Electronic Telegram No. 5644 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/2025 W3 (KRESKEN) Marco Micheli, Frascati, Italy, reports the discovery by Rainer Kresken (Darmstadt, Germany) of a comet with a noticeable straight tail on CMOS images obtained on Nov. 29 with the 0.80-m Schmidt telescope at Calar Alto in the course of a survey undertaken by the the Near-Earth-Object Coordination Centre of the European Space Agency's Planetary Defence Office. Micheli suspected this to be an active main-belt asteroid from the linear morphology and the lack of a radial coma, but he identified a possible brighter condensation near the tail tip as the likely nuclear condensation (or actual nucleus). F. Ocana also confirmed the activity and tail. Micheli adds that follow-up CCD observations with the Las Cumbres Observatory (LCO) 1.0-m reflector at Sutherland, South Africa, obtained on Dec. 1.01 UT showed a hint of tail; additional follow-up CCD images were obtained with the LCO 1.0-m reflector at Tenerife on Dec. 1.2 that show a clear tail. The discovery observations are tabulated below. 2025 UT R.A. (2000) Decl. Mag. Nov. 29.15968 7 30 47.94 +30 04 13.0 22.1 29.17063 7 30 47.66 +30 04 17.5 21.7 29.18160 7 30 47.40 +30 04 21.9 21.7 After the object was posted on the Minor Planet Center's PCCP webpage, R. Weryk (Department of Physics and Astronomy, University of Western Ontario) identified pre-discovery images of the comet in four 45-s w-band survey exposures obtained on Nov. 26.5 UT with the Pan-STARRS1 1.8-m Ritchey-Chretien reflector at Haleakala, HI, USA, in 1".1-1".3 seeing; the head showed a size of 2".0 (full-width-at-half-maximum) with a 60" tail toward p.a. 270 degrees in stacked images. Four stacked 45-s w-band survey images taken on Nov. 30.5 with the Pan-STARRS2 1.8-m Ritchey-Chretien reflector at Haleakala in 1".3- 1".4 seeing show a head of size 2".6 (FWHM) that was difficult to measure, with a straight 60" tail in p.a. 270 deg. The astrometry on MPEC 2025-X48 is missing the following astrometry noted above from Nov. 26: 2025 UT R.A. (2000) Decl. Mag. Nov. 26.47671 7 31 41.32 +29 46 23.7 21.7 26.48777 7 31 41.07 +29 46 28.3 22.2 26.49890 7 31 40.89 +29 46 32.6 22.0 26.51006 7 31 40.68 +29 46 37.4 22.2 The following orbital elements by S. Nakano (Central Bureau) are from 14 observations spanning 2025 Nov. 26-Dec. 1 (mean residual 0".3). The formal uncertainty in the orbital period is +/- 56.8 days. There are no close approaches to major planets. T = 2025 Mar. 18.19630 TT Peri. = 311.77383 e = 0.1863048 Node = 60.43904 2000.0 q = 2.0938541 AU Incl. = 9.85030 a = 2.5732660 AU n = 0.23876851 P = 4.13 years The following ephemeris by the undersigned from the above orbital elements uses photometric power-law parameters H = 18.0 and 2.5n = 7.5 for the magnitudes. Date TT R. A. (2000) Decl. Delta r Elong. Phase Mag. 2025 11 21 07 32.68 +29 11.5 1.665 2.403 127.9 18.9 22.0 2025 12 01 07 30.04 +30 16.7 1.595 2.423 138.5 15.7 21.9 2025 12 11 07 23.88 +31 25.9 1.543 2.443 149.5 11.8 21.9 2025 12 21 07 14.65 +32 32.5 1.514 2.463 160.4 7.7 21.8 2025 12 31 07 03.40 +33 28.8 1.512 2.483 168.4 4.6 21.9 2026 01 10 06 51.66 +34 09.0 1.538 2.504 166.0 5.5 21.9 2026 01 20 06 41.00 +34 30.9 1.592 2.524 156.3 9.0 22.0 NOTE: These 'Central Bureau Electronic Telegrams' are sometimes superseded by text appearing later in the printed IAU Circulars. (C) Copyright 2025 CBAT 2025 December 6 (CBET 5644) Daniel W. E. Green