After returning its sample of asteroid (101955) Bennu in September 2023, the OSIRIS-REx spacecraft will have sufficient fuel to rendezvous with asteroid (99942) Apophis shortly after that object’s close encounter with Earth in 2029. This Extended Mission opportunity will allow us to study a tumbling asteroid that has recently undergone a strong tidal perturbation. The close Earth encounter is likely to significantly change the rotation state of Apophis, modifying local slopes and possibly exposing subsurface material. While Apophis no longer presents a short-term hazard to the Earth, it remains an object of significant interest for planetary defense.
We will be able to study this stony (S-type) asteroid using the unparalleled OSIRIS-REx instrument suite, with spectroscopy from 0.4–4.7 and 7–50 microns (OVIRS and OTES spectrometers, respectively), visible wavelength color imagery (OCAMS imagers), a scanning lidar (OLA laser altimeter), and a radio science experiment using the high- and low-gain antennas.
By applying lessons learned from the OSIRIS-REx primary mission at Bennu, we will characterize Apophis with a combination of survey stations, orbital observations, and targeted reconnaissance flybys to produce global image mosaics at 10-cm pixel scale, with 1-cm imaging of sites of particular interest. Similarly, we will obtain global spectroscopy with ~30-m footprints, with regional observations with ~5-m footprints. We will map the distribution of minerals, search for water and organic materials, and characterize any exogenic material on the surface. We will search for particles ejected from the surface, as was seen at Bennu. Such particle ejections have not been observed by other smally-body missions, but OSIRIS-REx orbital images are particularly well-suited for their detection.
The Yarkovsky effect is a critical driver of orbital evolution of near-Earth objects and is dependent on the thermal and rotation state. As the spacecraft orbits Apophis, we will be in the unique position of being able to detect the change in the Yarkovsky effect due to the close Earth encounter, validating not just the existence of this effect, but also characterizing its dependence on the rotation and thermal state of the asteroid.