Using a discrete element method to investigate seismic response and spin change of 99942 Apophis during its 2029 tidal encounter with Earth

• Published in: Icarus (New York, N.Y. 1962) N.Y. 1962), 2019-08, Vol.328, p.93-103
• Main Authors: DeMartini, Joseph V., Richardson, Derek C., Barnouin, Olivier S., Schmerr, Nicholas C., Plescia, Jeffrey B., Scheirich, Petr, Pravec, Petr
• Format: Article
• Language: English
• Subjects: Asteroids; Composition; Dynamics; Rotation
• ISSN: 0019-1035; 1090-2643
• DOI: 10.1016/j.icarus.2019.03.015

Near-Earth Asteroid 99942 Apophis presents a unique opportunity to study the dynamics, bulk properties, and interior structure of a rubble-pile asteroid when it makes its close encounter with the Earth in 2029. In order to better understand the potential outcomes of a tidal encounter between Earth and Apophis, and to support a potential future mission to Apophis, we perform numerical simulations of the encounter. We represent Earth as a single rigid sphere and the target body as a cohesionless, self-gravitating granular aggregate of identical spheres in a hexagonal-close-packed configuration subject only to gravitational and soft-sphere (elastic) contact forces. We use a radar-derived shape model for the asteroid, along with current best estimates for the orbital solution of Apophis to simulate the encounter trajectory, and perform a large parameter sweep over different potential encounter orientations and bulk densities for the body. We find that the median change in the rotational period for Apophis, sampled for a range of different initial body and spin orientations, is −1.9 h (mean −0.1 ± 6.0 (1-σ) hours) during the encounter. Additionally, we measure that the mean of the largest change in axis length among the 3 primary body axes, also sampled over trials with different initial body and spin orientations, is 0.132 ± 0.066 mm during the encounter, assuming a bulk Young's Modulus of 106 Pa. Such strains on the timescale of peak stress during the encounter may be large enough to be detected by an in-situ seismometer. •We use a soft-sphere discrete element method to model tidal forces from Earth on Apophis.•We constrain bulk spin change during the encounter.•We constrain seismic response over a range of possible encounter and body parameters.•We compare the signal with Earth and Lunar seismic models.