Electronic Telegram No. 5338 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/2023 Y3 = P/2017 BQ_100 (ATLAS) A. Fitzsimmons reported on 2023 Dec. 19 that L. Denneau found an object discovered on CCD images taken on 2023 Dec. 19.0 UT with a 0.5-m f/2 Schmidt reflector at Sutherland, South Africa, in the course of the "Asteroid Terrestrial-Impact Last Alert System" (ATLAS) search program to be possibly diffuse in four individual 30-s exposures; Fitzsimmons measured the head to have a size of 8".5 (FWHM) in 5".3 seeing, with an extension visible toward p.a. 310 degrees in a co-added image. The discovery observations are tabulated below: 2023 UT R.A. (2000) Decl. Mag. Dec. 18.98291 7 21 11.73 +13 13 03.9 18.9 18.98472 7 21 11.63 +13 13 04.2 18.7 18.98787 7 21 11.56 +13 13 04.3 18.9 19.00861 7 21 10.82 +13 13 09.7 18.8 The Minor Planet Center subsequently identified three ATLAS observations of this comet at mag 19.0-19.2 that had been made on 2023 Dec. 13 with a 0.5-m f/2 Schmidt reflector at Rio Hurtado, Chile, in the MPC's "isolated tracklet file". After the comet was posted to the MPC's PCCP webpage, other observers commented on the object's cometary appearance to the Central Bureau, as listed below. R. Weryk, Department of Physics and Astronomy, University of Western Ontario, reports finding the comet in CCD images obtained on 2023 Nov. 17.6 UT with the Pan-STARRS2 1.8-m Ritchey-Chretien reflector at Haleakala; two 45-s w-band survey images taken in 1".4 seeing show a very condensed head of size 1".7 (FWHM) with a clear 4" tail in p.a. 275 degrees. Weryk adds that three 45-s w-band survey images taken on 2023 Nov. 20.5 with the Pan-STARRS1 1.8-m Ritchey-Chretien reflector at Haleakala in 1".4 seeing show a very condensed coma of size 1".7 (FWHM) and a clear 4" tail in p.a. 275 degrees. Weryk then identified the comet with an apparently asteroidal object that was discovered on 2017 Jan. 28 with the Pan-STARRS1 telescope (discovery observation tabulated below) and had been designated 2017 BQ_100 when it was announced on MPS 766268 on 2017 Feb. 12, when it had a five-day arc (four nights) and produced an un-noteworthy orbit; he adds that four 45-s w-band survey images taken in 1".0 seeing on 2017 Jan. 28 show a very sharp point-spread function (very condensed head of size 1".0 FWHM), but the comet also exhibits a clear tail 10" long in p.a. 270 degrees. The 2017 discovery observations are tabulated below. 2017 UT R.A. (2000) Decl. Mag. Jan. 28.49125 8 46 14.92 +18 06 09.7 20.8 28.50412 8 46 14.35 +18 06 16.4 20.8 28.51709 8 46 13.78 +18 06 23.2 20.7 28.53002 8 46 13.22 +18 06 29.8 20.9 Eight stacked 60-s CCD exposures taken remotely on 2023 Dec. 20.3 UT by Hidetaka Sato (Bunkyo-ku, Tokyo, Japan) with a "Deep Sky Chile" 0.51-m f/6.8 astrograph located at Rio Hurtado, Chile, show a strongly condensed coma 5" in diameter with a 10" tail extending toward p.a. 285 degrees; the magnitude was 19.1 as measured within a circular aperture of radius 2".8. Two-hundred stacked 30-s CCD exposures taken at Varese, Italy, on 2023 Dec. 24.9 UT by A. Aletti (and measured by Aletti and M. Auteri) with a 0.36-m f/7.5 reflector in 3".8 seeing show a slightly condensed coma of size 4".7 (FWHM), extended toward the west. Hirohisa Sato has measured sixteen stacked 60-s CCD exposures taken on 2024 Jan. 11.66 UT at Shinshiro, Japan, by T. Ikemura with a 0.38-m f/4.2 reflector, which reveal a condensed coma about 13" in diameter of total mag 18.2 with no tail. S. Deen (Simi Valley, CA, USA) has identified the comet in two publicly available 90-s i-band DECam exposures obtained on 2017 Jan. 28.24 UT by S. Bocquet, J. Frieman, and T. Li with the Cerro Tololo 4-m reflector. Deen sees no apparent coma in the images, but there is a very faint, straight 11" tail toward p.a. 267 degrees. The available astrometry appears on MPEC 2024-B139. The following linked orbital elements by S. Nakano (Central Bureau) are from 436 observations spanning 2015 Sept. 18-2024 Jan. 22 (mean residual 0".4). There are no close approaches to major planets. Epoch = 2017 Feb. 16.0 TT T = 2017 Mar. 7.09161 TT Peri. = 10.78533 e = 0.3186256 Node = 128.52705 2000.0 q = 2.5488954 AU Incl. = 12.49113 a = 3.7408147 AU n = 0.13622436 P = 7.24 years Epoch = 2024 May 10.0 TT T = 2024 May 1.97317 TT Peri. = 13.18078 e = 0.3434593 Node = 127.27143 2000.0 q = 2.3936061 AU Incl. = 12.52524 a = 3.6457850 AU n = 0.14158508 P = 6.96 years Epoch = 2031 May 14.0 TT T = 2031 Apr. 25.20362 TT Peri. = 13.43030 e = 0.3426015 Node = 127.21007 2000.0 q = 2.3992822 AU Incl. = 12.51986 a = 3.6496620 AU n = 0.14135954 P = 6.97 years Epoch = 2038 Apr. 7.0 TT T = 2038 Apr. 14.83698 TT Peri. = 13.11416 e = 0.3406883 Node = 127.16133 2000.0 q = 2.4142214 AU Incl. = 12.51308 a = 3.6617302 AU n = 0.14066128 P = 7.01 years The following ephemeris by the undersigned from the above orbital elements uses photometric power-law parameters H = 13.0 and 2.5n = 10 for the magnitudes. Date TT R. A. (2000) Decl. Delta r Elong. Phase Mag. 2024 01 11 07 04.69 +15 26.8 1.523 2.500 171.7 3.2 17.9 2024 01 21 06 57.02 +16 40.2 1.526 2.482 162.6 6.8 17.9 2024 01 31 06 50.71 +17 55.7 1.556 2.466 151.4 11.0 17.9 2024 02 10 06 46.67 +19 09.0 1.609 2.452 140.5 14.8 17.9 2024 02 20 06 45.50 +20 16.7 1.682 2.439 130.1 18.1 18.0 2024 03 01 06 47.43 +21 16.5 1.770 2.427 120.5 20.6 18.1 2024 03 11 06 52.36 +22 07.2 1.869 2.417 111.5 22.5 18.2 2024 03 21 07 00.08 +22 47.8 1.977 2.409 103.3 23.7 18.3 2024 03 31 07 10.23 +23 17.7 2.089 2.403 95.7 24.4 18.4 2024 04 10 07 22.47 +23 36.6 2.204 2.398 88.5 24.7 18.5 2024 04 20 07 36.46 +23 43.9 2.320 2.395 81.9 24.5 18.6 2024 04 30 07 51.84 +23 39.6 2.435 2.394 75.7 24.1 18.7 2024 05 10 08 08.32 +23 23.6 2.548 2.394 69.8 23.3 18.8 2024 05 20 08 25.66 +22 56.2 2.658 2.396 64.1 22.3 18.9 2024 05 30 08 43.61 +22 17.8 2.765 2.401 58.7 21.2 19.0 2024 06 09 09 01.99 +21 28.8 2.867 2.406 53.5 19.8 19.1 2024 06 19 09 20.64 +20 30.1 2.964 2.414 48.5 18.4 19.2 NOTE: These 'Central Bureau Electronic Telegrams' are sometimes superseded by text appearing later in the printed IAU Circulars. (C) Copyright 2024 CBAT 2024 January 27 (CBET 5338) Daniel W. E. Green