Electronic Telegram No. 5220 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 PE_20 (ATLAS) An apparently asteroidal object discovered on CCD images taken on 2021 Aug. 12 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 was found in the following days to show cometary appearance by CCD astrometrists elsewhere after the object was placed on the Minor Planet Center's NEOCP webpage. Despite this fact, the MPC inadvertently gave the minor-planet designation 2021 PE_20 when it was announced on MPEC 2021-Q10 (dated 2021 Aug. 17). The discovery observations are tabulated below. 2021 UT R.A. (2000) Decl. Mag. Aug. 12.58905 3 41 39.29 - 8 34 59.0 19.0 12.59226 3 41 39.55 - 8 35 00.1 18.3 12.59998 3 41 40.32 - 8 35 00.9 18.9 12.61789 3 41 41.97 - 8 35 04.3 18.6 Ten stacked 60-s exposures taken remotely by H. Sato (Bunkyo-ku, Tokyo, Japan) with a 0.51-m f/6.8 astrograph located at Siding Spring, NSW, Australia, on 2021 Aug. 13.8 UT shows a strongly condensed coma 10" in diameter and no tail; the magnitude was 18.2 as measured within a circular aperture of radius 6".6. Eight stacked 60-s follow-up exposures by Sato on 2021 Aug. 15.7 show a strongly condensed, disk-like coma 12" in diameter with no tail; the magnitude was 18.4 as measured within a circular aperture of radius 8".2. L. Buzzi, Varese, Italy, wrote that one-hundred 15-s exposures taken in good seeing on 2021 Aug. 14.1 with a 0.84-m f/3.5 reflector (measured by Buzzi and A. Aletti), but at low altitude (20-24 deg) show a condensed 8" coma with no apparent tail (perhaps a slight extension toward the west, only suspected); the head of the comet was then clearly extended with respect to the images of nearby stars (5".8 vs. 4".1 full-width-at-half-maximum). Buzzi also reports that twenty-four stacked 90-s exposures taken by L. Demetz and himself with a 0.51-m f/6.7 astrograph of the Skygems Telescope Network in Namibia on 2021 Aug. 16.2 show a condensed 6" coma extending for at least 6" in p.a. 250 degrees; the comet's head was clearly larger than images of nearby stars (4".2 vs. 2".8 FWHM). N. Erasmus, on behalf of the ATLAS team, subsequently reported that four stacked 30-s images obtained on 2021 Aug. 18.56-18.58 UT with a 0.5-m f/2 Schmidt ATLAS reflector at Mauna Loa, Hawaii, show a diffuse coma extending 10" toward p.a. 320 degrees (with the coma have a size of 9".6 FWHM in 4".9 seeing). R. Weryk, Department of Physics and Astronomy, University of Western Ontario, reported that images obtained with the Pan-STARRS1 1.8-m Ritchey- Chretien reflector at Haleakala on 2021 Aug. 25.55-25.59 UT show that 2021 PE_20 is clearly a comet; four 45-s i-band survey images show a condensed head of size 2".1 (FWHM) in 1".0 seeing, and a broad 6" tail spanning p.a. 215-280 degrees. Weryk added that four 45-s w-band survey images taken on 2021 Sept. 28.52-28.55 in poor seeing show a diffuse coma of size 3".4 (FWHM) in 2".0 seeing, witn no tail. And four 45-s w-band survey images taken with the Pan-STARRS1 reflector on 2021 Oct. 31.5 show a diffuse coma of size 2".4 (FWHM) in 1".0 seeing, with an east-west asymmetry that suggests the presence of a short tail. M. Micheli and B. Gray measured images taken with the Mt. Lemmon 1.5-m reflector by H. Groeller on 2021 Sept. 13.45-13.46 that show a coma of diameter about 10" (size 3".9 FWHM in 2".4 seeing) and mag 19.1-19.7 with a tail about 30" long in p.a. about 260 degrees. S. Deen (Simi Valley, CA, USA) reports on his analyses of CCD images taken with the 1.2-m Schmidt telescope at Palomar (as part of the "Zwicky Transient Facility" survey). On 2021 Aug. 9.47 UT, a pre-discovery image showed a vague appearance of a stubby tail around p.a. 240 degrees (with the anti-solar direction being p.a. 254 degrees); the condensed head showed a size of about 2" (FWHM) in 1" seeing. Another pre-discovery image taken on 2021 Aug. 12.46 shows clear cometary activity with a slightly condensed coma of size about 3" (FWHM) compared to 2" (FWHM) for surrounding stars, with no tail visible. On 2021 Aug. 14.44, there was a moderately condensed coma of size 2".5 (FWHM) in 1".5 seeing, again with a vague appearance of a stubby tail around p.a. 260 degrees (anti-solar directino p.a. 256 deg). An image taken on 2021 Aug. 17.44 shows a slightly diffuse coma of size about 4" (FWHM) in 2" seeing. Two images from 2021 Aug. 27.5 show clearly cometary features, with a very condensed coma of size about 2".5 (FWHM) in 1" seeing and a clear 10"-long broad tail at spanning p.a. 260-290 deg (anti-solar direction 261 deg). Two images on 2021 Aug. 30.45-30.48 again clearly show a very condensed coma of size about 3" (FWHM) in 1".5 seeing, with a clear 15"-long broad tail spanning p.a. 240-290 degrees (anti-solar directioon p.a. 262 deg). G. Blasco Gil, Guadalajara, Jalisco, Mexico, reports on his analyses of images obtained with the DECam camera on the Cerro Tololo 4-m reflector in data that was scheduled for public release this past month. On 2021 Sept. 23.29 UT, the comet showed an ellipsoidal shape of size 5" with a pronounced central condensation and a broad tail about 25" long spanning p.a. 255-265 degrees; cometary activity was also seen on Sept. 20.30 and 21.32 with slightly smaller coma size and shorter tail. Following a discussion with M. Micheli, there is agreement that there was a brightness increase between Aug. 13 and Sept. 27 that is compatible with cometary activity. DECam images from 2022 Dec. 19 and 24 show nothing at the predicted position to limiting mag 23. Blasco also looked at the ZTF public-domain images from 2021 Aug. 30 (see Deen's report, above), measuring a coma diameter of 5" and a 6" tail in p.a. 240 deg. Blasco forwards a report from Micheli and R. Kresken, who observed a condensed 8" coma and a 15" tail in p.a. 250 degrees on images obtained with a 1.0-m telescope at Tenerife on 2021 Sept. 7.20. Micheli also searched the Mt. Lemmon Survey archive (1.5-m reflector) and noticed the comet to show a clear tail on 2021 Sept. 13.5; images from 2021 Nov. 12.24-12.26 show only a faint, apparently stellar appearance. Blasco also measured twelve stacked 60-s w-filtered images obtained on 2021 Aug. 13.8 by T. Santana-Ros with a Las Campanas Observatory 1-m reflector at Siding Spring, which show a 6" coma and a 6" tail in p.a. 240 deg. The available astrometry appears on MPEC 2023-C66 (which include pre-discovery Palomar observations from 2021 Aug. 9 and 12 that were reported after the object was announced). The following elliptical orbital elements by S. Nakano (Central Bureau) are from 101 observations spanning 2021 Aug. 9- Nov. 12 (mean residual 0".5), which indicate that the comet will pass 0.16 AU from Jupiter on 2037 Sept. 4 UT; it passed 0.19 AU from Jupiter in 1942 Nov. and 3.64 AU from Saturn in 1919 Feb. Nakano was unable to identify previous observations of the comet in archival astrometry files. Epoch = 2006 Dec. 11.0 TT T = 2006 Dec. 1.58604 TT Peri. = 210.38686 e = 0.6709812 Node = 99.68877 2000.0 q = 1.2315719 AU Incl. = 20.04305 a = 3.7431658 AU n = 0.13609604 P = 7.24 years Epoch = 2014 Mar. 4.0 TT T = 2014 Mar. 5.73588 TT Peri. = 210.46656 e = 0.6697544 Node = 99.59292 2000.0 q = 1.2393262 AU Incl. = 20.00087 a = 3.7527410 AU n = 0.13557549 P = 7.27 years Epoch = 2021 May 26.0 TT T = 2021 June 7.18824 TT Peri. = 210.45229 e = 0.6707880 Node = 99.63068 2000.0 q = 1.2334233 AU Incl. = 20.01846 a = 3.7465933 AU n = 0.13590932 P = 7.25 years Epoch = 2028 Aug. 17.0 TT T = 2028 Sept. 1.86341 TT Peri. = 210.93534 e = 0.6726777 Node = 99.31678 2000.0 q = 1.2226477 AU Incl. = 19.95288 a = 3.7353027 AU n = 0.13652600 P = 7.22 years Epoch = 2035 Nov. 9.0 TT T = 2035 Nov. 21.62134 TT Peri. = 211.02040 e = 0.6735465 Node = 99.24001 2000.0 q = 1.2177488 AU Incl. = 19.95745 a = 3.7302362 AU n = 0.13680425 P = 7.20 years The following ephemeris by the undersigned from the above orbital elements uses photometric power-law parameters H = 16.0 and 2.5n = 10 for the magnitudes, given here for the discovery apparition in hopes that other pre-discovery images of the comet might be identified. Date TT R. A. (2000) Decl. Delta r Elong. Phase Mag. 2021 01 26 16 43.96 -08 18.7 2.343 1.978 56.5 24.5 20.8 2021 02 15 17 33.56 -10 03.9 2.026 1.819 63.7 29.1 20.1 2021 03 07 18 29.35 -11 20.9 1.729 1.664 69.5 33.9 19.4 2021 03 27 19 33.05 -12 09.0 1.466 1.520 73.4 39.0 18.7 2021 04 16 20 46.26 -12 25.8 1.254 1.395 75.4 44.1 17.9 2021 05 06 22 08.72 -12 05.8 1.107 1.298 75.5 48.8 17.4 2021 05 26 23 35.66 -11 06.4 1.033 1.243 74.8 51.9 17.0 2021 06 15 00 58.28 -09 44.2 1.021 1.237 74.8 52.4 17.0 2021 07 05 02 08.94 -08 34.7 1.046 1.282 76.9 50.5 17.2 2021 07 25 03 04.39 -08 08.8 1.078 1.371 81.8 47.2 17.5 2021 08 14 03 43.90 -08 38.9 1.097 1.491 89.8 42.8 17.9 2021 09 03 04 06.72 -09 59.4 1.099 1.632 101.4 37.3 18.3 2021 09 23 04 11.53 -11 44.0 1.095 1.784 116.4 30.2 18.7 2021 10 13 03 58.40 -12 58.5 1.113 1.943 133.9 21.7 19.1 2021 11 02 03 33.19 -12 33.8 1.193 2.103 148.4 14.3 19.6 2021 11 22 03 07.94 -10 05.0 1.364 2.263 148.2 13.3 20.2 2021 12 12 02 52.46 -06 16.0 1.629 2.422 134.3 16.9 20.9 NOTE: These 'Central Bureau Electronic Telegrams' are sometimes superseded by text appearing later in the printed IAU Circulars. (C) Copyright 2023 CBAT 2023 February 10 (CBET 5220) Daniel W. E. Green