Dan Bahcall · CC BY-SA 3.0

John N. Bahcall helped identify the solar neutrino problem — the puzzling shortfall between predicted and observed neutrinos from the sun — a discrepancy whose eventual resolution reshaped physicists' understanding of neutrinos themselves. A driving force behind the development of the Hubble Space Telescope, he spent his career at the Institute for Advanced Study in Princeton, where he held the Richard Black professorship in astrophysics.