Speaker
Description
The upcoming close encounter of asteroid 99942 Apophis with Earth in 2029 presents a once-in-7000-year opportunity to study the dynamics, bulk properties, and interior structure of a potential rubble-pile asteroid as it passes through Earth’s gravitational field. Numerical modeling—including via Discrete Element Methods (DEMs)—has helped to develop our understanding of the dynamics and physical outcomes of the tidal encounter between Apophis and Earth, including the expected change in the bulk shape and spin of the body, and predictions of potentially measurable surface and seismic outcomes due to the short period of natural tidal forcing. These models have helped to plan, orient, and design missions to Apophis to ensure that we can make the most of the natural experiment that the Apophis encounter provides.
We will present new and ongoing DEM models of the full Apophis-Earth close encounter, making use of recent developments in modeling realistic particle shapes with both a “glued-sphere” approach and a level-set DEM approach in the N-body gravity and soft-sphere DEM code PKDGRAV. The glued-sphere method provides simpler spherical gravity and collision detection calculations but requires smaller timesteps and stiffer constituent particles. The level-set method provides a more realistic shape representation at the cost of increased memory requirements and the loss of precise gravitational torques (as we do not calculate polyhedral gravity). Here, we compare the results and performance of both techniques.
Modeling with irregular particle shapes (rather than spheres) allows us to increase the macroporosity of the resultant rubble pile while also increasing the body’s shear strength. This occurs naturally when packing irregular shapes due to the void spaces created by interlocking grains, and the physical strength of those interlocked structures, which cannot be replicated by spheres alone. These methods allow us to create a high macroporosity regolith body—like those investigated in recent missions to rubble-pile asteroids Bennu, Ryugu, and Itokawa—that is also more resistant to reshaping or disaggregation than previous spheres-only models.
In our simulations, we model Apophis as a rubble pile constructed of irregular particles in the several-meters size range, with different interior structure profiles, including contact-binary, large single-core, and rubble throughout. We present the influence of these different interior structure profiles and constituent grain shapes in the plausible range of close-approach distances, and compare the deformation, spin change, and induced seismicity measurements with previously published/presented spheres-only simulation results and analyses.