Libration-induced Orbit Period Variations Following the DART Impact

The Double Asteroid Redirection Test (DART) mission will be the first test of a kinetic impactor as a means of planetary defense. In late 2022, DART will collide with Dimorphos, the secondary in the Didymos binary asteroid system. The impact will cause a momentum transfer from the spacecraft to the binary asteroid, changing the orbit period of Dimorphos and forcing it to librate in its orbit. Owing to the coupled dynamics in binary asteroid systems, the orbit and libration state of Dimorphos are intertwined. Thus, as the secondary librates, it also experiences fluctuations in its orbit period. These variations in the orbit period are dependent on the magnitude of the impact perturbation, as well as the system’s state at impact and the moments of inertia of the secondary. In general, any binary asteroid system whose secondary is librating will have a nonconstant orbit period on account of the secondary’s fluctuating spin rate. The orbit period variations are typically driven by two modes: a long period and a short period, each with significant amplitudes on the order of tens of seconds to several minutes. The fluctuating orbit period offers both a challenge and an opportunity in the context of the DART mission. Orbit period oscillations will make determining the post-impact orbit period more difficult but can also provide information about the system’s libration state and the DART impact

Influence of the body composition on the evolution of ejecta in the Didymos-Dimorphos binary system

The DART spacecraft will impact Dimorphos (the secondary body of the Didymos binary asteroid) around September/October 2021 to test the kinetic impactor deflection method against possibly hazardous Near Earth Asteroids. A comprehensive model was developed to study the outcome of the impact. The goal is to model the short (within the DART-LICIACube framework) and medium (the HERA framework) term dynamics of the system and of the ejecta particles. The expected final output shall be useful to better understand the initial crater evolution and the characterization of the dust environment within the binary system at the time of HERA arrival, exploring also the possibility of the formation of long term stable particles. Starting from different iSALE cratering simulations of the DART impact, 3D representations of the crater are produced. The particles are then propagated with a dynamical model considering non-spherical shapes for both the primary and the secondary bodies, solar radiation pressure and third body direct and indirect perturbations. A first step in the project is the need to understand the influence on the system evolution of a set of sub-models and parameters. Beyond the fine tuning of the whole dynamical model itself, the results of such a study are important to assess the accuracy needed in the measurement of the studied parameters to achieve a good representation and understanding of the whole system. The overall study will concentrate on several different points, such as the influence of the gravity field both in terms of primary body representation (i.e., polihedron vs. analytical approach), of the secondary body shape, of the final post-impact orbit of Dimorphos (circular vs. elliptic), of the shape of the ejected particles (modeled as ellipsoids with different rotation rates), of the location of the impact, etc. In the present work particular attention will be devoted to the study of the effect of the composition of the target on the evolution of the ejecta particles. Different compositions of Dimorphos in terms of material and physical properties (e.g., porosity, cohesive strength of damaged target, layering, etc.) translate into different initial conditions (in terms of initial state vectors and size and numerosity of the ejecta). The effects of these parameters on the ejecta cloud evolution will be evaluated over different time scales. Once the sensitivity analysis will be completed and the relevant parameters identified along with their needed accuracy, a massive simulation campaign is foreseen to model the dynamics of a large number of ejecta over different time spans. Due to the significant computational effort required for this task, only a sample of these results will be reported here. This research was supported by the Italian Space Agency (ASI) within the LICIACube project (ASI-INAF agreement AC n. 2019-31-HH.0)