Preparation of the GINS Software for the Hera Radio Science Experiment

Hera is an ESA mission currently en route to the Didymos, which it will reach in October 2026. The mission comprises a mothership, Hera, accompanied by two CubeSats, Juventas and Milani. Its primary scientific objectives include the detailed characterization of the binary asteroid system, and, in particular, the investigation of the internal structure of the asteroids, especially Dimorphos, the smaller member of the system. The key observables used to infer the interior properties of the asteroids are their mass, gravity field, moments of inertia, and rotation state, derived from the orbit determination of the three spacecraft. The internal properties of Dimorphos will be further constrained through radar and gravimetric observations acquired by the JuRa and GRASS instruments, respectively, before and after the landing of the Juventas CubeSat on Dimorphos.

The main objective of the Radio Science Experiment (RSE) is the precise reconstruction of spacecraft trajectories and the estimation of geodetic parameters characterising the bodies, using classical two-way Doppler and range direct-to-Earth measurements provided by the X-band Deep Space Transponder (X-DST), combined with inter-satellite link (ISL) S-band two-way range measurements between Hera and the CubeSats. In addition to these radiometric data, Hera’s orbit determination will benefit from the inclusion of LIDAR observations from the Planetary Altimeter (PALT) and optical images collected by the Asteroid Framing Cameras (AFC), both onboard the Hera spacecraft.

Among other quantities, the expected precision in the determination of asteroid mass, gravity field coefficients, and rotation parameters achievable by the mission was previously quantified by Gramigna et al. (2024) and Tortora et al. (2025) through multi-arc covariance analyses performed with NASA JPL’s orbit determination software MONTE. Here, we try to reproduce part of these simulations using CNES’s orbit determination software GINS, further adapted at the Royal Observatory of Belgium for planetary geodesy applications. In addition, we use GINS to analyze the real tracking data acquired by Hera during its Mars flyby in March 2025, thereby demonstrating its readiness for the processing of operational mission data. This work pursues a dual objective: independently validating the predicted scientific return of the Radio Science Experiment and preparing GINS for the analysis of the forthcoming Hera mission data.