The post-impact rotation state of Dimorphos due to the DART Impact

On 26 September 2022, the Double Asteroid Redirection Test (DART) spacecraft successfully impacted Dimorphos, the secondary component of the Didymos binary asteroid [1]. The impact reduced the mutual orbit period by 33±1(3σ) min, corresponding to an instantaneous reduction in Dimorphos’s along-track speed of 2.7±0.1(1σ) mm/s [2,3]. The change in Dimorphos’s velocity corresponds to a possible momentum enhancement factor, β, of 2.2–4.9, mainly depending on the uncertain mass of Dimorphos [3]. The DART impact significantly perturbed both the orbital and rotational state of Dimorphos. Here, we use numerical simulations that fully account for spin-orbit coupling to study Dimorphos’s excited post-impact rotation state resulting from its perturbed mutual orbit. Previous studies demonstrated that Dimorphos’s post-impact spin state is highly dependent on its own shape (i.e., moments of inertia) and 𝛽, both of which were unknown quantities at the time [4]. Based on Dimorphos’s subsequently observed shape [1], the post-impact orbit period [2], and the momentum and torque transferred to Dimorphos [3], we find a wide range of possible outcomes, including a libration amplitude exceeding 20º and a distinct possibility of entering an attitude instability characterized by chaotic non-principal-axis rotation. We consider prospects for constraining the rotation state using ground-based observations. We also examine the possibility that Dimorphos was already in an excited rotational state prior to the DART impact. We discuss the added complication that immediate impact-induced changes to Dimorphos’s moments of inertia may significantly influence the system’s dynamics [5, 6]. The implications of Dimorphos’s excited orbital and rotational state are discussed, including their effects on the system’s secular evolution [7], interior structure [8], and granular motion on Dimorphos’s surface [9], which will then be characterized in 2027 by the ESA Hera mission [10]. [1] Daly, R. T. et al., Nature, 2023 (DOI: 10.1038/s41586-023-05810-5) [2] Thomas, C. et al., Nature, 2023 (DOI: 10.1038/s41586-023-05805-2) [3] Cheng, A. F. et al., Nature, 2023 (DOI: 10.1038/s41586-023-05878-z) [4] Agrusa, H. F. et al., Icarus 370, 114624 (2021) [5] Raducan, S. et al., A&A 665, L10 (2022) [6] Nakano, R. et al., Planet. Sci. J. 3, 148 (2022) [7] Meyer et al., Icarus, 391, 115323 (2023) [8] Agrusa, H. F. et al., Planet. Sci. J. 3, 158 (2022) [9] Agrusa, H. F. et al., A&A (2022) [10] Michel, P. et al., Planet. Sc. J. 3, 160 (2022)