Definition of Explorer I from the Oxford Advanced Learner's Dictionary

 

Explorer I

 
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the first US artificial satellite (= electronic device that goes around the earth) in space. It was sent up on 31 January 1958 from Cape Canaveral, Florida, by the US Army. Explorer I, which weighed 30 pounds/13.5 kilograms, discovered the Van Allen radiation belt. Several other Explorers were later sent up to study radiation and 'wind' from the sun.