Electronic Telegram No. 5673 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 C1 (TSUCHINSHAN) An apparently asteroidal object found on CCD images obtained on Feb. 11 with the 1.04-m F/1.8 Schmidt telescope at the XuYi of the Purple Mountain Observatory (discovery observations tabulated below) has been found to show cometary appearance by observers elsewhere. 2026 UT R.A. (2000) Decl. Mag. Feb. 11.82603 12 50 29.79 + 5 34 18.6 19.3 11.84017 12 50 29.56 + 5 34 19.9 19.4 11.85432 12 50 29.32 + 5 34 21.3 19.3 13.85810 12 49 55.96 + 5 37 41.7 18.9 13.86075 12 49 55.89 + 5 37 41.7 19.2 13.87224 12 49 55.69 + 5 37 43.1 19.2 13.87490 12 49 55.65 + 5 37 43.1 19.3 13.88639 12 49 55.45 + 5 37 44.4 19.3 13.88904 12 49 55.42 + 5 37 44.7 19.4 R. Weryk, Department of Physics and Astronomy, University of Western Ontario, writes that it appears that this object was placed on the Minor Planet Center's NEOCP webpage after it first went into the MPC's "isolated tracklet file". Weryk identified pre-discovery images of the object from four 45-s stacked w-band survey images taken in 1".3-1".5 seeing on 2025 Dec. 22.62- 22.65 UT with the Pan-STARRS1 1.8-m Ritchey-Chretien reflector at Haleakala, which show no coma and no tail but with a head of size 1".7 (full-width-at- half-maximum). Weryk adds that that a similar appearance was found on four stacked 45-s w-band survey images taken on 2026 Jan. 20.6 with the Pan-STARRS2 1.8-m Ritchey-Chretien reflector at Haleakala in 1".1-1".5 seeing, with the head showing a size of 1".6 (FWHM). Weryk then moved the object from the NEOCP webpage to the PCCP webpage. The object was accidentally re-discovered on four 45-s Pan-STARRS2 w-band survey images taken on Feb. 27.6 taken in 1".0-1".1 seeing that show a very condensed head of size 1".5 (FWHM) with a possible hint of asymmetry towards the south-southeast. Eight stacked 60-s CCD exposures taken remotely on Feb. 25.36 UT by H. Sato (Tokyo, Japan) with a 0.51-m f/6.8 astrograph located at Rio Hurtado, Chile, show only a stellar appearance; the magnitude was 19.1 as measured within a circular aperture of radius 2".9. CCD images obtained remotely by T. Prystavski (Lviv, Ukraine) on Feb. 28.8 with an iTelescope 0.51-m f/6.8 Corrected Dall-Kirkham telescope located at Siding Spring, NSW, Australia, show no obvious coma but with a possible tail about 3".4 long in p.a. 357 degrees (and total mag 18.8). S. Deen (Simi Valley, CA, USA) notes that he searched for pre-discovery observations of this object and found several deep (185-s) exposures taken with the 4-m reflector at Cerro Tololo (+ DECam) on 2025 May 5 (when the object would have been at r = 11.9 AU, but he found no sign of the comet down to V = 24.5, suggesting a comet-like brightening. The available astrometry appears on MPEC 2026-F119. The following orbital elements by S. Nakano (Central Bureau) are from 434 observations spanning 2025 Dec. 22-2026 Mar. 21 (mean residual 0".3), with corresponding "original" and "future" values of 1/a being -0.000028 and +0.000461 (+/ -0.000235) AU**-1, respectively. The comet will pass 2.04 AU from Jupiter on 2027 Nov. 27 UT. Epoch = 2028 Nov. 5.0 TT T = 2028 Nov. 6.62403 TT Peri. = 310.78183 e = 0.9998858 Node = 3.64136 2000.0 q = 1.1502392 AU Incl. = 99.70813 The following ephemeris by Nakano from the above orbital elements uses photometric power-law parameters H = 5.5 and 2.5n = 8 for the magnitudes. Date TT R. A. (2000) Decl. Delta r Elong. Phase Mag. 2025 11 01 12 45.97 +05 58.3 11.537 10.705 31.7 2.8 19.0 2025 11 11 12 48.50 +05 39.6 11.367 10.635 40.6 3.5 19.0 2025 11 21 12 50.78 +05 23.8 11.175 10.564 49.8 4.1 18.9 2025 12 01 12 52.72 +05 11.1 10.965 10.493 59.1 4.6 18.9 2025 12 11 12 54.26 +05 02.0 10.740 10.422 68.7 5.0 18.8 2025 12 21 12 55.31 +04 56.7 10.504 10.351 78.4 5.3 18.7 2025 12 31 12 55.81 +04 55.5 10.262 10.280 88.3 5.5 18.7 2026 01 10 12 55.69 +04 58.2 10.018 10.208 98.4 5.5 18.6 2026 01 20 12 54.90 +05 05.1 9.779 10.137 108.6 5.3 18.5 2026 01 30 12 53.41 +05 15.7 9.550 10.064 119.0 4.9 18.4 2026 02 09 12 51.24 +05 29.8 9.336 9.992 129.5 4.4 18.3 2026 02 19 12 48.39 +05 46.7 9.142 9.920 140.1 3.7 18.3 2026 03 01 12 44.93 +06 05.6 8.972 9.847 150.5 2.8 18.2 2026 03 11 12 40.97 +06 25.6 8.832 9.774 160.5 1.9 18.2 2026 03 21 12 36.62 +06 45.7 8.722 9.701 168.6 1.2 18.1 2026 03 31 12 32.05 +07 04.6 8.645 9.627 168.9 1.1 18.1 2026 04 10 12 27.42 +07 21.3 8.600 9.554 161.1 1.9 18.0 2026 04 20 12 22.91 +07 35.0 8.587 9.480 151.3 2.9 18.0 2026 04 30 12 18.68 +07 44.9 8.602 9.406 141.0 3.9 18.0 2026 05 10 12 14.88 +07 50.3 8.641 9.331 130.7 4.7 17.9 2026 05 20 12 11.62 +07 51.1 8.702 9.257 120.5 5.4 17.9 2026 05 30 12 08.99 +07 47.2 8.778 9.182 110.5 5.9 17.9 2026 06 09 12 07.03 +07 38.6 8.864 9.106 100.6 6.3 17.9 2026 06 19 12 05.77 +07 25.6 8.957 9.031 91.0 6.5 17.9 2026 06 29 12 05.20 +07 08.4 9.049 8.955 81.5 6.4 17.9 2026 07 09 12 05.30 +06 47.6 9.138 8.879 72.2 6.3 17.9 2026 07 19 12 06.04 +06 23.4 9.217 8.803 63.0 5.9 17.9 2026 07 29 12 07.35 +05 56.3 9.284 8.726 54.0 5.4 17.9 2026 08 08 12 09.18 +05 26.7 9.335 8.649 45.1 4.8 17.8 2026 11 16 12 40.46 -00 08.5 8.545 7.864 44.0 5.0 17.3 2026 11 26 12 43.08 -00 36.1 8.333 7.784 53.3 5.8 17.2 2026 12 06 12 45.22 -01 00.8 8.104 7.703 62.8 6.5 17.1 2026 12 16 12 46.79 -01 22.0 7.862 7.622 72.4 7.1 17.0 2026 12 26 12 47.65 -01 39.2 7.610 7.541 82.2 7.4 16.9 2027 01 05 12 47.71 -01 52.1 7.354 7.459 92.3 7.6 16.8 2027 01 15 12 46.87 -02 00.1 7.099 7.377 102.6 7.5 16.7 2027 01 25 12 45.05 -02 03.1 6.850 7.294 113.2 7.1 16.6 2027 02 04 12 42.20 -02 00.8 6.613 7.211 124.0 6.5 16.5 2027 02 14 12 38.31 -01 53.4 6.395 7.128 135.1 5.6 16.4 2027 02 24 12 33.42 -01 41.3 6.199 7.044 146.4 4.5 16.2 2027 03 06 12 27.64 -01 25.0 6.032 6.960 157.9 3.1 16.1 2027 03 16 12 21.16 -01 05.7 5.896 6.876 169.5 1.5 16.1 2027 03 26 12 14.21 -00 44.5 5.794 6.791 178.6 0.2 16.0 2027 04 05 12 07.07 -00 22.9 5.727 6.706 167.2 1.9 15.9 2027 04 15 12 00.04 -00 02.7 5.694 6.620 155.6 3.6 15.8 2027 04 25 11 53.41 +00 14.8 5.692 6.534 144.2 5.2 15.8 2027 05 05 11 47.44 +00 28.3 5.717 6.447 133.0 6.6 15.8 2027 05 15 11 42.32 +00 36.7 5.765 6.360 122.1 7.7 15.7 2027 05 25 11 38.19 +00 39.4 5.830 6.272 111.5 8.6 15.7 2027 06 04 11 35.11 +00 36.0 5.906 6.184 101.2 9.3 15.7 2027 06 14 11 33.10 +00 26.5 5.988 6.096 91.3 9.6 15.7 2027 06 24 11 32.13 +00 10.8 6.070 6.007 81.6 9.6 15.6 2027 07 04 11 32.14 -00 10.7 6.148 5.917 72.2 9.4 15.6 2027 07 14 11 33.07 -00 37.9 6.216 5.827 63.1 9.0 15.6 2027 07 24 11 34.82 -01 10.4 6.271 5.737 54.2 8.3 15.6 2027 08 03 11 37.30 -01 48.0 6.310 5.646 45.5 7.4 15.5 2027 08 13 11 40.41 -02 30.2 6.329 5.554 37.0 6.3 15.5 NOTE: These 'Central Bureau Electronic Telegrams' are sometimes superseded by text appearing later in the printed IAU Circulars. (C) Copyright 2026 CBAT 2026 March 21 (CBET 5673) Daniel W. E. Green