Electronic Telegram No. 5043 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 HS (PANSTARRS) An apparently asteroidal object discovered on Apr. 16 on images taken with the Pan-STARRS1 1.8-m Ritchey-Chretien reflector at Haleakala, Hawaii, has been found to show cometary activity by observers elsewhere when the comet was close to perihelion. Pre-discovery images taken with the Pan-STARRS2 1.8-m Ritchey-Chretien reflector at Haleakala on Apr. 10 were also identified. The first two nights of Pan-STARRS astrometry are tabulated below. The object was given the minor-planet designation 2021 HS when announced by the Minor Planet Center last April on MPEC 2021-H60. 2021 UT R.A. (2000) Decl. Mag. Apr. 10.46451 12 43 13.80 -28 36 26.3 20.9 10.48125 12 43 11.99 -28 36 20.7 20.9 16.36723 12 32 37.79 -27 54 33.3 20.6 16.37933 12 32 36.38 -27 54 27.0 20.8 16.39140 12 32 34.98 -27 54 20.6 20.7 16.40347 12 32 33.58 -27 54 14.3 20.9 Following a request for information by the Central Bureau, R. Weryk (Department of Physics and Astronomy, University of Western Ontario) notes that the four 45-s w-band discovery images obtained on Apr. 16.4 UT do not show obvious cometary appearance (the head had size 1".3 full-width-at-half- maximum in 1".1-1".2 seeing); likewise, four 45-s w-band Pan-STARRS1 survey taken on May 14.3 show the size of the head as 1".4 (FWHM) in 1".2-1".3 seeing. Pan-STARRS2 w-band images from Apr. 10.5 and 18.3 in poor seeing also are of insufficient quality to show any activity; i-band Pan-STARRS2 images from Apr. 22 and Pan-STARRS1 images from July 18 and 20 also show the comet near the limit of detection. Seven stacked 60-s CCD exposures taken remotely on July 4.40 UT by H. Sato, Tokyo, Japan, with a 0.51-m f/6.8 astrograph located at Siding Spring, NSW, Australia, show a strongly condensed coma 8" in diameter with no tail; the magnitude was 19.0 as measured within a circular aperture of radius 4".9. Ten stacked 60-s follow-up exposures by Sato on July 6.39 show a strongly condensed, asymmetrical coma 12" in diameter with no tail; the magnitude was 18.8 as measured within a circular aperture of radius 8".2. In response to a notification by Sato, L. Buzzi and L. Demetz obtained thirty stacked 60-s CCD exposures using a 0.51-m f/6.7 astrograph of the Skygems Telescope Network in Namibia on each of two nights in July (measured by A. Aletti, Buzzi, and G. Galli) that show cometary activity: on July 9.76-9.78, there was a 6" coma of mag 19.5 with a possible very faint, broad, fan-shaped extension 45" long in p.a. 220-280 degrees, while on July 10.74-10.80, there was a 8" coma surrounded by a faint outer coma about 15" wide with a possible faint extension in p.a. 200-250 degrees. M. S. P. Kelley, University of Maryland, writes that his review of "Zwicky Transient Facility" survey images of 2021 HS that were obtained with the 1.2-m Schmidt telescope at Palomar reveal cometary activity when the comet was near perihelion on Aug. 1, 2, and 4 UT, when nightly stacks reveal a condensation of sizes of 7".7, 4".9, and 6".7 (FWHM) in seeing of 1".9, 2".6, and 2".1, respectively; the diffuse coma yielded diameter estimates of 24", 15", and 18", respectively. The r-band magnitudes were 18.3, 18.6, and 18.5 in an aperture of 5" radius on the three nights. In ZTF images taken on Sept. 10, 15, and 16, the comet had nearly returned to being a point source with size 2".6, 2".8, and 3".2 (FWHM) in 1".8, 2".0, and 1".9 seeing, respectively, with r-band magnitudes 19.7, 20.0, and 19.7. Additional observations appear on MPEC 2021-S44. The following orbital elements by S. Nakano (CBAT) are from 137 observations spanning 2021 Apr. 10- Sept. 12 (mean residual 0".4), indicating that the comet passed 0.26 AU from the earth on 2021 Aug. 13 UT also also (with noted uncertainty) that it would have passed 0.35 AU from Jupiter in 1954 Aug. A search for additional observations in archival astrometry produced nothing. Epoch = 2021 Aug. 14.0 TT T = 2021 Aug. 6.38953 TT Peri. = 46.08571 e = 0.8102182 Node = 262.39432 2000.0 q = 0.7964465 AU Incl. = 12.17112 a = 4.1966426 AU n = 0.11464394 P = 8.60 years Epoch = 2030 Feb. 28.0 TT T = 2030 Mar. 1.26090 TT Peri. = 47.14396 e = 0.8086967 Node = 260.88479 2000.0 q = 0.8058510 AU Incl. = 11.98649 a = 4.2124268 AU n = 0.11400018 P = 8.65 years The following ephemeris by the undersigned from the above orbital elements uses photometric power-law parameters H = 22.0 and 2.5n = 6 for the magnitudes. This comet has a very shallow light curve (n << 2), suggesting that something strange is happening to the nucleus. Date TT R. A. (2000) Decl. Delta r Elong. Phase Mag. 2021 09 03 07 08.65 +59 15.7 0.328 0.925 66.0 95.1 19.4 2021 09 08 06 33.40 +58 28.8 0.350 0.970 73.7 86.0 19.6 2021 09 13 06 04.94 +57 17.4 0.372 1.018 81.2 77.7 19.9 2021 09 18 05 41.01 +55 54.4 0.392 1.069 88.5 70.0 20.1 2021 09 23 05 19.82 +54 24.5 0.412 1.122 95.7 62.8 20.4 2021 09 28 05 00.17 +52 48.1 0.431 1.177 103.1 56.0 20.6 2021 10 03 04 41.38 +51 03.7 0.451 1.234 110.6 49.4 20.8 2021 10 08 04 23.20 +49 09.2 0.471 1.291 118.3 43.0 21.0 2021 10 13 04 05.75 +47 03.8 0.493 1.348 126.0 36.8 21.2 2021 10 18 03 49.29 +44 48.3 0.519 1.406 133.8 30.7 21.5 2021 10 23 03 34.13 +42 25.0 0.548 1.464 141.5 25.0 21.7 2021 10 28 03 20.51 +39 57.6 0.583 1.522 148.8 19.8 21.9 2021 11 02 03 08.59 +37 30.2 0.623 1.580 155.5 15.1 22.2 NOTE: These 'Central Bureau Electronic Telegrams' are sometimes superseded by text appearing later in the printed IAU Circulars. (C) Copyright 2021 CBAT 2021 September 20 (CBET 5043) Daniel W. E. Green