Time sequence of NASA's Fermi Gamma-ray Space Telescope images showing an improved view of the Moon's gamma ray glow. The images, showing gamma rays with energies above 31 million electron volts, became clearer with Fermi's increased exposure to the radiation over time. The time period of the eight images range from 2 (2nd image, top row) to 128 months (4th image, bottom row). Scientists are studying the Moon's gamma ray radiation in order to understand cosmic ray radiation caused by high-energy particles that move at speeds close to the speed of light.