The Caltech Mission to Apophis: Accelerating Planetary Science and Planetary Defense

The close approach of asteroid (99942) Apophis in 2029 offers a rare opportunity to observe the reaction of a small, suspected rubble-pile body to planetary tidal forces, providing insight into its surface and interior properties and contributing valuable knowledge for planetary defense. The California Institute of Technology (Caltech) is pioneering a mission to rendezvous with Apophis before and escort it through its Earth closest approach (ECA). The mission consists of a Mothership and two radar-equipped CubeSats. Mission science objectives include observing potential resurfacing due to ECA and studying the asteroid's interior using low-frequency radar, producing three-dimensional maps of Apophis's internal composition and porosity and providing unprecedented insight into the interiors of rubble piles. Here, we compile and summarize Apophis modeling and data, focusing on knowns, unknowns, current science knowledge gaps, and how Caltech's mission can fill those gaps. We discuss current knowledge regarding the surface dynamics and interior structure of rubble-pile asteroids, implications for asteroid formation and evolution, predicted responses to Earth encounters via numerical modeling, and how these predictions inform mission requirements. We outline how Caltech's mission aligns with the landscape of other Apophis missions, such as OSIRIS-APEX and RAMSES, and how its unique bistatic radar data set complements the data returned by other missions.