The Compton Gamma Ray Observatory (CGRO) was the high-energy astrophysics mission that was one of NASA’s four Great Observatories in space. CGRO was launched on April 5, 1991, and started working perfectly, conducting more than nine years of pioneering scientific observations.

The CGRO had four major scientific instruments, all of which produced significant scientific discoveries in high-energy X-ray and gamma-ray astronomy. The Burst and Transient Source Experiment (BATSE) monitored the entire sky for stellar explosions and gamma-ray bursts, and helped prove that gamma-ray bursts are hugely powerful explosions that usually occur in galaxies other than the Milky Way. The Compton Telescope (COMPTEL) imaged nearly a tenth of the sky at a time, and the Oriented Scintillation Spectrometer Experiment (OSSE) made more detailed observations in smaller areas of the sky. Together, they made the most sensitive and detailed gamma-ray maps and studies ever of the Sun, our galaxy, and the entire sky. (OSSE even found evidence of streams of antimatter in the Milky Way!) Finally, the Energetic Gamma Ray Experiment Telescope (EGRET) gathered data on very high-energy gamma rays. Its observations led to the discovery of blazars.