Rotational And Collisional Evolution Mechanisms Of Rubble-pile Asteroids

Asteroids are thought to contain an abundance of valuable mineral resources,and the original composition of the early solar system.The information contained in these small bodies could provide us with critical clues about how the planets formed and life started on the Earth.With the help of observations and several successful asteroid missions,the progression of asteroid research has been expanded from understanding their orbits,shapes,rotation states and spectral classes to exploring more details about their internal structures and how they evolve in response to external effects.The observations has showed general confirmation that most asteroids above 300 m size scale possess rubblepile structures.However,the discrete and low-strength characteristics of this structure raises many open questions to understanding its evolution and designing future asteroid missions.As an applying basic research,with the development of a physically-based high-efficiency parallel tree-framework gravitational N-body algorithm,the broad goal of this thesis is to investigate the relevant effects that affects the structural stability of rubble-pile asteroids and reveal their dynamical evolution mechanisms subject to these effects.In the study of the YORP spin-up effect,the spin limit and failure mode of rubble-pile asteroids with given material parameters are derived from the stress analyses based on two different methods,i.e.,the analytical continuum theory and the discrete element modeling.A technique is proposed to estimate a rubble pile’s friction angle and bulk cohesion from spin-up numerical experiments,which provides the opportunity to make quantitatively comparisons with the continuum theory.The results show that the continuum theory fails to explain some findings,while the discrete element modeling has great potential in predicting the behaviors and estimating the structural strengths of rubble-pile asteroids.The further study takes the near-Earth binary asteroid 65803 Didymos as a representative,and investigates the creep stability of the Didymos primary by representing it as a self-gravitating granular aggregate subject to rotational acceleration.The dependencies of the failure condition and failure mode on the arrangement and size distribution of constituent particles are established.Some constraints on the possible physical properties of the Didymos primary are given,and a plausible formation mechanism for this binary system is proposed.In the study of collisional evolution of rubble-pile asteroids,the effects of orbital perturbations on the motion of collisional ejecta are explored.The results indicate that the perturbations are enhanced when the impact is taken place near perihelion,leading to a larger mass loss and a lower catastrophic disruption threshold in collision events.Therefore,highly-dispersed collisional remnants are expected to be obtained for nearperihelion impact,which could be beneficial to deflecting a hazardous near-Earth asteroids.Base on this,the process of hyper-velocity impact for kinetically deflecting asteroids with two different structures is investigated with a combination of the material point method and the gravitational N-body method.The impact hazard of the fragments on the Earth is analyzed and the role of asteroid interior structures is explored.The results show that the structure of the simulated body is easily partially destroyed by the kinetic impactor.Some of the resulting fragments move backward along the impact direction,enhancing the deflection efficiency.Furthermore,the collision outcomes proved to be very dependent on the internal structure of the asteroid.The expected damage caused by the deflected monolithic target is larger than the rubble-pile target because of the exist of numerous small dangerous fragments.The method presented in this study can be used to infer the impact condition and outcomes in future planetary defense missions.