BIRDY is a 3-Unit CubeSat that is piggy-backed on a host mission to Mars and jettisoned at the beginning of the journey. Then it operates in full autonomy: no assistance, no communication but a beacon signal. The mission profile is a new contribution in Space Weather monitoring and an opportunity to assess the risks in the manned missions to Mars. It counts energetic particles in the maximum range 1 MeV/nucleon to 1 GeV/nucleon. The ground segment prepares a finetuned trajectory to be stored on-board, on the basis of the planed trajectory of the host mission that provides the main delta-V but not the ideal path. It makes the CubeSat compatible with almost all missions going to Mars. During the cruise, the CubeSat relies on an optical planet tracking system to locate itself and on small electrical thrusters to adapt its trajectory and perform the exact flyby at Mars that permits to come back to the Earth. The science data are collected all along the journey and only uploaded once in Mars' vicinity to one of the existing Martian orbiters or rovers, and once at the arrival back to the Earth. More widely than its own scientific mission, BIRDY demonstrates a new way to gather data from distant locations in the solar system. The project is an educational space mission, essentially leaded and designed by students from different educational levels in France and in Taiwan.