Electronic Telegram No. 5578 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 N1 (ATLAS) = 3I/ATLAS An apparently asteroidal object discovered on CCD images taken on July 1 UT with a 0.5-m f/2 Schmidt reflector at Rio Hurtado, Chile, in the course of the "Asteroid Terrestrial-Impact Last Alert System" (ATLAS) search program has been found to show cometary appearance by CCD astrometrists elsewhere. The discovery observations are tabulated below: 2025 UT R.A. (2000) Decl. Mag. July 1.21888 18 07 27.67 -18 41 40.1 17.7 1.22206 18 07 27.29 -18 41 40.1 17.7 1.22920 18 07 26.40 -18 41 39.7 17.8 1.26424 18 07 22.06 -18 41 37.8 17.9 The Central Bureau first received word of possible cometary activity from S. Deen (Simi Valley, CA, USA), who reports that several 10-min luminance CMOS exposures taken on July 2.0 UT with a 0.43-m reflector at Rio Hurtado, Chile, in roughly 1" seeing shows it to have a very condensed, potentially stellar head with a possible faint, very short and broad tail roughly 1" long in p.a. 270 degrees. Deen has also found numerous pre-discovery ATLAS images that were not given on MPEC 2025-N12, as tabulated below; those on June 5 and 25 are from the 0.5-m f/2 Schmidt ATLAS reflector at Sutherland, South Africa, while the rest are from Rio Hurtado. Deen notes that, "although the object wasn't exceptionally faint for most of this time, it went unseen for around a month as it crossed over the dense galactic center star fields, only being discovered now as it is currently passing over several very thick dust lanes in the center of the galactic plane". 2025 UT R.A. (2000) Decl. Mag. June 5.97049 18 48 31.49 -18 44 14.1 5.97230 18 48 31.44 -18 44 13.7 7.31883 18 46 51.12 -18 44 27.5 8.34129 18 45 32.88 -18 44 37.6 17.29098 18 32 43.28 -18 45 33.8 17.29375 18 32 43.12 -18 45 33.0 24.38744 18 20 40.19 -18 44 44.8 24.39077 18 20 39.89 -18 44 44.7 24.40658 18 20 38.09 -18 44 44.3 18.6 25.08410 18 19 23.70 -18 44 34.4 25.09429 18 19 22.56 -18 44 32.8 A. Hale (Cloudcroft, NM, USA) remotely obtained CCD exposures with a 0.35-m f/3 Cassegrain reflector located at Tenerife on July 1.94 UT that show only a stellar appearance with no tail. Twelve stacked 30-s CCD exposures obtained remotely by H. Sato (Tokyo, Japan) with a 0.51-m f/6.8 astrograph located at Rio Hurtado on July 2.19-2.20 show only a stellar appearance; the magnitude was 18.2 as measured within a circular aperture of radius 2".2. Sixteen stacked 30-s exposures taken by Sato with the same instrumentation on July 2.3 again show only a stellar appearance; the magnitude was 17.6 as measured within a circular aperture of radius 2".2. F. Romanov (Yuzhno-Morskoy, Nakhodka, Russia) writes that he stacked five 20-s luminance-filtered CCD exposures obtained remotely using an iTelescope 0.51-m f/6.8 reflector at Rio Hurtado Valley in Chile on July 2.32 that yield G-magnitude 17.5 with no tail. R. Weryk, Department of Physics and Astronomy, University of Western Ontario, reports that R. Wainscoat and he obtained three 60-s gri-band images on July 2.41-2.42 UT with the 3.6-m Canada-France-Hawaii Telescope on Mauna Kea (queue observer J. Silva; queue coordinator N. Manset); the best image, taken in 0".7 seeing, shows a very condensed head of size 1".3 (full-width-at- half-maximum) with a short tail about 3"-4" long toward p.a. 278 degrees. The following orbital elements by S. Nakano (Central Bureau) are from 122 observations spanning June 5-July 2 (mean residual 0".4), with corresponding "original" and "future" values of 1/a being -3.802066 and -3.803953 (+/- 0.004024) AU**-1, respectively. The comet has passed r = 1000 AU around 1944, and it will pass 1000 AU from the sun again around 2105. The comet will pass 0.20 AU from Mars on 2025 Oct. 3 and 0.34 AU from Jupiter on 2026 Mar. 16 UT. This is the third known interstellar object observed to pass through the inner solar system, and thus given the designation 3I. Epoch = 2025 Oct. 12.0 TT T = 2025 Oct. 29.41062 TT Peri. = 127.93850 e = 6.1801697 Node = 322.18759 2000.0 q = 1.3620397 AU Incl. = 175.11382 The following ephemeris by the undersigned from the above orbital elements uses photometric power-law parameters H = 9.5 and 2.5n = 8 for the magnitudes. Date TT R. A. (2000) Decl. Delta r Elong. Phase Mag. 2025 03 26 19 21.04 -19 01.0 7.977 7.798 76.1 7.1 21.1 2025 04 05 19 21.09 -18 54.6 7.462 7.459 86.0 7.7 20.8 2025 04 15 19 20.14 -18 49.2 6.945 7.119 96.0 8.1 20.5 2025 04 25 19 17.95 -18 45.1 6.430 6.780 106.2 8.2 20.2 2025 05 05 19 14.26 -18 42.5 5.923 6.441 116.8 8.0 19.8 2025 05 15 19 08.75 -18 41.6 5.432 6.102 127.7 7.5 19.5 2025 05 25 19 01.09 -18 42.2 4.961 5.764 139.1 6.6 19.1 2025 06 04 18 50.87 -18 43.9 4.517 5.426 151.0 5.2 18.7 2025 06 14 18 37.74 -18 45.4 4.108 5.090 163.3 3.3 18.2 2025 06 24 18 21.38 -18 44.9 3.741 4.754 174.7 1.1 17.8 2025 07 04 18 01.64 -18 39.5 3.422 4.420 167.5 2.9 17.3 2025 07 14 17 38.72 -18 25.8 3.155 4.087 153.0 6.5 16.9 2025 07 24 17 13.19 -18 00.6 2.945 3.758 137.6 10.5 16.4 2025 07 29 16 59.75 -17 43.0 2.860 3.594 129.6 12.6 16.2 2025 08 03 16 46.07 -17 22.0 2.789 3.431 121.6 14.6 16.0 2025 08 08 16 32.32 -16 57.8 2.730 3.269 113.5 16.5 15.8 2025 08 13 16 18.64 -16 30.6 2.682 3.109 105.5 18.3 15.6 2025 08 18 16 05.18 -16 00.8 2.644 2.951 97.4 19.9 15.4 2025 08 23 15 52.06 -15 29.1 2.615 2.794 89.4 21.2 15.2 2025 08 28 15 39.37 -14 55.8 2.593 2.640 81.5 22.2 14.9 2025 09 02 15 27.16 -14 21.4 2.576 2.489 73.7 22.9 14.7 2025 09 07 15 15.46 -13 46.3 2.563 2.341 65.9 23.1 14.5 2025 09 12 15 04.25 -13 10.8 2.552 2.197 58.3 22.9 14.3 2025 09 17 14 53.51 -12 35.0 2.542 2.059 50.8 22.2 14.0 2025 09 22 14 43.19 -11 59.1 2.531 1.928 43.3 20.9 13.8 2025 11 21 12 34.09 -02 22.3 2.040 1.593 49.8 28.3 12.7 2025 11 26 12 18.72 -01 01.8 1.978 1.694 58.9 29.9 12.8 2025 12 01 12 01.83 +00 27.9 1.920 1.808 68.4 30.5 13.0 2025 12 06 11 43.30 +02 06.8 1.871 1.932 78.4 30.0 13.1 2025 12 11 11 23.10 +03 54.3 1.833 2.063 88.8 28.5 13.3 2025 12 16 11 01.34 +05 48.4 1.811 2.202 99.7 26.1 13.5 2025 12 21 10 38.31 +07 45.8 1.808 2.345 110.8 23.1 13.7 2025 12 26 10 14.50 +09 42.1 1.828 2.493 122.1 19.5 14.0 2025 12 31 09 50.52 +11 32.4 1.872 2.645 133.3 15.7 14.2 2026 01 05 09 27.06 +13 12.7 1.941 2.799 144.3 11.8 14.5 2026 01 10 09 04.75 +14 40.1 2.035 2.956 155.0 8.1 14.8 2026 01 15 08 44.07 +15 53.7 2.153 3.114 165.1 4.6 15.1 2026 01 20 08 25.34 +16 53.8 2.293 3.274 174.5 1.7 15.4 2026 01 25 08 08.70 +17 41.8 2.454 3.436 175.2 1.4 15.7 2026 01 30 07 54.15 +18 19.6 2.632 3.599 167.0 3.5 16.1 2026 02 04 07 41.57 +18 49.1 2.825 3.762 159.0 5.4 16.4 2026 02 09 07 30.81 +19 12.0 3.032 3.927 151.4 6.9 16.7 2026 02 19 07 14.01 +19 43.5 3.478 4.258 137.4 9.0 17.2 2026 03 01 07 02.31 +20 02.7 3.956 4.591 124.5 10.2 17.8 2026 03 11 06 54.50 +20 14.4 4.457 4.926 112.7 10.7 18.3 2026 03 21 06 49.62 +20 21.7 4.971 5.263 101.6 10.7 18.8 2026 03 31 06 46.94 +20 26.0 5.491 5.600 91.1 10.3 19.2 2026 04 10 06 45.92 +20 28.1 6.011 5.938 81.0 9.6 19.6 2026 04 20 06 46.14 +20 28.6 6.526 6.276 71.3 8.7 20.0 2026 04 30 06 47.29 +20 27.7 7.031 6.615 61.8 7.7 20.3 2026 05 10 06 49.11 +20 25.7 7.521 6.954 52.6 6.6 20.6 2026 05 20 06 51.40 +20 22.6 7.994 7.293 43.5 5.5 20.9 NOTE: These 'Central Bureau Electronic Telegrams' are sometimes superseded by text appearing later in the printed IAU Circulars. (C) Copyright 2025 CBAT 2025 July 2 (CBET 5578) Daniel W. E. Green