Italian-born physicist who was awarded one of the three Nobel Prizes for Physics in 2002 for his seminal discoveries of cosmic sources of X-rays, which helped lay the foundations for the field of X-ray astronomy. Giacconi received a Ph.D. from the University of Milan in 1954. In 1959 he joined the research firm American Science and Engineering, and in 1973 he moved to the Harvard-Smithsonian Center for Astrophysics. He later directed the Space Telescope Science Institute (198193) and the European Southern Observatory (199399).