Like long-distance runners, atomic nuclei, from light helium to heavy iron, race toward us across the Galaxy. At the finish line, in a detector on a high-altitude balloon, we see that heavy nuclei arrive a bit more exhausted than light ones. The difference is small, but it repeats from flight to flight.

Scientists took data from the ATIC instrument, which flew over Antarctica, and 'rewound' the particles' path back to their sources. It turned out that this difference in tiredness is not random: the probability of error is less than three hundredths of a percent. It seems that the stars that accelerate the nuclei do so slightly differently for different elements.

This means that particle accelerators in space, such as supernova explosions, do not 'kick' different nuclei equally. Perhaps light and heavy elements are born in different conditions or escape from their stars differently. This conclusion helps us peek into the kitchen where cosmic rays are cooked.