Electronic Telegram No. 5599 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 Q1 (ATLAS) H. Weiland reports the discovery of a comet with a diffuse 6".6 coma and a 44" straight tail in p.a. 122 degrees on CCD images taken on Aug. 27 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. The discovery observations are tabulated below: 2025 UT R.A. (2000) Decl. Mag. Aug. 27.23877 20 25 12.93 -79 05 41.2 19.8 27.24730 20 25 11.87 -79 05 38.2 19.9 27.25582 20 25 11.24 -79 05 37.2 19.9 27.26433 20 25 10.92 -79 05 33.7 19.7 After the comet was posted to the Minor Planet Center's PCCP webpage, other astrometrists reported physical observations. S. Deen (Simi Valley, CA, USA) obtained CMOS observations of the comet using a 0.43-m reflector at Rio Hurtado on Aug. 27.4 and Sept. 2.3 UT, in which the object appeared distinctly cometary but was involved with stars. Deen also found several ATLAS archival images of the comet during 2020-2023 (tabulated below). The comet appears fully stellar on images taken on 2020 Jan. 31 in 1".2 seeing. On 2022 Feb. 24 in 1".4 seeing, the comet appears compact with a size of 1".7 (full-width-at- half-maximum) and no tail. On 2022 Nov. 10, the comet was heavily involved with stars. On 2023 Apr. 29, there was a very compact coma of size 1".4 (FWHM) in 1".0 seeing with a bright tail extending roughly 3" in p.a. 45-90 degrees, curving north into a much fainter tail extending 12" in total from the nuclear condensation toward p.a. 10-25 degrees. Deen adds that the comet is not visible in z-band images taken with the 8.2-m Subaru telescope on Mauna Kea on 2015 Mar. 16 (limiting mag 23.5). 2020 UT R.A. (2000) Decl. Mag. Jan. 31.35817 10 15 25.95 -32 30 18.9 24.1 2022 UT R.A. (2000) Decl. Mag. Feb. 24.11947 10 18 27.97 -56 55 04.1 20.4 Nov. 10.35532 11 05 52.58 -60 39 41.4 20.6 2023 UT R.A. (2000) Decl. Mag. Apr. 29.07326 9 42 53.55 -70 29 38.0 19.3 A. Hale Cloudcroft, NM, USA) reported that he obtained a marginal detection on CCD images obtained remotely on Aug. 31.9 UT with a 0.35-m f/3 Cassegrain reflector located at Sutherland, South Africa; three 300-s exposures showed a condensed head of size 7" of mag 19.5-19.9 with a possible very faint, diffuse coma and no tail. The available astrometry appears on MPEC 2025-R19. The following orbital elements by S. Nakano (Central Bureau) are from 18 observations spanning 2020 Jan. 31-2025 Sept. 2 (mean residual 0".6), with corresponding "original" and "future" values of 1/a being +0.000609 and +0.000663 (+/- 0.000033) AU**-1, respectively. There are no close approaches to major planets. Epoch = 2024 July 29.0 TT T = 2024 July 12.22514 TT Peri. = 280.23759 e = 0.9912193 Node = 326.97870 2000.0 q = 10.1000600 AU Incl. = 66.14939 The following ephemeris by Nakano from the above orbital elements uses photometric power-law formula Mag. = 6.5 + 5 log Delta + 8.0 log r. Date TT R. A. (2000) Decl. Delta r Elong. Phase Mag. 2025 07 24 21 27.52 -80 27.0 9.772 10.302 118.9 5.0 19.6 2025 08 03 21 07.33 -80 18.6 9.808 10.312 117.2 5.0 19.6 2025 08 13 20 48.07 -79 57.1 9.860 10.323 114.6 5.1 19.6 2025 08 23 20 31.31 -79 23.3 9.926 10.334 111.2 5.2 19.6 2025 09 02 20 18.01 -78 39.0 10.005 10.346 107.1 5.3 19.6 2025 09 12 20 08.49 -77 46.3 10.095 10.358 102.4 5.4 19.6 2025 09 22 20 02.54 -76 47.6 10.194 10.370 97.3 5.5 19.7 2025 10 02 19 59.79 -75 44.8 10.300 10.382 91.9 5.5 19.7 2025 10 12 19 59.74 -74 39.5 10.410 10.394 86.3 5.5 19.7 2025 10 22 20 01.88 -73 33.3 10.523 10.407 80.6 5.4 19.7 2025 11 01 20 05.82 -72 27.0 10.634 10.420 75.0 5.3 19.8 2025 11 11 20 11.16 -71 21.8 10.741 10.434 69.4 5.1 19.8 2025 11 21 20 17.60 -70 18.1 10.842 10.447 64.0 4.9 19.8 2025 12 01 20 24.88 -69 16.7 10.934 10.461 59.0 4.6 19.9 2025 12 11 20 32.77 -68 17.9 11.015 10.475 54.6 4.4 19.9 2025 12 21 20 41.09 -67 22.2 11.083 10.489 50.8 4.2 19.9 2025 12 31 20 49.67 -66 30.1 11.137 10.504 48.0 4.0 19.9 2026 01 10 20 58.38 -65 41.8 11.175 10.519 46.3 3.9 19.9 2026 01 20 21 07.09 -64 57.7 11.196 10.534 45.8 3.8 19.9 2026 01 30 21 15.69 -64 18.1 11.200 10.549 46.7 3.9 19.9 2026 02 09 21 24.07 -63 43.4 11.187 10.564 48.9 4.0 19.9 2026 02 19 21 32.14 -63 13.8 11.158 10.580 52.2 4.2 19.9 2026 03 01 21 39.80 -62 49.6 11.113 10.596 56.3 4.5 19.9 2026 03 11 21 46.97 -62 31.0 11.055 10.612 61.2 4.7 19.9 2026 03 21 21 53.55 -62 18.2 10.983 10.629 66.7 4.9 19.9 2026 03 31 21 59.45 -62 11.3 10.902 10.646 72.6 5.1 19.9 NOTE: These 'Central Bureau Electronic Telegrams' are sometimes superseded by text appearing later in the printed IAU Circulars. (C) Copyright 2025 CBAT 2025 September 2 (CBET 5599) Daniel W. E. Green