Electronic Telegram No. 5229 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/2018 S3 (TESS) An apparently asteroidal object that was discovered on images obtained in 2018 with the Transiting Exoplanet Survey Satellite (TESS; cf. CBET 5163), and reported belatedly in 2022 and then posted to the Minor Planet Center's PCCP webpage due to its comet-like orbit, has been found to be cometary by S. Deen (Simi Valley, CA, USA) on publicly available archival images obtained with the 4-m Cerro Tololo reflector (+ DECam) on eleven nights spanning 2014 Feb. 15-2018 Dec. 1 UT. The astrometry appears on MPEC 2023-D54; the first two and last two TESS observations are tabulated below: 2018 UT R.A. (2000) Decl. Mag. Sept.22.38519 2 33 33.27 -43 59 29.1 18.8 22.48936 2 33 29.22 -43 59 45.0 19.0 30.82269 2 28 46.51 -44 10 52.3 18.7 Oct. 1.05186 2 28 39.07 -44 10 57.1 18.7 Deen writes that he first found the 2018 Sept. 9 pre-discovery DECam image and then used that with the TESS astrometry to compute orbits and find the rest of the 2018-opposition observations; Deen then worked backwards year-by-year through the archive to find the additional pre-discovery DECam observations. Deen's descriptive comments from the DECam observations are as follows. On 2014 Dec. 15.3 UT, the comet appeared "possibly cometary" with a moderately condensed head of size 0".8 (full-width-at-half-maximum), red mag 22.0, and no tail in 0".5 seeing. On 2016 Feb. 14.33, the comet was obviously cometary with a moderately condensed coma of size 1" (FWHM) and red mag 19.2 in 0".8 seeing, with a 16"-long broad tail spanning p.a. 25-70 degrees. On 2016 Feb. 15.23, the comet was again obviously cometary, with a slightly condensed coma of size 1".5 (FWHM) and red mag 19.0 in 0".7 seeing, with an 18"-long broad tail spanning p.a. 40-95 deg. On 2016 Feb. 29.11, Deen reports a moderately condensed coma of size 1".5 (FWHM) and g mag 19.2-19.4 in 0".6 seeing, with a 8"-long broad tail spanning p.a. 45-90 degrees. On 2017 Oct. 3.32, the comet showed a slightly diffuse coma of size 2" (FWHM) and z magnitude 20.0 in 0".8 seeing, with a 6"-long broad tail spanning p.a. 100-150 degrees. On 2017 Dec. 11.19, the cometary nature continued to be obvious, with a very condensed coma of size 1".2 (FWHM) and red mag 20.4 in 0".5 seeing, with a 30"-long broad tail spanning p.a. 140-175 degrees. On 2018 Dec. 5.13, there was a condensed coma of size 1".0 (FWHM) and z mag 19.9 in 0".4 seeing, with a 6"-long broad tail spanning p.a. 135-170 degrees. On 2018 Sept. 9.31, the comet had a very condensed coma of size 0".9 (FWHM) and z magnitude 20.1 in 0".6 seeing, with an 11"-long broad tail spanning p.a. 150-195 degrees. On 2018 Oct. 22.20, the noise was too poor to determine cometary nature. On 2018 Nov. 17.07-17.19, there was a slightly diffuse coma of size 1".7 (FWHM) and z mag 20.5-20.6 in 0".5 seeing, with an 11"-long, fairly thin tail spanning p.a. 180-210 degrees. On 2018 Dec. 1.21, there was a slightly condensed coma of size 1".5 (FWHM) and g magnitude 21.5 in 0".5 seeing, with a 9"-long broad tail spanning p.a. 150-200 degrees. The following orbital elements by S. Nakano (Central Bureau) are from 24 observations spanning 2014 Feb. 15-2018 Dec. 1 (weighted mean residual 1".8), with corresponding "original" and "future" values of 1/a being +0.000070 and +0.000031 (+/- 0.000006) AU**-1, respectively. The comet passed 3.70 AU from Saturn on 2012 Mar. 22 UT. All of the TESS observations were given 0.1 weighting. Epoch = 2016 June 21.0 TT T = 2016 June 29.33766 TT Peri. = 263.84087 e = 1.0004268 Node = 24.98676 2000.0 q = 7.9998357 AU Incl. = 78.41812 The reported magnitudes from the two telescopes are discordant; Deen's magnitudes are 1-3 magnitudes fainter than indicated by power-law parameters of H = 6.5 and 2.5n = 8, which represent the TESS magnitudes in 2018. NOTE: These 'Central Bureau Electronic Telegrams' are sometimes superseded by text appearing later in the printed IAU Circulars. (C) Copyright 2023 CBAT 2023 March 1 (CBET 5229) Daniel W. E. Green