On stress, strength, and failure in asteroids during planetary entry

Abstract

Efforts to characterize the danger posed by asteroids have motivated an effort to model their entry and breakup in Earth’s atmosphere. These models, crucial to planetary defense efforts, necessitate an understanding of the physics underlying fragmentation — including knowledge of key governing physical properties such as strength. Recovered meteorites provide some of the best evidence for these properties. However, their measured strengths are often orders of magnitude higher than those inferred from meteor observations. In this thesis, we seek to provide a full-field description of the stresses that develop in monolithic meteors as they enter the atmosphere and deform, to shed light on the fragmentation process. To quantify those stresses, we develop a simple model of meteor entry that treats the bolide as a deformable body subject to suitable aerodynamic, inertial, and centrifugal loads. We apply these external loads via the Meteor Equations in conjunction with modified Newtonian aerodynamic theory at high Mach numbers. First, we compute an analytical series-solution to the stress field in an idealized case and show that, unlike what is classically assumed, the tensile stresses in asteroids may be as much as 20 times lower than the ram pressure. Then, we conduct finite-element simulations of meteor falls attendant to non-ideal asteroids, and show that our conclusions hold for all but the most irregularly shaped bodies, where geometric stress concentrations may cause early fragmentation. Finally, we simulate the breakup process in select cases by recourse to the discontinuous Galerkin / Cohesive Zone method, confirming that cracks nucleate in accordance with our analytical predictions. We conclude that this factor is an important parameter in the modeling of asteroid entry and fragmentation and that, in combination with Weibull-type size-strength scaling laws, may help shed some light on the observed discrepancy between meteor and meteorite strengths.