Electronic Telegram No. 5031 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 R2 (PANSTARRS) R. Weryk, Department of Physics and Astronomy, University of Western Ontario, reports the discovery of another comet in three 45-s w-band survey images obtained with the Pan-STARRS2 1.8-m Ritchey-Chretien reflector at Haleakala, Hawaii, on Sept. 5.34-5.37 UT as tabulated below; all the other observations below were made with the Pan-STARRS1 1.8-m Ritchey-Chretien reflector at Haleakala and identified subsequently by Weryk. The discovery images in poor seeing (1".7) show a diffuse coma of size 2".5 (full-width- at-half-maximum) and no tail. The cometary appearance was not obvious on Aug. 28 or Sept. 3 (with seeing around 2".6 on the latter date). 2021 UT R.A. (2000) Decl. Mag. Aug. 28.57899 23 38 31.89 +18 39 31.6 20.6 28.57899 23 38 31.89 +18 39 31.6 20.6 28.59064 23 38 31.28 +18 39 31.1 20.7 28.59064 23 38 31.28 +18 39 31.1 20.7 28.61390 23 38 30.05 +18 39 30.2 20.7 28.61390 23 38 30.05 +18 39 30.2 20.7 Sept. 3.51479 23 33 17.85 +18 34 38.6 20.4 3.51479 23 33 17.85 +18 34 38.6 20.4 3.52750 23 33 17.17 +18 34 38.0 20.4 3.52750 23 33 17.17 +18 34 38.0 20.4 3.54022 23 33 16.45 +18 34 37.4 20.4 3.54022 23 33 16.45 +18 34 37.4 20.4 3.55298 23 33 15.77 +18 34 36.4 20.4 3.55298 23 33 15.77 +18 34 36.4 20.4 5.33882 23 31 39.70 +18 32 32.5 20.4 5.36205 23 31 38.43 +18 32 30.7 20.4 5.37370 23 31 37.78 +18 32 30.0 20.3 5.59594 23 31 25.65 +18 32 13.2 20.9 5.59679 23 31 25.60 +18 32 13.2 20.9 Weryk adds that R. Wainscoat and he obtained three 60-s gri-band follow-up exposures with the 3.6-m Canada-France-Hawaii Telescope on Mauna Kea on Sept. 6.45 UT (queue observer L. Wells; queue coordinator N. Manset) in 0".7 seeing, showing a very condensed coma of size 1".0 (FWHM) and a straight tail 30" long in p.a. 105 degrees. After the comet was posted on the Minor Planet Center's PCCP webpage, M. Jaeger (Vienna) reported on the "ICQ Comet Observations" Facebook page that eighteen stacked 180-s exposures taken with a 30-cm reflector at Stixendorf on Sept. 4.95-4.97 show a nearly stellar central condensation of mag 20.4 and a very faint tail 15" long in p.a. 116 degrees. The available astrometry appears on MPEC 2021-R151. The following preliminary parabolic orbital elements by S. Nakano (CBAT) are from 35 observations spanning 2021 Aug. 28-Sept. 7 (mean residual 0".5). T = 2022 Jan. 4.51475 TT Peri. = 33.05991 Node = 16.29934 2000.0 q = 7.3050231 AU Incl. = 134.49592 The following ephemeris by the undersigned from the above orbital elements uses photometric power-law parameters H = 8.5 and 2.5n = 8 for the magnitudes. Date TT R. A. (2000) Decl. Delta r Elong. Phase Mag. 2021 08 14 23 50.49 +18 38.6 6.635 7.362 132.9 5.8 19.5 2021 08 24 23 42.45 +18 41.2 6.525 7.354 142.6 4.8 19.5 2021 09 03 23 33.76 +18 35.2 6.446 7.347 151.4 3.8 19.5 2021 09 13 23 24.70 +18 20.7 6.399 7.341 157.7 3.0 19.5 2021 09 23 23 15.59 +17 58.5 6.389 7.335 159.0 2.8 19.5 2021 10 03 23 06.75 +17 30.1 6.415 7.329 154.3 3.4 19.5 2021 10 13 22 58.50 +16 57.4 6.475 7.324 146.1 4.4 19.5 2021 10 23 22 51.07 +16 22.5 6.567 7.320 136.4 5.4 19.5 2021 11 02 22 44.64 +15 47.8 6.686 7.316 126.2 6.3 19.5 2021 11 12 22 39.34 +15 15.3 6.827 7.313 115.8 7.0 19.6 2021 11 22 22 35.18 +14 46.7 6.984 7.310 105.5 7.5 19.6 2021 12 02 22 32.14 +14 23.2 7.151 7.308 95.3 7.7 19.7 2021 12 12 22 30.16 +14 05.9 7.321 7.307 85.3 7.7 19.7 NOTE: These 'Central Bureau Electronic Telegrams' are sometimes superseded by text appearing later in the printed IAU Circulars. (C) Copyright 2021 CBAT 2021 September 8 (CBET 5031) Daniel W. E. Green