Explorer 3

Explorer 3 was a 1958 Earth satellite whose radiation measurements helped establish the existence of the Van Allen radiation belts. Like Explorer 1, it carried a Geiger-counter experiment, but its recording system allowed measurements to be stored and transmitted later. That improved orbital coverage beyond the periods when a spacecraft was directly visible to a receiving station.^[ref][ref]

The interpretation illustrates an important detector limitation. A Geiger counter can cease to report the expected events when an intense particle flux overwhelms its counting system. An apparent absence of counts is then not evidence for an empty region. Repeated measurements at different orbital positions helped connect these anomalous responses to a trapped charged-particle population around Earth.

Thus the satellite's scientific contribution depended on the combination of detector response, orbital sampling and data storage. A map of raw count rates needs calibration and saturation checks before it becomes a map of radiation intensity.