Refined rotational state and shape model of (98943) Torifune ahead of the Hayabusa2# flybyOPEN ACCESS
P. Fatka, P. Pravec, P. Scheirich, P. Kušnirák, K. Hornoch, H. Kučáková, M. Hirabayashi
“The Hayabusa2# mission will perform a high-speed flyby of near-Earth asteroid (98943) Torifune on 5 July 2026, offering a rare opportunity to compare a pre-encounter spin and shape model derived from convex light-curve inversion with spacecraft imaging. We refined Torifune’s rotational state and shape model using previously published dense light curves, new photometry data obtained in late 2025, and selected sparse photometry data from the Asteroid Terrestrial-impact Last Alert System (ATLAS). A key methodological contribution is a per-measurement weighting scheme that accounts for heterogeneous data quality and cadence through light-curve scatter, rotational-phase-dependent brightness, and correlation-time downweighting of closely spaced measurements. We also estimated uncertainties with an apparition-constrained block bootstrap, resampling complete light curves while preserving the dense/sparse composition of the dataset and retaining coverage of each apparition. This procedure provides realistic uncertainties for the determined sidereal period, pole direction, and shape parameters. We refine the sidereal period to Psid = 5.0215221 +0.0000011 −0.0000007 h and confirm a prograde spin state with the axis near the north ecliptic pole. The nominal pole solution is (λ, β) = (314◦, +84◦), with a 5.4◦ angular uncertainty. The dynamically equivalent ellipsoid has a/b = 1.66 +0.03 −0.07, whereas its polar axis remains less well constrained, with b/c = 1.49 +0.44 −0.29.. All quoted uncertainties correspond to the two-sided 95% intervals of the block-bootstrap distributions. The forthcoming Hayabusa2# flyby should enable direct evaluation of which elements of this pre-encounter convex inversion model are robust, particularly the pole orientation and global silhouette, and may help constrain the remaining uncertainty in the polar dimension of the body.”
