Modeling the response of a multi-component model of Apophis to its close Earth Approach

On Friday, April 13, 2029, the Near-Earth Asteroid (99942) Apophis will have a close approach to Earth. This event holds considerable significance, as such close encounters are rarely known so far in advance. Numerous plans have been proposed to observe or rendezvous with Apophis, including NASA’s initiative to dispatch the OSIRIS-REx spacecraft, renamed the OSIRIS-APEX mission, to rendezvous with Apophis after its close approach to Earth (DellaGuistina, et al. 2022).

The primary consequence of this flyby is expected to be a significant alteration in the asteroid’s spin state (Benson et al., Icarus 2023). Although the flyby distance is not close enough to dislodge material from the surface, it is plausible that the stresses exerted by Earth during the encounter could cause a shift in Apophis’ components. In anticipation of these observations, the asteroid’s spin state and shape model have been estimated through combined photometric and radar observations (Pravec et al., Icarus 2014, Brozovic et al., Icarus 2018). Radar data indicates that Apophis might possess a non-homogeneous structure, characterized by a neck and potentially being a bilobate or even a contact binary asteroid (Brozovic et al., Icarus 2018). This implies that Apophis may consist of disjoint components resting upon each other, unlike the commonly used homogeneous rubble pile model. While the rubble pile models exhibit minor variations in mass distribution on the surface (Hirabayashi et al. Icarus 2021, Demartini et al. Icarus 2019), if Apophis is a bilobate body the contact regions between components would be particularly sensitive to stress variations (Hirabayashi & Scheeres, 2019). Consequently, larger-scale shape changes or deformations, akin to those observed in comet 67P/Churyumov–Gerasimenko due to changes in its spin state (Hirabayashi et al. Nature, 2016), could be anticipated in these areas.

To investigate the dynamics of Apophis during its flyby, this study employs rigid polyhedral bodies, bilobate shapes, and few component configurations generated using a convex decomposition approach. These rigid polyhedral models, derived from the radar shape model, will be used in Contact Dynamics simulations to study the response of the asteroid. Contact Dynamics, a class of discrete element methods (DEM) founded on non-smooth mechanics and implemented in the LMGC90, deviates from the classical Spherical DEM approach by accommodating non-spherical particle systems. This characteristic allows for a more precise estimate of Apophis’ response that can model potential reconfigurations in Apophis.