The Compton Gamma-Ray Observatory (GRO) is one of NASA's four "Great Observatories". GRO's specific mission is to study the sources and astrophysical processes (e.g., nuclear reactions, electron bremsstrahlung, matter-antimatter annihilation, elementary particle producation and decay, Compton scattering, and synchrotron radiation) that produce that highest energy electromagnetic radiation from the cosmos.