The Gamma Ray Observatory (GRO) is designed to survey gamma ray sources such as novas, supernovas, and neutron stars. Objects that emit gamma rays cannot be photographed, but images can be derived through painstaking reconstruction of detector data.

Gamma rays are similar to visible light, except with 10,000 times more energy. This intense energy allows gamma ray photons to penetrate matter very much like X rays and they are not scattered like visible light photons are scattered by our atmosphere. However, most cosmic gamma radiation is absorbed by the earth's atmosphere and it must be detected by satellites that are above the atmosphere. Gamma photons are very rare; for example, the Crab Nebula pulsar produces one gamma photon for every 10,000 visible light photons.