Electronic Telegram No. 5698 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 220P/McNAUGHT This comet appears to have undergone a major outburst of up to 8-10 magnitudes within a week. Astrometry submitted to the Minor Planet Center (cf. MPEC 2026-K26) gave (presumed total) magnitudes near 17.5 in May (as late as May 28.2 UT from CCD imaging with a 0.5-m f/3.6 reflector at La Palma by K. Hills). T. Prystavski (Lviv, Ukraine) measured total mag about 18.1 from CCD imaging obtained on 2026 Apr. 27.41 UT with a 51-cm f/7 reflector at Rio Hurtado, Chile. M. Kelley posted to the comets-ml online forum that a rise of about 6 magnitudes is seen in imaging obtained at Palomar through an r-band filter between May 26.47 and 31.47, with other imaging obtained with 0.5-m f/2 Schmidt reflectors in the course of the "Asteroid Terrestrial-Impact Last Alert System" (ATLAS) search program showing that the outburst occurred between May 30.59 and 31.15. K. Yoshimoto, Yamaguchi, Japan, reports that CCD imaging obtained remotely with a 0.11-m f/5.0 astrograph located at the Utah Desert Remote Observatory (near Beryl Junction, UT, USA) on June 3.46 UT show a condensed bright coma of total mag 8.3 and diameter 6'.5 with a 2' tail in p.a. 240 degrees. On the "ICQ Comet Observations" Facebook forum, numerous photos of comet 220P in outburst have been posted this week despite bright moonlight in the morning sky. Stacked CMOS exposures taken on June 2.11 by G. Rhemann and M. Jaeger using a 30-cm f/3.6 reflector at the Tivoli Farm in Namibia show a long, faint-but-obvious ion tail apparently exceeding 2 degrees in length. Visual total-magnitude and coma-diameter estimates, all in bright moonlight: June 3.32 UT, 8.5, 3' (J. G. de S. Aguiar, Campinas, Brazil, 0.27-m reflector); 3.33 8.3, 3' (M. Goiato, Aracatuba, Brazil, 20x100 binoculars; moonlight); 4.10, 9.1, -- (S. J. O'Meara, Maun, Botswana, 7.6-cm refractor; star close to a star of mag 8.9, making estimate difficult; bright moonlight); 4.32, 8.4, 3' (Aguiar), S. Nakano (Central Bureau) provides the following linked orbital elements from 505 observations spanning 2004 May 20-2026 May 28 (mean residual 0".6), with associated non-gravitational parameters A1 = +0.01 +/- 0.01 and A2 = -0.0008 +/- 0.0000. The comet will pass 0.96 AU from Jupiter on 2051 Feb. 28 UT. Epoch = 2004 June 4.0 TT T = 2004 June 16.81388 TT Peri. = 180.77280 e = 0.5012703 Node = 150.12363 2000.0 q = 1.5544464 AU Incl. = 8.12284 a = 3.1168114 AU n = 0.17911761 P = 5.50 years Epoch = 2010 Jan. 4.0 TT T = 2009 Dec. 15.42859 TT Peri. = 180.75725 e = 0.5026822 Node = 150.11772 2000.0 q = 1.5486195 AU Incl. = 8.13265 a = 3.1139433 AU n = 0.17936513 P = 5.50 years Epoch = 2015 June 27.0 TT T = 2015 June 13.75174 TT Peri. = 180.66953 e = 0.5014312 Node = 150.11318 2000.0 q = 1.5539767 AU Incl. = 8.12521 a = 3.1168748 AU n = 0.17911214 P = 5.50 years Epoch = 2020 Dec. 17.0 TT T = 2020 Dec. 10.62758 TT Peri. = 180.53117 e = 0.5018315 Node = 150.10209 2000.0 q = 1.5523509 AU Incl. = 8.12870 a = 3.1161165 AU n = 0.17917753 P = 5.50 years Epoch = 2026 June 9.0 TT T = 2026 June 14.12025 TT Peri. = 180.56390 e = 0.5003473 Node = 150.08382 2000.0 q = 1.5593034 AU Incl. = 8.12078 a = 3.1207746 AU n = 0.17877651 P = 5.51 years Epoch = 2031 Nov. 30.0 TT T = 2031 Dec. 18.28971 TT Peri. = 180.44028 e = 0.4983380 Node = 150.02453 2000.0 q = 1.5678595 AU Incl. = 8.11144 a = 3.1253304 AU n = 0.17838576 P = 5.52 years Epoch = 2037 July 1.0 TT T = 2037 June 30.77342 TT Peri. = 180.55783 e = 0.4975173 Node = 149.98343 2000.0 q = 1.5721211 AU Incl. = 8.10893 a = 3.1287070 AU n = 0.17809706 P = 5.53 years Epoch = 2043 Jan. 31.0 TT T = 2043 Jan. 23.01457 TT Peri. = 180.83245 e = 0.4896119 Node = 149.75275 2000.0 q = 1.6063466 AU Incl. = 8.07309 a = 3.1473045 AU n = 0.17652082 P = 5.58 years Epoch = 2048 Sept. 1.0 TT T = 2048 Aug. 26.70381 TT Peri. = 180.97747 e = 0.4892230 Node = 149.70967 2000.0 q = 1.6082570 AU Incl. = 8.07233 a = 3.1486480 AU n = 0.17640785 P = 5.59 years The following ephemeris by the undersigned from the above orbital elements uses photometric power-law parameters H = 5.5 and 2.5n = 10 for the magnitudes, based on the current outburst. Note that this is about 9.5 magnitudes brighter in H than was the case for previous apparitions of comet 220P, and the outburst occurring now near perihelion should be expected to fade rapidly in the coming weeks and months accordingly. Date TT R. A. (2000) Decl. Delta r Elong. Phase Mag. 2026 06 09 00 20.50 +02 37.4 1.537 1.560 72.1 38.2 8.4 2026 06 19 00 47.11 +04 28.5 1.479 1.560 74.8 38.9 8.3 2026 06 29 01 12.94 +06 05.7 1.425 1.566 77.8 39.4 8.2 2026 07 09 01 37.76 +07 26.5 1.374 1.578 81.1 39.5 8.2 2026 07 19 02 01.23 +08 28.4 1.325 1.596 84.8 39.4 8.1 2026 07 29 02 22.98 +09 10.0 1.277 1.619 89.1 38.8 8.1 2026 08 08 02 42.62 +09 30.7 1.231 1.647 93.9 37.9 8.1 2026 08 18 02 59.67 +09 30.3 1.185 1.679 99.4 36.5 8.1 2026 08 28 03 13.68 +09 09.6 1.141 1.716 105.7 34.5 8.1 2026 09 07 03 24.19 +08 30.0 1.100 1.757 112.8 31.9 8.2 2026 09 17 03 30.75 +07 33.9 1.064 1.800 120.9 28.6 8.2 2026 09 27 03 33.12 +06 25.2 1.036 1.847 129.9 24.6 8.2 2026 10 07 03 31.34 +05 09.6 1.020 1.895 139.6 20.0 8.3 2026 10 17 03 25.84 +03 54.4 1.019 1.946 149.8 14.9 8.4 2026 10 27 03 17.66 +02 48.9 1.037 1.998 159.3 10.1 8.6 2026 11 06 03 08.23 +02 01.1 1.078 2.052 164.9 7.2 8.8 2026 11 16 02 59.11 +01 36.5 1.143 2.106 162.0 8.3 9.0 2026 11 26 02 51.65 +01 37.1 1.232 2.161 153.7 11.7 9.3 2026 12 06 02 46.67 +02 00.6 1.342 2.217 144.1 15.1 9.6 2026 12 16 02 44.52 +02 43.1 1.472 2.273 134.5 18.0 9.9 2026 12 26 02 45.18 +03 40.0 1.618 2.329 125.3 20.1 10.2 NOTE: These 'Central Bureau Electronic Telegrams' are sometimes superseded by text appearing later in the printed IAU Circulars. (C) Copyright 2026 CBAT 2026 June 4 (CBET 5698) Daniel W. E. Green