Supernova explosions continue to shower Earth with cosmic rays or radioactive debris, according to a new study, bolstering beliefs that supernovas have previously affected our own planet.

At least two massive stars not far from the solar system exploded in the past few million years, as suggested by observations from the NASA’s Advanced Composition Explorer (ACE) spacecraft, which is about 930,000 miles from Earth.

The “smoking gun” that indicated a supernova in the planet's neighborhood: cosmic rays and high-energy electromagnetic radiation bombarding the planet and its vicinity.

“The new data also show the source of galactic cosmic rays is nearby clusters of massive stars, where supernova explosions occur every few million years,” said study author and physics professor Martin Israel of Washington University.

Since the 1997 launch of the spacecraft, it has detected 15 cosmic rays composed of radioactive isotope iron-60. This maintains a half life of 2.6 million years, meaning half of the iron nuclei in any sample will decay into other elements during that time period.

The relatively short half-life of iron-60 implies that it has been produced in the past few million years. The radioactive iron is thought to be created from core-collapse supernovae, or massive explosions that mark massive stars’ death and largely occur in massive star clusters known as OB associations.

Based on an analysis of the radioactive iron nuclei and their diffusion and spread throughout space, the two supernovas probably took place within 2,000 light-years or so of Earth.

“[This] is telling us the supernova explosion that accelerated these cosmic rays must not have been too far away, and must not have been too long ago,” Israel told Gizmodo.

More than 20 OB associations fall into this range of distance or close enough to shower Earth with cosmic rays, including Upper Scorpius with 83 stars, Upper Centaurus Lupus with 134 stars and Lower Centaurus Crux with 97 stars. Owing to their proximity and size, they emerge as the highly likely sources of the ACE-detected 60Fe.

Another recent research coincidentally demonstrated iron-60 in Pacific, Atlantic, and Indian ocean floor samples. The authors concluded that these vouch for two supernova explosions, the first occurring 6.5 to 8.7 million years earlier and the second 3.2 to 1.7 million years ago.

Just last week, a study arrived at the same conclusion after examining debris of iron-60 on the moon – nine core samples returned by the Apollo crews.

Israel noted, however, that cosmic rays hail from all directions and it would be almost impossible to figure out their exact source. The team is now gearing up to use other instruments for searching rarer and heavier cosmic ray isotopes to enlighten them on the matter.

These results continue to intrigue scientists, particularly with supernova connections such as the occurrence of the planet’s most recent ice age.

The findings were published on April 21 in the journal Science.

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