The OSIRIS-APEX mission will reprise the discoveries of the OSIRIS-REx spacecraft at a second asteroid, Apophis. An hour after Apophis’s dramatic close approach to Earth on April 13, 2029, The OSIRIS-APEX spacecraft will use Earth’s gravity to put itself on a course to rendezvous with the asteroid to begin an 18-month campaign of investigation and discovery. Having already challenged our understanding of “carbonaceous” (C-complex) asteroids during its exploration of Bennu, the spacecraft instrument suite will provide first-of-its-kind high-resolution data of a “stony” (S-complex) asteroid—dramatically advancing our knowledge of this asteroid class and its connection to the meteorite collection. After 15 months orbiting Apophis, APEX will use its thrusters to dig into the surface. This will allow us to observe subsurface material, which will provide otherwise inaccessible insight into space weathering and the surface strength of stony asteroids.

Although scientific discovery is APEX’s prime motivation, Apophis’ bulk structure and surface strength have critical implications for planetary defense. Shortly after its discovery in 2004, there was concern that Apophis could hit Earth in the 2029 encounter. Further observations ruled out that possibility, and we now know that it does not present any danger for at least 100 years. Nevertheless, as an S-complex object, Apophis represents the most common class of potentially hazardous asteroids (PHAs) and knowledge of its properties can inform mitigation strategies. Monitoring Apophis during and after Earth approach provides the first opportunity to witness any change in the surfaces and orbits of an asteroid that could influence its likelihood of striking Earth.