Cosmic Rays

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High-energy subatomic particles mainly originating outside the Solar System that continuously bombard the Earth from all directions. They represent one of the few direct samples of matter from outside our solar system and travel through space at nearly the speed of light. These charged particles – positively charged protons or nuclei, or negatively charged electrons – are composed mainly of (~85%) protons – nuclei of hydrogen, the lightest and most common element in the universe – but they also include nuclei helium alpha particles (~14%) and heavier nuclei (~1%), all the way up to lead or, according to some scientists, even uranium.

When they collide with atoms and molecules in the upper atmosphere, they generate cosmic ray “showers.” The initial collision produces pions (π), which quickly decay into muons (m) and γ-rays. Muons decay further into electrons (e-), positrons (e+), and neutrinos (n). Deceleration of the electrons and positrons in the atmosphere produces a flash of light that can be observed from the ground with special telescopes; however, most of the secondary cosmic ray particles that reach sea-level are undecayed muons.

The Earth’s atmosphere and magnetic field shield the biosphere from some of the most damaging high-energy cosmic ray particles.

Cosmic rays are used to determine the amount of time a meteorite or return mission samples have spent in space – the cosmic ray exposure age.

Some or all content above used with permission from J. H. Wittke.