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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.

The Van Allen radiation belts are regions of energetic charged particles trapped around Earth by its magnetic field. Particle motion combines gyration around a field line, repeated magnetic reflection between hemispheres and slower drift around the planet. The inner belt is especially associated with energetic protons, while energetic electrons are prominent in the outer belt; their boundaries and intensities vary.

The discovery grew from cosmic-ray detector measurements aboard Explorer 1 and follow-up observations by Explorer 3 in 1958. A Geiger counter sometimes returned unexpectedly low counts because intense radiation saturated the detector, rather than because particles were absent. Repeated orbital observations established a trapped population associated with Earth's magnetosphere.^[ref]

A magnetic field can redirect a particle without doing work on it, so trapping alone does not explain where its energy came from. Measuring particle energy spectra together with waves and fields is necessary to investigate acceleration and losses. This distinction matters both for space-plasma physics and for the radiation exposure of spacecraft electronics.