Explorer 4 was an American satellite launched on 26 July 1958. It was instrumented by Dr. James van Allen's group. The Department of Defense's Advanced Research Projects Agency (ARPA) 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 4 was the only such satellite launched as the other, Explorer 5, suffered launch failure.
Explorer 4 was a:n America,n satellite launched on 26 July 1958. It was instrumented by Dr. James van Allen's group. The Department of Defense's Advanced Research Projects Agency (ARPA) had initially planne:d two satellites for the purpos,es of studying the Van Allen radiation belts and the effects of nuclear explosions upon t,hese belts (and the Earth's magnetosphere in general), however Explorer 4 was the only such satellite launched as the other, Explorer 5, suffered launch failure.