深空探测承载了人类对宇宙奥秘的孜孜追寻，而小天体是其中不可或缺的重要目标。实施小天体接触探测，对探索行星形成、寻找生命起源、防御其撞击地球、以及空间资源开发利用具有重要的科学意义。面对弱引力小天体的接触探测这一世界性难题，各大航天机构已实施的多次小天体任务中，接触、巡视、采样等关键操作迄今还没有完全成功的先例，而对任务返回的地质探测数据的深入解析也仍有诸多困难。小天体地表颗粒碎石系统表现出的结构无序、高维度、强非线性动力学行为是导致这一难题的主要因素。本文基于对风化层颗粒物质力学性质和相互作用的认识，建立了大规模颗粒演化的精细动力学模型，结合离散数值模拟与多尺度分析方法，对小天体接触探测中涉及到的颗粒动力学问题进行了研究。 针对探测器与风化层颗粒介质的接触动力学过程，本文分别基于两种典型着陆场景，即保持恒定低速状态的准静态侵入，与相对高速的动态侵彻，分析了侵入阻力的基本形式与参数依赖关系，根据颗粒系统从介观结构到宏观运动的跨尺度动力学行为，揭示了探测器与风化层接触作用的本质机制。 针对探测器在小天体地表的复杂操作，本文关注其与颗粒风化层的动力学相互作用。 首先，对于小天体表面弹跳移动问题，发现了巡视器宏观状态的动力学特征以及颗粒层的介观尺度响应，量化了风化层力学性质对弹跳宏介观动力学过程的影响。随后，对于采样器在小天体表面的表壤采样问题，揭示了颗粒介质在射弹冲击下的宏观输运机理，给出了冲击溅射采样机构的设计参照。 针对Phobos表面沟壑网络的形成机制，本文重现了Phobos的轨道衰减和潮汐应力场历史，并通过离散元模拟给出了其结构异质的风化层在潮汐应力作用下的次表层破碎与地表渗漏过程，详细讨论了凝聚力系数对颗粒介质失效模式的影响，并通过模拟得到的拉伸撕裂结构与Phobos表面沟壑地形的一致性分析，证实了轨道衰减—潮汐撕裂机制可以有效解释火卫一沟壑网络的起源。 针对陀螺型小行星的整体形貌与局部地形特征，本文立足于小行星接触探测任务的最新地质测绘数据，构建了风化层颗粒介质及表面巨石在YORP效应引起的自旋加速下耦合演化的离散元模型，发现了颗粒层滑坡不仅导致了鼓起赤道脊的形成，还引起了地表巨石构造的丰富演化路径，解释了陀螺型小行星的主要地质特征，显示了YORP效应在小天体地貌演变中的重要作用。
Exploring the vast universe is the shared dream of humankind, and small bodies are indispensable targets. Touchdown exploration on those celestial bodies would provide crucial information regarding the formation of the Solar System, the emergence of life on the early Earth, defending our planet from impact hazards, and utilizing the planetary resource. However, so far, there is no successful touchdown operation in previous small body missions, and we still do not fully understand the geological data returned by these missions. The unique amorphous structure, high-dimensional behavior, and strong nonlinearity of the granular regolith on small bodies remain unknown, which is the main factor that results in the difficulty. Based on the understanding of the mechanical properties and physical interactions within small body regolith, this thesis develops a detailed dynamic model for large-scale granular systems and investigates the granular dynamics issues involved in touchdown exploration through numerical simulations and multi-scale analysis.In the study of the interaction between the spacecraft and the regolith layer, this thesis focuses on two typical landing mechanisms, i.e., slow intrusion with quasi-static state and dynamical penetration at high velocity. The universal law and the physical origin of the resistance force are revealed by relating the mesoscopic structure and the macroscopic rheology of the granular system under intrusion.In the study of the surface operation on small bodies, this thesis first focuses on the hopping locomotion on the regolith surface. Simulations demonstrate the hopping outcomes of the lander and the grain-scale response of the granular layer. The influence of physical properties of the regolith particles is also systematically explored. In addition, the low-speed impact sampling on granular regolith is further analyzed. The results demonstrate that the projectile shape significantly influences the collected mass, which provides crucial information on the optimal designs of impact sampling devices for future sample return missions.In the study of the formation mechanism of Phobos grooves, the orbital decay and tidal stress history are first reconstructed based on the proposed internal structure. Simulations of the extension of the regolith shell show that the subsurface cohesive layer undergoes rupture, while the upper loose regolith drains subsequently into these open fissures. The regimes of granular failure landscapes are explored by considering a range of potential cohesion strengths. The geometry and pattern of simulated grooves are comparable to the geological striations on Phobos, which lends support to the tidal disruption model as the origin of surface grooves on Martian moon Phobos.In the study of the formation history of top-shaped asteroids, the discrete element model of the boulder-regolith evolution driven by YORP spin-up is established based on the observation of the geological landforms on top-shaped asteroids by two recent space missions. The simulations show that their equatorial bulges can be explained by surface regolith flows under rotational failure. In addition, during the regolith migration, the surface boulders coevolve with the underlying regolith and exhibit diverse dynamical behaviors. The predominant geological features commonly observed on top-shaped asteroids are commensurate with this coevolution scenario, which provides a prevalent mechanism for the formation history of top-shaped asteroids.
