Goals: Similar to Pioneer 0/Able 1, this mission was to study the ionizing radiation, cosmic rays, magnetic fields and micrometeorites in the vicinity of Earth and the enter lunar orbit.

Accomplishments: A malfunction of the launch vehicle prevented the spacecraft from reaching the escape velocity needed for a trip to the Moon. It reached a peak altitude of 115,400 km (about 71,700 miles) and re-entered Earth's atmosphere about 43 hours after launch. However, during its flight the spacecraft did return data on the near-Earth space environment, including confirmation of the existence of the Van Allen belts.

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