Astronaut Scott Kelly spent nearly a year aboard the International Space Station while his identical twin brother Mark stayed on Earth, creating an unprecedented experiment to see how extreme environments rewrite human biology. This episode follows his path from teenage EMT and naval aviator to commander of the ISS, and the engineering mindset that taught him to trust data over instinct.

The story covers his investigation into an F-14 crash, a harrowing foam strike during a shuttle launch, and the profound isolation of commanding a station while his sister-in-law was shot on Earth. At its core is the Twins Study and its surprising findings about telomeres, gene expression, and the biological whiplash of returning to gravity, raising hard questions about future missions to Mars.

• How early EMT work built his cognitive tolerance for high-stakes chaos
• The digital flight control system he championed and was first to fly
• Spaceflight Associated Neuro-ocular Syndrome and his fight to be reselected
• Why his telomeres lengthened in orbit then rapidly shrank on return
• His later activism, including sending back a Russian medal over Ukraine