Every second, dozens of particles from space fly through each of us. Most are light protons, but there are also heavier atomic nuclei: carbon, oxygen, iron. They are born in stellar explosions and travel to Earth for millions of years. Their energy can be staggering: a single particle can carry as much as a tennis ball on a serve.

But there is a strange kink in this particle stream. At energies around 3 PeV (peta-electronvolts: that's like 3 million billion electronvolts), there are noticeably fewer particles than expected. No one has known exactly why for half a century. Perhaps they lose energy along the way, or their sources work differently.

To figure this out, the HERO instrument is being built. It's a calorimeter — a device that slows down a particle and measures how much heat it releases. The heavier the calorimeter, the more precise the measurement. HERO will have a version up to 70 tons, like ten elephants. It will fly into space no earlier than 2029 and over five years will collect data on all common nuclei in cosmic rays. The accuracy will be up to 3%—as if you measured your weight with an accuracy of 30 grams.

If the kink in the spectrum turns out to be the same for all nuclei, then the cause lies in how particles travel through the galaxy. If it's different, then their sources are to blame. The answer to this question will hint at where the most energetic particles in the universe are born.