Most of the galactic cosmic rays reaching Earth come from nearby clusters of massive stars, according to new observations from NASA's ACE spacecraft. The distance between the cosmic rays' point of origin and Earth is limited by the survival of a radioactive isotope of iron, Fe-60, which has a half life of 2.6 million years. These tiny clocks indicate there was a source within spitting distance of Earth within the past few million years.
via Science Daily
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