At the meeting in Naples (20-23 September 1999), the European Space Agency's Science Programme Committee recognised the achievements of the late Giuseppe Colombo of the University of Padua by adopting his name for the Mercury project which was then under consideration. Almost everything known until now about the planet Mercury comes from three passes by NASA's Mariner 10 in 1974-75, which were inspired by Colombo's calculations. He suggested how to put that spacecraft into an orbit that would bring it back repeatedly to Mercury. The Italian scientist also explained, as an unsuspected resonance, Mercury's peculiar habit of rotating three times in every two revolutions of the Sun.