Electronic Telegram No. 5550 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 TR_81 (LEMMON) An apparently asteroidal object discovered with the Mt. Lemmon survey's 1.5-m reflector on 2021 Oct. 8, and given the minor-planet designation 2021 TR_81 when announced on MPS 1513132, has been found to show cometary appearance by observers elsewhere. The discovery observations are tabulated below. 2021 UT R.A. (2000) Decl. Mag. Oct. 8.39029 3 57 00.98 +15 49 50.5 21.2 8.39602 3 57 00.98 +15 49 45.9 21.4 8.40116 3 57 00.99 +15 49 41.7 20.9 8.40632 3 57 00.90 +15 49 36.5 21.5 When re-discovered on images obtained on 2022 Jan. 8.23-8.25 UT with the Pan-STARRS2 1.8-m Ritchey-Chretien reflector at Haleakala, Y. Ramanjooloo (Institute for Astronomy, University of Hawaii) quickly saw that it was 2021 TR_81 and noted slight fuzziness in the images; three 45-s w-band exposures taken in 1".6-2".2 seeing showed a diffuse coma of size about 2"-2".3 with a 2"-wide tail that was about 2" long in p.a. 315 degrees. R. Weryk recently looked at these same Pan-STARRS2 images again and remarked on a broad, short tail spanning p.a. 250-350 deg. H. Sato (Bunkyo-ku, Tokyo, Japan) writes that twelve stacked 60-s CCD exposures taken remotely on 2021 Dec. 29.45 UT with a 0.51-m f/6.8 astrograph at Siding Spring, NSW, Australia, show only a stellar appearance; the magnitude was 19.7-19.8 as measured within a circular aperture of radius 4".9. Twelve stacked 60-s exposures taken with the same telescope on 2022 Jan. 23.46 show again a stellar appearance; the magnitude was 18.7 as measured within a circular aperture of radius 4".9. Six stacked 60-s CCD exposures taken by Sato on 2022 Mar. 22.39 show a strongly condensed head with an outer coma 1'.6 in diameter and no tail; the magnitude was 15.0 as measured within a circular aperture of radius 49".2. Six stacked 60-s exposures, again taken by Sato with the same telescope, on 2022 Apr. 12.39 in moonlight show a strongly condensed head with a smaller coma 30" in diameter and no tail; the magnitude was 16.9 as measured within a circular aperture of radius 15".3. Sato also obtained ten stacked 60-s CCD exposures with a 0.43-m f/6.8 astrograph at Siding Spring on 2022 Mar. 21.4 that show a strongly condensed head with an outer coma 50" in diameter and no tail; the magnitude was 15.0 as measured within a circular aperture of radius 26".5. Thirty stacked 90-s CCD exposures taken on 2022 Jan. 5.9 UT by L. Buzzi and L. Demetz with a Skygems Telescope Network 0.51-m f/6.7 astrograph in Namibia (and measured by Buzzi, A. Aletti, and G. Galli) following an alert Sato, were reviewed and show a stellar head of mag 20.1, but there was then a trace of a faint, broad, fan-shaped tail toward the north. One hundred 15-s CCD exposures taken by Buzzi with a 0.84-m f/3.5 reflector at Varese, Italy, on 2022 Mar. 22.8 in good conditions (but low altitude) show a condensed 8" coma of mag 18.1 that was clearly extended toward the north, with an apparent faint, broad 6" tail centered at p.a. about 20 deg. Seventy-six 15-s images obtained by Buzzi with the same telescope at Varese on 2022 Mar. 24.8 in good conditions (but low altitude) show a condensed 9" coma of mag 18.0 with a sign of a faint outer coma toward the north. Buzzi also measured sixty stacked 60-s CCD exposures taken by Aletti, F. Bellini, Galli, and himself with a 0.36-m f/8.4 reflector located at the Schiaparelli Southern Observatory in Hakos, Namibia, on 2022 May 1.75-1.78 that show its clear cometary appearance, with a moderately diffuse coma 5" wide (mag 18.6) and with a broad, fan-shaped tail extending at least 12" in p.a. about 90 deg. The available astrometry appears on MPEC 2025-H93 and includes pre- discovery observations obtained with the with the Pan-STARRS2 1.8-m Ritchey- Chretien reflector at Haleakala on 2021 Sept. 11.5 UT (mag 21.6-22.4) and Sept. 29.45-29.47 (mag 21.5-21.7) that were reported later. The following orbital elements by S. Nakano (Central Bureau) are from 112 observations spanning 2021 Sept. 11-2022 May 27 (mean residual 0".5). The comet passed 1.69 AU from Jupiter in 1971 August. Epoch = 2022 Apr. 11.0 TT T = 2022 Apr. 3.02891 TT Peri. = 272.04547 e = 0.8324968 Node = 220.20599 2000.0 q = 1.4685820 AU Incl. = 36.82071 a = 8.7674870 AU n = 0.03796570 P = 25.96 years Epoch = 2048 Mar. 25.0 TT T = 2048 Apr. 9.99203 TT Peri. = 271.77580 e = 0.8321595 Node = 219.80168 2000.0 q = 1.4913422 AU Incl. = 36.93286 a = 8.8854722 AU n = 0.03721203 P = 26.49 years The photometric power-law parameters H = 15.0 and 2.5n = 10 suggest that the comet is currently near mag 29. NOTE: These 'Central Bureau Electronic Telegrams' are sometimes superseded by text appearing later in the printed IAU Circulars. (C) Copyright 2025 CBAT 2025 April 30 (CBET 5550) Daniel W. E. Green