This is a single spark chamber element from the stack of 28 modules used in the gamma-ray detector called EGRET (Energetic Gamma-Ray Experiment Telescope), one of four major instruments that flew on the Compton Gamma Ray Observatory satellite (CGRO). EGRET was responsible for producing an all-sky map of the gamma ray sky, locating new sources of gamma rays in the 20 to 30 billion electron volt energy range for closer study. Gamma-rays consist of energetic electromagnetic radiation that arises from a number of exotic processes that occur in particularly violent regions of the universe. These places include solar flares, nuclear reactions resulting from supernovae core collapse, the decay of radioactive particles in interstellar space, collisions of cosmic rays and interstellar gases and grains, annihilation events when matter and antimatter interact in the vicinity of neutron stars and black holes, and regimes in the cores of galaxies where supermassive black holes create intense gravitational acceleration. GRO was launched 1991 from the Space Shuttle Atlantis and provided data on celestial gamma-ray sources until it was commanded to re-enter the earth's atmosphere in June 2000 to minimize the chance of injury from the remnants of the 17-ton satellite, the largest civilian scientific payload ever flown on the Shuttle. This flight spare unit was manufactured by Ideas Inc., under contract to NASA's Goddard Space Flight Center. It was transferred to NASM in 1993.