Insights into cratering events on small asteroids from experimental and numerical simulation results

Asteroids are the building blocks of the Solar System and gaining insights into their mechanical characteristics and internal structures is not only key to understanding the Solar System formation, but also essential in safeguarding Earth from potential asteroid-related hazards. Recent space missions to small asteroids, including JAXA’s Hayabusa2, NASA’s OSIRIS-Rex and DART [1] have revealed that cratering events on these bodies occur in a regime that is not yet fully understood, where the interplay of low gravity and material strength (cohesion) influences the outcomes. 

The craters on asteroid surfaces are key to understanding their surface properties and evolutionary history. Impact simulations using so-called shock physics codes have been previously used to determine the target properties on asteroids based on observed craters [2]. When validated against laboratory experiments, these models become invaluable for interpreting the history of asteroids. However, the low-gravity, low-strength conditions on rubble-pile asteroids pose significant challenges for both experimental investigations and numerical modelling. In this regime, the craters formed can grow to approximately a hundred times the size of the impacting projectile over extended periods, needing considerable computational resources to accurately simulate the impact physics and replicate the final crater. Our recent work introduces a novel approach that directly employs shock physics code calculations to model the entire process of impacts in this challenging regime. This method has shown promise, notably in replicating the SCI artificial impact experiment conducted by Hayabusa2 on asteroid Ryugu [3], which helped determine the asteroid's surface mechanical properties2. However, to enhance the credibility and accuracy of our simulations, further validation through detailed laboratory impact experiments is essential.

This research focuses on the effect of gravity on the size and shape of impact craters on rubble-pile asteroids. We used the Bern SPH [4] shock physics codes to model the outcomes of recent laboratory cratering experiments conducted under simulated low-gravity environments and performed at the Institute of Space and Astronautical Science (ISAS) in Japan [5]. By validating our numerical simulations with laboratory experiments, we aim to deepen our understanding of the physical processes involved in low-gravity impacts, including the interaction between particles and the material's crushing behaviour.

The findings of this study will aid in interpreting the cratering history on asteroids Didymos and Dimorphos, targets of the upcoming ESA Hera mission in late 2026 [6]. This work is part of the Hera Impact Physics Working Group (IWG) and aims to develop new modelling strategies by integrating different numerical codes for simulating cratering on small, rubble-pile asteroids. The results from this research, and other benchmark and validation studies carried out within the Hera IWG  [e.g., 7], will provide quantitative and reliable predictions about impact outcomes, which can be measured using spaceborne and in-situ instruments by the Hera mission. 

Acknowledgements: S.D.R. and M.J. acknowledge support from the Swiss National Science Foundation (project number 200021_207359). S.D.R. gratefully acknowledges the support received from the Swiss Society for Astrophysics and Astronomy (SSAA) MERAC Travel Award. M. K. and A. M. N. acknowledge support by JSPS KAKENHI (grant number, JP21H01148 and JP24K17116).

References: 

[1] Daly, R. T. et al. Successful Kinetic Impact into an Asteroid for Planetary Defense. Nature 1–3 (2023) doi:10.1038/s41586-023-05810-5

[2] Jutzi, M., Raducan, S. D., Zhang, Y., Michel, P. & Arakawa, M. Constraining surface properties of asteroid (162173) Ryugu from numerical simulations of Hayabusa2 mission impact experiment. Nat. Commun. 13, 7134 (2022).

[3] Arakawa, M. et al. An artificial impact on the asteroid 162173 Ryugu formed a crater in the gravity-dominated regime. Science (2020) doi:10.1126/science.aaz1701.

[4] Jutzi, M., Benz, W. & Michel, P. Numerical simulations of impacts involving porous bodies: I. Implementing sub-resolution porosity in a 3D SPH Hydrocode. Icarus 198, 242–255 (2008).

[5] Kiuchi, M. et al. Impact experiments on granular materials under low gravity: Effects of cohesive strength, internal friction, and porosity of particle layers on crater size. Icarus 404, 115685 (2023).

[6] Michel, P. et al. The ESA Hera Mission: Detailed Characterization of the DART Impact Outcome and of the Binary Asteroid (65803) Didymos. Sci. J. 3, 160 (2022).

[7] Luther R., et al. Impact Experiments and Model Validation in the frame of the Hera mission. EPSC (2024).