Optical Navigation Image Processing and Orbit Determination for Hera’s Mars Flyby

Hera is the ESA’s contribution to an international collaboration project named Asteroid Impact and Deflection Assessment (AIDA). Last 26^th September 2022, NASA’s DART mission performed a kinetic impact on Dimorphos, the Didymos secondary asteroid. At the end of 2026, Hera will arrive at the binary system, following up for a detailed post-impact survey, to fully characterize this planetary defense technique. In addition, Hera will deploy two CubeSats, Juventas and Milani, which will contribute to asteroid science. In March 2025, Hera performed a Mars flyby, during which Earth-based radiometric data and optical images of the planet and its moons, Phobos and Deimos, were acquired. This flyby provided a valuable opportunity to test and validate Optical Navigation (OPNAV) image-processing pipelines and orbit determination algorithms and tools developed within the Hera mission.

In this context, optical measurements derived from Hera Asteroid Framing Camera (AFC) images of Deimos and Phobos allow the extraction of surface normal points, which can be used to refine the moons’ ephemerides. Images of Mars, on the other hand, are primarily exploited for optical navigation purposes: limb and center-of-figure measurements provide angular constraints on the spacecraft–planet geometry, contributing to the improvement of the spacecraft’s trajectory estimation during the flyby.

This work presents an OPNAV image-processing pipeline developed for the Hera mission. The pipeline begins with the calibration of the Hera’s AFCs using star-field images, allowing the estimation of the optical distortion parameters of both cameras. The subsequent processing focuses on the extraction of Phobos and Deimos center-of-figure from the acquired images of the Mars flyby. For both calibration and centroid extraction, we use the ARAGO software, developed at the Observatoire de Paris, to find star centroids and then determine the moons’ astrometric points. Finally, these OPNAV measurements are then incorporated into the orbit determination process to assess their impact on the spacecraft trajectory estimation. Preliminary results are presented to demonstrate the performance of the proposed approach.