The possibility of trapped charged particles had previously been investigated by Kristian Birkeland, Carl Størmer, and Nicholas Christofilos prior to the Space Age. The existence of the belt was confirmed by the Explorer 1 and Explorer 3 missions in early 1958, under Dr. James Van Allen at the University of Iowa. The trapped radiation was first mapped out by Sputnik 3, Explorer 4, Pioneer 3 and Luna 1.