Electronic Telegram No. 5056 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/2021 T3 = P/2015 K6 (PANSTARRS) R. Weryk, Department of Physics and Astronomy, University of Western Ontario, reports the discovery of another comet in images obtained with the Pan-STARRS1 1.8-m Ritchey-Chretien reflector at Haleakala on 2021 Oct. 2, with the discovery astrometry tabulated below; four 45-s w-band survey images show a very condensed coma of size 1".3 (full-width-at-half-maximum) in 1".2 seeing and a "very obvious" broad tail > 8" long spanning p.a. 285-315 deg. 2021 UT R.A. (2000) Decl. Mag. Oct. 2.40387 1 24 05.29 -28 44 48.4 20.5 2.41354 1 24 04.81 -28 44 50.1 20.4 2.42329 1 24 04.32 -28 44 51.9 20.4 2.43296 1 24 03.83 -28 44 53.6 20.5 2.51495 1 23 59.70 -28 45 07.3 20.3 2.51560 1 23 59.67 -28 45 07.3 20.3 2.52487 1 23 59.20 -28 45 08.9 20.4 2.52553 1 23 59.17 -28 45 09.0 20.5 Weryk adds that eight additional 60-s w-band follow-up Pan-STARRS1 images obtained on Oct. 5.6 UT in 1".2 seeing show the object clearly to be a comet, with a very condensed coma of size 1".4 (FWHM) with a broad 8" tail spanning p.a. 285-315 deg. After the comet was posted on the Minor Planet Center's PCCP webpage, H. Sato, Tokyo, Japan, writes that twelve stacked 60-s CCD exposures taken remotely on Oct. 3.69 UT with a 0.51-m f/6.8 astrograph located at Siding Spring, NSW, Australia, show a strongly condensed coma 6" in diameter with no tail; the magnitude was 20.0 as measured within a circular aperture of radius 4".9. Following are tabulated the first of numerous observations identified following the 2021 discovery from the previously unknown 2015-2017 apparition. The 2015 Feb. 24 observations were found by, and measured by, S. Deen (Simi Valley, CA, USA) in publicly available images obtained with the DECam camera on the Cerro Tololo 4-m reflector. The 2015 May 18 and 19 Pan-STARRS1 observations were reported to the MPC first, and they are the basis of the customary pre-numbering second-appartion designation P/2015 K6. Other Pan- STARRS1 and Pan-STARRS2 observations were identified subsequent to the 2021 discovery for 2015 June 10 and 19, 2016 Oct. 10 and Nov. 1, and 2017 Jan. 20 (with peak brightness of mag 21-22 given for the Oct. and Nov. astrometry). 2015 UT R.A. (2000) Decl. Mag. Feb. 24.31664 16 29 02.22 - 6 02 46.9 23.8 24.31869 16 29 02.35 - 6 02 47.1 May 18.54468 16 24 53.41 - 4 02 23.5 22.7 18.55524 16 24 52.85 - 4 02 23.7 22.8 19.42821 16 24 06.45 - 4 02 56.4 22.5 19.43887 16 24 05.87 - 4 02 56.8 22.8 19.46026 16 24 04.69 - 4 02 57.5 22.6 Additional single-night observations from 2002 were found and measured by Deen in publicly available images obtained with the Apache Point 2.5-m reflector as part of the Sloan Digital Sky Survey, as tabulated below: 2002 UT R.A. (2000) Decl. Mag. Jan. 9.35170 7 54 22.60 +27 51 31.6 9.35253 7 54 22.55 +27 51 32.2 22.1 9.35502 7 54 22.39 +27 51 33.1 23.3 The available astrometry appears on MPEC 2021-T184, which include Pan- STARRS1 and Pan-STARRS2 pre-discovery observations identified on five nights spanning 2020 Apr. 20-May 21 (magnitude 22-23). The following linked orbital elements by S. Nakano (CBAT) are from 82 observations spanning 2002-2021 (mean residual 0".2). A search of archival astrometry produced no additional observations, meaning that two apparitions were missed between 2002 and 2015. The comet passed 1.98 AU from Jupiter on 2002 Jan. 8 UT, just when the available 2002 observations were made. Epoch = 2000 Aug. 4.0 TT T = 2000 July 25.45740 TT Peri. = 244.59401 e = 0.3095096 Node = 98.82438 2000.0 q = 2.0927127 AU Incl. = 18.88557 a = 3.0307629 AU n = 0.18679969 P = 5.28 years Epoch = 2005 Nov. 6.0 TT T = 2005 Nov. 1.91903 TT Peri. = 244.76244 e = 0.3165627 Node = 98.63674 2000.0 q = 2.0661729 AU Incl. = 18.87140 a = 3.0232078 AU n = 0.18750035 P = 5.26 years Epoch = 2011 Feb. 8.0 TT T = 2011 Feb. 3.29789 TT Peri. = 244.86173 e = 0.3181464 Node = 98.60074 2000.0 q = 2.0599884 AU Incl. = 18.87310 a = 3.0211592 AU n = 0.18769109 P = 5.25 years Epoch = 2016 May 12.0 TT T = 2016 Apr. 30.82359 TT Peri. = 244.48330 e = 0.3183601 Node = 98.56229 2000.0 q = 2.0584609 AU Incl. = 18.88370 a = 3.0198656 AU n = 0.18781171 P = 5.25 years Epoch = 2021 Aug. 14.0 TT T = 2021 July 27.53180 TT Peri. = 244.16418 e = 0.3190246 Node = 98.55664 2000.0 q = 2.0563538 AU Incl. = 18.88241 a = 3.0197180 AU n = 0.18782547 P = 5.25 years Epoch = 2026 Oct. 7.0 TT T = 2026 Oct. 26.57615 TT Peri. = 244.15727 e = 0.3174260 Node = 98.52286 2000.0 q = 2.0629196 AU Incl. = 18.87368 a = 3.0222651 AU n = 0.18758808 P = 5.25 years Epoch = 2032 Jan. 9.0 TT T = 2032 Jan. 26.99092 TT Peri. = 243.95395 e = 0.3142085 Node = 98.40473 2000.0 q = 2.0745481 AU Incl. = 18.87923 a = 3.0250419 AU n = 0.18732985 P = 5.26 years Epoch = 2037 May 22.0 TT T = 2037 May 4.94829 TT Peri. = 244.18263 e = 0.3135286 Node = 98.35529 2000.0 q = 2.0776898 AU Incl. = 18.87204 a = 3.0266225 AU n = 0.18718312 P = 5.26 years The following ephemeris by the undersigned from the above orbital elements uses photometric power-law parameters H = 17.0 and 2.5n = 8 for the magnitudes. Date TT R. A. (2000) Decl. Delta r Elong. Phase Mag. 2021 09 23 01 30.81 -28 00.4 1.193 2.093 144.5 16.2 19.9 2021 10 03 01 23.61 -28 46.3 1.204 2.106 145.4 15.7 20.0 2021 10 13 01 15.44 -28 50.3 1.234 2.121 143.5 16.3 20.1 2021 10 23 01 07.76 -28 10.0 1.284 2.138 139.3 17.7 20.2 2021 11 02 01 01.78 -26 49.2 1.350 2.157 133.5 19.5 20.3 2021 11 12 00 58.27 -24 55.2 1.434 2.178 127.0 21.3 20.5 2021 11 22 00 57.55 -22 36.9 1.531 2.200 120.1 22.9 20.7 2021 12 02 00 59.56 -20 02.7 1.641 2.223 113.2 24.0 20.9 2021 12 12 01 04.05 -17 19.0 1.761 2.248 106.4 24.8 21.0 2021 12 22 01 10.71 -14 31.1 1.890 2.274 99.7 25.2 21.2 2022 01 01 01 19.17 -11 42.6 2.026 2.301 93.2 25.3 21.4 2022 01 11 01 29.16 -08 56.0 2.166 2.329 86.9 24.9 21.6 2022 01 21 01 40.41 -06 13.3 2.308 2.358 80.7 24.3 21.8 2022 01 31 01 52.70 -03 35.8 2.453 2.388 74.6 23.4 22.0 2022 02 10 02 05.88 -01 04.4 2.597 2.418 68.6 22.3 22.1 2022 02 20 02 19.78 +01 19.9 2.739 2.449 62.8 21.0 22.3 2022 03 02 02 34.30 +03 36.8 2.877 2.481 57.0 19.6 22.5 NOTE: These 'Central Bureau Electronic Telegrams' are sometimes superseded by text appearing later in the printed IAU Circulars. (C) Copyright 2021 CBAT 2021 October 9 (CBET 5056) Daniel W. E. Green