Electronic Telegram No. 5538 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 F2 (SWAN) On Mar. 29, Vladimir Bezugly (Dnipro, Ukraine) informed the Central Bureau that he found an apparent moving object (promising candidate for a possible comet) on low-resolution public website hydrogen Lyman-alpha daily images obtained during Mar. 22-28 UT with the Solar Wind Anisotropies (SWAN) camera on the Solar and Heliospheric Observer (SOHO) spacecraft (see CBETs 4068, 4136, 4939; and website URL http://swan.projet.latmos.ipsl.fr/), SWAN creates daily full-sky scans at the wavelength of hydrogen Lyman_alpha, but its images are at a very poor resolution (not better than about 1 degree). Bezugly provided the following approximate astrometry for the first and last dates (at that time) as follows: 2025 UT R.A. (2000) Decl. Mag. Mar. 22.5 22 39 +14 34 11.5 23.5 22 43 +15 34 24.5 22 49 +16 53 27.5 22 57 +18 12 28.5 23 01 +19 15 11.5 Also on Mar. 29, M. Mattiazzo (Swan Hill, Vic., Australia) reported "a pretty obvious comet" near the edge of the SWAN frames with the following approximate positions provided. He noted approximate magnitude 11 "and apparently brightening". Mattiazzo later reported that five stacked 30-s CCD exposures taken on Apr. 5.48 UT with a 0.50-m f/3 Planewave Delta Rho 500 telescope located at Rio Hurtado Valey in Chile show a condensed coma of size 3' with a total GAIA DR2 G magnitude of 9.6, having a straight 22'-long tail in p.a. 304 degrees. 2025 UT R.A. (2000) Decl. Mar. 22.5 22 41.4 +15 12 23.5 22 44.5 +15 51 24.5 22 49.2 +16 42 26.5 22 52.1 +17 27 27.5 22 56.1 +18 13 28.5 22 59.8 +18 56 29.5 23 04.4 +19 53 On Mar. 31, R. D. Matson (Irvine, CA, USA) reported that he had been following this object independently in SWAN imagery, reporting the following astrometry: 2025 UT R.A. (2000) Decl. Mar. 22.5 22 42.1 +15 34 23.5 22 44.8 +15 51 24.5 22 48.1 +16 30 26.5 22 51.5 +17 10 27.5 22 55.6 +18 12 28.5 23 00.4 +19 01 29.5 23 02.5 +19 32 The comet was posted on Apr. 1 to the comets-ml discussion group, eventually leading to lots of confirming observations after a great deal of effort of searching the area given by the approximate SWAN positions. G.-y. Sun reports that he identified images of the comet showing a coma of diameter about 30" (V mag 14.0-14.4) with no tail on Xingming Observatory CCD images obtained on Mar. 26.93 UT with a 0.11-m f/3.6 reflector. M. Jaeger writes that he measured an image composed of twenty-four 15-s CMOS exposures taken during Apr. 3.13-3.14 by G. Rhemann, E. Prosperi, and himself with a 0.28-m f/2.2 reflector located at Stixendorf, Austria. Jaeger reports that the comet was about 2 degrees off of the predictions at comets-ml from the SWAN positions; it was strongly condensed with a coma of size 200" and total magnitude of around 10.0, and a faint tail about 10" long in p.a. 283 degrees. Jaeger's astrometry is provided below: 2025 UT R.A. (2000) Decl. Mag. Observer Apr. 3.13479 23 15 12.22 +22 43 36.2 12.3 Jaeger 3.13617 23 15 12.60 +22 43 38.9 12.4 " 3.13703 23 15 12.83 +22 43 40.8 12.5 " 3.13782 23 15 12.99 +22 43 42.8 12.6 " Twelve stacked 30-s CMOS exposures taken on Apr. 3.83 UT by K. Yoshimoto (Kumage, Yamaguchi, Japan) with a 0.20-m f/8 Ritchey-Chretien reflector show a condensed coma of size 2'.2 and total mag 10.2 with a straight tail more than 35' long in p.a. 295 deg. D. Buczynski (Portmahomack, Scotland) reports that CCD images obtained on Apr. 4.15-4.16 UT with a 0.36-m f/6.2 Schmidt-Cassegrain reflector, while the comet was only 10 degrees above the horizon in twilight, show a central condensation surrounded by a 10" inner coma and a 1'.6-diameter outer coma. Thirty stacked 15-s CCD exposures taken on Apr. 4.16-4.17 by A. Aletti and M. Auteri with a 0.36-m f/7.5 reflector (+ R_c filter) at Varese, Italy, show a condensed coma of size 20". T. Prystavski (Lviv, Ukraine) writes that images obtained remotely with an iTelescope 0.11-m f/5 astrograph located at the Utah Desert Remote Observatory (near Beryl Junction, UT, USA) on Apr. 4.5 UT show a coma of diameter about 1'.4 and total mag about 10.3 with a tail about 7'.3 long in p.a. 303 degrees; despite morning twilight (the comet was at altitude 17 deg), the coma was visible evenon single images. Additional CMOS exposures taken by Prystavski on Apr. 7.49 with a 0.51-m f/3.0 Cassegrain reflector at Beryl Junction show a coma of diameter about 3'.2 and total mag about 8.5 with a tail > 33' long (extends beyond the edge of the field-of-view) in p.a. 298 deg; despite morning twilight, he remarks that the comet was much brighter on Apr. 7 compared to three nights earlier. Twelve stacked 30-s CCD exposures taken on Apr. 4.5 UT by Hidetaka Sato (Bunkyo-ku, Tokyo, Japan) with a 0.51-m f/6.8 astrograph located at Beryl Junction show a strongly condensed object with an outer coma 80" in diameter and no tail; the magnitude was 10.3 as measured within a circular aperture of radius 41".0. Twelve stacked 30-s CMOS exposures taken by him on Apr. 5.5 with a 0.51-m f/3 astrograph at Beryl Junction show a strongly condensed comet with an outer coma 1'.5 in diameter and a straight tail 12' long toward p.a. 300 deg; the magnitude was 9.2 as measured within a circular aperture of radius 45".9. CCD exposures taken on Apr. 4.79 UT by K. Kadota (Ageo, Japan) with a 0.25-m f/5 reflector show a strong central condensation inside a coma of diameter 2'.8 and total mag 10.5 with no tail. Exposures taken by Kadota on Apr. 7.80 show a strong central condensation inside a coma of diameter 3'.2 and total mag 9.1, with a straight, narrow tail at least 18' long in p.a. 298 deg. Nine stacked 45-s CCD exposures taken in unfavorable conditions on Apr. 4.8 UT by Hirohisa Sato at Sukagawa, Japan, with a 0.25-m f/4 reflector show a strongly condensed coma about 2'.2 in diameter with total mag 10.3; twenty- four stacked 30-s exposures taken with the same instrumentation on Apr. 7.8 show a strongly condensed coma about 3'.0 in diameter (total mag 8.8), and a tail about 14' long (extending beyond the field-of-view) toward p.a. about 302 degrees. Exposures taken on Apr. 4.80 UT by T. Yusa (Osaki, Miyagi, Japan) with a 0.05-m f/5 ZWO Seestar All-in-One Smart Telescope show an "emerald green" color of mag 10.5 and coma diameter 2'.1. Additional images obtained on Apr. 6.79 show a coma of total mag 8.6 and diameter 2'.5 (Yusa noted the 2-mag brightening from Apr. 4). Fifty-six stacked 15-s CCD exposures taken with a 0.36-m f/7.5 reflector (+ R_c filter) at Varese, Italy, by A. Aletti (measured by Aletti and M. Auteri) 0.36-m f/7.5 reflector on Apr. 5.15 UT show a condensed 25" coma wide and (despite some haze) a faint straight tail at least 2' long in p.a. 305 degrees. Eighty 10-s unfiltered CCD exposures taken during Apr. 6.14-6.16 UT by J.-F. Soulier with a 0.30-m f/3.8 Newtonian reflector located at Maisoncelles, France, show a coma of size about 4'.2 and a tail 27'.5 long in p.a. 310 deg; the magnitude was 9.1 in an aperture of radius 0'.81. R. Weryk, Department of Physics and Astronomy, University of Western Ontario, writes that he has identified pre-discovery images of this comet as a soft or slightly "fuzzy" object on images obtained with the Pan-STARRS1 1.8-m Ritchey-Chretien reflector at Haleakala on 2024 Oct. 6.24-6.28 UT (mag 21.3-23.1) in 1".1-1".2 seeing (no tail). A similar "slightly fuzzy" appearance is present in images taken in 0".9-1".1 seeing with the Pan-STARRS2 1.8-m reflector at Haleakala on 2024 Oct. 4.2 (mag 21.4-22.4). Weryk also identified pre-discovery Pan-STARRS1 images of the comet from 2024 Sept. 3.4 UT (mag 22.1-22.3), Sept. 24.25 (mag 21.0), and Sept. 27.3 (mag 21.8-22.9), as well as Pan-STARRS1 images from 2024 Sept. 9.3 (mag 20.9-22.5). After Weryk reported the above pre-discovery observations, S. Deen (Simi Valley, CA, USA) identified a pre-discovery image of the comet taken in 1".0 seeing on 2024 Sept. 8.01 UT with the 4-m reflector (+ DECam) at Cerro Tololo, in which the comet appeared stellar at mag i = 22.4 with a possible faint tail about 2" long toward p.a. 60-120 degrees. M. Meyer (Limburg, Germany) reports that he observed the comet visually on Apr. 4.13 UT with a 25-cm f/5 Dobsonian reflector at low altitude; at 73x, it resembled a globular cluster, very well condensed with a coma diameter of 2' and total mag 9.2. Additional visual total-magnitude and coma-diameter estimates reported to the Central Bureau and ICQ: Apr. 4.13, 9.4, 2'.5 (R. J. Bouma, Groningen, The Netherlands, 25-cm reflector); Apr. 6.02, 8.1, 2'.5 (A. Novichonok, Karelia, Russia, 8-cm refractor); 6.13, 8.3, 2'.5 (Bouma); 7.13, 7.8, 2'.7 (Bouma); 7.18, 7.9, 4' (J. J. Gonzalez Suarez, Cueto Rosales, Leon, Spain, 15x70 binoculars); 7.35, 8.3, 3' (V. Gonano, Udine, Italy, 20x80 binoculars; 1.5-deg tail in p.a. 300 deg); 8.09, 7.3, 4' (M. Reszelski, Ostrorog, Poland, 15x70 binoculars). The available astrometry appears on MPEC 2025-G101. The following orbital elements by S. Nakano (Central Bureau) are from 295 observations spanning 2024 Sept. 3-2025 Apr. 8 (mean residual 0".5), with corresponding "original" and "future" values of 1/a being +0.000593 and +0.000119 (+/- 0.000014) AU**-1, respectively. The comet passsed 0.86 AU from Saturn on 2023 Jan. 29 UT. Epoch = 2025 May 5.0 TT T = 2025 May 1.15885 TT Peri. = 153.85050 e = 0.9999717 Node = 329.84375 2000.0 q = 0.3334179 AU Incl. = 90.37246 The following ephemeris by the undersigned from the above orbital elements uses photometric power-law parameters H = 8.5 and 2.5n = 10 for the magnitudes. This is a difficult light curve to assess. The comet has clearly brightened by much more than 10 log r since last October, but observations are lacking from early October to March 20 (and since January due to small solar elongations). Observers have commented on a clear outburst in brightness having occurred around April 5. Predicting the comet's brightness as it approaches perihelion is necesssarily highly uncertain. Date TT R. A. (2000) Decl. Delta r Elong. Phase Mag. 2025 03 26 22 48.22 +17 47.7 1.750 0.978 27.3 27.9 9.6 2025 03 27 22 51.08 +18 21.0 1.724 0.958 27.5 28.8 9.5 2025 03 28 22 54.04 +18 55.1 1.698 0.937 27.7 29.7 9.4 2025 03 29 22 57.12 +19 30.1 1.671 0.917 27.9 30.6 9.2 2025 03 30 23 00.33 +20 05.9 1.645 0.897 28.1 31.6 9.1 2025 03 31 23 03.68 +20 42.7 1.618 0.876 28.2 32.6 9.0 2025 04 01 23 07.17 +21 20.3 1.591 0.856 28.4 33.7 8.8 2025 04 02 23 10.83 +21 58.8 1.565 0.835 28.5 34.8 8.7 2025 04 03 23 14.67 +22 38.2 1.538 0.815 28.6 35.9 8.5 2025 04 04 23 18.69 +23 18.5 1.511 0.794 28.6 37.1 8.4 2025 04 05 23 22.92 +23 59.7 1.484 0.773 28.7 38.4 8.2 2025 04 06 23 27.38 +24 41.7 1.457 0.752 28.7 39.7 8.1 2025 04 07 23 32.09 +25 24.5 1.430 0.731 28.6 41.0 7.9 2025 04 08 23 37.07 +26 07.9 1.403 0.710 28.6 42.4 7.8 2025 04 09 23 42.34 +26 52.0 1.377 0.689 28.5 43.9 7.6 2025 04 10 23 47.94 +27 36.5 1.350 0.668 28.4 45.4 7.4 2025 04 11 23 53.89 +28 21.3 1.324 0.647 28.2 47.1 7.2 2025 04 12 00 00.23 +29 06.3 1.298 0.626 28.0 48.7 7.0 2025 04 13 00 06.98 +29 51.0 1.272 0.605 27.8 50.5 6.8 2025 04 14 00 14.20 +30 35.3 1.246 0.584 27.5 52.4 6.6 2025 04 15 00 21.91 +31 18.6 1.221 0.563 27.1 54.3 6.4 2025 04 16 00 30.16 +32 00.4 1.197 0.543 26.8 56.4 6.2 2025 04 17 00 38.98 +32 40.2 1.172 0.522 26.3 58.5 6.0 2025 04 18 00 48.41 +33 17.2 1.149 0.502 25.9 60.7 5.8 2025 04 19 00 58.47 +33 50.5 1.126 0.483 25.4 63.1 5.6 2025 04 20 01 09.20 +34 19.1 1.104 0.463 24.8 65.5 5.4 2025 04 21 01 20.58 +34 41.7 1.083 0.445 24.2 68.0 5.2 2025 04 22 01 32.61 +34 57.1 1.064 0.427 23.6 70.5 4.9 2025 04 23 01 45.25 +35 03.8 1.045 0.410 23.0 73.1 4.7 2025 04 24 01 58.40 +35 00.4 1.028 0.394 22.3 75.7 4.5 2025 04 25 02 11.98 +34 45.4 1.012 0.380 21.7 78.2 4.3 2025 04 26 02 25.81 +34 17.6 0.999 0.367 21.1 80.6 4.1 2025 04 27 02 39.72 +33 36.0 0.987 0.356 20.5 82.8 4.0 2025 04 28 02 53.50 +32 40.2 0.977 0.346 20.0 84.8 3.8 2025 04 29 03 06.94 +31 30.2 0.970 0.340 19.7 86.3 3.7 2025 04 30 03 19.82 +30 06.8 0.965 0.335 19.4 87.4 3.7 2025 05 01 03 31.98 +28 31.4 0.963 0.333 19.3 87.9 3.6 2025 05 02 03 43.27 +26 45.9 0.963 0.334 19.4 88.0 3.7 2025 05 03 03 53.63 +24 52.7 0.965 0.338 19.6 87.4 3.7 2025 05 04 04 03.02 +22 54.0 0.969 0.344 19.9 86.4 3.8 2025 05 05 04 11.46 +20 52.3 0.976 0.352 20.4 85.1 3.9 2025 05 07 04 25.71 +16 47.9 0.994 0.375 21.6 81.5 4.2 2025 05 09 04 37.00 +12 51.8 1.017 0.405 23.0 77.4 4.6 2025 05 11 04 45.99 +09 10.7 1.044 0.439 24.6 73.3 5.0 2025 05 13 04 53.26 +05 47.4 1.074 0.476 26.2 69.4 5.4 2025 05 15 04 59.26 +02 41.8 1.105 0.516 27.8 65.9 5.8 2025 05 17 05 04.34 -00 07.2 1.137 0.557 29.3 62.8 6.2 2025 05 19 05 08.72 -02 41.3 1.169 0.598 30.8 59.9 6.6 2025 05 21 05 12.59 -05 02.3 1.201 0.640 32.2 57.4 7.0 2025 05 23 05 16.08 -07 12.0 1.232 0.682 33.6 55.2 7.3 2025 05 25 05 19.28 -09 11.9 1.263 0.724 35.0 53.3 7.6 2025 05 27 05 22.25 -11 03.4 1.294 0.766 36.3 51.5 7.9 2025 05 29 05 25.05 -12 47.7 1.323 0.808 37.6 49.9 8.2 2025 05 31 05 27.72 -14 25.8 1.352 0.849 38.9 48.5 8.4 2025 06 02 05 30.28 -15 58.6 1.380 0.890 40.1 47.2 8.7 2025 06 04 05 32.75 -17 26.8 1.407 0.931 41.3 46.1 8.9 2025 06 06 05 35.16 -18 51.0 1.434 0.971 42.6 45.0 9.2 2025 06 08 05 37.51 -20 11.9 1.460 1.011 43.8 44.0 9.4 2025 06 10 05 39.83 -21 29.8 1.485 1.050 45.0 43.1 9.6 2025 06 12 05 42.11 -22 45.3 1.509 1.089 46.2 42.3 9.8 2025 06 14 05 44.36 -23 58.6 1.533 1.128 47.4 41.5 9.9 2025 06 19 05 49.89 -26 54.5 1.589 1.223 50.3 39.8 10.4 2025 06 24 05 55.34 -29 43.0 1.641 1.315 53.3 38.3 10.8 2025 06 29 06 00.71 -32 26.8 1.690 1.406 56.2 36.9 11.1 2025 07 04 06 06.01 -35 08.1 1.737 1.495 59.0 35.7 11.4 2025 07 09 06 11.22 -37 48.1 1.782 1.581 61.9 34.5 11.7 2025 07 14 06 16.35 -40 28.0 1.825 1.667 64.7 33.5 12.0 2025 07 19 06 21.36 -43 08.3 1.867 1.750 67.4 32.4 12.3 2025 07 24 06 26.24 -45 49.5 1.910 1.832 70.1 31.4 12.5 2025 08 03 06 35.39 -51 15.3 1.996 1.992 75.1 29.5 13.0 2025 08 13 06 43.35 -56 44.0 2.087 2.147 79.5 27.6 13.4 2025 08 23 06 49.39 -62 12.6 2.187 2.297 83.1 25.9 13.8 2025 09 02 06 52.13 -67 36.5 2.298 2.444 85.9 24.3 14.2 2025 09 12 06 48.77 -72 49.0 2.423 2.586 87.7 22.9 14.5 2025 09 22 06 32.78 -77 41.7 2.561 2.726 88.5 21.6 14.9 2025 10 02 05 45.78 -81 56.5 2.714 2.863 88.2 20.5 15.2 2025 10 12 03 42.15 -84 37.1 2.879 2.996 87.0 19.4 15.6 NOTE: These 'Central Bureau Electronic Telegrams' are sometimes superseded by text appearing later in the printed IAU Circulars. (C) Copyright 2025 CBAT 2025 April 8 (CBET 5538) Daniel W. E. Green