The Influence of Internal Structure on Physical Outcomes of the 2029 Apophis Close Approach with Earth

Abstract The 2029 close approach between asteroid 99942 Apophis and Earth represents a unique opportunity to study the influence of tidal encounters on the deformation and rotational evolution of an S-complex near-Earth object in the 0.1–1 km size range—a class of object of particular interest to the planetary defense community. The encounter may give insight into the internal structure of the body, presenting an opportunity for our first direct probe of an asteroid’s interior. We present results from numerical simulations of the Apophis–Earth tidal encounter, modeling Apophis as a self-gravitating granular aggregate of meter-sized spherical particles, with the aim of constraining measurable rotation change, deformation, and seismicity induced in Apophis during the encounter, in support of potential rendezvous missions. We vary particle size distributions, particle shapes, packing configurations, and interior structures (with large embedded cores) to examine the effects that internal structure has on these measurable encounter outcomes. We find that the packing configuration with the lowest shear strength exhibits the most deformation by a factor of 4 over the most compact configuration, and the presence of internal cores in a low shear strength configuration can reduce the stress felt by the asteroid. We also find that short-period seismic events are likely to occur in the shallow subsurface layers of the body in the hours just following the time of perigee. These events have amplitudes on the order of 10 −4 m s −2 Hz −1/2 in the 0.1–1 Hz range, which should be measurable with the latest seismic instruments developed for asteroid interior characterization.