Soon after the space shuttle atlantis launched a new observatory into orbit in 1991, Gerald Fishman of the NASA Marshall Space Flight Center realized that something very strange was going on. The Compton Gamma Ray Observatory (CGRO), designed to detect gamma rays from distant astrophysical objects such as neutron stars and supernova remnants, had also begun recording bright, millisecond-long bursts of gamma rays coming not from outer space but from Earth below.
Astrophysicists already knew that exotic phenomena such as solar flares, black holes and exploding stars accelerate electrons and other particles to ultrahigh energies and that these supercharged particles can emit gamma rays—the most energetic photons in nature. In astrophysical events, however, particles accelerate while moving almost freely in what is essentially a vacuum. How, then, could particles in Earth's atmosphere—which is certainly nowhere close to being a vacuum—be doing the same thing?