Which early spacecraft discovered the Van Allen radiation belts while mapping the Earth's magnetic field?

The correct answer is Explorer I, which was the first successful satellite launched by the United States in January 1958. As part of its mission, Explorer I was designed to study cosmic rays and, while in orbit, it discovered the existence of the Van Allen radiation belts. These belts are zones of charged particles held in place by Earth's magnetic field, which can be harmful to spacecraft and astronauts if not properly accounted for. The data collected by Explorer I contributed significantly to our understanding of the Earth's magnetosphere and were critical for future space missions and advancements in space science.

The significance of Explorer I’s findings was recognized by the scientific community, leading to further research and exploration into understanding radiation environments in space.