Electronic Telegram No. 5163 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 S2 (TESS) K. Ly, El Monte, CA, USA, reports that he has identified cometary images in publicly available archival data with three different ground-based telescopes of an apparently asteroidal object that was discovered with the Transiting Exoplanet Survey Satellite (TESS); the object had been posted on the Minor Planet Center's PCCP webpage due to its comet-like orbit. The TESS, which is in a highly elliptical high-earth orbit (in a lunar-resonant orbit) has four 10-cm f/1.4 camera lenses with a 24 deg x 24 deg field-of-view and a 600- to 1000-nm bandpass. The discovery observations are from 2018, were apparently only reported recently, and are tabulated below. 2018 UT R.A. (2000) Decl. Mag. Sept.23.78103 4 23 03.99 -48 01 14.7 18.5 23.88519 4 23 02.62 -48 01 42.4 18.7 23.90603 4 23 02.43 -48 01 48.9 18.8 23.92686 4 23 01.96 -48 01 56.6 18.6 23.96853 4 23 01.61 -48 02 04.4 18.7 Ky identified two single-night pre-discovery observations (at magnitudes i = 19.5 and z = 19.8) obtained on 2017 Aug. 9.64 UT with the 1.35-m f/4.8 "SkyMapper" telescope at Siding Spring, NSW, Australia, and a single-night image obtained on 2017 Nov. 7.11 (at mag i = 19.1) with a 2.65-m Ritchey- Chretien reflector at Cerro Paranal, along with DECam images obtained on five nights with the Cerro Tololo 4-m reflector during 2017 Aug. 2-2019 Mar. 10. In an extremely dense star field in four 140-s r-band DECam exposures taken by A. Zenteno on 2017 Aug. 2.4 and a single 140-s exposure on Aug. 3.4, the comet showed a condensed coma 3" in diameter (full-width-at-half-maximum) at magnitude r = 19.7 with a broad, slightly curved tail 10" long and pointing toward p.a. 260 degrees (250 deg on Aug. 3.4). In a pair of 100-s i-band and z-band exposures by SkyMapper on 2017 Aug. 9.6, no obvious tail was visible, and the coma appeared presumably condensed; in the i-band exposure, the comet had a distinctly "soft" appearance with a slightly wider FWHM diameter of 5" compared to the surrounding stars' FWHM diameters of 4". The coma magnitudes in the SkyMapper observations were i = 19.5 and z = 19.8. In the single 240-s i-band OMEGACAM exposure taken by G. Piotto on 2017 Nov. 7.1, the comet had a condensed coma with an FWHM diameter of 3" and a 3"-long broad, fan-like tail pointing toward p.a. 30-290 deg (northward); the coma magnitude was i = 19.1. In a single 90-s z-band DECam exposure by D. Gruen and A. Plazas on 2018 Oct. 21.3, the comet had a condensed coma with an FWHM diameter of 3" and a 50"-long tail pointing toward p.a. 210-230 deg; the coma magnitude was z = 18.9 as measured in a 6" circular aperture. In a single 90-s g-band DECam exposure taken by L. M. Nikolay and K. M. Murphy on 2018 Dec. 2.3, the comet had a very condensed coma with an FWHM diameter of 2" and a 50"-long tail pointing toward p.a. 210-225 deg; the coma magnitude was g = 19.5 as measured in a 6" circular aperture. In a stack of six i-, r-, and g-band DECam exposures taken by W. Dawson on 2019 Mar. 10.0 (480-s integration time), the comet had a condensed coma with an FWHM diameter of 5" and a 60"-long curved tail pointing toward p.a. 190-195 deg; the measured 6"-aperture magnitudes were i = 19.7, r = 19.7-19.8, and g = 20.4. Ly adds that the comet's activity apparently subsided after Mar. 2019, because the comet is not visible in Zwicky Transient Facility (ZTF) observations past August 2019. No conclusive sign of the comet was found in a stack of three 90-s i-band DECam exposures on 2020 Feb. 6.1, implying that the comet's nuclear condensation must have been fainter than i = 24 mag on that date. S. Deen (Simi Valley, CA, USA) assisted Ly in the research and reporting. The astrometry appears on MPEC 2022-Q126. The following elliptical orbital elements by S. Nakano (Central Bureau) are from 342 observations spanning 2017 Aug. 2-2019 Mar. 10 (mean residual 1".1). The formal error on the orbital period is +/- 2.48 days. Epoch = 2018 Nov. 18.0 TT T = 2018 Nov. 5.37739 TT Peri. = 290.70446 e = 0.6162382 Node = 85.11779 2000.0 q = 5.4712113 AU Incl. = 64.22196 a = 14.2567910 AU n = 0.01830929 P = 53.83 years Epoch = 2072 Dec. 24.0 TT T = 2072 Dec. 15.83893 TT Peri. = 290.67297 e = 0.6183394 Node = 85.17272 2000.0 q = 5.4823921 AU Incl. = 64.16908 a = 14.3645748 AU n = 0.01810360 P = 54.44 years NOTE: These 'Central Bureau Electronic Telegrams' are sometimes superseded by text appearing later in the printed IAU Circulars. (C) Copyright 2022 CBAT 2022 August 26 (CBET 5163) Daniel W. E. Green