Effect of material strength and heterogeneity on small asteroid cratering events

<p><strong>Keywords:</strong> Impact; asteroid surfaces; SSDEM; SPH.</p> <p><strong>Introduction:</strong> Impacts can modify the physical state of a substantial fraction of a target body. Studying the hypervelocity impact process and outcome is crucial in the interpretation of the history of a planetary body (Jutzi et al., 2015) and the design of asteroid deflection strategies based on the kinetic impactor technique (Raducan et al., 2019). Images returned by space missions show that small asteroids have complex surface morphologies with heterogeneous distributions of fine regolith and large boulders (e.g., Watanabe et al., 2019). To properly decipher the crater imprints on asteroid surfaces, we carried out numerical investigations to understand the effect of surface material properties (i.e., friction, cohesion) and the presence of large boulders on cratering processes.</p> <p><strong>Methods:</strong> We used a hybrid SPH-SSDEM framework to model the high-speed impact cratering (Zhang et al., 2021). The Smooth Particle Hydrodynamics (SPH) is used to simulate the initial shock propagation and fragmentation stage (Jutzi & Michel, 2015). The outcome is then transferred into a Soft-Sphere Discrete Element Method (SSDEM) code (Zhang et al., 2018), which solves the ejecta evolution and crater growth in the later stages. This modeling framework is capable of simulating impacts from the beginning to the later stages when all ejecta are settled down, allowing capturing the final morphology of the resulting crater.</p> <p>To make comparisons with the first impact experiment performed on an asteroid by the Hayabusa2 Small Carry-on Impactor (SCI; Arakawa et al., 2020), we conducted SCI-like cratering tests using the same impact condition (except using an impact angle of 0º) and Ryugu’s gravity field. The target is modeled as a 15-meter-radius granular bed held by a hemispherical ball. The particle-ball contact parameters are the same as those used for particle-particle contacts.</p> <p><strong>Results:</strong> As the SCI cratering analyses show consistencies with a very low-strength scaling law (Arakawa et al., 2020), we considered modeling the surface properties with three types of low cohesion (i.e., 0 Pa, 0.01 Pa, and 0.1 Pa) and four types of low to moderate friction angles (20°, 25°, 30°, and 33°). The results show that, in a monotonic manner, the diameter and depth of the resulting crater and rim decrease with a larger friction or cohesion (Fig. 1). Compared with the crater morphology of the SCI impact (i.e., crater diameter 14.5 ± 0.8 m and depth ~2.3 m, rim diameter 17.6 ± 0.7 m and depth 0.4 m), the case with <em>C</em> = 0.1 Pa and

Assessment of Near-Earth Asteroid Deflection Techniques via Spherical Fuzzy Sets

Extensions of fuzzy sets to broader contexts constitute one of the leading areas of research in the context of problems in artificial intelligence. Their aim is to address decision-making problems in the real world whenever obtaining accurate and sufficient data is not a straightforward task. In this way, spherical fuzzy sets were recently introduced as a step beyond to modelize such problems more precisely on the basis of the human nature, thus expanding the space of membership levels, which are defined under imprecise circumstances. The main goal in this study is to apply the spherical fuzzy set version of Technique for Order of Preference by Similarity to Ideal Solution (TOPSIS), a well-established multicriteria decision-making approach, in the context of planetary defense. As of the extraction of knowledge from a group of experts in the field of near-Earth asteroids, they rated four deflection technologies of asteroids (kinetic impactor, ion beam deflection, enhanced gravity tractor, and laser ablation) that had been previously assessed by means of the classical theory of fuzzy series. This way, a comparative study was carried out whose most significant results are the kinetic impactor being the most suitable alternative and the spherical fuzzy set version of the TOPSIS approach behaves more sensitively than the TOPSIS procedure for triangular fuzzy sets with regard to the information provided by our group of experts.

ESA’s Hera mission to asteroid Dimorphos

<p>The Hera mission contributes to the international effort towards the validation of the kinetic impactor asteroid deflection technique by retrieving all the physical and dynamical properties of Dimorphos in order to validate numerical impact codes. In particular, Hera’s core objectives from the point of view of the deflection demonstration are the following:</p> <ul> <li>(i) Measuring the mass of Dimorphos to determine the momentum transfer efficiency from DART to the asteroid;</li> <li>(ii) Investigating the crater in detail to improve our understanding of the cratering process and the mechanisms by which the crater formation drives the momentum transfer efficiency;</li> <li>(iii) Observing subtle dynamic effects (e.g. libration imposed by the impact, orbital and spin excitation of the secondary) that are difficult to detect for remote observers;</li> <li>(iv) Characterising the physical properties of Dimorphos (including size, shape, volume, density, porosity, size distribution of surface material) to allow scaling of the momentum transfer efficiency to different asteroids.</li> </ul> <p>In addition, the Hera mission will allow the demonstration of two key technologies for future deep-space missions:</p> <ul> <li>(v) The use of CubeSats for multipoint investigations operated via an inter-satellite network link via Hera;</li> <li>(vi) Autonomous visual-based navigation for very low altitude flybys over the surface of Dimorphos.</li> </ul> <p> </p> <p>The Hera spacecraft will launch in October 2024 onboard an Ariane 6 launcher with an 18-days launch window. The trajectory foresees a Mars swing-by in mid-March 2025 and the rendezvous phase with Didymos starting end-December 2026. Following the operationally safe capture sequence, the asteroid close-proximity operation phase will start from a gate position of about 30 km. Operations will continue for about 6 months and allow for detailed investigations of the Didymos surface down to few kilometres or less from the surface of Dimorphos depending on the performance of the onboard feature-tracking navigation system.</p> <p> </p> <p>The mission is currently in phase B2 with OHB System as prime contractor together with GMV, QinetiQ, Spacebel and OHB-I as core team members. The preliminary design review is schedules in October 2020. The paper will provide an overview of the mission together with its latest system and payload configuration.</p>

LICIACube observation capabilities of dust plume evolution after DART impact

<p>The NASA Double Asteroid Redirection Test (DART) mission will be the first test to check an asteroid deflection by a kinetic impactor. The target of DART mission is the secondary element of the (65803) Didymos binary asteroid system and the impact is expected in late September – early October, 2022. The DART S/C will carry a 6U cubesat called LICIACube (Light Italian Cubesat for Imaging of Asteroid), provided by the Italian Space Agency, with the aim to collect pictures of the impact’s effects. The impact of the 610 kg DART spacecraft at 6.58 km/s on the 163 m Didymos B will result in a change of the binary orbital period of about 10 minutes assuming momentum transfer efficiency β = 1. Values of β > 1 are expected because the produced ejecta carries momentum, primarily in the direction opposite the DART speed direction. The LICIACube mission profile consists in a flyby of Didymos system with closest approach about 3 minutes after the DART impact. LICIACube will be able to acquire the structure and evolution of the DART impact ejecta plume and will obtain high-resolution images and also in 3 colour of the surfaces of both bodies. The nominal mission foresees also imaging of the Dydymos B non-impact hemisphere. The contributions of LICIACube observations to the DART investigations are important for determination of the momentum transfer efficiency β, that is a crucial result of the planetary defence test. Moreover, captured images can enable scientific investigations about the main features of the asteroid system. </p><p>In order to check the imaging capability and to optimize the fast scientific phase of LICIAcube, the LICIA team performed several simulations of pictures’ acquisition. In these simulations, considering the specifications of the 2 optical payloads and the foreseen mission design, we reconstructed synthetic images mainly of the plume. As the plume evolution remains the most important uncertainty, since it depends on a very high number of impacting phase parameters, we simulated imaging of different expected evolution behaviours, to obtain instrument operative parameters and to prepare the data analysis.  </p>