Most of the cosmic rays that we detect at the Earth originated relatively recently in nearby clusters of massive stars, according to the researchers.
After analysing results from NASA's Advanced Composition Explorer (ACE) spacecraft, the research team determined the source of these cosmic rays by observing a very rare type of cosmic ray that acts like a tiny timer, limiting the distance the source can be from the Earth.
"Before these observations, we didn't know if this radiation was created a long time ago and far, far away, or relatively recently and nearby," said Eric Christian of NASA's Goddard Space Flight Center in Greenbelt, Maryland.
The Earth's atmosphere and magnetic field shield us from less-energetic cosmic rays which are the most common.
However, cosmic rays will present a hazard to unprotected astronauts traveling beyond the Earth's magnetic field because they can act like microscopic bullets, damaging structures and breaking apart molecules in living cells.
NASA is currently researching ways to reduce or mitigate the effects of cosmic radiation to protect astronauts travelling to Mars.
The galactic cosmic rays detected by ACE that allowed the team to estimate the age of the cosmic rays, and the distance to their source, contain a radioactive form of iron called Iron-60 (60Fe).
Some 60Fe in the debris from the destroyed star is accelerated to cosmic-ray speed when another nearby massive star in the cluster explodes and its shock wave collides with the remnants of the earlier stellar explosion.
"Our detection of radioactive cosmic-ray is a smoking gun indicating that there has likely been more than one supernova in the last few million years in our neighbourhood of the Galaxy," said Robert Binns from Washington University, St. Louis.
ACE was launched on August 25, 1997 to a point 900,000 miles away between the Earth and the Sun where it has acted as a sentinel, detecting space radiation from solar storms, the galaxy and beyond.
The research was published in the journal Science.
--IANS
na/vt
