Los Alamos National Laboratory · Public domain

A particle physicist who worked on uranium enrichment for the Manhattan Project, then found his physics career blacklisted for years after McCarthy-era scrutiny of his earlier Communist Party membership, unable to get a university teaching post until 1957. He rebuilt his career teaching high school science in Colorado before founding San Francisco's Exploratorium in 1969, a hands-on science museum built on the idea that people learn by touching and testing rather than reading placards, which he directed until his death.