Explorer 3

Explorer 3 was the third satellite of the Explorer mission series and the first successful follow-on to Explorer 1, which made history when in January 1958 it became the United States' first space satellite. Like it's predecessor, Explorer 3 collected data confirming the theory that radiation belts trapped by Earth's magnetic field exist around the planet.

The Explorer series consisted of more than fifty satellites launched over a period of about 15 years. In 1960, NASA's Jet Propulsion Laboratory, which designed, built and operated the first five satellites of the series, turned the project over to NASA's Goddard Space Flight Center and turned its attention to planetary exploration with its Mariner, Ranger and Surveyor spacecraft projects.

Data from Explorer 3 — combined with earlier measurements from Explorer 1 — confirmed Principal Investigator James Van Allen's theory that radiation belts trapped by Earth's magnetic field exist around the planet.