Double Asteroid Redirection Test (DART): Structural and Dynamic Interactions between Asteroidal Elements of Binary Asteroid (65803) Didymos

Abstract

NASA's Double Asteroid Redirection Test (DART) mission is the first full-scale planetary defense mission. The target is the binary asteroid (65803) Didymos, in which the smaller component Dimorphos (∼164 m equivalent diameter) orbits the larger component Didymos (∼780 m equivalent diameter). The DART spacecraft will impact Dimorphos, changing the system’s mutual orbit by an amount that correlates with DART's kinetic deflection capability. The spacecraft collision with Dimorphos creates an impact crater, which reshapes the body. Also, some particles ejected from the DART impact site on Dimorphos eventually reach Didymos. Because Didymos’s rapid spin period (2.26 hr) may be close to its stability limit for structural failure, the ejecta reaching Didymos may induce surface disturbance on Didymos. While large uncertainties exist, nonnegligible reshaping scenarios on Didymos and Dimorphos are possible if certain conditions are met. Our analysis shows that given a surface slope uncertainty on Dimorphos of 45°, with no other information about its local topography, and if the DART-like impactor is treated as spherical, the ejecta cone crosses Didymos with speeds ≳14 m s−1 in 13% of simulations. Additional work is necessary to determine the amount of mass delivered to Didymos from the DART impact and whether the amount of kinetic energy delivered is sufficient to overcome cohesive forces in those cases. If nonnegligible (but small) reshaping occurs for either of these asteroids, the resulting orbit perturbation and reshaping are measurable by Earth-based observations.

Predictions for the Dynamical States of the Didymos System before and after the Planned DART Impact

“…5. Models in which either component of Didymos is treated as a rubble pile are broadly consistent with the rigid-body modeling (Section 4; H. Agrusa 2022, in preparation), but they do reveal the sensitivity of the primary to shape deformation due to its near-or supercritical rotation state (Section 5, Hirabayashi et al 2022). A change of shape of either body due to the impact has implications for the final orbital period and, therefore, β.…”

NASA’s Double Asteroid Redirection Test (DART): Mutual Orbital Period Change Due to Reshaping in the Near-Earth Binary Asteroid System (65803) Didymos

“…Numerical N-body discrete element model simulations have shown that Dimorphos's shape is stable in the nominal relaxed preimpact state, both in case the body is a fully fragmented rubble pile and in case it has some structural coherence (Richardson et al 2022). The same is not true for Didymos, which has a much higher spin rate and requires structural coherence (e.g., cohesive strength of ∼25 Pa for the bulk density of 2170 kg m -3 ) to maintain its shape (Hirabayashi et al 2022). The physical parameters of the Didymos system and the dynamically relaxed, nominal initial conditions that approximately put the system on a circular orbit are summarized in Table 1.…”

“…Although its magnitude is uncertain, reshaping (i.e., shape change without significant mass loss) may also occur depending on Dimorphos's structural and impact conditions (Raducan & Jutzi 2022). Moreover, the excavated materials ejected from the impact site will evolve within the system (Yu et al 2017;Yu & Michel 2018;Hirabayashi et al 2022) and may eventually fall onto Didymos, delivering some amount of kinetic energy to the body. The fact that Didymos appears to have a spinning-top shape suggests that Didymos could have gone through a rotationally induced evolution, as seen on Ryugu and Bennu (e.g., Watanabe et al 2019;Scheeres et al 2019;Hirabayashi et al 2020).…”

Dynamical Evolution of the Didymos−Dimorphos Binary Asteroid as Rubble Piles following the DART Impact

“…Contact and collision interactions are handled using the Smooth Contact Method module of the open-source multiphysics code CHRONO (Tasora et al 2016). GRAINS has been used recently to study the stability and internal structure of Didymos (Ferrari & Tanga 2022;Hirabayashi et al 2022). In this work, we use GRAINS to model Dimorphos as a gravitational aggregate of irregularly shaped, meter-sized boulders.…”

“…In this work, we do not consider immediate deformation due to the DART impact itself. We refer the reader to the companion papers by Hirabayashi et al (2022) and Nakano et al (2022) that model the direct deformation of Didymos or Dimorphos due to the DART impact and propagate the resulting system as rigid bodies. The degree of shape deformation that DART will cause is unclear, as it depends on many unknowns such as the bulk density, cohesion, boulder distribution, among many other parameters.…”