Nowadays, space missions face a relentless increase in requirements for on-board computers. Higher computing capacities are needed, while the power consumption, mass and area must be reduced. Unfortunately, requirements evolve faster than the ability of manufacturers to develop better space-qualified processors, so techniques that allow designers to use COTS (commercial, off-the-shelf) components are needed. As such, collaborative hardening is a powerful an efficient technique to guarantee the reliability of the safety critical tasks of a satellite. However, the stringent dependability requirements of space missions call for comprehensive on-ground validation of any design using COTS components before it can be used in orbit. In this work, we present the collaborative hardening techniques developed for the OPTOS satellite, and how it was on-ground validated against the effects of radiation. We introduce a methodology for early SEU sensitivity assessment based on fault injection through an autonomous emulation system. Fault injection is performed at system level, not unit level, to validate the safety critical techniques implemented by the collaborative architecture. The experimental results show that, while single units are vulnerable to the effects of radiation, the reliability of the system as a whole is not compromised.