Explorer 4
Explorer 4 was a US satellite launched on July 26, 1958. It was instrumentated by Dr. James van Allen's group. The Department of Defense's Advanced Research Projects Agency had initially planned two satellites for the purposes of studying the Van Allen radiation belts and the effects of nuclear explosions upon these belts (and the Earth's magnetosphere in general), however Explorer IV was the only such satellite launched.See also: Operation Argus
