Explorer XI
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Explorer 11, also known as S15, was the orbital spacecraft that carried the first gamma ray telescope. This was the earliest beginnings of gamma-ray astronomy. Launched on 27 April 1961 by a Juno II rocket the satellite returned data until early September, when power supply problems eventually ended the mission. During the spacecraft's four month lifespan it detected twenty-two events from gamma-rays and approximately 22,000 events from cosmic radiation.
The telescope used a combination of a sandwich scintillator detector along with a Cherenkov counter to measure direction of events. Since the telescope could not be aimed the spacecraft was set in an end over end tumble to give a rough scan of the celestial sphere, with emphasis on the galactic plane, the galactic center, the Sun, and other known radio noise sources.
The gamma-ray events observed by Explorer 11 seemed to suggest a random distribution 'background' of gamma-rays. Later telescopes observed point sources in addition to the background noise of gamma-rays.
