Gamma Ray Observatory (GRO) in orbit, 1991.
© National Aeronautics & Space Administration / Science & Society
The GRO, launched on 5th April 1991, is the largest and most powerful gamma ray satellite ever built. The objective of the GRO mision was to map the entire sky comprehensively. Gamma rays are parcels of electromagnetic radiation that are produced in the most extreme physical conditions and allow scientists to study exotic objects such as neutron stars, black holes and quasars.