A review of the structural stability of Didymos, the target of the NASA DART and ESA Hera planetary defense missions

Abstract:

I review our detailed investigation of the stability and physical properties of the binary asteroid (65803) Didymos-Dimorphos, which is the target of the first asteroid deflection test involving NASA's DART and ESA's Hera missions. To support these missions and to gain a better understanding of the evolution of this binary, including what may happen after DART impact, careful theoretical and numerical explorations into the binary system based on current observations are required.

In a first study, we investigated the creep stability of Didymos assuming that it is a cohesioneless spinning self-gravitating aggregate. By using a soft-sphere discrete element model (SSDEM), we modeled the YORP rotational acceleration effect by spinning up the aggregate to Didymos's current spin period of 2.26 hr. Our results quantify the influence of the internal configuration (i.e., particle size, arrangement and density distribution) and material parameters on the aggregate’s structural stability and failure behaviors. In a second currently ongoing study, we investigate the role of cohesion in the creep stability of Didymos. Over the wide diversity of rubble-pile asteroids' properties (e.g., bulk density, packing configuration, spin rate, shape), the effect of cohesion is very complex and remains largely unknown. We estimate the minimum cohesive strength that is required to maintain the structural stability of Didymos at its current spin rate. We also perform a comparison with estimates from the continuum theory, and clarify the differences between the discrete element modeling and the continuum theory.

The combination of observed surface characteristics by Hera and our modeling can allow inferring the actual structural properties of Didymos, which will also shed light on possible binary formation scenarios. Given the fast spin rate of Didymos, perturbations caused by the DART impact may also induce some observable shape modifications to it. We explore this possibility using our modeling and provide constrains to the physical properties of Didymos accordingly.

Acknowledgement: We acknowledge funding from the Université Côte d'Azur "Individual grants for young researchers program" of IDEX JEDI and from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 870377 (NEO-MAPP project).