Design of the Vision-Based GNC Subsystem of Hera’s Milani Mission

Milani is a 6U CubeSat that will be released by ESA's Hera spacecraft near the binary asteroid system 65803 Didymos. Its scientific objectives are to acquire high-resolution images of the two asteroids and characterize the dust environment in their vicinity. This work describes in detail Milani's Guidance, Navigation & Control subsystem. Its main tasks are to compute a primary pointing direction and estimate the state of the spacecraft with respect to the asteroid system. To this aim, an onboard Extended Kalman Filter has been implemented. It uses image processing observables and lidar measurements. Furthermore, different pointing strategies have been introduced to increase the robustness of the attitude guidance. The GNC can select the appropriate mode autonomously, depending on the available data. The design has been validated through extensive analyses that covered nominal and off-nominal scenarios. The most significant results are reported in this work.