A Radio Science Experiment for the RAMSES Mission to Asteroid 99942 Apophis

On April 13, 2029, the asteroid 99942 Apophis will have a very close encounter with the Earth, transiting the GEO ring. This flyby represents a unique opportunity to observe a well-known potentially hazardous asteroid subject to strong tidal forces. The time-varying orbital and rotational environment can lead to changes in the surface slopes. Depending on the circumstances, this mechanism may drive significant property changes in the asteroid's internal structure and granular motion on its surface. In this context, characterizing the bulk density and its mass distribution within the asteroid nucleus before and after the encounter could represent a critical step towards understanding the evolution history of near-Earth asteroids.In this work, we present the outline of a possible Radio Science Experiment (RSE) onboard the Rapid Apophis Mission for Space Safety (RAMSES) proposed by the European Space Agency, which is expected to rendezvous with the asteroid in February 2029, right before the close encounter. The objectives of this experiment will include characterizing the overall mass, density, and porosity of the nucleus with an accuracy of less than 1%, determining its spin rate and orientation to less than 1% and 5, respectively, estimating the extended gravity field and internal structure of the nucleus, and improving its trajectory reconstruction.To reach the outlined objectives, the radio science experiment will combine Earth-based radiometric measurements, namely Doppler, range, and DOR, with optical images collected by the onboard narrow- and wide-angle cameras. Furthermore, the RSE will exploit the radiometric measurements collected through the Inter-Satellite Link with the CubeSats released by the RAMSES spacecraft, allowing us to collect high-accuracy Doppler measurements at closer orbital distances from the target and after an eventual landing. In this regard, the mission might serve as the first deep space application for a new X-band Inter-Satellite Link Transceiver (ISL-T) for CubeSats, which is being developed in the framework of the INNOVATOR project founded by the Italian Space Agency.Building on the experience gained with the RSE onboard the HERA mission, this work shows that the proposed concept of operations, involving a radio link between RAMSES and two deployable CubeSats, is fully capable of satisfying the mission objectives of characterizing Apophis with high accuracy before and after the encounter.Simulations for the orbit determination covariance analysis of RAMSES are conducted within JPL's Mission Analysis, Operations, and Navigation Toolkit Environment (MONTE) software, using the full-two body problem model to accurately describe the non-principal axis rotation of Apophis and its interactions with the Earths gravity field.As part of this study, we identify possible synergies with opportunity payloads to be embarked on either RAMSES or the CubeSats and coordinated campaigns involving ground-based observatories and the OSIRIS-APEX mission.