When Explorer I, America's first satellite, was launched in early 1958, it contained a payload of miniaturized instruments designed to study the regions surrounding the earth in space. James Van Allen, an expert on cosmic rays and skilled with electronic miniaturization technology, had designed the small instrument package that orbited over six hundred miles above the Earth.

One of the most intriguing findings of Explorer I involved radiation levels surrounding the Earth. As the satellite ascended, cosmic ray levels as measured on its Geiger counter rose to a peak, dropped off suddenly to zero, and then returned to high levels once again. Explorer 3 gave the same peculiar readings. Carl McIlwain, one of Van Allen's graduate students, suggested that a high enough level of radiation could jam a counter and send the apparent counting rate to zero. Van Allen speculated that the satellites had found a...