Explorer 11 (also known as S-15) was a NASA satellite that carried the first space-borne gamma-ray telescope. This marked the beginning of space gamma-ray astronomy. Launched on 27 April 1961 by a Juno II, the satellite returned data until 17 November 1961, when power supply problems ended the science mission. During the spacecraft's seven-month lifespan it detected twenty-two events from gamma-rays and approximately 22,000 events from cosmic radiation. Launch Explorer 11 was launched for the purpose of detecting the sources of high-energy gamma rays. The spacecraft achieved an orbit with an apogee of , a perigee of , a period of 108.10 minutes, and an inclination of 28.90°. In addition to detecting gamma rays, Explorer 11 was designed to map their direction with emphasis on the plane of the galaxy, the galactic center, the Sun, and other known radio noise sources; to relate the measurements to the cosmic-ray flux density and the density of interstellar matter; and to measure the high-energy gamma-ray albedo of the Earth's atmosphere. The satellite was a spin-stabilized octagonal aluminum box () on a cylinder ( in diameter and long). Explorer 11 was constructed so that its stable motion was an end-over-end tumble about the transverse principal axis that had the largest moment of inertia. The gamma-ray telescope assembly was mounted so that its axis of sensitivity, which was parallel to the long axis of the satellite, would rotate in the plane of tumble. The orientation of this axis in space was determined to approximately 5° by means of optical aspect detectors and the use of the known radiation pattern of the vehicle antenna. Telemetry was provided only in real time by two PM transmitters, since the onboard tape recorder failed at launch. Instruments Crystal Sandwich / Cherenkov Counter The instrumentation for the Explorer 11 crystal sandwich Cherenkov detector experiment was the same as that used for the gamma-ray telescope experiment. The gamma-ray experiment was designed by researchers at the Massachusetts Institute of Technology (MIT) primarily to detect cosmic gamma rays greater than 50 MeV. Charged particle data were collected using the same instrument. This telescope was used to determine the intensity and pitch-angle distribution of geomagnetically trapped protons. The telescope consisted of an anticoincidence plastic shield, layers of Sodium iodide (NaI) and Caesium iodide (CsI) crystals, and a cylindrical Cherenkov detector. When the anticoincidence requirement of the plastic scintillator shield was removed, charged particle information