Explorer 1 to 5
The first generation satellites of the still-active Explorers program were among the very first spacecraft successfully launched into orbit by the United States. Although unable to beat Sputnik on being the first artificial object in orbit, the Explorers (particularly Explorer 1) discovered the existence of a region of charged particles around the Earth's magnetosphere called the Van Allen radiation belts. Five satellites were launched in 1958, only three were successful (1, 3 and 4). All of them operated for four months each, but stayed derelict in orbit for many years until 1970
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