Explorer 4

Explorer 4 was designed to further investigate the radiation belt around Earth, discovered during the Explorer 1 and 3 missions. The space satellite, which was part of the notbale Explorer mission series that launched the United States into the Space Age, made the first detailed measurements of charged particles (protons and electrons) trapped in the terrestrial radiation belts.

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.