Electronic Telegram No. 5689 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/2026 H3 (BOK) An apparently asteroidal object found on CCD images obtained on Apr. 27 UT by R. A. Mastaler with the 2.25-m Bok reflector at Kitt Peak (discovery observations tabulated below) has been found to be cometary elsewhere. 2026 UT R.A. (2000) Decl. Mag. Apr. 27.46483 17 29 28.79 +17 40 02.2 20.9 27.47178 17 29 28.50 +17 40 04.0 27.48002 17 29 28.12 +17 40 06.5 21.6 27.48560 17 29 27.93 +17 40 07.9 21.7 After the object was posted to the Minor Planet Center's PCCP webpage, R. Weryk (Department of Physics and Astronomy, University of Western Ontario) reported cometary activity from various follow-up observations. Three 60-s gri-band exposures obtained on May 12.5 UT (queue observer L. Wells; queue coordinator H. Flewelling) with the 3.6-m Canada-France-Hawaii Telescope at Mauna Kea in 0".5-0".6 seeing show a condensed head of size about 1".1 (full-width-at-half-maximum) with a likely short tail about 2" long in p.a. 180 degrees (problematic because of involvement with a background galaxy). Additional CFHT images obtained on May 13.5 (queue observer A. Acohido; queue coordinator J. Ou) in 0".8-0".9 seeing show a condensed head of size 1".2 (FWHM) with a clear 6" tail in p.a. 135 degrees. Weryk also noticed the comet in images obtained with the Pan-STARRS2 1.8-m Ritchey-Chretien reflector at Haleakala on 2025 July 27.4 (which had been put in the MPC's "isolated tracklet file"), noting that it was then too faint to discern any activity (magnitude 22.3-22.4). Weryk also identified additional pre-discovery Pan-STARRS2 observations from 2025 July 20.3 (mag 22.1-22.4), 2025 Aug. 1.4 (mag 21.4-23.7), 2025 Aug. 23.3 (mag 20.5-22.4), and 2025 Aug. 29.3 (mag 21.9-22.0). H. Sato (Bunkyo-ku, Tokyo, Japan) writes that twenty-four stacked 60-s CCD exposures taken on 2026 May 19.3 UT with a 0.51-m f/6.8 astrograph located at Rio Hurtado, Chile, show a strongly condensed, asymmetrical coma 4" in diameter with no tail; the magnitude was 21.3 as measured within a circular aperture of radius 2".2. The available astrometry appears on MPEC 2026-K33. The following orbital elements by S. Nakano (Central Bureau) are from 59 observations spanning 2025 July 20-2026 May 19 (mean residual 0".4), with corresponding values of "original" and "future" 1/a being +0.000064 and +0.000094 (+/- 0.000004) AU**-1, respectively. The comet passed 3.85 AU from Saturn on 2022 Oct. 10 UT. Epoch = 2026 June 9.0 TT T = 2026 May 22.98583 TT Peri. = 98.49284 e = 1.0034458 Node = 352.49604 2000.0 q = 7.7282218 AU Incl. = 142.56725 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. 2026 04 30 17 27.71 +17 51.0 7.107 7.730 125.0 6.1 21.4 2026 05 10 17 20.19 +18 30.0 7.026 7.729 131.2 5.6 21.3 2026 05 20 17 11.93 +19 01.0 6.971 7.728 135.7 5.2 21.3 2026 05 30 17 03.19 +19 22.1 6.946 7.728 138.0 5.0 21.3 2026 06 09 16 54.25 +19 32.5 6.950 7.729 137.5 5.1 21.3 2026 06 19 16 45.39 +19 31.8 6.985 7.730 134.4 5.4 21.3 2026 06 29 16 36.92 +19 20.1 7.048 7.732 129.2 5.8 21.3 2026 07 09 16 29.08 +18 58.6 7.137 7.734 122.7 6.4 21.4 2026 07 19 16 22.06 +18 28.6 7.248 7.736 115.2 6.8 21.4 2026 07 29 16 16.01 +17 51.8 7.377 7.739 107.3 7.2 21.4 2026 08 08 16 10.99 +17 10.1 7.519 7.743 99.0 7.4 21.5 2026 08 18 16 07.03 +16 25.2 7.668 7.747 90.7 7.5 21.5 2026 08 28 16 04.10 +15 38.8 7.820 7.752 82.4 7.4 21.6 2026 09 07 16 02.14 +14 52.3 7.971 7.757 74.2 7.2 21.6 2026 09 17 16 01.08 +14 06.9 8.114 7.762 66.1 6.8 21.7 2026 09 27 16 00.82 +13 23.7 8.247 7.768 58.4 6.3 21.7 2026 10 07 16 01.26 +12 43.7 8.365 7.775 51.0 5.7 21.7 2026 10 17 16 02.29 +12 07.4 8.465 7.782 44.2 5.1 21.8 2026 10 27 16 03.80 +11 35.7 8.545 7.789 38.3 4.5 21.8 2026 11 06 16 05.68 +11 09.0 8.601 7.797 33.8 4.1 21.8 2026 11 16 16 07.81 +10 47.8 8.633 7.806 31.3 3.8 21.8 NOTE: These 'Central Bureau Electronic Telegrams' are sometimes superseded by text appearing later in the printed IAU Circulars. (C) Copyright 2026 CBAT 2026 May 19 (CBET 5689) Daniel W. E. Green