Galactic cosmic rays (GCRs) are the high-energy particles that flow into our solar system from far away in the Galaxy. GCRs are mostly pieces of atoms: protons, electrons, and atomic nuclei which have had all of the surrounding electrons stripped during their high-speed (almost the speed of light) passage through the Galaxy. Cosmic rays provide one of our few direct samples of matter from outside the solar system. The magnetic fields of the Galaxy, the solar system, and the Earth have scrambled the flight paths of these particles so much that we can no longer point back to their sources in the Galaxy. If you made a map of the sky with cosmic ray intensities, it would be completely uniform. So we have to determine where cosmic rays come from by indirect means.
One of the indirect observations we can make, the "composition" of GCRs, can tell us a lot about the sources and the cosmic rays' trip through the Galaxy. The "composition" of cosmic rays is the way in which the cosmic rays are divided up into each of the different types, what fraction is protons, what fraction is helium nuclei, etc. All of the natural elements in the periodic table are present in cosmic rays, in roughly the same proportion as they occur in the solar system. But detailed differences provide a "fingerprint" of the cosmic ray's source. Measuring the quantity of each different element is relatively easy, since the different charges of each nucleus give very different signatures. Harder to measure, but a better fingerprint, is the isotopic composition (nuclei of the same element but with different numbers of neutrons). To tell the isotopes apart involves, in effect, weighing each atomic nucleus that enters the cosmic ray detector.
About 90% of the cosmic ray nuclei are hydrogen (protons), about 9% are helium (alpha particles), and all of the rest of the elements make up only 1%. Even in this one percent there are very rare elements and isotopes. These require large detectors to collect enough particles to say something meaningful about the "fingerprint" of their source. The HEAO Heavy Nuclei Experiment, launched in 1979, collected only about 100 cosmic rays between element 75 and element 87 (the group of elements that includes platinum, mercury, and lead), in almost a year and a half of flight, and it was much bigger than most scientific instruments flown by NASA today. To make better measurements requires an even larger instrument, and the bigger the instrument, the greater the cost.
Where do they come from?
Most galactic cosmic rays are probably accelerated in the blast waves of supernova remnants. This doesn't mean that the supernova explosion itself gets the particles up to these speeds. The remnants of the explosions, expanding clouds of gas and magnetic field, can last for thousands of years, and this is where cosmic rays are accelerated. Bouncing back and forth in the magnetic field of the remnant randomly lets some of the particles gain energy, and become cosmic rays. Eventually they build up enough speed that the remnant can no longer contain them, and they escape into the Galaxy.
Because the cosmic rays eventually escape the supernova remnant, they can only be accelerated up to a certain maximum energy, which depends upon the size of the acceleration region and the magnetic field strength.
However, cosmic rays have been observed at much higher energies than supernova remnants can generate, and where these ultra-high-energies come from is a big question. Perhaps they come from outside the Galaxy, from active galactic nuclei, quasars or gamma ray bursters. Or perhaps they're the signature of some exotic new physics: superstrings, exotic dark matter, strongly-interacting neutrinos, or topological defects in the very structure of the universe. Questions like these tie cosmic-ray astrophysics to basic particle physics and the fundamental nature of the universe.
Source for an earlier version of the above article: NASA