Electronic Telegram No. 5651 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 A/2025 Y1 An apparently asteroidal object discovered on CCD images taken on 2025 Dec. 26 UT with a 0.5-m f/2 Schmidt reflector at Haleakala, Hawaii, in the course of the "Asteroid Terrestrial-Impact Last Alert System" (ATLAS) search program has been given the designation A/2025 Y1 when announced by the Minor Planet Center on MPEC 2026-A16 after it was posted to its PCCP webpage, with no reports of cometary activity despite its near-parabolic orbit. The discovery observations are tabulated below. 2025 UT R.A. (2000) Decl. Mag. Dec. 26.56991 13 06 26.46 - 1 25 34.7 18.7 26.58492 13 06 26.71 - 1 25 37.2 18.8 26.58811 13 06 26.80 - 1 25 36.4 18.8 26.58992 13 06 26.85 - 1 25 35.4 18.9 26.59544 13 06 26.98 - 1 25 36.7 18.6 26.59636 13 06 27.00 - 1 25 36.8 18.7 26.61333 13 06 27.37 - 1 25 37.5 18.4 MPEC 2026-A16 contains three single-night pre-discovery observations at mag 20.0-20.2 obtained with a 2.5-m reflector at Lenghu, China. S. Deen (Simi Valley, CA, USA) writes that he searched for archival images of this object and found additional CCD images obtained on 2025 Dec. 1 with the ATLAS 0.5-m f/2 Schmidt reflector at Mauna Loa, Hawaii, and further ATLAS images taken with the reflector at Haleakala on Dec. 9, as tabulated below. Deen adds that it appears "generally cometary" but was hard to say more due to its faintness on the exposures. 2025 UT R.A. (2000) Decl. Mag. Dec. 1.63434 12 55 49.38 - 0 54 00.5 19.0 1.63849 12 55 49.50 - 0 54 00.1 18.9 1.64601 12 55 49.74 - 0 54 00.8 19.0 1.65254 12 55 49.86 - 0 54 02.4 19.0 9.63207 12 59 35.50 - 1 07 22.5 18.6 9.64898 12 59 35.99 - 1 07 26.0 19.2 9.65445 12 59 36.15 - 1 07 25.5 19.0 9.65993 12 59 36.33 - 1 07 27.1 19.2 Deen also found r-band DECam images from last May taken with the 4-m reflector at Cerro Tololo (when the comet was at r = 8.3-8.4 AU), as tabulated below. 2025 UT R.A. (2000) Decl. Mag. May 1.98342 11 39 32.27 + 5 46 36.2 23.6 2.09044 11 39 30.96 + 5 46 45.8 4.97974 11 38 57.28 + 5 51 03.1 23.7 5.00977 11 38 56.95 + 5 51 05.4 Deen adds that a pair of deep 8.2-m Subaru telescope images from 2023 Mar. 21 show nothing at the predicted position from an orbit by Deen, down to magnitude r = 25.5 or 26. CCD images obtained remotely by H. Sato (Bunkyo-ku, Tokyo, Japan) with a 0.51-m f/6.8 astrograph located at Rio Hurtado, Chile, on 2025 Dec. 30.35 UT show only a stellar appearance; the magnitude was 18.8 as measured within a circular aperture of radius 2".9. The following orbital elements by S. Nakano (Central Bureau) are from 106 observations spanning 2025 May 1-2026 Jan. 6 (mean residual 0".4), with corresponding "original" and "future" values of 1/a being +0.000232 and -0.000488 (+/- 0.000100) AU**-1, respectively. The comet will pass 2.66 AU from Saturn on 2030 Mar. 27 UT. Epoch = 2027 Oct. 2.0 TT T = 2027 Oct. 13.87451 TT Peri. = 124.31139 e = 1.0000920 Node = 166.54837 2000.0 q = 2.6005456 AU Incl. = 14.55737 The following ephemeris by the undersigned from the above orbital elements uses photometric power-law parameters H = 7.0 and 2.5n = 8 for the magnitudes. Date TT R. A. (2000) Decl. Delta r Elong. Phase Mag. 2025 12 31 13 07.93 -01 27.8 6.685 6.637 83.0 8.5 17.7 2026 01 10 13 10.75 -01 28.5 6.447 6.563 92.5 8.6 17.6 2026 01 20 13 12.75 -01 22.9 6.210 6.490 102.2 8.5 17.5 2026 01 30 13 13.84 -01 10.7 5.979 6.416 112.1 8.2 17.3 2026 02 09 13 13.97 -00 51.9 5.760 6.341 122.3 7.6 17.2 2026 02 19 13 13.10 -00 26.7 5.556 6.267 132.6 6.7 17.1 2026 03 01 13 11.27 +00 04.3 5.372 6.193 143.1 5.5 17.0 2026 03 11 13 08.54 +00 39.9 5.213 6.118 153.5 4.1 16.9 2026 03 21 13 05.06 +01 18.8 5.081 6.044 163.7 2.7 16.8 2026 03 31 13 01.04 +01 59.0 4.979 5.969 171.5 1.4 16.7 2026 04 10 12 56.72 +02 38.4 4.908 5.894 168.9 1.9 16.6 2026 04 20 12 52.40 +03 15.0 4.867 5.819 159.6 3.5 16.6 2026 04 30 12 48.36 +03 46.8 4.855 5.744 149.3 5.1 16.5 2026 05 10 12 44.87 +04 12.2 4.869 5.669 139.0 6.7 16.5 2026 05 20 12 42.15 +04 30.1 4.905 5.594 128.8 8.1 16.4 2026 05 30 12 40.39 +04 39.8 4.958 5.519 118.8 9.3 16.4 2026 06 09 12 39.67 +04 41.4 5.024 5.444 109.2 10.1 16.4 2026 06 19 12 40.06 +04 34.9 5.099 5.368 99.9 10.7 16.4 2026 06 29 12 41.57 +04 20.8 5.178 5.293 90.9 11.1 16.4 2026 07 09 12 44.17 +03 59.7 5.257 5.217 82.2 11.1 16.3 2026 07 19 12 47.81 +03 32.4 5.333 5.142 73.8 10.9 16.3 2026 07 29 12 52.44 +02 59.6 5.401 5.067 65.6 10.5 16.3 2026 08 08 12 57.98 +02 22.0 5.460 4.991 57.6 9.9 16.3 2026 08 18 13 04.36 +01 40.5 5.506 4.916 49.9 9.1 16.2 2026 08 28 13 11.51 +00 55.7 5.539 4.841 42.4 8.1 16.2 NOTE: These 'Central Bureau Electronic Telegrams' are sometimes superseded by text appearing later in the printed IAU Circulars. (C) Copyright 2026 CBAT 2026 January 7 (CBET 5651) Daniel W. E. Green