This work is framed within the Systems Engineering domain, and in particular in the area of approaches and methodologies for the early design stages of space systems and missions. This thesis aims at defining a set of tools for enabling the design and sizing of space missions developed by nanosatellites. The tools will support the design from phase 0 through phase B of the life cycle, making use of the MBSE (Model Based System Engineering) and Concurrent Design approaches. The Model-Based Systems Engineering (MBSE) is an approach currently used in the industry to improve the efficiency and effectiveness of the design and development process.