Electronic Telegram No. 5596 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 3I/2025 N1 (ATLAS) This interstellar comet has been brightening close to the prediction on CBET 5578. Many images of comet 3I have been posted at the "ICQ Comet Observations" forum on Facebook. There appears to be a very faint extension of material in the general direction of the sun. M. Jaeger (Vienna) reports a very faint 20" fan of material on seventeen 80-s images obtained on Aug. 18.82 UT with a 0.40-m f/3.2 reflector at Martinsberg, Oed, Austria. Jaeger reported a 17" coma from images taken on Aug. 14.82. E. Bryssinck, Kruibeke, Belgium obtained CCD images on Aug. 18.85 with a 40-cm reflector (+ Bessel R filter) that show total mag 15.3 and coma diameter 0'.5. Optical-bandpass CCD imagery of comet 3I obtained by D. Jewitt with the Hubble Space Telescope on July 21.69 (two 40-s exposures) and 21.75 (two 25-s exposures) revealed a broad fan of material in p.a. 270-290 degrees, indicating dust ejected on the solar side of the object's nucleus and no tail on the anti-sunward side (see website URL https//arxiv.org/pdf/2508.02934). The following updated orbital elements to the orbit on CBET 5578 (which was well-determined already) by S. Nakano (Central Bureau) are from 2882 observations spanning 2025 May 21-Aug. 13 (mean residual 0".4), with corresponding "original" and "future" values of 1/a being -3.788016 and -3.789898 (+/- 0.000361) AU**-1, respectively. (The residuals from the previous CBET orbit are +14" in R.A. and -6" in Decl.) The comet will pass 0.19 AU from Mars on 2025 Oct. 3 and 0.36 AU from Jupiter on 2026 Mar. 16 UT. Epoch = 2025 Oct. 12.0 TT T = 2025 Oct. 29.48137 TT Peri. = 128.01113 e = 6.1402764 Node = 322.16051 2000.0 q = 1.3565627 AU Incl. = 175.11327 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 08 13 16 18.66 -16 30.7 2.678 3.106 105.5 18.3 15.6 2025 08 18 16 05.20 -16 00.9 2.640 2.947 97.4 19.9 15.4 2025 08 23 15 52.09 -15 29.2 2.611 2.791 89.4 21.2 15.2 2025 08 28 15 39.41 -14 56.0 2.589 2.637 81.5 22.3 14.9 2025 09 02 15 27.21 -14 21.7 2.573 2.485 73.7 22.9 14.7 2025 09 07 15 15.52 -13 46.6 2.559 2.338 66.0 23.2 14.5 2025 09 12 15 04.32 -13 11.2 2.548 2.194 58.3 23.0 14.3 2025 09 17 14 53.59 -12 35.5 2.538 2.056 50.8 22.3 14.0 2025 09 22 14 43.27 -11 59.5 2.527 1.924 43.3 21.0 13.8 2025 09 27 14 33.29 -11 23.3 2.513 1.801 36.0 19.1 13.5 2025 10 02 14 23.56 -10 46.5 2.496 1.687 28.6 16.5 13.3 2025 11 11 13 01.10 -04 39.9 2.162 1.432 32.7 21.9 12.4 2025 11 16 12 48.20 -03 35.1 2.098 1.500 41.1 25.7 12.5 2025 11 21 12 34.14 -02 22.9 2.033 1.586 49.8 28.4 12.6 2025 11 26 12 18.73 -01 02.3 1.970 1.687 58.9 30.1 12.8 2025 12 01 12 01.78 +00 27.6 1.912 1.801 68.5 30.6 13.0 2025 12 06 11 43.18 +02 06.9 1.862 1.925 78.4 30.1 13.1 2025 12 11 11 22.88 +03 54.8 1.824 2.056 88.9 28.6 13.3 2025 12 16 11 01.02 +05 49.4 1.802 2.195 99.7 26.2 13.5 2025 12 21 10 37.87 +07 47.3 1.799 2.338 110.9 23.2 13.7 2025 12 26 10 13.92 +09 44.1 1.818 2.486 122.2 19.6 14.0 2025 12 31 09 49.81 +11 34.8 1.862 2.637 133.5 15.7 14.2 2026 01 05 09 26.24 +13 15.3 1.931 2.791 144.5 11.8 14.5 2026 01 10 09 03.83 +14 42.7 2.026 2.948 155.2 8.0 14.8 2026 01 15 08 43.08 +15 56.2 2.144 3.106 165.4 4.6 15.1 2026 01 20 08 24.30 +16 56.1 2.285 3.266 174.7 1.6 15.4 2026 01 25 08 07.63 +17 43.9 2.446 3.427 175.0 1.4 15.7 2026 01 30 07 53.06 +18 21.5 2.624 3.590 166.8 3.6 16.0 2026 02 04 07 40.49 +18 50.7 2.818 3.753 158.8 5.5 16.3 2026 02 09 07 29.74 +19 13.3 3.024 3.918 151.2 7.0 16.6 NOTE: These 'Central Bureau Electronic Telegrams' are sometimes superseded by text appearing later in the printed IAU Circulars. (C) Copyright 2025 CBAT 2025 August 20 (CBET 5596) Daniel W. E. Green