An integrated DEM code for tracing the entire regolith mass movement on asteroids

The mass movement on asteroid surfaces is governed by their weak but complex gravitational fields and is highly coupled with the asteroid's surface topography. Consequently, unlike mass movement on Earth, the movement range of regolith particles on asteroid surfaces can expand from the granular scale to the asteroid scale [1–5]. For instance, during YORP spin-up, local landslides of regolith materials may be initiated when the local slope angle exceeds the friction angle. Depending on the spin rate of asteroids, the scale of movement of these regolith particles can continuously expand from meters during surface local landslides to tens of kilometers during orbital motion after mass-shedding. The DEM (Discrete Element Method) simulation of such regolith movement is confronted with 1) the interactive dynamics between the regolith particles and the asteroid; 2) the modeling of high-resolution asteroid surface topography and the efficient algorithm for computing particle-surface contact forces; 3) the precise computation of irregular gravitational fields on asteroid surfaces; and 4) the scale-span simulation of the regolith migration. To address these challenges, this paper presents a DEM modeling strategy for tracking the scale-span evolution of the asteroid regolith materials, which can efficiently track the scale-span movement process of asteroid regolith materials with high-resolution surface topography at the particle sizes approaching the actual sizes of regolith grains on asteroid surfaces. Using the strategy, a specific DEM code that integrates key mechanical models, including the irregular gravitational fields, the inter-particle and particle-surface interactions, and the coupled dynamics between the particles and the asteroid, is developed to track the asteroid regolith mass movement processes.
Using this code, we investigated the landslide of sand piles on asteroid surfaces during spin-up. The results indicate that: 1) landslides on asteroids are similar to that on Earth, exhibiting distinct particle size segregation, with small particles at the bottom and large particles on the surface; 2) due to the influence of asteroid surface topography, small particles tend to deposit in stable regions on the asteroid surface, resulting in a particle size sieving effect in the landslide-shedding process of the regolith; 3) at the spin rate near the shedding failure limit, the cohesionless surface regolith grains flow toward the equator from the middle latitudes regions and these particles sliding to the equator continues to slide towards the minimum geopotential area and ultimately shed from the near minimum geopotential area; and 4) the centrifugal force significantly deflects the direction of the surface particle flow caused by landslides.
The code has great potential in both theoretical and applied studies. It can serve as a powerful tool for high-resolution investigation of surface activities on asteroids, offering insights into the fine geological processes that shape the landscapes of these celestial bodies. For instance, it can be used to analyze the possible effects of Apophis’ 2029 close encounter with Earth on Apophis' surface and nearby dynamics. Additionally, for the engineering designs in asteroid surface exploration missions, it can provide a reliable way to model the tricky maneuvers of spacecraft landing or contacting the asteroid surface. For example, the code can predict the global cruise trajectory of a hopping lander with high fidelity, providing strong theoretical support for asteroid surface exploration mission design.

Acknowledgements
This work is supported by the National Natural Science Foundation of China under Grant 12272018.
References
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