Electronic Telegram No. 5252 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/2018 HT_3 (NEOWISE) On 2023 Apr. 18, Robert D. Matson (Irvine, CA, USA) reported finding a moving object suspected of being a comet in low-resolution public website hydrogen Lyman-alpha images obtained during Apr. 5-16 with the Solar Wind Anisotropies (SWAN) camera (cf. IAUCs 8619, 8587, 8344, 8346) on the Solar and Heliospheric Observer (SOHO) spacecraft (see CBETs 4068, 4136, 4939; and website URL http://swan.projet.latmos.ipsl.fr/). Matson's discovery observations are tabulated below (with his estimated magnitude based on correlations of ground-based magnitudes with SWAN brightness from previous comets). 2023 UT R.A. (2000) Decl. Mag. Apr. 5.5 2h59m30s - 3d07' 6.5 3 01 42 - 4 38 11.5 3 09 12 -11 21 11.0 13.5 3 10 36 -14 37 14.5 3 11 48 -15 22 15.5 3 13 36 -16 38 16.5 3 15 24 -18 11 Matson sent a search ephemeris to, and requested ground-based confirming images from, M. Mattiazzo near Exmouth, Western Australia (where Mattiazzo was preparing to observe the total solar eclipse). Using a Celestron 11-inch f/2.2 Schmidt telescope (+ Canon 6D camera), Mattiazzo was able to get confirming images of the comet near NGC 1232 and close to the predictions sent by Matson, measuring astrometry and magnitude 14.3-14.7 on Apr. 18.5 UT (see MPEC 2023-J12), apparentlly with a tail in p.a. 150 degrees. Following Matson's posting of information on this apparent new SWAN comet at the online "Comets-ML" discussion forum, M. Masek reported his confirming CCD observations made with a 7.5-cm f/1.8 lens of the F(/Ph)otometric Robotic Atmospheric Monitor (FRAM) located near Cerro Paranal, Chile, on 2023 Apr. 19.0 and with a 30-cm FRAM telescope located at the Pierre Auger Observatory near Malargue, Argentina, on Apr. 20.0, with a measured coma diameter of 1'.3 and total mag 14.0, elongated in p.a. 136 deg (a short tail about 0'.5 long) in the 30-cm reflector images. H. Sato (Bunkyo-ku, Tokyo, Japan) writes that twelve stacked 60-s CMOS exposures obtained on Apr. 20.0 with a "Deep Sky Chile" 6.5-cm f/3.5 refractor at Rio Hurtado, Chile, show a strongly condensed coma 30" in diameter with a fan-like tail 1'.4 long spanning p.a. 90-135 degrees; the magnitude was 14.7 as measured within a circular aperture of radius 18".2. At the same time as reporting his observation above, Sato also suggested identification of this SWAN comet with an apparently asteroidal Apollo-type object that had been discovered in infrared images obtained on 2018 Apr. 22 UT (discovery observations tabulated below from MPEC 2018-K06, in which the listed optical-wavelength magnitude was roughly estimated based on past WISE and NEOWISE cometary observations) with the Near-Earth Object Wide-field Infrared Survey Explorer (or NEOWISE; formerly the WISE earth-orbiting satellite; cf. CBET 4225). The object was designated 2018 HT_3 in May 2018. 2018 UT R.A. (2000) Decl. Mag. Apr. 22.96098 20 46 25.56 -39 48 59.5 19 23.22278 20 46 11.89 -39 48 15.4 23.28823 20 46 08.43 -39 48 03.5 23.35356 20 46 05.09 -39 47 52.7 23.41901 20 46 01.39 -39 47 42.4 23.48446 20 45 58.01 -39 47 31.4 23.54991 20 45 54.51 -39 47 20.9 23.68082 20 45 47.49 -39 47 00.6 23.81172 20 45 40.26 -39 46 36.5 Astrometric ground-based observations from 2018 May placed the object's brightness at mag 20.5-20.8. Pre-discovery observations of 2018 HT_3 were subsequently reported on MPEC 2021-O60 as found in images obtained with the Pan-STARRS1 1.8-m Ritchey-Chretien reflector at Haleakala, Hawaii, on 2012 Sept. 6.6 (at mag 22.2-22.4) and Oct. 10.6 (mag 20.2-20.8) and on 2013 June 18.5 UT (mag 21.3-21.7), at the comet's previous return to perihelion. S. Deen (Simi Valley, CA, USA) has now identified two images of this comet in publicly available data obtained on 2012 Nov. 6.5 with the 1.2-m Oschin Schmidt telescope at Palomar, noting red mag 17.7-17.9 with a condensed coma of size 2".5 (full-width-at-half-maximum) in 2" seeing and an extremely broad 14" tail spanning p.a. 350-130 degrees (brightest near p.a. 0 degrees). Observations obtained with the Spacewatch 1.8-m f/2.7 reflector at Kitt Peak were identified at the current return to perihelion on 2022 Nov. 30 (no brightness provided). The following linked orbital elements by S. Nakano (Central Bureau) are from 65 observations spanning 2012-2023 (mean residual 0".6). The comet passed 0.97 AU from Jupiter on 1973 May 29 UT. Epoch = 2007 Oct. 27.0 TT T = 2007 Oct. 23.55829 TT Peri. = 214.95405 e = 0.8224451 Node = 284.86853 2000.0 q = 0.5266026 AU Incl. = 30.51826 a = 2.9658570 AU n = 0.19296510 P = 5.10 years Epoch = 2012 Dec. 19.0 TT T = 2012 Dec. 13.08878 TT Peri. = 215.64663 e = 0.8221860 Node = 284.12932 2000.0 q = 0.5302778 AU Incl. = 29.62731 a = 2.9822050 AU n = 0.19138057 P = 5.15 years Epoch = 2018 Feb. 11.0 TT T = 2018 Feb. 7.98122 TT Peri. = 215.73507 e = 0.8221450 Node = 284.04233 2000.0 q = 0.5305996 AU Incl. = 29.58756 a = 2.9833274 AU n = 0.19127257 P = 5.15 years Epoch = 2023 Apr. 6.0 TT T = 2023 Mar. 29.61990 TT Peri. = 216.36728 e = 0.8255671 Node = 283.21155 2000.0 q = 0.5189595 AU Incl. = 29.27753 a = 2.9751235 AU n = 0.19206427 P = 5.13 years Epoch = 2028 May 29.0 TT T = 2028 May 15.42941 TT Peri. = 216.41461 e = 0.8260955 Node = 283.16696 2000.0 q = 0.5168991 AU Incl. = 29.27895 a = 2.9723155 AU n = 0.19233650 P = 5.12 years Epoch = 2033 June 12.0 TT T = 2033 June 28.72934 TT Peri. = 216.44174 e = 0.8261886 Node = 283.13915 2000.0 q = 0.5166351 AU Incl. = 29.25879 a = 2.9723896 AU n = 0.19232931 P = 5.12 years Epoch = 2038 Aug. 5.0 TT T = 2038 Aug. 11.40556 TT Peri. = 216.41800 e = 0.8268457 Node = 283.18062 2000.0 q = 0.5140943 AU Incl. = 29.28689 a = 2.9689951 AU n = 0.19265925 P = 5.12 years The following ephemeris by the undersigned from the above orbital elements uses photometric power-law parameters H = 17.0 and 2.5n = 8 for the magnitudes. Date TT R. A. (2000) Decl. Delta r Elong. Phase Mag. 2023 04 06 02 56.00 -03 50.2 0.695 0.543 31.2 107.3 14.1 2023 04 11 03 05.05 -10 06.0 0.626 0.583 32.7 111.9 14.1 2023 04 16 03 10.81 -16 52.6 0.568 0.636 35.8 112.7 14.2 2023 04 21 03 14.08 -24 02.2 0.520 0.699 40.8 110.2 14.3 2023 04 26 03 15.61 -31 33.5 0.480 0.767 47.4 105.2 14.5 2023 05 01 03 16.02 -39 30.2 0.447 0.838 55.4 98.6 14.6 2023 05 06 03 15.71 -47 58.7 0.421 0.911 64.5 90.8 14.8 2023 05 11 03 14.88 -57 04.8 0.401 0.984 74.8 82.1 15.0 2023 05 16 03 13.36 -66 48.7 0.390 1.056 85.8 72.6 15.1 2023 05 21 03 09.42 -77 00.9 0.388 1.128 97.2 62.8 15.4 2023 05 26 02 32.77 -87 17.6 0.397 1.199 108.5 53.2 15.6 2023 05 31 15 38.25 -82 40.4 0.417 1.269 118.9 44.3 15.9 2023 06 05 15 30.10 -73 32.3 0.450 1.338 127.8 36.8 16.3 2023 06 10 15 29.39 -65 28.7 0.494 1.406 134.5 31.0 16.7 2023 06 15 15 30.45 -58 34.9 0.548 1.472 138.7 27.1 17.0 2023 06 20 15 32.41 -52 47.6 0.611 1.537 140.5 24.8 17.4 2023 06 25 15 35.00 -47 59.3 0.682 1.601 140.3 23.9 17.8 2023 07 05 15 41.51 -40 44.3 0.843 1.726 136.0 24.2 18.5 2023 07 15 15 49.34 -35 46.2 1.026 1.846 129.3 25.2 19.2 2023 07 25 15 58.26 -32 19.1 1.226 1.962 121.8 26.1 19.8 2023 08 04 16 08.06 -29 53.2 1.438 2.073 114.3 26.5 20.3 2023 08 14 16 18.57 -28 08.5 1.660 2.181 106.9 26.4 20.8 2023 08 24 16 29.69 -26 52.2 1.889 2.286 99.6 25.9 21.3 2023 09 03 16 41.32 -25 55.2 2.122 2.387 92.4 25.0 21.7 2023 09 13 16 53.36 -25 11.5 2.356 2.485 85.3 23.8 22.0 NOTE: These 'Central Bureau Electronic Telegrams' are sometimes superseded by text appearing later in the printed IAU Circulars. (C) Copyright 2023 CBAT 2023 May 3 (CBET 5252) Daniel W. E. Green