Once considered radiation but now known to be charged particles, cosmic rays constantly bombard Earth, but ultrahigh-energy cosmic rays—mostly protons accelerated to near light speed by some mysterious mechanism—are extremely rare. Arriving on Earth at a rate of one particle per square kilometer per century, they collide with the atmosphere, producing a cascade of billions of secondary particles. “By observing the tracks particles make in the atmosphere,” explains UChicago astrophysicist Angela Olinto, “we can look back onto the universe to search for their origins.”

Hoping to discover the source of the cosmic rays, 100 million times more energetic than anything humans can produce, Olinto leads the US branch of a 13-country collaboration to build a 2.5-meter ultraviolet telescope called the Extreme Universe Space Observatory, which will be deployed aboard the Japanese Experiment Module of the International Space Station in 2017. Instead of looking out into space, the telescope will face Earth to observe cosmic ray collisions—detecting not the rays but the UV light produced when nitrogen molecules, excited by the particle shower, return to ground state. For this mission, Earth’s atmosphere is the particle detector.

Olinto expects the telescope, from an altitude of 400 kilometers, to observe ten times the number of showers ground-based observatories are capable of detecting, helping to map “hot spots” where cosmic rays seem to originate. Astrophysicists can then look in those directions to find possible sources—maybe supermassive black holes, rapidly spinning neutron stars, or something else entirely—and better understand the universe’s dynamics.