Electronic Telegram No. 5534
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 F1 (ATLAS)
     A. Fitzsimmons reported that a fast-moving discovered on four 30-s CCD
images taken on Mar. 22 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 appeared cometary with a diffuse 50" coma.  The
discovery observations are tabulated below:

     2025 UT             R.A. (2000) Decl.       Mag.
     Mar. 22.03985    3 58 36.25   -43 38 45.2   19.1
          22.05050    3 58 39.25   -43 37 40.6   19.3
          22.06084    3 58 41.77   -43 36 34.4   19.1
          22.06412    3 58 42.70   -43 36 14.4   19.2

After the comet was posted on the Minor Planet Center's PCCP webpage, other
astrometrists commented on the cometary appearance.  S. Deen (Simi Valley, CA,
USA) finds a very condensed, near-stellar coma and a tail about 4".5 long in
p.a. 125-130 degrees, with a much fainter circular haze extending roughly 5"
in all directions from the pseudonucleus in CCD images taken by A. Schnabel
and D. Ruhela with a 0.43-m reflector at Rio Hurtado, Chile on Mar. 23.0 UT.
Additional images taken on Mar. 24.0 show a similar appearance with an R
filter; in G and especially B filters, the appearance is increasingly
dominated by a roughly 16"-diameter coma.  Twenty-four stacked 10-s CCD
exposures taken remotely on Mar. 24.0 by H. Sato (Tokyo, Japan) using a 0.51-m
f/6.8 astrograph located at Rio Hurtado, Chile, show a strongly condensed coma
5" in diameter with no tail; the magnitude was 18.9 as measured within a
circular aperture of radius 2".8.
     The astrometry appears on MPEC 2025-G03.  The following parabolic orbital
elements by S. Nakano (Central Bureau) are from 40 observations spanning
Mar. 22-Apr. 1 (mean residual 0".5).  There are no close approaches to major
planets.

     T = 2025 Apr. 20.40811 TT        Peri. = 334.14616
                                      Node  = 122.67843 2000.0
     q = 1.0705028 AU                 Incl. = 109.22553

The following ephemeris by the undersigned from the above orbital elements
uses photometric power-law parameters H = 17.5 and 2.5n = 7.5 for the
magnitudes.

Date    TT    R. A. (2000) Decl.     Delta      r     Elong.  Phase  Mag.
2025 03 26    04 15.09   -36 41.0    1.083    1.150    67.0    53.0  18.1
2025 03 31    04 32.67   -27 41.9    1.089    1.123    64.9    53.7  18.1
2025 04 05    04 47.49   -18 55.7    1.120    1.101    62.3    53.6  18.1
2025 04 10    05 00.22   -10 44.3    1.171    1.084    59.3    52.6  18.1
2025 04 15    05 11.35   -03 20.2    1.239    1.074    56.1    50.8  18.2
2025 04 20    05 21.25   +03 13.0    1.321    1.071    52.7    48.3  18.3
2025 04 25    05 30.20   +08 57.4    1.411    1.073    49.3    45.3  18.5
2025 04 30    05 38.44   +13 58.2    1.506    1.082    45.9    41.9  18.6
2025 05 05    05 46.13   +18 21.7    1.603    1.098    42.5    38.4  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 1                     (CBET 5534)              Daniel W. E. Green