Cassini-Huygens, a large mission sponsored by NASA and ESA, will fully explore the Saturnian system at the beginning of next century. In view of its great scientific interest and the involvement of the European Space Agency and the Agenzia Spaziale Italiana, Il Nuovo Cimento is publishing five papers dealing with the main features of the mission. This is a general introduction, with some attention to the planning of interplanetary space mission.