Small bodies interior characterization through gravity inversion in preparation for ESA’s RAMSES mission to asteroid Apophis

The Rapid Apophis Mission for Space Safety (RAMSES) is an ESA’s mission to study the near-Earth asteroid 99942 Apophis before, during and after its close encounter with the Earth on April 13, 2029. Given the very small closest approach distance, strong tidal forces may induce significant changes in Apophis’s rotational state, surface morphology, and internal structure. This event offers a unique opportunity to observe and quantify these effects, advancing our understanding of rubble-pile asteroids’ formation and evolution.In this regard, a detailed reconstruction of Apophis’ internal structure will be crucial to characterize the internal alterations induced by Earth’s strong gravitational pull during the flyby, giving important information about the asteroid’s internal cohesion and evolution history.Among the different payloads and experiments currently foreseen, the Radio Science Experiment (RSE) will allow to estimate the asteroid’s gravity field, rotational state, and heliocentric orbit through the precise orbit determination of the RAMSES spacecraft and two CubeSats that will be released before the Apophis closest approach. The gravity field can be employed in the so-called Gravity Inversion (GI) problem, i.e. the process aimed at inferring the internal structure of the body starting from its gravitational field. A wide variety of techniques have been proposed and employed to address the GI problem. For example, global GI relies on the inversion of the direct problem, which links the density distributions with the generalized moments of inertia. This approach yields continuous density distributions that exactly reproduce the observed gravitational field, but suffers from an infinite number of possible solutions. Alternatively, the Markov Chain Monte Carlo (MCMC) algorithm is a novel approach that allows to estimate the size and density of pre-defined regions within the body’s volume starting from its gravitational field. This technique proved to be very versatile since it has been used for large, differentiated bodies, such as Vesta, and smaller rubble piles like Bennu.This work describes the gravity inversion methodologies currently under development in preparation for the RAMSES mission. Particular focus is placed on the MCMC techniques, given their flexibility and capability of integrating multiple data types beyond gravity, leading to a more comprehensive analysis.