Every second, invisible high-speed particles born in the deaths of distant stars — and possibly in sources far more extreme still — are raining down through Earth's atmosphere and passing straight through your body. This episode traces cosmic rays from their accidental discovery aboard a hot air balloon to the atmospheric air showers they trigger, the muons that reach the ground, and the still-unsolved mystery of the highest-energy particles ever detected.

You'll come away understanding what a cosmic ray actually is, how Victor Hess discovered them, where they come from, what happens in an atmospheric air shower, why muons reaching sea level are a real-world demonstration of relativity, and why aircrew, astronauts and electronics engineers all have to take cosmic radiation seriously.

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