Abstract
Context. Slow interactions on small body surfaces occur both naturally and through human intervention. The resettling of grains and boulders following a cratering event, as well as observations made during small body missions, can provide clues regarding the material properties and the physical evolution of a surface. In order to analyze such events, it is necessary to understand how gravity influences granular behavior. Aims. In this work, we study slow impacts into granular materials for different collision velocities and gravity levels. Our objectives are to develop a model that describes the penetration depth in terms of the dimensionless Froude number and to use this model to understand the relationship between collision behavior, collision velocity, and gravity. Methods. We used the soft-sphere discrete element method to simulate impacts into glass beads under gravitational accelerations ranging from 9.81 m s−2 to 0.001 m s−2. We quantified collision behavior using the peak acceleration, the penetration depth, and the collision duration of the projectile, and we compared the collision behavior for impacts within a Froude number range of 0–10. Results. The measured penetration depth and collision duration for low-velocity collisions are comparable when the impact parameters are scaled by the Froude number, and the presented model predicts the collision behavior well within the tested Froude number range. If the impact Froude number is low (0 < Fr < 1.5), the collision occurs in a regime that is dominated by a depth-dependent quasi-static friction force. If the impact Froude number is high enough (1.5 < Fr < 10), the collision enters a second regime that is dominated by inertial drag. Conclusions. The presented collision model can be used to constrain the properties of a granular surface material using the penetration depth measurement from a single impact event. If the projectile size, the collision velocity, the gravity level, and the final penetration depth are known and if the material density is estimated, then the internal friction angle of the material can be deduced.
Highlights
Two recent small body missions have involved direct interactions with asteroid surfaces: the JAXA Hayabusa2 mission (Watanabe et al 2017) and the NASA OSIRIS-REx mission (Lauretta et al 2017)
We introduce a framework that can be used to analyze the outcome of slow impacts and interactions with granular materials, independent of gravity level
The objective of this study is to develop a theoretical collision model that links a projectile’s final penetration depth to its impact velocity, the gravity level of the system, and the properties of the surface material
Summary
Two recent small body missions have involved direct interactions with asteroid surfaces: the JAXA Hayabusa mission (Watanabe et al 2017) and the NASA OSIRIS-REx mission (Lauretta et al 2017). From December 2018 to June 2021, the OSIRIS-REx spacecraft surveyed and mapped the surface of asteroid (101955) Bennu. Ryugu and Bennu, similar to most asteroids, are covered with boulders and a layer of loose granular material, referred to as regolith (Murdoch et al 2015; Hestroffer et al 2019). The electrostatic and cohesive forces that are often ignored on Earth can become nonnegligible in low-gravity (Scheeres et al 2010; Hestroffer et al 2019), and the bulk granular system can experience regime changes under different external triggers (Brucks et al 2007; Murdoch et al 2017). The nonintuitive nature of granular flow in low-gravity, paired with frequent unknowns regarding target body surface materials, makes it difficult to plan and analyze spacecraft operations similar to those listed above. It makes it very challenging to deduce the origins of the various surface features observed on asteroids
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