(CN) — Most of the cosmic rays bombarding the Earth originated in nearby clusters of massive stars in the Milky Way Galaxy, according to new results from NASA's Advanced Composition Explorer spacecraft.
The Cosmic Ray Isotope Spectrometer — which allows astronomers to determine the age, temperature and speed of the light emitted by a given space object - aboard the spacecraft enabled a research team to determine the source of the cosmic rays. The findings were published Thursday in the journal Science.
The researchers were able to observe a rare type of cosmic ray that acts like a tiny clock, which limits the distances the source can be from Earth.
"Before the Advanced Composition Explorer observations, we didn't know if this radiation was created a long time ago and far, far away, or relatively recently and nearby," Eric Christian, a co-author of the paper with NASA's Goddard Space Flight Center, said in a statement.
Cosmic rays are high-speed atomic nuclei that have a wide range of energy. The most powerful type of cosmic rays travel at nearly the speed of light.
They present a hazard to unprotected astronauts because they can act like microscopic bullets, damaging structures and breaking down molecules in living cells. Earth's magnetic field and atmosphere protect people on Earth from less energetic cosmic rays.
NASA is actively researching methods for mitigating or reducing the effects of cosmic radiation on astronauts traveling to Mars.
The rare type of cosmic ray the researchers observed was a radioactive isotope of iron, Iron-60, with a half life — the length of time it takes for the radioactivity of an isotope to fall to half its original value - of 2.6 million years. Half of the iron nuclei decay into other elements during that span of time.
Isotopes have different amounts of neutrons, which can lead to multiple different forms and atomic masses of the same element
Cosmic rays are produced by a variety of violent events in space. Most cosmic rays that originate within our solar system have relatively low energy and stem from explosive events on the sun, such as coronal mass ejections and flares.
The highest energy cosmic rays are very rare, and scientists believe that they are powered by massive black holes consuming matter at the center of other galaxies.
The cosmic rays that are the subject of the research, which are referred to as galactic cosmic rays, come from outside our solar system but within the Milky Way galaxy. They are likely produced by shock waves from exploding stars called supernovas.
Iron-60 is created inside massive stars when they explode and is blasted into space by the shock waves from the supernova.
The galactic rays detected by the Advanced Composition Explorer allowed the team to estimate the age of the cosmic rays and the distance to their source.
"In 17 years of observing, the Advanced Composition Explorer detected about 300,000 galactic cosmic rays of ordinary iron, but just 15 of the radioactive Iron-60. The fact that we see any Iron-60 at all means that these cosmic ray nuclei must have been created fairly recently (within the last few million years) and that the source must be relatively nearby, within about 3,000 light years," Christian said.
Iron-60 cosmic rays travel through space at half the speed of light, roughly 90,000 miles per second. However, they typically do not travel far on a galactic scale.
They do not travel in straight lines, as they respond to magnetic forces since they are electrically charged. This forces them to take complicated paths along magnetic fields within the galaxy.
These radioactive cosmic rays also self-destruct and decay into other elements: Cobalt-60, after which they become Nickel-60. If they originate from too far away or were created too long ago, the Cosmic Ray Isotope Spectrometer will not be able to detect them.
"Our detection of radioactive cosmic-ray iron nuclei is a smoking gun indicating that there has likely been more than one supernova in the last few million years in our neighborhood of the galaxy," W. Robert Binns, a research professor at Washington University in St. Louis and lead author of the study, said in a statement.
Photo: NASA/ESA/Arizona State University
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