On 13 April 1970, an oxygen tank on board the Apollo-13 spacecraft exploded, seriously jeopardizing the lives of three astronauts. NASA mission controllers tinkered with the 15 training simulators and, despite the zero margin for error, quickly came up with fool-proof procedures that brought back all three astronauts safely to earth. Although the term ?digital twin? was not in use back then, this was perhaps its first application. That?s just about how useful even rudimentary digital twins were half a century ago. ?Rudimentary? because simulators are not digital twins by themselves. The tinkering to replicate conditions in space brought them close to digital twins