The present thesis is about the design of an on board computer system for the Spei Satelles space mission. Turin Polytechnic has a rich history of CubeSat launches and on 2023 it launched a 3 units (3U) CubeSat mission under the name of Spei Satelles, promoted by the Dicaster for Communication of Vatican City in collaboration with the Italian Space Agency (ASI) and Consiglio Nazionale delle Ricerche (CNR). The system in question has the goal to collect telemetry and scientific data about the spacecraft and space environment, store it inside non volatile memory and prepare it to be sent back to earth, where it will be used to check the spacecraft health and verify thermal and attitude models theorized during the design phase; a secondary achievement is to demonstrate the survivability in the space environement of a low-cost system completely built from Components Off The Shelf (COTS).