Electronic Telegram No. 5424 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 O1 (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 July 28 (discovery observations tabulated below). Four 45-s w-band survey images taken in variable seeing of 1".0-1".5 show a possible assymetry toward p.a. 115 degrees but no obvious coma and no obvious tail. 2024 UT R.A. (2000) Decl. Mag. July 28.39663 21 02 48.52 +14 26 10.6 21.0 28.41936 21 02 47.85 +14 26 13.8 21.1 28.43073 21 02 47.51 +14 26 15.7 21.1 Weryk adds that three 60-s gri-band confirming observations were made on July 29.48 UT with the 3.6-m Canada-France-Hawaii Telescope atop Mauna Kea (with R. Wainscoat's observing time; queue observer J. Silva; queue coordinator H. Flewelling) in 0".9 seeing show a very broad tail 5" long spanning p.a. 80-180 degrees with a head of size 1".9 (full-width-at-half-maximum). Additional pre-discovery Pan-STARRS1 observations from 2023 June 18.5 (mag 21.7-21.8) and 2023 Sept. 12.3 (mag 21.5-21.7) were subsequently found in the Minor Planet Center's "isolated tracklet file" (which is accessible publicly), as were observations from 2023 Sept. 15.3 (at mag 21.5-21.6) obtained with the Pan-STARRS2 1.8-m Ritchey-Chretien reflector at Haleakala. The comet was posted on the MPC's PCCP website, leading to other observations. S. Deen (Simi Valley, CA, USA) then identified two observations each on two nights that were obtained with the 4-m Cerro Tololo reflector (+ DECam) on 2023 July 16.14-16.15 (magnitude g = 21.5-21.7) and on 2023 July 28.2 (mag z = 20.6-20.7). On July 16, the comet showed slight activity with a coma of size 1".5 (FWHM) in 1".0 seeing and no tail; on July 28, it showed a coma of size 1".4 in 0".9 seeing with no tail. Deen adds that he was unable to find the comet in any 2022 images, noting that a 2022 Sept. 19 DECam frame of the comet's predicted position shows nothing to limiting mag g = 23.4; an image from 2022 Aug. 16 might show the comet at the limit (i = 23.1), but Deen was not confident in the detection. Seventeen stacked 60-s CCD exposures taken remotely by H. Sato (Tokyo, Japan) with a 0.51-m f/6.8 astrograph at Siding Spring, NSW, Australia, on 2024 July 30.6 UT show a moderately condensed coma 5" in diameter with no tail; the magnitude was 20.9 as measured within a circular aperture of radius 3".2. The available astrometry appears on MPEC 2024-P21. The following orbital elements by S. Nakano (Central Bureau) are from 25 observations spanning 2023 June 18-2024 July 29 (mean residual 0".2); the corresponding "original" and "future" values of 1/a are +0.000027 and +0.000143 (+/- 0.000002) AU**-1, respectively. There are no close approaches to major planets. Epoch = 2023 Jan. 16.0 TT T = 2023 Jan. 29.15824 TT Peri. = 349.93217 e = 1.0024928 Node = 303.32958 2000.0 q = 6.6084350 AU Incl. = 59.05398 The following ephemeris by the undersigned from the above orbital elements uses photometric power-law parameters H = 10.0 and 2.5n = 8 for the magnitudes. Date TT R. A. (2000) Decl. Delta r Elong. Phase Mag. 2024 07 29 21 02.51 +14 27.6 6.671 7.535 146.0 4.3 21.1 2024 08 08 20 57.60 +14 47.4 6.680 7.566 148.8 4.0 21.2 2024 08 18 20 52.76 +14 59.0 6.716 7.598 148.5 4.0 21.2 2024 08 28 20 48.24 +15 03.5 6.778 7.630 145.2 4.3 21.2 2024 09 07 20 44.22 +15 01.8 6.866 7.662 139.6 4.9 21.3 2024 09 17 20 40.88 +14 55.4 6.978 7.695 132.7 5.5 21.3 2024 09 27 20 38.33 +14 46.0 7.109 7.728 125.0 6.1 21.4 2024 10 07 20 36.67 +14 35.1 7.258 7.761 116.9 6.6 21.4 2024 10 17 20 35.91 +14 24.5 7.420 7.795 108.6 7.0 21.5 2024 10 27 20 36.06 +14 15.4 7.591 7.830 100.3 7.2 21.6 2024 11 06 20 37.08 +14 09.1 7.767 7.864 92.0 7.2 21.6 2024 11 16 20 38.92 +14 06.7 7.944 7.899 83.8 7.2 21.7 2024 11 26 20 41.48 +14 08.8 8.118 7.934 75.8 6.9 21.7 2024 12 06 20 44.69 +14 16.1 8.285 7.970 68.1 6.6 21.8 2024 12 16 20 48.46 +14 29.0 8.442 8.005 60.6 6.1 21.9 2024 12 26 20 52.68 +14 47.7 8.587 8.041 53.6 5.6 21.9 2025 01 05 20 57.25 +15 12.4 8.716 8.078 47.0 5.1 22.0 NOTE: These 'Central Bureau Electronic Telegrams' are sometimes superseded by text appearing later in the printed IAU Circulars. (C) Copyright 2024 CBAT 2024 August 1 (CBET 5424) Daniel W. E. Green