Electronic Telegram No. 5675 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/2026 A1 (MAPS) Further to CBET 5663, Z. Sekanina (La Canada Flintridge, CA, USA) forwards the following orbital elements computed by M. Krolikowska (Centrum Badan Kosmicznych Polskiej Akademii Nauk, Warsaw) from 916 observations spanning 2025 Dec. 18-2026 Mar. 17 (mean error 0".5). She has also derived a set of elements with non-gravitational parameters with a slightly lower mean residual that yields "original" 1/a = +0.007024 AU^(-1) tentatively. Epoch = 2025 Nov. 21.0 TT T = 2026 Apr. 4.60065 TT Peri. = 86.23013 e = 0.9999634 Node = 7.75709 2000.0 q = 0.0057421 AU Incl. = 144.49501 Sekanina adds that the meaningful orbital period is the original barycentric period, which in case of C/2026 A1 determines the previous perihelion time to within a few years, and given the sub-kilometer size of the comet's nucleus (about 0.4 km; cf. Zhang et al. 2026, Research Notes AAS 10, 57), the sublimation-driven effects on the nucleus are likely to be significant. A gravitational orbit can still appear adequate, but inclusion of non- gravitational parameter(s) may improve the fit to the observed positions, decreasing the mean residual. Most importantly, it may erase the inexplicable disparity between the orbital periods derived from different orbital arcs. The small size of the nucleus of C/2026 A1 works against splitting into two fragments of comparable dimensions, so the issue is cohesion of the material, which is unknown. Large nuclei (elongated in particular) are more likely to split, because the tidal force is larger, while smaller nuclei are more likely to disintegrate right away rather than split. As for tidal force vs/ heat-wave penetration effects, comparison of close-approaching comets to the sun and Jupiter suggests that tides are more significant. The long period of C/2026 A1 is important because it shows the incredibly rapid mixing power of perihelion fragmentation; any potential relation between comets C/2026 A1 and other Kreutz sungrazers observed in the past will have to be revisited and may lead to revision of the evolutionary paths because of unaccounted nongravitational effects. The following updated orbital elements by S. Nakano (Central Bureau) are from 898 observations spanning 2025 Dec. 18-2026 Mar. 20 (mean residual 0".6), with corresponding "original" and "future" values of 1/a being +0.006911 and +0.006841 (+/- 0.000018) AU**-1, respectively. The "original" barycentric value of 1/a corresponds to P = 1740.6 years (or a previous perihelion in the second half of 285 AD). Nakano suggests that any non-gravitational effects from the available arc of observations are very small at this time. Epoch = 2026 Mar. 21.0 TT T = 2026 Apr. 4.60046 TT Peri. = 86.33409 e = 0.9999633 Node = 7.87967 2000.0 q = 0.0057343 AU Incl. = 144.49256 Many photos of comet C/2026 A1 have been posted on the "ICQ Comet Observations" Facebook forum, showing the comet's development over the past month or two as it now is reaching small solar elongations that will make observations increasingly difficult. G. Rhemann and M. Jaeger have been keeping a regular image monitoring of this comet using a 0.30-m f/3.6 ASA astrograph located at the Tivoli Farm observatory in Namibia, with the comet showing a strong condensation and a thin gas tail and Jaeger's derived total wide-band optical magnitudes, coma diameters, and tail lengths are as follows: Mar. 6.80 UT, 11.9, 6', 15' in p.a. 94 degrees; 7.80, 11.3, 8', 15' in p.a. 89 deg; 9.80, 10.5, 9', 15' in p.a. 93 deg; 14.77, 10.1, 6'.5, 25' in p.a. 96 deg; 22.76, 9.5, 4'.5, > 75' in p.a. 98 deg; 23.76, 9.3, 4', > 50' in p.a. 101 deg. T. Lovejoy reports that four 180-s green-filtered images obtained remotely on Mar. 14.40 UT with an iTelescope 0.51-m f/6.8 "Corrected Dall- Kirkham" Astrograph at Siding Spring (NSW, Australia) show total mag about 10.2 with a coma diameter of 6'; the ion tail was 10' or longer in p.a. 95 degrees. Selected CCD total magnitudes and coma diameters: Feb. 27.51 UT, 12.3, 3'.2 (A. Pearce, Perth Observatory, Bickley, 0.35-m Schmidt-Cassegrain reflector); 28.44, 12.5, 2'.0 (Y. Nagai, Saitama, Japan, 0.05-m refractor); Mar. 1.52, 12.1, 3.5' (Pearce); 2.51, 11.9, 3' (Pearce); 4.42, 12.2, 1'.3 (Nagai); 5.51, 11.3, 2'.3 (Pearce); 6.51, 11.3, 2'.8 (Pearce); 10.48, 11.6, 3' (Pearce, Nedlands, Perth, W. Australia, 0.05-m refractor); 11.48, 11.1, 3'.2 (Pearce); 12.48, 11.1, 3'.3 (Pearce); 13.77, 10.9, 4' (Pearce, Skygems Namibia Remote Observatory, 0.1-m refractor; 8' tail in p.a. 96 deg); 14.48, 11.1, 3'.7 (Pearce, Nedlands, 0.05-m refractor). Selected visual total-magnitude and coma-diameter estimates for P/2026 A1: Mar. 1.92, 11.6, 1' (J. G. de S. Aguiar, Campinas, Brazil, 0.30-m reflector; moonlight); 4.92, 11.1, 1' (Aguiar); 6.09, 11.9, 1'.2 (A. Hale, Cloudcroft, NM, USA, 0.41-m reflector); 9.40, 11.8, 2'.8 (C. Stoitsis, Tarnagulla, Vic., Australia, 25x100 binoculars); 10.79, 10.9, -- (S. J. O'Meara, Maun, Botswana, 0.08-m refractor); 11.74, 10.8, -- (O'Meara); 12.84, 9.8, 6' (J. J. Gonzalez, Leon, Spain, 25x100 binoculars); 13.76, 10.4, -- (O'Meara); 14.92, 10.4, 2' (Aguiar); 15.92, 10.2, 2' (Aguiar); 16.83, 9.2, 7' (Gonzalez, Alto del Castro, Aralla, Leon, Spain, 0.10-m reflector); 17.74, 9.6, -- (O'Meara); 19.83, 8.3, 8' (Gonzalez, Alto de la Camperona, Leon, Spain, 0.10-m reflector; altitude 5 degrees); 22.43, 9.6, -- (M. Mattiazzo, Swan Hill, Vic., Australia, 25x100 binoculars); 23.39, 9.7, 2'.4 (C. Wyatt, Walcha, NSW, Australia, 0.25-m reflector; moonlight; 4'.5 tail in p.a. approximately 114 deg); 23.76, 8.6, -- (O'Meara); 24.91, 8.9, 2' (Aguiar); 25.38, 8.5, 1'.5 (Wyatt, 15x70 binoculars; moonlight); 25.39, 8.7, 1'.5 (Wyatt, 0.25-m reflector; altitude 4-5 degrees; tail 7'.0 in p.a. approximately 115 deg). V-band CMOS-camera total magnitudes and coma diameters for C/2026 A1 from K. Yoshimoto, Yamaguchi, Japan, using a 0.077-m f/4.3 refractor (* = corrected for atmospheric extinction): Mar. 10.44 UT, 10.8, 4'.0; 13.45, 10.7, 3'.8 (tail 10' long in p.a. 98 deg); 14.44, 10.4*, 3'.3 (tail 7' long in p.a. 104 deg); 17.44, 10.7*, 3'.0 (tail 10' long in p.a. 101 deg); 21.44, 10.6*, 1'.9 (tail 5' long in p.a. 101 deg). He notes that the color index for the entire coma on Mar. 14 was B-V = +0.9. The following ephemeris by Nakano from his orbital elements above uses photometric power-law parameters H = 11.0 and 2.5n = 15, following CBET 5663 (as observed total magnitudes have been following that ephemeris for the past month). As noted also on CBET 5658, caution must be used when discussing these ephemeris magnitudes, especially given the likelihood of this comet's nucleus being disrupted as it approaches perihelion. The ephemeris is extended beyond perihelion in the event that there is some survival of the comet (or material from the comet). Date TT R. A. (2000) Decl. Delta r Elong. Phase Mag. 2026 03 01 02 51.42 -17 11.1 1.270 1.162 60.3 47.8 12.5 2026 03 06 02 41.34 -14 38.0 1.259 1.047 53.8 49.9 11.8 2026 03 11 02 31.44 -12 00.1 1.245 0.925 47.2 52.0 11.0 2026 03 16 02 21.20 -09 15.9 1.224 0.794 40.3 54.1 9.9 2026 03 21 02 09.79 -06 21.8 1.197 0.651 33.0 56.3 8.6 2026 03 22 02 07.27 -05 45.3 1.190 0.621 31.4 56.8 8.3 2026 03 23 02 04.62 -05 08.2 1.183 0.590 29.9 57.3 7.9 2026 03 24 02 01.84 -04 30.2 1.176 0.558 28.3 57.8 7.6 2026 03 25 01 58.90 -03 51.3 1.168 0.525 26.6 58.2 7.1 2026 03 26 01 55.77 -03 11.4 1.160 0.491 24.9 58.7 6.7 2026 03 27 01 52.42 -02 30.3 1.151 0.456 23.1 59.2 6.2 2026 03 28 01 48.80 -01 47.8 1.142 0.420 21.3 59.7 5.6 2026 03 29 01 44.85 -01 03.7 1.132 0.382 19.4 60.2 5.0 2026 03 30 01 40.50 -00 17.6 1.121 0.342 17.3 60.6 4.2 2026 03 31 01 35.63 +00 30.9 1.109 0.299 15.2 60.9 3.4 2026 04 01 01 30.07 +01 22.4 1.097 0.253 12.8 61.1 2.2 2026 04 02 01 23.53 +02 18.3 1.082 0.203 10.2 60.8 0.8 2026 04 03 01 15.43 +03 20.6 1.066 0.145 7.2 59.5 -1.4 2026 04 04 01 04.12 +04 36.0 1.042 0.073 3.4 53.1 -5.9 2026 04 05 01 04.94 +05 24.9 0.969 0.054 2.6 123.4 -3.2 2026 04 06 01 27.34 +04 48.4 0.961 0.132 7.3 103.8 -2.3 2026 04 07 01 45.20 +04 23.3 0.965 0.191 11.0 95.3 0.1 2026 04 08 02 00.85 +04 03.4 0.975 0.243 14.0 89.1 1.7 2026 04 09 02 14.96 +03 46.7 0.989 0.289 16.7 84.1 2.9 2026 04 10 02 27.86 +03 32.6 1.005 0.333 19.1 79.9 3.8 2026 04 11 02 39.75 +03 20.2 1.024 0.373 21.2 76.1 4.6 2026 04 12 02 50.77 +03 09.5 1.044 0.412 23.1 72.7 5.3 2026 04 13 03 01.03 +03 00.0 1.066 0.449 24.8 69.6 5.9 2026 04 14 03 10.60 +02 51.5 1.089 0.484 26.3 66.8 6.5 2026 04 15 03 19.55 +02 44.0 1.114 0.518 27.7 64.2 6.9 2026 04 16 03 27.95 +02 37.3 1.139 0.551 28.9 61.8 7.4 2026 04 17 03 35.84 +02 31.2 1.164 0.583 30.1 59.5 7.8 2026 04 18 03 43.28 +02 25.8 1.191 0.615 31.1 57.4 8.2 2026 04 19 03 50.30 +02 20.8 1.218 0.645 31.9 55.5 8.6 2026 04 20 03 56.93 +02 16.3 1.245 0.675 32.7 53.6 8.9 2026 04 25 04 25.40 +01 58.6 1.386 0.815 35.6 45.9 10.4 2026 04 30 04 47.99 +01 45.8 1.530 0.944 36.9 39.9 11.5 2026 05 05 05 06.54 +01 35.2 1.674 1.065 37.3 35.1 12.5 2026 05 10 05 22.24 +01 25.3 1.816 1.179 37.1 31.1 13.4 2026 05 15 05 35.85 +01 15.2 1.955 1.288 36.3 27.7 14.1 2026 05 20 05 47.88 +01 04.3 2.090 1.393 35.3 24.8 14.8 2026 05 25 05 58.69 +00 52.4 2.221 1.494 34.0 22.3 15.3 2026 05 30 06 08.51 +00 39.1 2.348 1.591 32.6 20.1 15.9 2026 06 04 06 17.55 +00 24.5 2.470 1.686 31.2 18.2 16.4 2026 06 09 06 25.92 +00 08.4 2.586 1.778 29.7 16.5 16.8 2026 06 14 06 33.75 -00 09.2 2.698 1.867 28.3 15.0 17.2 2026 06 19 06 41.10 -00 28.4 2.804 1.955 27.1 13.7 17.6 2026 06 24 06 48.02 -00 49.2 2.905 2.041 26.0 12.6 18.0 2026 06 29 06 54.56 -01 11.5 3.000 2.125 25.2 11.7 18.3 2026 07 04 07 00.76 -01 35.3 3.090 2.207 24.6 11.1 18.6 2026 07 09 07 06.65 -02 00.6 3.174 2.288 24.5 10.6 18.9 2026 07 14 07 12.23 -02 27.5 3.252 2.367 24.7 10.3 19.2 2026 07 19 07 17.53 -02 55.9 3.325 2.445 25.3 10.2 19.4 NOTE: These 'Central Bureau Electronic Telegrams' are sometimes superseded by text appearing later in the printed IAU Circulars. (C) Copyright 2026 CBAT 2026 March 25 (CBET 5675) Daniel W. E. Green