Electronic Telegram No. 5671 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/2025 Y1 (ATLAS) The object A/2025 Y1 (cf. CBET 5651) has been found to show cometary appearance. T. Prystavski (Lviv, Ukraine) writes that images obtained remotely with an iTelescope 0.51-m f/6.8 Corrected Dall-Kirkham telescope located at Rio Hurtado, Chile, on 2026 Jan. 30.35 and 31.34 UT show a tail about 7" long in p.a. about 280 degrees, showing a distinct teardrop shape, though a coma is not clearly visible; the total magnitude was given as 17.8. Additional observations obtained by Prystavski on Feb. 15.3 yield total mag 17.5 with a condensed coma and a tail approximately 0'.1 long in p.a. 279 degrees. Additional CCD images by Prystavski on Feb. 28.3 show a 0'.2-long tail in p.a. 286 degrees (total mag 17.1). Stacked 300-s images obtained by A. Hale (Cloudcroft, NM, USA) on 2026 Feb. 12.44 UT with a 1-m reflector at McDonald Observatory show a very condensed coma (essentially stellar) with a definite stubby tail 8" long in p.a. 280 degrees. One-hundred-five 30-s unfiltered CMOS images taken by A. Aletti, F. Bellini, L. Buzzi, and G. Galli with a 0.36-m f/8.6 reflector located at Hakos, Namibia, on Feb. 15.0 show a very condensed coma 6" wide with a weak, straight tail 10" long in p.a. 280 degrees. Thirty-six stacked 120-s CMOS exposures taken on Feb. 20.64-20.67 by K. Yoshimoto (Kumage, Yamaguchi, Japan) with a 0.3-m f/4 reflector show a faint tail 10" long in p.a. 280 deg with total V mag 17.6. Additional astrometry appears on MPEC 2026-F33. The following orbital elements by S. Nakano (Central Bureau) are from 413 observations spanning 2025 May 1-2026 Mar. 12 (mean residual 0".3), with corresponding "original" and "future" values of 1/a being +0.000105 and -0.000616 (+/- 0.000011) AU**-1, respectively. The comet will pass 2.67 AU from Saturn on 2030 Mar. 26 UT. Epoch = 2027 Oct. 2.0 TT T = 2027 Oct. 13.84095 TT Peri. = 124.28500 e = 1.0004241 Node = 166.54669 2000.0 q = 2.6017187 AU Incl. = 14.55582 The following ephemeris by Nakano 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 6.638 83.0 8.5 17.7 2026 01 10 13 10.75 -01 28.5 6.447 6.564 92.5 8.6 17.6 2026 01 20 13 12.75 -01 22.9 6.211 6.490 102.2 8.5 17.5 2026 01 30 13 13.84 -01 10.7 5.980 6.416 112.1 8.2 17.3 2026 02 09 13 13.97 -00 51.9 5.760 6.342 122.3 7.6 17.2 2026 02 19 13 13.11 -00 26.7 5.556 6.268 132.6 6.7 17.1 2026 03 01 13 11.27 +00 04.3 5.373 6.193 143.1 5.5 17.0 2026 03 11 13 08.55 +00 39.9 5.214 6.119 153.5 4.1 16.9 2026 03 21 13 05.07 +01 18.7 5.082 6.044 163.7 2.7 16.8 2026 03 31 13 01.05 +01 58.9 4.980 5.970 171.5 1.4 16.7 2026 04 10 12 56.73 +02 38.4 4.909 5.895 168.9 1.9 16.6 2026 04 20 12 52.41 +03 15.0 4.868 5.820 159.6 3.5 16.6 2026 04 30 12 48.37 +03 46.7 4.856 5.745 149.3 5.1 16.5 2026 05 10 12 44.88 +04 12.1 4.869 5.670 139.0 6.7 16.5 2026 05 20 12 42.16 +04 30.0 4.905 5.595 128.8 8.1 16.4 2026 05 30 12 40.40 +04 39.8 4.958 5.519 118.8 9.3 16.4 2026 06 09 12 39.68 +04 41.3 5.025 5.444 109.2 10.1 16.4 2026 06 19 12 40.08 +04 34.8 5.100 5.369 99.9 10.7 16.4 2026 06 29 12 41.59 +04 20.7 5.179 5.293 90.9 11.1 16.4 2026 07 09 12 44.18 +03 59.6 5.258 5.218 82.2 11.1 16.3 2026 07 19 12 47.83 +03 32.3 5.333 5.142 73.8 10.9 16.3 2026 07 29 12 52.45 +02 59.5 5.402 5.067 65.6 10.5 16.3 2026 08 08 12 57.99 +02 22.0 5.460 4.992 57.6 9.9 16.3 2026 08 18 13 04.37 +01 40.4 5.507 4.916 49.9 9.1 16.2 2026 08 28 13 11.52 +00 55.7 5.539 4.841 42.4 8.1 16.2 2026 12 16 15 04.21 -07 21.8 4.744 4.026 38.9 8.8 15.2 2026 12 26 15 15.99 -07 48.2 4.574 3.955 46.0 10.3 15.1 2027 01 05 15 27.71 -08 08.3 4.393 3.883 53.1 11.7 14.9 2027 01 15 15 39.26 -08 21.4 4.203 3.813 60.3 12.9 14.8 2027 01 25 15 50.51 -08 27.0 4.006 3.743 67.6 14.1 14.6 2027 02 04 16 01.34 -08 24.4 3.805 3.674 74.9 15.0 14.4 2027 02 14 16 11.59 -08 13.4 3.601 3.606 82.4 15.7 14.2 2027 02 24 16 21.10 -07 53.8 3.398 3.539 90.0 16.2 14.0 2027 03 06 16 29.69 -07 25.4 3.198 3.473 97.7 16.4 13.8 2027 03 16 16 37.16 -06 48.6 3.004 3.408 105.6 16.3 13.6 2027 03 26 16 43.33 -06 04.1 2.818 3.344 113.6 15.9 13.4 2027 04 05 16 48.02 -05 12.9 2.644 3.283 121.7 15.0 13.2 2027 04 15 16 51.06 -04 16.8 2.485 3.222 130.0 13.8 13.0 2027 04 25 16 52.37 -03 18.4 2.342 3.164 138.2 12.2 12.8 2027 05 05 16 51.97 -02 20.8 2.219 3.107 146.1 10.4 12.7 2027 05 15 16 50.00 -01 28.1 2.117 3.052 153.0 8.7 12.5 2027 05 25 16 46.78 -00 44.8 2.039 3.000 157.5 7.4 12.4 2027 06 04 16 42.80 -00 15.0 1.985 2.950 157.9 7.4 12.2 2027 06 14 16 38.68 -00 02.1 1.955 2.903 154.0 8.8 12.2 2027 06 24 16 35.11 -00 08.0 1.948 2.858 147.5 11.0 12.1 2027 07 04 16 32.70 -00 32.7 1.962 2.817 139.9 13.5 12.1 2027 07 14 16 31.96 -01 14.9 1.994 2.778 131.9 15.8 12.0 2027 07 24 16 33.26 -02 11.7 2.042 2.743 124.0 17.9 12.1 2027 08 03 16 36.74 -03 19.8 2.104 2.711 116.4 19.6 12.1 2027 08 13 16 42.47 -04 35.7 2.176 2.684 109.1 20.9 12.1 2027 08 23 16 50.37 -05 55.7 2.257 2.660 102.1 21.8 12.2 2027 09 02 17 00.32 -07 16.8 2.344 2.640 95.5 22.4 12.2 2027 09 12 17 12.17 -08 36.0 2.437 2.624 89.2 22.6 12.3 2027 09 22 17 25.71 -09 50.7 2.535 2.612 83.1 22.4 12.4 2027 10 02 17 40.76 -10 58.8 2.635 2.605 77.3 22.0 12.4 2027 10 12 17 57.11 -11 58.4 2.738 2.602 71.6 21.4 12.5 2027 10 22 18 14.56 -12 48.0 2.843 2.603 66.1 20.5 12.6 2027 11 01 18 32.91 -13 26.5 2.948 2.609 60.6 19.4 12.7 2027 11 11 18 51.96 -13 52.9 3.053 2.619 55.3 18.1 12.8 2027 11 21 19 11.51 -14 07.1 3.158 2.633 50.0 16.7 12.9 2027 12 01 19 31.39 -14 08.9 3.261 2.652 44.7 15.2 13.0 2027 12 11 19 51.41 -13 58.7 3.362 2.674 39.4 13.5 13.1 2027 12 21 20 11.42 -13 37.2 3.459 2.701 34.1 11.8 13.1 2028 04 19 23 38.70 -01 47.6 4.036 3.260 34.7 10.1 14.1 2028 04 29 23 51.84 -00 49.9 4.015 3.321 41.0 11.5 14.2 2028 05 09 00 04.15 +00 02.3 3.983 3.384 47.5 12.7 14.2 2028 05 19 00 15.60 +00 48.2 3.940 3.449 54.3 13.8 14.3 2028 05 29 00 26.09 +01 26.9 3.888 3.514 61.2 14.6 14.3 2028 06 08 00 35.55 +01 57.6 3.828 3.581 68.4 15.3 14.3 2028 06 18 00 43.89 +02 19.6 3.761 3.649 75.9 15.7 14.4 2028 06 28 00 50.99 +02 32.1 3.688 3.717 83.7 15.8 14.4 2028 07 08 00 56.76 +02 34.6 3.614 3.787 91.9 15.6 14.4 2028 07 18 01 01.10 +02 26.7 3.541 3.857 100.5 15.0 14.4 2028 07 28 01 03.92 +02 08.2 3.472 3.928 109.5 14.1 14.5 2028 08 07 01 05.17 +01 39.4 3.411 4.000 118.9 12.8 14.5 2028 08 17 01 04.89 +01 01.1 3.362 4.072 128.7 11.2 14.5 2028 08 27 01 03.15 +00 14.7 3.330 4.145 138.9 9.2 14.6 2028 09 06 01 00.15 -00 37.5 3.320 4.218 149.3 7.0 14.6 2028 09 16 00 56.16 -01 32.7 3.334 4.292 159.8 4.7 14.7 2028 09 26 00 51.55 -02 27.6 3.377 4.366 169.3 2.5 14.8 2028 10 06 00 46.76 -03 18.8 3.448 4.440 171.6 1.9 14.9 2028 10 16 00 42.20 -04 03.3 3.550 4.514 163.4 3.6 15.0 2028 10 26 00 38.25 -04 38.7 3.680 4.589 153.1 5.6 15.1 2028 11 05 00 35.21 -05 03.9 3.837 4.664 142.6 7.4 15.3 2028 11 15 00 33.28 -05 18.2 4.017 4.739 132.2 8.9 15.4 2028 11 25 00 32.55 -05 21.9 4.216 4.814 122.1 10.0 15.6 2028 12 05 00 33.06 -05 15.8 4.430 4.889 112.3 10.7 15.7 2028 12 15 00 34.75 -05 01.1 4.654 4.965 102.7 11.2 15.9 2028 12 25 00 37.53 -04 38.8 4.885 5.040 93.4 11.2 16.1 2029 01 04 00 41.30 -04 10.3 5.117 5.115 84.4 11.0 16.2 2029 01 14 00 45.94 -03 36.7 5.348 5.191 75.6 10.6 16.4 2029 01 24 00 51.32 -02 59.3 5.573 5.266 67.0 9.9 16.5 NOTE: These 'Central Bureau Electronic Telegrams' are sometimes superseded by text appearing later in the printed IAU Circulars. (C) Copyright 2026 CBAT 2026 March 17 (CBET 5671) Daniel W. E. Green