Electronic Telegram No. 5681 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) Z. Sekanina, La Canada Flintridge, CA, USA, reports results of his preliminary modeling of a cloud of particulate debris of comet C/2026 A1, as seen in images of coronagraphs onboard space observatories starting a few hours after the comet's predicted perihelion time (Apr. 4.60 UT; cf. CBET 5675). In the Solar and Heliospheric Observatory's LASCO C3 coronagraph, the feature showed up for the first time in an image taken on Apr. 4 at 20h42m UTC, expanding rapidly in the direction radially away from the sun at a position angle near 280 deg and acquiring a geyser-like appearance. The time of its first appearance in the LASCO C2 coronagraph was less well defined, between 16h24m and 16h48m UTC. Analysis was difficult because of nearly edge-on view, as the earth had crossed the comet's orbital plane only about a week earlier. The most obvious quantity to measure was the solar elongation of the outer tip of the expanding geyser feature as a function of time. In images, taken with the CCOR-1 coronagraph onboard the Geostationary Operational Environmental Satellite-19, this elongation was crudely measured to equal 1.5 degrees on Apr. 4.88 UTC, 0.28 day after perihelion; 1.9 deg on Apr. 4.98; 2.8 deg on Apr. 5.28; and 4.2 deg on Apr. 5.71, 1.11 day after perihelion. The motions of dust particles relative to the comet's nucleus from which they were released are essentially determined by the solar-radiation pressure accelerations (b, in units of the sun's gravitational acceleration) to which the particles have been subjected and by the times of their release (t, measured in days from the comet's perihelion time). If the comet disintegrates between the times of release and observation, the particle motions refer to the predicted positions of the defunct nucleus. Given the peculiar geometry, model computations showed that the measured tip of the debris feature was populated by dust grains subjected to a peak radiation-pressure acceleration and released at the end of an event, regardless of its duration. Over the range of tip-measurement times, the position of the debris feature was relative to the sun almost exactly 180 deg from the predicted position of the defunct comet, so that the measured solar elongation of the cloud's tip essentially equaled its angular distance from the comet's predicted position minus the comet's predicted solar elongation. The measured solar elongations of the cloud's tip were found to be consistent with b = 0.67 and t = -0.09 day (or at Apr. 4.51 UTC). Shortly before this time, the comet's head began to hide behind the occulting disk of the C2 coronagraph. Some of the images of the cloud did tend to show that it may have consisted of streamers, suggesting that the disintegration event may have been made up of a number of isolated fragmentation episodes, extending over a period of at least several hours. An apparent trait of the debris cloud was the isolation of its brightest part (aided by forward-scattering effect), possibly indicating that most mass was released at the very end, 0.09 day before perihelion, and that --- given the value of b --- the cloud's cross- sectional area was dominated by submicron-sized grains of silicate composition (presumably olivine and pyroxene). Such grains moved in concave hyperbolic orbits with perihelion distances greater than the comet's predicted perihelion distance, thus avoiding harsh effects of sublimation. NOTE: These 'Central Bureau Electronic Telegrams' are sometimes superseded by text appearing later in the printed IAU Circulars. (C) Copyright 2026 CBAT 2026 April 6 (CBET 5681) Daniel W. E. Green