碎石堆小行星的不规则散体建模与N体动力学仿真研究

Asteroid exploration will be the spotlight of the research in aerospace science in upcoming decade. Various asteroid exploration missions, such as asteroid close flyby, orbiting, landing, sampling and returning were proposed by other counties. China has also planned to carry out the asteroid exploration mission. The present data indicates that most of the asteroids are gravitational aggregates, which has zero tensile strength and is easy to be disrupted by external force produced by exploration missions. Therefore, in order to successfully carry out the asteroid exploration mission, it is essential to build the dynamics model of the asteroid and explore the asteroid evolution’s interaction effect. Currently, the numerical modeling method for rubble-pile asteroids which are made of irregular particulate matter is rarely seen. In order to solve the problem, this research will adopt both characteristic of granular dynamics and gravitational N-body numerical integration method to analyze the dynamic properties of rubble-pile asteroids. The work intended to be carried out is illustrated as follows. First of all, based on discrete element method, the contact model and algorithm of sphere and polyhedron particle would be built. Then, on the basis of the comparison of irregular gravitational field modeling method, the gravitational interaction model between multiple irregular particles would be studied for developing the full model of rubble pile asteroids. Besides, the data structure and overall architecture combining gravitational N-body numerical integration method and contact model would be designed, which are both precise and efficient. Finally, the dynamics behavior of rubble-pile structure under impact and various space effects will be elaborately investigated. This research will lay a theoretical foundation for the asteroid exploration and defense in our country, and enhance our understanding on how the solar system formed.

探测小行星是未来几十年内航天界的研究热点。各国相继提出并部分实施了小行星近距飞越、环绕着陆、采样返回等项目，我国也将实施小行星探测任务。资料表明小行星多为碎石堆结构，易受探测活动中产生的力或振动的影响而破碎瓦解，因此探测前必须进行动力学建模，研究探测活动与小行星自身演化的相互作用，确保探测任务顺利执行。目前国际上尚未形成针对不规则碎石堆小行星力学特性的建模方法，本项目结合散体动力学与引力N体动力学，拟开展以下工作：1) 建立球体与多面体散体颗粒的接触模型，构建处理多面体接触的高效算法；2) 比较不规则颗粒引力场建模方法，研究不规则颗粒的相互引力耦合作用模型，建立碎石堆引力场高精度模型，并为其设计并行树结构的引力N体数值积分方法；3) 建立空间环境作用力模型，研究碎石堆结构在空间环境中的冲击响应特性和长期演化规律。此项目将为我国小行星探测和防御奠定理论基础，并促进太阳系起源和演化的研究。

本项目从碎石堆结构的物理特性出发，建立了高效并行树结构引力 N 体-离散动力学模型，结合连续介质理论与冲击动力学方法，对碎石堆小行星的结构稳定性和动力学过程进行了研究。针对 YORP 加速自旋效应，分别采用连续介质理论分析方法和离散动力学数值模拟方法，从不同角度分析了给定材料参数的碎石堆小行星在一定旋转状态下的应力分布特征，对碎石堆结构的失效模式和极限转速进行推断。提出了评估碎石堆离散元模型的极限转速和结构强度的方法。以近地双小行星系统65803Didymos为例，通过离散动力学自旋加速模拟，研究其结构的动力学响应特性和蠕变稳定性，建立了该系统主星的颗粒分布构型与失效条件、失效模式间的联系，给出了物理性质和材料参数的取值范围，讨论了类Didymos型双小行星系统的形成机制。研究小行星轨道摄动力对撞击喷射物质运动的作用，发现摄动力在近日点处增强，能够提高撞击事件中小行星的质量损失率。结合物质点法对两种不同结构的小行星开展高速撞击演化模拟，分析了撞击后碎片对地球的威胁指数。结果显示，与完整结构相比，针对碎石堆结构小行星的撞击防御的总体效果更好。所建立的研究方法有望用于未来小行星防御任务的撞击条件选择和撞击结果预估。