coma 10" in diameter with no tail; the magnitude was 17.9 as measured within a circular aperture of radius 5".4. Eleven stacked 120-s exposures taken remotely by K. Yoshimoto (Kumage, Yamaguchi, Japan) on Jan. 25.6 with a 0.50-m f/6.8 reflector at Siding Spring show a condensed coma 11" in diameter with total magnitude 17.7 and a tail 10" long in p.a. 290 deg. E. Bryssinck, Kruibeke, Belgium, writes that twenty stacked 90-s exposures taken by M. Rocchetto, E. Guido, G. Milani, A. Valvasori, and himself remotely with a "Telescope Live" 0.61-m f/6.6 reflector located at Rio Hurtado, Chile, on Feb. 2.3 show a strong central condensation with a small coma of size 6" and hint of a tail 6" long in p.a. 302"; it was difficult to measure a reliable magnitude due to a dense starfield. The available astrometry appears on MPEC 2023-C44. The following parabolic orbital elements by S. Nakano (Central Bureau) are from 52 observations spanning 2022 Nov. 16-2023 Feb. 2 (mean residual 0".4), but Nakano cautions about the heavy dependence upon the one-night Pan-STARRS1 observations from Nov. 16; an orbit based only on observations from Jan. 21- Feb. 2 gives residuals of +16" in R.A. and -37" in Decl. for the Nov. 16 astrometry. T = 2023 Mar. 10.38246 TT Peri. = 317.27711 Node = 218.18751 2000.0 q = 1.7450451 AU Incl. = 40.78166 The following ephemeris by the undersigned from the above orbital elements uses photometric power-law parameters H = 15.0 and 2.5n = 8 for the magnitudes. Date TT R. A. (2000) Decl. Delta r Elong. Phase Mag. 2023 01 16 10 34.36 -51 29.5 1.454 1.877 98.9 31.2 18.0 2023 01 21 10 41.40 -52 49.5 1.409 1.854 100.1 31.5 17.9 2023 01 26 10 48.26 -54 00.0 1.365 1.834 101.4 31.8 17.8 2023 01 31 10 54.94 -54 59.8 1.321 1.815 102.8 32.0 17.7 2023 02 05 11 01.41 -55 47.8 1.278 1.798 104.4 32.1 17.6 2023 02 10 11 07.64 -56 22.7 1.235 1.784 106.3 32.1 17.5 2023 02 15 11 13.61 -56 43.0 1.193 1.771 108.3 32.0 17.4 2023 02 20 11 19.30 -56 47.2 1.151 1.761 110.6 31.7 17.3 2023 02 25 11 24.70 -56 33.4 1.111 1.754 113.1 31.3 17.2 2023 03 02 11 29.86 -55 59.9 1.071 1.748 115.9 30.6 17.1 2023 03 07 11 34.79 -55 05.2 1.033 1.746 119.1 29.8 17.0 2023 03 12 11 39.52 -53 47.3 0.997 1.745 122.5 28.7 16.9 2023 03 17 11 44.10 -52 05.0 0.964 1.747 126.3 27.3 16.9 2023 03 22 11 48.58 -49 56.8 0.934 1.752 130.3 25.7 16.8 2023 03 27 11 53.03 -47 22.6 0.909 1.758 134.5 23.9 16.8 2023 04 01 11 57.53 -44 23.4 0.889 1.768 138.8 21.9 16.7 2023 04 06 12 02.10 -41 01.5 0.875 1.779 143.0 19.8 16.7 2023 04 11 12 06.79 -37 21.0 0.869 1.793 146.8 17.8 16.7 2023 04 16 12 11.61 -33 27.4 0.870 1.809 149.7 16.3 16.8 2023 04 21 12 16.57 -29 27.3 0.880 1.827 151.3 15.3 16.8 2023 04 26 12 21.71 -25 28.0 0.899 1.847 151.4 15.1 16.9 2023 05 01 12 27.01 -21 36.6 0.927 1.869 149.9 15.7 17.0 2023 05 06 12 32.47 -17 58.7 0.964 1.892 147.2 16.8 17.1 2023 05 11 12 38.08 -14 38.3 1.009 1.918 143.6 18.2 17.3 2023 05 16 12 43.81 -11 37.8 1.061 1.945 139.6 19.7 17.4 2023 05 21 12 49.66 -08 58.3 1.121 1.974 135.4 21.1 17.6 2023 05 26 12 55.64 -06 39.5 1.186 2.004 131.2 22.4 17.8 2023 05 31 13 01.72 -04 40.5 1.258 2.036 127.0 23.4 18.0 2023 06 05 13 07.90 -02 59.7 1.334 2.068 122.9 24.3 18.2 2023 06 10 13 14.15 -01 35.3 1.414 2.102 119.0 25.0 18.3 2023 06 15 13 20.48 -00 25.6 1.498 2.137 115.2 25.5 18.5 2023 06 20 13 26.88 +00 31.2 1.585 2.174 111.5 25.8 18.7 2023 06 25 13 33.34 +01 16.6 1.675 2.211 107.9 25.9 18.9 NOTE: These 'Central Bureau Electronic Telegrams' are sometimes superseded by text appearing later in the printed IAU Circulars. (C) Copyright 2023 CBAT 2023 February 7 (CBET 5217) Daniel W. E. Green