Electronic Telegram No. 5539 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/2023 RS_61 The Centaur-type minor planet 2023 RS_61 was discovered on images obtained on 2023 Sept. 15 with the Pan-STARRS2 1.8-m Ritchey-Chretien reflector at Haleakala (discovery observations below from MPEC 2023-T108). 2023 UT R.A. (2000) Decl. Mag. Sep. 15.42559 1 20 42.39 +22 58 32.8 21.9 15.43851 1 20 42.15 +22 58 32.8 21.8 15.45130 1 20 41.99 +22 58 32.8 21.6 15.46438 1 20 41.75 +22 58 32.8 21.7 Earlier observations were identified later from 2015 Sept. 5 (mag V = 24.6 on DECam images obtained with the Cerro Tololo 4-m reflector, 2020 Sept. 15 (w = 22.1-22.9 from images obtained with the Pan-STARRS1 1.8-m reflector at Haleakala), and 2021 Sept. 5 and 6 (mag 22.5 on images obtained with the 8.2-m f/2.2 Subaru Telescope at Mauna Kea) -- published on MPEC 2023-U271. 2015 UT R.A. (2000) Decl. Mag. Sept. 5.07174 21 51 41.98 -22 13 00.5 24.6 5.14618 21 51 40.93 -22 13 03.1 5.21860 21 51 39.94 -22 13 05.5 2020 UT R.A. (2000) Decl. Mag. Sep. 15.39639 23 40 30.53 + 1 43 09.8 22.0 15.42341 23 40 30.10 + 1 43 08.3 22.9 15.43691 23 40 29.88 + 1 43 07.3 22.5 2021 UT R.A. (2000) Decl. Mag. Sep. 5.60040 0 11 07.64 + 8 21 10.3 22.4 6.60688 0 10 52.66 + 8 20 24.0 22.5 R. Weryk, Department of Physics and Astronomy, University of Western Ontario, writes that he was alerted to cometary activity in the Centaur-type minor planet 2023 RS_61 via E. Lilly et al. [2025, *Research Notes AAS* 9(3), 67], who reported that seven r'-band images obtained on 2025 Jan. 2 with the 4.3-m Lowell Discovery Telescope show a coma and a broad, fan-like tail about 5".9 long in p.a. 100-170 degrees (with total mag 21.5 in a 3" aperture). Weryk identified the object in four 45-s i-band Pan-STARRS1 images from 2017 Oct. 10 taken in 1".0 seeing that show a diffuse coma of size 1".7 (full- width-at-half-maximum) with no tail; his astrometry is given below. 2017 UT R.A. (2000) Decl. Mag. Oct. 10.34362 22 25 51.68 -14 22 07.5 19.0 10.35473 22 25 51.56 -14 22 07.7 19.1 10.36579 22 25 51.46 -14 22 07.9 19.0 10.37703 22 25 51.35 -14 22 08.1 19.1 Additional astrometry appears on MPEC 2025-H113. The following orbital elements by S. Nakano (Central Bureau) are from 67 observations spanning 2015 Sept. 5-2024 Dec. 2 (mean residual 0".3). There have been no recent close approaches to major planets. Epoch = 2028 Dec. 15.0 TT T = 2028 Dec. 10.48389 TT Peri. = 119.87138 e = 0.3262472 Node = 347.04193 2000.0 q = 7.9886770 AU Incl. = 19.94800 a = 11.8569843 AU n = 0.02414030 P = 40.83 years Epoch = 2069 June 23.0 TT T = 2069 June 29.19308 TT Peri. = 119.72313 e = 0.3237458 Node = 346.83915 2000.0 q = 7.9829707 AU Incl. = 19.97756 a = 11.8046886 AU n = 0.02430090 P = 40.56 years 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. If the astrometric magnitudes from 2015-2020 can be trusted, there appears to have been an outburst of some five magnitudes in intrinsic brightness between 2015 and 2017, fading again by 2020. Date TT R. A. (2000) Decl. Delta r Elong. Phase Mag. 2025 06 14 02 51.12 +33 04.6 9.862 9.059 35.9 3.8 19.6 2025 06 24 02 56.10 +33 37.8 9.755 9.045 43.5 4.4 19.6 2025 07 04 03 00.69 +34 11.2 9.630 9.030 51.3 5.0 19.6 2025 07 14 03 04.81 +34 44.4 9.490 9.015 59.4 5.6 19.5 2025 07 24 03 08.40 +35 17.1 9.338 9.001 67.6 6.0 19.5 2025 08 03 03 11.34 +35 49.1 9.177 8.986 76.0 6.3 19.4 2025 08 13 03 13.58 +36 19.8 9.011 8.972 84.6 6.5 19.4 2025 08 23 03 15.04 +36 48.8 8.842 8.958 93.3 6.5 19.4 2025 09 02 03 15.65 +37 15.6 8.675 8.944 102.2 6.3 19.3 2025 09 12 03 15.40 +37 39.4 8.515 8.929 111.3 6.0 19.3 2025 09 22 03 14.27 +37 59.6 8.365 8.915 120.4 5.6 19.2 2025 10 02 03 12.30 +38 15.5 8.230 8.901 129.6 5.0 19.2 2025 10 12 03 09.58 +38 26.3 8.114 8.887 138.6 4.3 19.1 2025 10 22 03 06.23 +38 31.7 8.021 8.873 147.1 3.5 19.1 2025 11 01 03 02.45 +38 31.3 7.954 8.860 154.4 2.8 19.1 2025 11 11 02 58.44 +38 25.1 7.915 8.846 158.9 2.3 19.1 2025 11 21 02 54.45 +38 13.8 7.905 8.832 158.6 2.3 19.1 2025 12 01 02 50.72 +37 58.0 7.925 8.819 153.6 2.9 19.1 2025 12 11 02 47.47 +37 39.0 7.972 8.805 145.9 3.6 19.1 2025 12 21 02 44.88 +37 18.0 8.046 8.792 137.1 4.4 19.1 2025 12 31 02 43.11 +36 56.7 8.142 8.778 127.7 5.1 19.1 2026 01 10 02 42.22 +36 36.2 8.257 8.765 118.2 5.7 19.1 2026 01 20 02 42.27 +36 17.7 8.386 8.752 108.7 6.1 19.2 2026 01 30 02 43.26 +36 02.2 8.525 8.739 99.3 6.4 19.2 2026 02 09 02 45.15 +35 50.2 8.669 8.726 90.0 6.5 19.2 2026 02 19 02 47.89 +35 42.2 8.813 8.713 80.9 6.4 19.2 2026 03 01 02 51.42 +35 38.3 8.953 8.700 72.1 6.2 19.3 2026 03 11 02 55.64 +35 38.5 9.086 8.687 63.4 5.9 19.3 2026 03 21 03 00.48 +35 42.5 9.207 8.674 55.0 5.4 19.3 2026 07 19 04 16.55 +39 41.5 9.157 8.529 49.3 5.2 19.3 2026 07 29 04 21.88 +40 09.5 9.026 8.517 57.1 5.7 19.2 2026 08 08 04 26.62 +40 37.9 8.883 8.506 65.1 6.2 19.2 2026 08 18 04 30.70 +41 06.5 8.729 8.495 73.3 6.6 19.1 2026 08 28 04 34.00 +41 35.0 8.569 8.484 81.8 6.8 19.1 2026 09 07 04 36.43 +42 03.2 8.406 8.472 90.4 6.8 19.0 2026 09 17 04 37.92 +42 30.5 8.243 8.461 99.1 6.7 19.0 2026 09 27 04 38.39 +42 56.3 8.086 8.451 108.1 6.5 19.0 2026 10 07 04 37.83 +43 19.8 7.937 8.440 117.1 6.1 18.9 2026 10 17 04 36.23 +43 40.2 7.802 8.429 126.2 5.5 18.9 2026 10 27 04 33.67 +43 56.5 7.684 8.419 135.2 4.8 18.8 2026 11 06 04 30.28 +44 07.8 7.588 8.408 143.7 4.0 18.8 2026 11 16 04 26.23 +44 13.4 7.518 8.398 151.2 3.3 18.8 2026 11 26 04 21.80 +44 12.9 7.474 8.388 156.4 2.7 18.8 2026 12 06 04 17.25 +44 06.1 7.459 8.377 157.4 2.6 18.7 2026 12 16 04 12.88 +43 53.6 7.474 8.367 153.7 3.0 18.7 2026 12 26 04 08.99 +43 36.4 7.516 8.357 146.9 3.7 18.8 NOTE: These 'Central Bureau Electronic Telegrams' are sometimes superseded by text appearing later in the printed IAU Circulars. (C) Copyright 2025 CBAT 2025 April 24 (CBET 5539) Daniel W. E. Green