NASA Really Did The Twin Experiment
The Rest Is Science
Sep 13, 2026 21:00
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Does space travel make astronauts younger?
Professor Hannah Fry and Michael Stevens explore Einstein’s twin paradox, time dilation and NASA’s famous Twins Study with astronauts Scott and Mark Kelly.
After Scott Kelly spent 340 days aboard the International Space Station, he returned milliseconds younger than his identical twin. But microgravity had also affected his eyesight, bones, blood, gut microbiome, body temperature, genes and telomeres.
So what exactly happens to the human body in space, and which changes remain after astronauts return to Earth?
In this episode, Hannah and Michael discover how deeply the human body depends on gravity. Along the way, they uncover why your head ages faster than your feet, what a 63 day cave experiment reveals about our sense of time, and whether future humans could ever truly adapt to life beyond Earth.
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