High above the Antarctic ice sheet, a balloon with a detector floated for twenty days. It caught tiny particles that had been traveling toward us for millions of years from distant stellar explosions. Now scientists have sorted through the catch and reported how much energy the protons and nuclei of different elements carried.

It turns out that not all particles are the same. Protons (hydrogen nuclei) lose energy faster than helium nuclei: their spectrum is steeper. But for carbon nuclei, above 10 TeV (that's like the energy of a flying mosquito, but packed into a single subatomic particle), the slope changes: the spectrum becomes flatter. This behavior doesn't fit a simple power law and hints at different sources or different paths for particles in the Galaxy.

The detector also measured the average mass of the particles. As energy increases, it behaves in a complex way: first it rises, then falls, then rises again. This 'humpy' curve indicates that the composition of cosmic rays changes with energy, and different elements come from different places. Perhaps heavy nuclei are born closer to us, while light protons come from farther away, in more powerful accelerators.

The balloon over Antarctica became a window into the far reaches of the Galaxy. Now we better understand what cosmic rays are made of and how they travel.