Abstract The near-Earth asteroid 2024 YR4 was reported to have a non-negligible Earth-impact probability in 2032 December. Although the impact probability has since been reduced to nearly zero, this study investigates a hypothetical planetary defense scenario in which the Earth collision risk remained high. We conducted a rapid-response feasibility study in which the in-space Hayabusa2 spacecraft was retargeted from its originally planned rendezvous target, asteroid 1998 KY26, to 2024 YR4 in order to induce orbit deflection by a kinetic impact prior to a potential Earth impact. The analyses show that a Hayabusa2 impact could impart a significant velocity change of a few cm s −1 to 2024 YR4 and, under certain conditions, could effectively reduce the impact probability. Based on this hypothetical case study and the estimated temporal evolution of orbit-determination accuracy, we further propose a decision-making framework that integrates orbit determination updates, impact probability assessment, kinetic-impact feasibility, and hazard evaluation. This framework highlights the importance of iterative assessments as observational knowledge improves and provides insight into decision processes for future planetary defense scenarios.
NAKAGAWA et al. (2026) studied this question.