An astronaut refused to fly until this mathematician checked the computer's math by hand

Katherine Johnson calculated the trajectories for America's first crewed spaceflights at NASA, working the orbital mechanics with a mechanical desk calculator. Before his 1962 orbital flight, John Glenn asked engineers to have Johnson personally re-verify the new electronic computer's numbers, reportedly saying he wouldn't fly until she signed off. She went on to calculate trajectories for Apollo 11's Moon landing and its return.

— NASA, Katherine Johnson Biography — NASA Langley Research Center