There is a particular kind of iron that cannot be made on Earth. It forms only inside massive stars, in the extraordinary pressures of their cores, and it reaches us only one way, when those stars explode. Iron-60 is a radioactive isotope with a half life of 2.6 million years, that’s short enough, on cosmic timescales, that every atom present when our Solar System formed 4.5 billion years ago has long since decayed to nothing. Any iron-60 found on Earth today came from somewhere else. Scientists have known for years that our Solar System was struck by iron-60 from nearby supernovae on at least two occasions millions of years ago, the evidence is written in deep sea sediments and in rocks on the Moon. But then something puzzling turned up, iron-60 in Antarctic surface snow less than twenty years old. There had been no nearby stellar explosion to explain it. So where was it coming from?…