On February 26, 1964, a 40-year-old man slipped in a hotel bathroom and clocked his head on the tub. The painful tumble would end up altering how the entire world approached space exploration. Why? Because that man was John Glenn, the first American to orbit the Earth, and that fall triggered a medical mystery that pushed to the forefront research into what spaceflight might do to the human body.

In the latest Popular Sciencevideo, we dig into the intriguing backstory of Glenn’s bathroom spill.

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