On December 14, 2009, while NASA's Fermi flew over Egypt, the spacecraft intercepted a particle beam from a terrestrial gamma-ray flash (TGF) that occurred over its horizon. Fermi's Gamma-ray Burst Monitor detected the signal of positrons annihilating on the spacecraft тАФ not once, but twice. After passing Fermi, some of the particles reflected off of a magnetic "mirror" point and returned.