Electronic Telegram No. 5687
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/2026 H2 (LEONARD)
     Gregory J. Leonard, Lunar and Planetary Laboratory, University of 
Arizona, reports his discovery on CCD images obtained on Apr. 16 UT with the
Mt. Lemmon Survey's 1.5-m reflector (discovery observations tabulated below).
The comet showed a strongly condensed coma and a thin diffuseness extending 
about 8" across, with no apparent tail, in four 30-s stacked images of a 
crowded star field.

     2026 UT             R.A. (2000) Decl.       Mag.   Observer   
     Apr. 16.45429   17 15 48.43   + 9 47 46.6   19.6   Leonard
          16.45953   17 15 48.02   + 9 47 48.6   19.8     "
          16.46478   17 15 47.62   + 9 47 51.4            "
          16.47001   17 15 47.24   + 9 47 52.6   19.8     "

After the comet was posted to the Minor Planet Center's PCCP webpage, other
observers reported on the cometary appearance.  Twelve stacked 60-s CCD 
exposures obtained remotely on Apr. 18.7 UT by H. Sato (Bunkyo-ku, Tokyo, 
Japan) with a 0.51-m f/6.8 astrograph located at Siding Spring, NSW, 
Australia, show a strongly condensed coma 10" in diameter with no tail; the 
magnitude was 19.2 as measured within a circular aperture of radius 5".4.
Four 8-s unfiltered CCD exposures taken on Apr. 20.4 with the Steward 
Observatory 1-m reflector at Mt. Lemmon by K. W. Wierzchos show a diffuse 
head of size 6" with no tail.  Fifty stacked 60-s CCD exposures taken during 
Apr. 22.04-22.15 by E. Bryssinck (Kruibeke, Belgium) using a 0.4-m f/3.8 
reflector show a central condensation with a faint coma 10" in diameter,
with a total magnitude of 18.9 in an aperture of radius 5".0.
     One-hundred-four stacked 30-s unfiltered CCD images taken by A. Aletti 
during Apr. 24.03-24.05 in 3".8 seeing with a 0.36-m f/7.5 reflector at 
Varese, Italy, show a moderately condensed coma 7" wide, elongated towards the 
east; the magnitude was measured to be 19.3-19.4.  R. Weryk, Department of 
Physics and Astronomy, University of Western Ontario, reports that three 
stacked 60-s gri-band follow-up images that were taken in 0".6-0".7 seeing 
with the 3.6-m Canada-France-Hawaii telescope at Mauna Kea on Apr. 24.6 (queue 
observer L. Wells; queue coordinator V. Khatu) show a very condensed head of 
size 1".1 (full-width-at-half-maximum) with a broad tail > 8" long spanning 
p.a. 90-150 degrees.  T. Prystavski (Lviv, Ukraine) reports that CCD images 
obtained remotely with an iTelescope 0.51-m f/6.8 "corrected Dall-Kirkham"
astrograph at Siding Spring on Apr. 28.8 show total mag 19.2 and a tail 0'.2 
long in p.a. 112 deg.
     The available astrometry appears on MPEC 2026-J15.  The following 
parabolic orbital elements by S. Nakano (Central Bureau) are from 222 
observations spanning 2026 Apr. 15-May 6 (mean residual 0".4).  The comet 
passed 3.30 AU from Uranus in 2016 Oct. and 3.08 AU from Saturn in 2023 Sept., 
and it and will pass 2.02 AU from Jupiter on 2028 Mar. 14.

     T = 2026 Mar. 17.36227 TT        Peri. = 111.56995
                                      Node  =   5.47125 2000.0
     q = 4.5455767 AU                 Incl. = 148.64759

The following ephemeris by the undersigned from the above orbital elements
uses photometric power-law parameters H = 11.0 and 2.5n = 8 for the
magnitudes.

Date    TT    R. A. (2000) Decl.     Delta      r    Elong.  Phase   Mag.
2026 04 30    16 56.90   +11 10.0    3.792    4.559   134.9     9.0  19.2
2026 05 10    16 40.76   +11 58.7    3.724    4.566   142.4     7.8  19.1
2026 05 20    16 23.41   +12 32.3    3.698    4.574   146.3     7.0  19.1
2026 05 30    16 05.70   +12 47.6    3.715    4.584   145.2     7.3  19.1
2026 06 09    15 48.48   +12 43.6    3.776    4.595   139.5     8.2  19.2
2026 06 19    15 32.53   +12 21.8    3.875    4.608   131.1     9.6  19.2
2026 06 29    15 18.42   +11 44.7    4.009    4.622   121.5    10.8  19.3
2026 07 09    15 06.45   +10 56.3    4.169    4.637   111.4    11.8  19.4
2026 07 19    14 56.70   +10 00.0    4.348    4.653   101.2    12.4  19.5
2026 07 29    14 49.11   +08 58.9    4.539    4.671    91.2    12.6  19.6
2026 08 08    14 43.48   +07 55.6    4.734    4.690    81.4    12.3  19.7
2026 08 18    14 39.59   +06 51.9    4.927    4.711    71.8    11.8  19.8
2026 08 28    14 37.20   +05 49.1    5.112    4.732    62.5    10.9  19.9


NOTE: These 'Central Bureau Electronic Telegrams' are sometimes
      superseded by text appearing later in the printed IAU Circulars.

                         (C) Copyright 2026 CBAT
2026 May 6                       (CBET 5687)              Daniel W. E. Green