Radiation Forces and Trajectory of Hayabusa2♯ Target 1998 KY ₂₆

Abstract Small near-Earth asteroid 1998 KY 26 has recently become an object of interest because it is the target of the Hayabusa2 ♯ mission and because challenges in fitting its astrometric data have led to classifying it as a dark comet. Data acquired during the favorable apparition in 2024 helped significantly improve the physical characterization of 1998 KY 26 but also led to increased orbit determination challenges. We develop a high-fidelity force model of solar radiation pressure and thermal emission of 1998 KY 26 that accounts for a nonspherical shape of the asteroid and results in an excellent orbital fit. Thanks to its close ties to the physical properties of the asteroid, the model removes the degeneracy in the orientation of the rotation pole stemming from lightcurve analysis and constrains 1998 KY 26 ’s bulk density to <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mn>2</mml:mn> <mml:mo>.</mml:mo> <mml:msubsup> <mml:mrow> <mml:mn>8</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>1.0</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>1.6</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> g cm −3 (90% confidence level), which is consistent with the asteroid’s taxonomic type. Our model does not require the contribution of undetected cometary activity of 1998 KY 26 , which alleviates safety concerns for the Hayabusha2 ♯ mission and has broader implications for the dark comet hypothesis.