The probe entered a special low-altitude orbit for about five months to allow the GRAND instrument to analyze emissions from Vesta's surface that result from collisions between cosmic ray particles and elements in the asteroid's dirt.

MSN: Surprisingly, giant asteroid Vesta covered in hydrogen

Collisions between the cosmic rays and the slower-moving protons that exist in the gas and the dust around a supernova should produce subatomic particles called neutral pions.

BBC: Cosmic rays: Fermi telescope settles mystery of origin

In 1966 Kenneth Greisen, Vadim Kuzmin and Georgiy Zatsepin showed that high-energy charged particles (cosmic rays are mostly atomic nuclei, and thus positively charged) should be slowed by collisions with the photons of the cosmic microwave background (radiation left over from the Big Bang that permeates all space).

ECONOMIST: Cosmic rays: They came from outer space | The