Igor Kurchatov called him a "genius" and Andrei Sakharov named him "a man of universal scientific interests." After the first meeting in Moscow, Stephen W. Hawking wrote to Zeldovich: "now I know that you are a real person and not a group of scientists like Bourbaki." He was a member of the American Academy of Arts and Sciences (1975), the United States National Academy of Sciences (1979), and the American Philosophical Society (1979).