Electronic Telegram No. 5173 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 P/2020 S6 = P/1987 A2 = P/2013 Y3 (LEONARD) M. Meyer, Limburg, Germany, has suggested the identify of comet P/2020 S6 (cf. CBET 4868) with the comet X/1987 A2, which was discovered by Malcolm Hartley as a trail on a single plate taken in early 1987 with the 1.2-m Schmidt telescope at Siding Spring (and reported by R. McNaught a couple of months later; cf. IAUC 4355) with a 2' tail. Meyer has re-measured the 1987 plate trail ends, as tabulated below, and with the confirming observations identified by S. Deen (Simi Valley, CA, USA) in 2013-2014, the formerly ambiguous motion in 1987 has shown to be reversed from the positions published on IAUC 4355. The designation X/1987 A2 was published in Marsden and Williams' *Catalogue of Cometary Orbits 1995*, p. 108, as part of their first listing of X/ comets following the introduction of the then-new comet-designation system. 1987 UT R.A. (2000) Decl. Mag. Observer Jan. 5.66764 9 42 17.34 - 8 43 16.7 17.5 Hartley 5.73361 9 42 15.35 - 8 43 36.9 " Following notification of Meyer's suspected 1987 identification, Deen found very diffuse images of this comet in publicly available CCD images obtained apparently as part of B. Gladman's "Outer Solar System Origins Survey" with the 3.58-m Canada-France-Hawaii Telescope at Mauna Kea, and Deen's measures are tabulated below. Deen was unable to find further images of this comet in other publicly available data. 2013 UT R.A. (2000) Decl. Mag. Dec. 29.30479 1 16 34.40 +12 19 27.5 22.2 29.30927 1 16 34.75 +12 19 27.0 2014 UT R.A. (2000) Decl. Mag. Jan. 3.24625 1 24 19.08 +12 07 06.4 22.5 3.30818 1 24 24.88 +12 07 00.2 22.7 The following linked orbital elements by S. Nakano (Central Bureau) are from 273 observations spanning 1987-2021 (mean residual 0".6), and include the non-gravitational parameters A_1 = -0.12 +/- 0.15, A2 = -0.0576 +/- 0.0008. These indicate that the comet has passed 0.064 AU from Jupiter on 1983 July 1 UT; prior to that close approach, the comet had a much-longer orbital period with approximate orbital elements T = 1972 Oct. 22, q = 4.355 AU, e = 0.221, Peri. = 93.0 deg, Node= 250.6 deg, i= 15.7 deg, a= 5.587 AU, P = 13.2 years. An orbit from the 2013-2021 observations without non-gravitational effects shows residuals of +0.08 deg in R.A. and -0.02 deg in Decl. for the 1987 observations. Epoch = 1986 June 19.0 TT T = 1986 June 19.90883 TT Peri. = 176.44411 e = 0.4756270 Node = 242.85094 2000.0 q = 1.8938364 AU Incl. = 15.41548 a = 3.6116209 AU n = 0.14359881 P = 6.86 years Epoch = 1993 May 13.0 TT T = 1993 May 7.68658 TT Peri. = 176.60068 e = 0.4743935 Node = 242.78598 2000.0 q = 1.9017623 AU Incl. = 15.39908 a = 3.6182241 AU n = 0.14320589 P = 6.88 years Epoch = 2000 Apr. 6.0 TT T = 2000 Mar. 25.15888 TT Peri. = 176.49485 e = 0.4734029 Node = 242.76582 2000.0 q = 1.9092300 AU Incl. = 15.39852 a = 3.6255993 AU n = 0.14276914 P = 6.90 years Epoch = 2007 Mar. 1.0 TT T = 2007 Feb. 18.72033 TT Peri. = 176.42851 e = 0.4717632 Node = 242.76913 2000.0 q = 1.9180625 AU Incl. = 15.37893 a = 3.6310654 AU n = 0.14244689 P = 6.92 years Epoch = 2014 Jan. 23.0 TT T = 2014 Jan. 14.57774 TT Peri. = 176.46445 e = 0.4736222 Node = 242.73597 2000.0 q = 1.9061258 AU Incl. = 15.40716 a = 3.6212125 AU n = 0.14302865 P = 6.89 years Epoch = 2020 Nov. 7.0 TT T = 2020 Nov. 23.61273 TT Peri. = 176.65407 e = 0.4789979 Node = 242.57109 2000.0 q = 1.8747918 AU Incl. = 15.45944 a = 3.5984340 AU n = 0.14438888 P = 6.83 years Epoch = 2027 Oct. 2.0 TT T = 2027 Sept.25.02828 TT Peri. = 176.81959 e = 0.4792873 Node = 242.49520 2000.0 q = 1.8732271 AU Incl. = 15.46572 a = 3.5974296 AU n = 0.14444936 P = 6.82 years Epoch = 2034 Oct. 5.0 TT T = 2034 Sept.25.36610 TT Peri. = 178.82234 e = 0.4532423 Node = 241.20966 2000.0 q = 2.0201521 AU Incl. = 15.61468 a = 3.6947849 AU n = 0.13877791 P = 7.10 years Epoch = 2041 Nov. 17.0 TT T = 2041 Nov. 6.83830 TT Peri. = 178.88616 e = 0.4516340 Node = 241.17337 2000.0 q = 2.0310982 AU Incl. = 15.59415 a = 3.7039096 AU n = 0.13826540 P = 7.13 years The photometric power-law parameters for the 2020-2021 apparitions are something like H = 15.0, 2.5n = 10.0 (or H = 13.5, 2.5n = 15.0), but the 2020 parameters are about three magnitudes too faint for the 1987 post-perihelion magnitude estimated by Hartley/McNaught and are about four magnitudes brighter than Deen's magnitudes for the near-perihelion observations in 2013-2014. NOTE: These 'Central Bureau Electronic Telegrams' are sometimes superseded by text appearing later in the printed IAU Circulars. (C) Copyright 2022 CBAT 2022 October 6 (CBET 5173) Daniel W. E. Green