Cosmic rays are atomic particles flying nearly at light speed, constantly bombarding the Earth. Lower-energy particles come from the Sun. Mid- and higher-energy cosmic rays, such as protons and heavier atomic nuclei, may be produced in stellar explosions. What is most perplexing are the highest-energy cosmic rays. Not only is their origin unknown, these particles possess an energy level that seems implausible.

"These highest-energy cosmic rays are a Catch-22," said Dr. Robert Streitmatter, a Goddard astrophysicist who works on NIGHTGLOW. "Anything that energetic had to have come from within 150 million light-years of Earth, because anything traveling farther would have lost its energy during the long trip. Yet there are no obvious sources within 150 million light years that could produce a particle this energetic."

Right: At energies greater than a few times 10-19 eV, the number of cosmic rays is expected to decline sharply because they interact with the omnipresent 2.7 K cosmic microwave background. Observations show an initial drop off (known as the ankle), but then the spectrum becomes more shallow, meaning there are a lot more particles at these energies than was expected. These are subatomic particles with more kinetic energy than a major league fastball! Where do these ultra energetic particles come from? It's a mystery that NIGHTGLOW measurements will help solve. [more information from Goddard Space Flight Center]

When these highest-energy cosmic rays strike the Earth's atmosphere, they produce low-energy UV radiation in the NIGHTGLOW range. A proposed NASA satellite mission called OWL (Orbiting Wide-angle Light-collectors) would detect this radiation from a low-earth orbit and help us understand their origins. The highest-energy cosmic rays are rare, and a device such as OWL is needed to search for them simultaneously over wide stretches of the atmosphere, as wide as 400,000 square miles.