Electronic Telegram No. 5323 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/2023 X2 (LEMMON) 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 Dec. 4.3 UT, in which four 45-s w-band survey images taken in 1".0 seeing show a condensed head of size 1".7 (full-width-at-half-maximum) with a tail about 6" long toward p.a. 210 degrees. The object was reported independently as an apparently asteroidal object on images obtained shortly thereafter (Dec. 4.4) with the 1.5-m Mt. Lemmon Survey reflector, but the Mt. Lemmon observations were posted to the Minor Planet Center's NEOCP website before the Pan-STARRS1 observations arrived, and Weryk immediately noticed the identity. The discovery observations are tabulated below. 2023 UT R.A. (2000) Decl. Mag. Observer Dec. 4.31664 4 13 21.52 +16 02 18.3 20.5 Pan-STARRS1 4.33254 4 13 20.94 +16 02 20.6 20.8 " 4.34673 4 13 20.41 +16 02 22.5 20.6 " 4.35197 4 13 20.17 +16 02 22.6 19.7 Mt. Lemmon Survey 4.35709 4 13 19.97 +16 02 23.2 20.2 " 4.36088 4 13 19.89 +16 02 24.2 20.5 Pan-STARRS1 4.36221 4 13 19.78 +16 02 24.1 20.1 Mt. Lemmon Survey 4.36733 4 13 19.60 +16 02 24.4 19.8 " Pan-STARRS1 observations made in Oct. 12.5 UT (when the comet was at mag 21.3-21.6) were subsequently identified in MPC's "isolated tracklet file", together with observations on Oct. 17.5 (mag 21.5-21.8) made with the Pan-STARRS2 1.8-m Ritchey-Chretien reflector at Haleakala. Weryk then found additional Pan-STARRS2 observations on Aug. 18.6 (mag 21.7-22.1), Sept. 20.6 (mag 21.4-21.8), and Oct. 7.5 (mag 21.1-21.5). Y. Ramanjooloo (Institute for Astronomy, University of Hawaii) writes that four 45-s w-band Pan-STARRS2 images obtained on Dec. 7.33-7.37 show a diffuse and faint 2".2 tail towards p.a. 227 degrees, with a condensed coma of size 2".1. After the comet was posted to the MPC's PCCP webpage, other CCD astrometrists commented on the object's appearance. Ten stacked 120-s 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 Dec. 6.2 UT show only a stellar appearance; the magnitude was 20.2 as measured within a circular aperture of radius 3".8. Ninety-nine stacked 30-s exposures taken by L. Buzzi and G. Galli at Varese, Italy, with a 0.84-m f/3.5 reflector on Dec. 7.0 (measured by A. Aletti, Buzzi, and Galli) show a condensed coma 7" wide (mag 20.5) with a faint, straight tail 12" long in p.a. 265 degrees. The available astrometry appears on MPEC 2023-X226. The following orbital elements by S. Nakano (Central Bureau) are from 75 observations spanning 2023 Aug. 18-Dec. 11 (mean residual 0".3). The corresponding "original" and "future" values of 1/a are +0.000333 and +0.000712 (+/- 0.000044) AU**-1, respectively. The comet will pass 1.82 AU from Jupiter on 2024 Aug. 9 UT. Epoch = 2025 Dec. 31.0 TT T = 2025 Dec. 28.47605 TT Peri. = 64.79176 e = 0.9997663 Node = 66.30112 2000.0 q = 5.0889123 AU Incl. = 76.97848 The following ephemeris by the undersigned from the above orbital elements uses photometric power-law parameters H = 8.5 and 2.5n = 8 for the magnitudes. Date TT R. A. (2000) Decl. Delta r Elong. Phase Mag. 2023 12 02 04 14.78 +15 57.0 6.541 7.521 173.1 0.9 19.6 2023 12 12 04 08.69 +16 20.2 6.522 7.471 163.3 2.2 19.6 2023 12 22 04 02.86 +16 44.9 6.536 7.421 152.2 3.5 19.5 2024 01 01 03 57.55 +17 11.1 6.581 7.371 141.0 4.8 19.5 2024 01 11 03 52.97 +17 39.0 6.651 7.321 129.9 5.9 19.5 2024 01 21 03 49.29 +18 08.9 6.744 7.272 119.0 6.8 19.5 2024 01 31 03 46.61 +18 40.9 6.852 7.223 108.4 7.4 19.5 2024 02 10 03 44.98 +19 14.9 6.970 7.174 98.0 7.8 19.6 2024 02 20 03 44.39 +19 50.9 7.093 7.125 87.9 8.0 19.6 2024 03 01 03 44.81 +20 28.9 7.215 7.077 78.0 7.9 19.6 2024 03 11 03 46.18 +21 08.7 7.332 7.029 68.5 7.6 19.6 2024 03 21 03 48.42 +21 50.1 7.438 6.981 59.2 7.0 19.6 2024 03 31 03 51.44 +22 33.1 7.530 6.933 50.2 6.4 19.6 2024 07 29 04 54.21 +32 34.2 6.980 6.386 50.8 7.1 19.2 2024 08 08 04 58.67 +33 34.6 6.802 6.343 59.2 7.9 19.1 2024 08 18 05 02.45 +34 37.8 6.613 6.301 67.8 8.6 19.0 2024 08 28 05 05.41 +35 44.2 6.416 6.258 76.5 9.0 18.9 2024 09 07 05 07.34 +36 54.0 6.214 6.217 85.5 9.3 18.8 2024 09 17 05 08.08 +38 07.3 6.012 6.175 94.6 9.3 18.7 2024 09 27 05 07.43 +39 23.5 5.815 6.135 104.0 9.1 18.6 2024 10 07 05 05.21 +40 42.1 5.627 6.094 113.5 8.7 18.5 2024 10 17 05 01.25 +42 01.4 5.453 6.055 123.1 7.9 18.4 2024 10 27 04 55.49 +43 19.6 5.299 6.016 132.5 7.0 18.4 2024 11 06 04 47.93 +44 33.9 5.170 5.977 141.4 5.9 18.3 2024 11 16 04 38.75 +45 41.5 5.069 5.939 149.1 4.9 18.2 2024 11 26 04 28.31 +46 39.5 4.999 5.901 153.9 4.2 18.2 2024 12 06 04 17.10 +47 25.9 4.962 5.865 154.2 4.2 18.1 2024 12 16 04 05.81 +47 59.8 4.958 5.828 149.7 4.9 18.1 2024 12 26 03 55.08 +48 21.9 4.985 5.793 142.2 6.0 18.1 2025 01 05 03 45.54 +48 34.3 5.039 5.758 133.3 7.1 18.1 NOTE: These 'Central Bureau Electronic Telegrams' are sometimes superseded by text appearing later in the printed IAU Circulars. (C) Copyright 2023 CBAT 2023 December 12 (CBET 5323) Daniel W. E. Green