Massachusetts Institute of Technology · CC BY-SA 3.0

Bruno Rossi built the instruments that let physicists actually see cosmic rays, inventing an improved coincidence circuit and traveling to Eritrea to prove cosmic ray intensity varied by direction — a foundational result for the field. Italy's 1938 racial laws forced him out of the country; he worked with Niels Bohr in Denmark, then Patrick Blackett in Manchester, before settling in the United States, where he joined the Manhattan Project at Los Alamos and later helped found X-ray astronomy, with instruments he built detecting the magnetopause and discovering the first extra-solar X-ray source.