Electronic Telegram No. 5035 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 R5 (RANKIN) David Rankin reports his discovery of a comet with a coma of size about 8" on CCD images obtained on Sept. 9.3 UT with the Mt. Lemmon 1.5-m reflector (discovery observations tabulated below). 2021 UT R.A. (2000) Decl. Mag. Observer Sept. 9.28644 0 34 13.28 +10 32 39.6 19.3 Rankin 9.29208 0 34 13.13 +10 32 38.3 19.2 " 9.29865 0 34 12.96 +10 32 36.9 19.3 " 9.30527 0 34 12.78 +10 32 35.6 19.2 " 9.38955 0 34 10.57 +10 32 18.1 19.4 " 9.39600 0 34 10.39 +10 32 16.9 19.4 " 9.40265 0 34 10.24 +10 32 15.5 19.3 " Single-night observations that had not been flagged as unusual were subsequently identified in the Minor Planet Center's "isolated tracklet file" (present there at the time of discovery): observations obtained on Sept. 5.4 UT (at mag 19.9-20.1) with the Pan-STARRS2 1.8-m Ritchey-Chretien reflector at Haleakala, Hawaii, and on Sept. 7.5 (at mag 19.4-19.6) with a 0.5-m f/2 Schmidt reflector at Mauna Loa, Hawaii, in the course of the "Asteroid Terrestrial-Impact Last Alert System" (ATLAS) search program. R. Weryk, Department of Physics and Astronomy, University of Western Ontario, writes that the Sept. 5 Pan-STARRS2 observations consist of four 45-s w-band survey images taken in 1".2 seeing that show a condensed coma of size 1".9 (full-width-at-half-maximum) and no tail; it was not initially seen to show cometary appearance. After the comet was posted to the MPC's PCCP webpage, other CCD observers also commented on the cometary appearance. Nineteen stacked 60-s exposures taken by K. Yoshimoto, Kumage, Yamaguchi, Japan, with a 0.20-m f/8 reflector on Sept. 9.7 UT show a coma about 14" in diameter with total mag 19.3; Yoshimoto also obtained ten stacked 60-s exposures remotely on Sept. 9.4 with a 0.50-m f/6.8 reflector located near Mayhill, NM, USA, that show a compact coma 15" in diameter that extends to the southwest (total mag 19.4). Twelve stacked 60-s exposures taken remotely by H. Sato (Tokyo, Japan) on Sept. 9.43 with a 0.51-m f/6.8 astrograph located near Mayhill show a strongly condensed coma 12" in diameter with a fan-like tail 15" toward p.a. 150-300 degrees; the magnitude was 18.8 as measured within a circular aperture of radius 6".5. The available astrometry appears on MPEC 2021-R257 and includes pre-discovery observations that were identified later from images obtained with the Pan-STARRS1 1.8-m Ritchey-Chretien reflector at Haleakala (2021 June 22.6 UT, mag 21.0-21.8; Aug. 11.5, mag 20.0-20.2) and with the Pan-STARRS2 reflector (Sept. 1.4, mag 20.0), as well as observations made at Catalina on Sept. 6.4 (mag 19.6-20.3). The following orbital elements by S. Nakano (CBAT) are from 66 observations spanning 2021 June 22-Sept. 11 (mean residual 0".4); he notes with caution the dependence upon single-night observations on June 22 and Aug. 11. This orbit suggests that the comet passed 0.54 AU from Jupiter in 2007 August. No observations of this comet were identified in a search of archival astrometry. T = 2022 Jan. 9.64905 TT Peri. = 166.86240 e = 0.3075763 Node = 219.99123 2000.0 q = 3.3256213 AU Incl. = 7.84755 a = 4.8028707 AU n = 0.09363813 P = 10.53 years The following ephemeris by the undersigned from the above orbital elements uses photometric power-law parameters H = 12.0 and 2.5n = 10 for the magnitudes. Date TT R. A. (2000) Decl. Delta r Elong. Phase Mag. 2021 08 14 00 40.11 +11 16.4 2.705 3.414 127.2 13.7 19.5 2021 08 19 00 39.84 +11 15.3 2.647 3.408 132.1 12.7 19.4 2021 08 24 00 39.16 +11 10.9 2.593 3.403 137.0 11.7 19.4 2021 08 29 00 38.06 +11 03.0 2.544 3.397 142.0 10.5 19.3 2021 09 03 00 36.58 +10 51.7 2.500 3.392 147.2 9.3 19.3 2021 09 08 00 34.74 +10 37.0 2.462 3.387 152.4 7.9 19.3 2021 09 13 00 32.60 +10 19.1 2.430 3.383 157.7 6.5 19.2 2021 09 18 00 30.21 +09 58.3 2.404 3.378 163.0 5.0 19.2 2021 09 23 00 27.64 +09 35.1 2.385 3.373 168.1 3.5 19.2 2021 09 28 00 24.96 +09 09.7 2.373 3.369 172.5 2.2 19.2 2021 10 03 00 22.23 +08 42.7 2.368 3.365 174.1 1.8 19.1 2021 10 08 00 19.55 +08 14.7 2.370 3.361 171.1 2.6 19.1 2021 10 13 00 17.00 +07 46.4 2.380 3.358 166.3 4.0 19.1 2021 10 18 00 14.64 +07 18.4 2.396 3.354 161.0 5.5 19.2 2021 10 23 00 12.55 +06 51.2 2.419 3.351 155.6 7.0 19.2 2021 10 28 00 10.79 +06 25.5 2.449 3.348 150.2 8.5 19.2 2021 11 02 00 09.40 +06 01.8 2.485 3.345 144.8 9.8 19.2 2021 11 07 00 08.42 +05 40.4 2.526 3.342 139.5 11.1 19.3 2021 11 12 00 07.88 +05 21.7 2.573 3.340 134.3 12.3 19.3 2021 11 17 00 07.79 +05 06.1 2.624 3.337 129.2 13.3 19.3 2021 11 22 00 08.17 +04 53.6 2.679 3.335 124.1 14.2 19.4 2021 11 27 00 09.02 +04 44.4 2.738 3.333 119.2 15.0 19.4 2021 12 02 00 10.31 +04 38.5 2.800 3.332 114.5 15.6 19.5 2021 12 07 00 12.05 +04 35.8 2.865 3.330 109.8 16.2 19.5 2021 12 12 00 14.22 +04 36.4 2.932 3.329 105.2 16.6 19.6 2021 12 17 00 16.80 +04 40.1 3.001 3.328 100.7 16.9 19.6 2021 12 22 00 19.77 +04 46.8 3.071 3.327 96.4 17.1 19.7 2021 12 27 00 23.10 +04 56.2 3.141 3.326 92.1 17.2 19.7 2022 01 01 00 26.78 +05 08.4 3.213 3.326 87.9 17.2 19.8 2022 01 06 00 30.78 +05 23.0 3.284 3.326 83.9 17.1 19.8 2022 01 11 00 35.08 +05 39.9 3.355 3.326 79.9 16.9 19.8 2022 01 16 00 39.67 +05 58.9 3.425 3.326 75.9 16.7 19.9 2022 01 21 00 44.50 +06 19.9 3.494 3.326 72.1 16.4 19.9 NOTE: These 'Central Bureau Electronic Telegrams' are sometimes superseded by text appearing later in the printed IAU Circulars. (C) Copyright 2021 CBAT 2021 September 12 (CBET 5035) Daniel W. E. Green