On Dec. 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 -- not once, but twice. After passing Fermi, some of the particles reflected off of a magnetic "mirror" point and returned. Credit: NASA's Goddard Space Flight Center

On Dec. 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 — not once, but twice. After passing Fermi, some of the particles reflected off of a magnetic "mirror" point and returned. Credit: NASA’s Goddard Space Flight Center

On Dec. 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 — not once, but twice. After passing Fermi, some of the particles reflected off of a magnetic "mirror" point and returned. Credit: NASA’s Goddard Space Flight Center