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This paper presents the design of an autonomous GNC system for the rendezvous phase of low-cost missions to very small, faint Near Earth Object (NEO). The optical navigation problem presents a strong relation between the approach strategy and the best combination of sensors, algorithms and actuators. First of all the problem is globally analyzed and the rendezvous strategy defined, from detection of the asteroid to insertion into a desired bounded orbit, considering the observability of the full spacecraft state relative to the asteroid. Then, for each sub-phase the GNC algorithms, including the Image Processing, are described for different propulsion systems (impulsive and low-thrust), and including or not an altimeter. Monte Carlo simulations for different NEO (Apophis and 2003 SM84) are run to tune the algorithms and to asses the system performances. The simulation tool includes high-fidelity models of the optical sensors (navigation camera and star-tracker). The conclusions show the driving parameters and the feasibility of the proposed GNC system.

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