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The European spacecraft mission HERA is currently flying towards the binary asteroid system Didymos. The spacecraft will arrive in the vicinity of the asteroids at the end of 2026 and thanks to its autonomous vision-based GNC, will be able to fly autonomously down to a distance of a few hundred meters from them. The main purpose of the mission is to characterize the impact of DART (NASA mission) over the surface of the moon of the binary asteroid system. In order to achieve its goal, HERA GNC subsystem has been developed to be highly autonomous. Using a vision-based technology (the Asteroid Framing Camera is used both for Science and for navigation) the attitude guidance is autonomously computed on-board. In the experimental phase of the mission, also the maneuvers can be corrected autonomously, to get as close as possible to the asteroids in safe conditions.
Considering HERA success, ESA is planning another asteroid mission called RAMSES, in the frame of Planetary Defence. The mission objective is Apophis, an asteroid that, on 13 April 2029, will fly-by the Earth at a distance of less than 32,000 kilometers, rendering it visible to the naked eye. RAMSES currently entered its implementation phase and its purpose is to conduct a detailed characterization of the asteroid both pre and post the encounter, to be able to assess the effects of the tidal forces exerted by the Earth during the close passage. This endeavor will provide sufficient data to analyze and predict the likelihood of a potential impact with Earth in 2069.
This paper will focus on the HERA GNC design, and its evolution towards the RAMSES mission needs. A primary difference between the RAMSES and HERA missions is the more constrained dry weight of the spacecraft, as RAMSES needs considerably greater Delta-V for its deep space trajectory. Accordingly, a significant effort has been made to minimize the overall weight of the SC, influencing both the sensor suite available and the fuel budget during proximity operations. Consequently, the redundancy philosophy has been revisited leading to the removal of specific physical unit redundancies, which leads to the implementation of functional redundancies. Furthermore, the proximity operations have been designed to minimize the Delta-V consumption and increase on-board autonomy. As a key difference, Hovering has been proposed for RAMSES aiming the minimization of operational costs and ground effort, and benefiting from an increased AOCS/GNC autonomy.