Electronic Telegram No. 5029 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 P/2021 Q5 (ATLAS) According to P. Veres, Minor Planet Center, L. Denneau reported the discovery of a comet with a diffuse 6".1 coma on CCD images taken on Aug. 29.6 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; the discovery observations are tabulated below. 2021 UT R.A. (2000) Decl. Mag. Aug. 29.60677 6 50 43.47 +22 54 23.5 17.5 29.61227 6 50 44.73 +22 54 19.6 17.3 29.62884 6 50 48.62 +22 54 06.4 17.0 29.63683 6 50 50.50 +22 54 00.0 17.7 After the comet was posted on the MPC's PCCP webpage, other observers have also commented on the cometary appearance. J.-F. Soulier writes that two- hundred 30-s unfiltered CCD exposures taken on Sept. 1.09-1.15 UT with a 0.30-m f/3.8 Newtonian reflector at Maisoncelles, France, show a diffuse coma of size 23" x 17" with no tail; the red magnitude was 17.1 in an aperture of radius 6".5. Soulier adds that one-hundred-twenty 30-s exposures taken on Sept. 2.12-2.15 show a diffuse coma of size 20" x 12" with no tail; the magnitude was again 17.1 within an aperture of radius 6".5. Sixty stacked 10-s CCD exposures taken by L. Buzzi, Varese, Italy, on Sept. 1.15 with a 0.84-m f/3.5 reflector (and measured by A. Aletti), in twilight and with the waning crescent moon nearby, show a 12" coma of mag 17.6, elongated for about 12" in p.a. 280 degrees. A. Valvasori, Padulle, Italy reports that fifty- eight stacked 30-s CCD exposures taken by E. Guido and himself on Sept. 3.14- 3.15 with a 0.30-m f/4 reflector reveals a compact coma about 8" arcsec in diameter (mag 16.9-17.1) and a tail 10" long in p.a. 270 degrees. Ten stacked 30-s CCD exposures taken remotely by M. Mattiazzo with an iTelescope 0.32-m f/8 astrograph at Nerpio, Spain, on Sept. 4.2 show a diffuse 1' coma with a total GAIA-based broadband G magnitude of 15.5. J. J. Gonzalez Suarez reports a total visual magnitude of 11.5 and coma diameter 4' (wide, faint, uncondensed coma) on Sept. 5.19, using a 0.20-m f/10 Schmidt-Cassegrain reflector at Alto del Castro, near Leon, Spain, in very clear sky (limiting stellar magnitude near the comet was 13.4, and motion was followed for 85 min with field-star checking done via the Digitized Sky Survey). The available astrometry appears on MPEC 2021-R98. The following preliminary elliptical orbital elements by S. Nakano (CBAT) are from 89 observations spanning 2021 Aug. 29-Sept. 5 (mean residual 0".5); the orbital period is uncertain on the order of plus-or-minus a half year. Some initial orbits suggest that the comet may have passed 0.06-0.25 AU from Jupiter around late 2018 or early 2019, explaining why the comet may not have been discovered previously. Nakano was unable to definitively identify additional archival observations, though this is not unexpected, given the preliminary nature of this orbit. The mean residual on a parabolic orbit is only slightly worse (0".7), with T = 2021 Sept. 3.3288 TT, Peri. = 184.3804 deg, Node = 243.7349 deg, i = 10.9941 deg (equinox J2000.0), q = 1.339681 AU. T = 2021 Aug. 30.27439 TT Peri. = 180.74942 e = 0.6115346 Node = 239.62033 2000.0 q = 1.2307940 AU Incl. = 10.72482 a = 3.1683490 AU n = 0.17476504 P = 5.64 years If Gonzalez's visual observation above is correct, it makes the prediction of ephemeris magnitudes difficult. It is, of course, unknown (but suspected) that the comet is currently undergoing some kind of outburst in brightness. The following ephemeris by the undersigned from the above elliptical orbital elements uses photometric power-law parameters H = 12.0 and 2.5n = 10 for the magnitudes (assuming a current total magnitude of 13.5, splitting the difference of the visual and brightest CCD total magnitudes). Date TT R. A. (2000) Decl. Delta r Elong. Phase Mag. 2021 08 14 05 46.89 +25 30.7 1.531 1.246 54.1 41.2 13.9 2021 08 19 06 07.80 +24 52.1 1.519 1.238 54.2 41.5 13.8 2021 08 24 06 28.31 +24 02.3 1.510 1.233 54.3 41.8 13.8 2021 08 29 06 48.32 +23 02.3 1.503 1.231 54.5 41.9 13.8 2021 09 03 07 07.75 +21 53.2 1.498 1.232 54.7 42.0 13.8 2021 09 08 07 26.51 +20 36.0 1.495 1.235 55.1 42.0 13.8 2021 09 13 07 44.56 +19 12.1 1.493 1.242 55.6 41.9 13.8 2021 09 18 08 01.86 +17 42.8 1.493 1.251 56.1 41.8 13.8 2021 09 23 08 18.41 +16 09.1 1.493 1.264 56.8 41.7 13.9 2021 09 28 08 34.21 +14 32.3 1.494 1.279 57.7 41.5 13.9 2021 10 03 08 49.26 +12 53.3 1.495 1.296 58.6 41.3 14.0 2021 10 08 09 03.57 +11 13.2 1.496 1.316 59.8 41.0 14.1 2021 10 13 09 17.15 +09 32.6 1.497 1.338 61.0 40.7 14.1 2021 10 18 09 30.02 +07 52.5 1.497 1.361 62.4 40.5 14.2 2021 10 23 09 42.19 +06 13.3 1.496 1.387 64.0 40.1 14.3 2021 10 28 09 53.69 +04 35.6 1.495 1.415 65.7 39.8 14.4 2021 11 02 10 04.51 +02 59.8 1.492 1.443 67.6 39.5 14.5 2021 11 07 10 14.66 +01 26.4 1.487 1.474 69.7 39.1 14.5 2021 11 12 10 24.14 -00 04.3 1.481 1.505 71.9 38.7 14.6 2021 11 17 10 32.94 -01 32.0 1.474 1.537 74.3 38.3 14.7 2021 11 22 10 41.07 -02 56.4 1.465 1.571 76.9 37.8 14.8 2021 11 27 10 48.50 -04 17.3 1.455 1.605 79.7 37.2 14.9 2021 12 02 10 55.22 -05 34.4 1.443 1.640 82.6 36.6 14.9 2021 12 07 11 01.20 -06 47.4 1.430 1.675 85.7 35.9 15.0 2021 12 12 11 06.40 -07 56.0 1.415 1.711 89.1 35.1 15.1 NOTE: These 'Central Bureau Electronic Telegrams' are sometimes superseded by text appearing later in the printed IAU Circulars. (C) Copyright 2021 CBAT 2021 September 6 (CBET 5029) Daniel W. E. Green