A very fast charged particle from interstellar space. The majority of these particles are protons (85%), helium nuclei (14%), electrons (1%) and other atomic nuclei.
Their energy is typically 1 GeV (the energy that an electron would have if accelerated by a voltage of a billion volts), but it can reach as high as 10 to the power 11 GeV. These particles could have been accelerated by supernova explosions. The Sun also emits high energy particles, but generally with very much lower values of GeV.
High energy cosmic rays coming from space and striking the Earth's atmosphere are very puzzling. These rays disintegrate, creating spectacular showers of particles that can be detected on Earth. Such high energy rays, higher than 7x10^19eV, are very rare: there are only a few events per square metre per century, and a few dozen observations have been made in the world since 1962. However, their mere existence is a challenge for theoretical physicists. These cosmic rays are assumed to be protons. But this assumption, the most natural, comes up against a problem. Very high energy protons travelling great distances in space are in a way braked by the very diffuse gas of photons that forms the well-known cosmic background radiation at 3 K, the echo of the "Big Bang". This effect, called the Greisen-Zatsepin-Kuz'min (GZK) limit, should completely eliminate the very high energy cosmic rays, or at least, those emitted by distant sources, spread out over so-called "cosmological" distances (distances measured in millions of light-years). But the few observations that we have do not show this limit at all, quite the contrary, in fact: there is a relative increase in these high energy events.