Electronic Telegram No. 5345 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/2024 B2 (LEMMON) An apparently asteroidal object discovered on Jan. 31 with the Mt. Lemmon Survey's 1.5-m reflector in Arizona has been found to show cometary appearance after posting on the Minor Planet Center's PCCP webpage. The discovery observations are tabulated below. 2024 UT R.A. (2000) Decl. Mag. Jan. 31.25544 8 42 46.00 +57 36 25.6 20.6 31.26070 8 42 45.07 +57 36 24.2 31.26596 8 42 44.25 +57 36 21.2 20.3 31.27122 8 42 43.27 +57 36 18.8 20.8 R. Weryk (Department of Physics and Astronomy, University of Western Ontario) writes that he found this object in pre-discovery exposures taken with both the Pan-STARRS1 and the Pan-STARRS2 1.8-m Ritchey-Chretien reflectors at Haleakala, Hawaii. Pan-STARRS1 images show the comet on 2023 Dec. 29.5 (mag 21.2-21.5) and 30.52-30.55 UT (mag 20.6-21.3), along with images from both Pan-STARRS1 and Pan-STARRS2 on 2023 Dec. 31.6 (mag 20.8-22.2) and 2024 Jan. 23.4 (mag 20.9-22.6), and from Pan-STARRS2 alone on 2024 Jan. 27.5 (mag 20.6- 21.5). Four 45-s Pan-STARRS2 w-band survey images taken on 2024 Feb. 3.4 in 1".5-1".7 seeing show a diffuse coma of size 2".1 (full-width-at-half-maximum) and no tail. S. Deen (Simi Valley, CA, USA) writes that he found pre-discovery images of this object in 300-s exposures taken on 2023 May 14.28-14.39 and 15.28- 15.35 UT with the 3.6-m Canada-France-Hawaii Telescope by J.-C. Cuillandre for the "Ultraviolet Near Infrared Optical Northern Survey". The comet is trailed but shows a coma, and in some images it appears to have a faint tail roughly 8" long in p.a. 90-135 degrees. The red magnitude was measured as 22.2-22.6 on May 14 and 22.3-22.5 on May 15. Twenty-four stacked 60-s CCD exposures taken remotely by H. Sato (Bunkyo-ku, Tokyo, Japan) with a 0.43-m f/6.8 astrograph located at the Utah Desert Remote Observatory (near Beryl Junction, UT, USA) on 2024 Feb. 4.3 UT show a strongly condensed coma 6" in diameter and no tail. The magnitude was 20.1 as measured within a circular aperture of radius 5".7. The astrometry appears on MPEC 2024-C87. The following nearly parabolic orbital elements by S. Nakano (Central Bureau) are from 66 observations spanning 2023 May 14-2024 Feb. 6 (mean residual 0".3), with corresponding "original" and "future" values of 1/a being +0.001009 and +0.001111 (+/- 0.000005) AU**-1, respectively. These indicate that the comet passed 4.47 AU from Saturn on 2019 Aug. 16 and will pass 3.11 AU from Jupiter on 2025 Apr. 1 UT. Epoch = 2023 Oct. 23.0 TT T = 2023 Oct. 5.96412 TT Peri. = 130.30602 e = 0.9994130 Node = 294.37687 2000.0 q = 4.0764471 AU Incl. = 99.84050 The following ephemeris by the undersigned from the above orbital elements uses photometric power-law parameters H = 12.5 and 2.5n = 8 for the magnitudes. Date TT R. A. (2000) Decl. Delta r Elong. Phase Mag. 2024 01 11 09 41.66 +58 38.9 3.395 4.159 136.1 9.4 20.1 2024 01 21 09 12.66 +58 30.0 3.375 4.177 139.9 8.7 20.1 2024 01 31 08 43.49 +57 38.3 3.393 4.196 140.0 8.7 20.1 2024 02 10 08 16.86 +56 05.6 3.449 4.217 136.2 9.3 20.2 2024 02 20 07 54.65 +54 00.9 3.541 4.239 129.5 10.4 20.3 2024 03 01 07 37.53 +51 36.7 3.664 4.263 121.2 11.5 20.4 2024 03 11 07 25.27 +49 04.5 3.815 4.288 112.1 12.4 20.5 2024 03 21 07 17.24 +46 32.7 3.986 4.315 102.7 13.0 20.6 2024 03 31 07 12.66 +44 07.0 4.171 4.343 93.2 13.3 20.7 2024 04 10 07 10.83 +41 49.9 4.363 4.373 84.0 13.2 20.8 2024 04 20 07 11.16 +39 42.4 4.558 4.404 74.9 12.7 20.9 2024 04 30 07 13.14 +37 44.3 4.749 4.436 66.0 12.0 21.1 2024 05 10 07 16.38 +35 54.8 4.933 4.470 57.4 11.0 21.2 NOTE: These 'Central Bureau Electronic Telegrams' are sometimes superseded by text appearing later in the printed IAU Circulars. (C) Copyright 2024 CBAT 2024 February 8 (CBET 5345) Daniel W. E. Green