Scientists studied rocks from several NASA moon landings between 1969 and '72, including Apollo missions 12, 15 and 16. Within those rocks, they found a radioactive iron isotope called 60Fe that is created almost exclusively in supernovas. The team was able to date the isotope to about 2 million years, based on its half-life of 2.62 million years. That corresponds well with the discovery of the same isotope here on Earth from Pacific Ocean crusts.

The moon's isotopes provide a better cosmic record than those found on earth, since they're not altered by passing through our atmosphere. As a result, scientists were able to deduce an upper limit for how much 60Fe reached the moon, and further calculate where it came from. "The measured 60Fe-flow corresponds to a supernova at a distance of about 300 light years," says Technical University of Munich researcher Dr. Gunther Korschinek.

That means that a Milky Way star not terribly far away went supernova, and the material actually hit our planet. Luckily, those particles were small when they arrived, and if it happens again, we will get a heads up. Supernova ejecta travel at one-tenth the speed of light, max, so if we happen to witness a similar explosion, the consequences won't hit us for at least 3,000 years.