Electronic Telegram No. 5027 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/2021 Q3 (ATLAS) An apparently asteroidal object discovered on CCD images taken on Aug. 26 UT with a 0.5-m f/2 Schmidt reflector at Haleakala, Hawaii, in the course of the "Asteroid Terrestrial-Impact Last Alert System" (ATLAS) search program has been found to show cometary appearance by CCD astrometrists elsewhere after the object was posted on the Minor Planet Center's PCCP webpage. The discovery observations are tabulated below. 2021 UT R.A. (2000) Decl. Mag. Aug. 26.29654 15 36 03.13 +82 47 43.0 19.0 26.29975 15 36 02.12 +82 47 41.6 19.2 26.30663 15 36 01.21 +82 47 37.1 19.2 26.31673 15 35 58.86 +82 47 31.7 18.7 Seventy-five stacked 30-s exposures taken on Aug. 26.9 UT by L. Buzzi, Varese, Italy, with a 0.84-m f/3.5 reflector (measured by A. Aletti) in 4".5 seeing show a condensed coma 7" wide and of mag 19.4 with a faint, straight tail 10" long in p.a. 95 degrees. Ten stacked 60-s exposures taken remotely by H. Sato (Tokyo, Japan) on Aug. 29.18 using a 0.43-m f/6.8 astrograph located near Mayhill, NM, USA, show a strongly condensed coma 10" in diameter with a hint of a tail 6" long toward p.a. 90 degrees; the magnitude was 18.9 as measured within a circular aperture of radius 5".7. E. Bryssinck, Kruibeke, Belgium, has stacked thirty-six 60-s exposures taken remotely on Aug. 29.87 by a large group of observers using a "Telescope Live" 0.7-m f/8 reflector in Oria, Spain, to find a central condensation surrounded by a 7" coma of magnitude 19.1 in an aperture of radius 5". A. Valvasori, Padulle, Italy, writes that twenty-four stacked 120-s exposures taken by E. Guido and himself on Aug. 29.9 with a 0.30-m f/4 reflector at the ALMO Observatory in Italy show a diffuse coma about 7".4 in diameter of mag 18.3-18.9. The available astrometry appears on MPEC 2021-Q97 (which includes pre-discovery observations identified by R. Weryk in images obtained with the Pan-STARRS1 1.8-m Ritchey-Chretien reflector at Haleakala on Aug. 19.3 UT, when the magnitude was given as 19.7). The following parabolic orbital elements by S. Nakano (CBAT) are from 69 observations spanning Aug. 19-30 (mean residual 0".6). T = 2021 Nov. 26.29013 TT Peri. = 106.79398 Node = 309.58150 2000.0 q = 5.3675841 AU Incl. = 77.65930 The following ephemeris by the undersigned from the above orbital elements uses photometric power-law parameters H = 9.0 and 2.5n = 8 for the magnitudes. Date TT R. A. (2000) Decl. Delta r Elong. Phase Mag. 2021 08 14 16 27.46 +84 27.1 5.562 5.423 76.9 10.5 18.6 2021 08 19 16 01.42 +83 49.3 5.547 5.418 77.4 10.5 18.6 2021 08 24 15 42.69 +83 07.6 5.532 5.413 78.0 10.5 18.6 2021 08 29 15 29.57 +82 24.0 5.516 5.408 78.7 10.5 18.6 2021 09 03 15 20.64 +81 40.2 5.499 5.404 79.3 10.6 18.6 2021 09 08 15 14.85 +80 57.2 5.482 5.400 80.1 10.6 18.6 2021 09 13 15 11.45 +80 15.7 5.464 5.396 80.8 10.6 18.5 2021 09 18 15 09.89 +79 36.2 5.445 5.392 81.7 10.6 18.5 2021 09 23 15 09.75 +78 59.3 5.426 5.389 82.5 10.6 18.5 2021 09 28 15 10.75 +78 25.1 5.407 5.386 83.5 10.7 18.5 2021 10 03 15 12.70 +77 53.9 5.386 5.383 84.5 10.7 18.5 2021 10 08 15 15.43 +77 26.1 5.365 5.380 85.5 10.7 18.5 2021 10 13 15 18.80 +77 01.8 5.344 5.378 86.6 10.7 18.5 2021 10 18 15 22.73 +76 41.2 5.322 5.376 87.7 10.7 18.5 2021 10 23 15 27.14 +76 24.4 5.300 5.374 88.9 10.7 18.5 2021 10 28 15 31.97 +76 11.5 5.278 5.372 90.1 10.7 18.5 2021 11 02 15 37.19 +76 02.7 5.256 5.371 91.3 10.6 18.4 2021 11 07 15 42.76 +75 58.1 5.234 5.370 92.5 10.6 18.4 2021 11 12 15 48.66 +75 57.8 5.212 5.369 93.8 10.6 18.4 2021 11 17 15 54.85 +76 01.8 5.190 5.368 95.0 10.6 18.4 2021 11 22 16 01.32 +76 10.1 5.170 5.368 96.2 10.5 18.4 2021 11 27 16 08.09 +76 22.8 5.150 5.368 97.4 10.5 18.4 2021 12 02 16 15.14 +76 39.9 5.131 5.368 98.6 10.5 18.4 2021 12 07 16 22.49 +77 01.4 5.113 5.368 99.7 10.4 18.4 2021 12 12 16 30.14 +77 27.2 5.098 5.369 100.7 10.4 18.4 NOTE: These 'Central Bureau Electronic Telegrams' are sometimes superseded by text appearing later in the printed IAU Circulars. (C) Copyright 2021 CBAT 2021 September 1 (CBET 5027) Daniel W. E. Green