What was Explorer 1 primarily noted for discovering?

Explorer 1 is primarily noted for discovering the Van Allen radiation belt, which is a zone of charged particles held in place by the Earth's magnetic field. This groundbreaking discovery occurred during its mission, launched in 1958 as the first satellite sent into orbit by the United States. The satellite provided crucial data on cosmic rays and contributed significantly to the understanding of the Earth's magnetosphere.

The detection of this radiation belt was pivotal in space science, as it initiated a new field of research regarding the interaction between solar wind and the Earth's magnetic field. This finding demonstrated the existence of areas around the Earth where charged particles are trapped, significantly enhancing our knowledge about space weather and its potential effects on satellites and astronauts.

The other options—The Moon, Mars atmosphere, and the International Space Station—are not related to Explorer 1's mission or discoveries. The Moon has been studied through various missions and manned lunar landings, the Mars atmosphere has been explored primarily by missions specifically targeting the planet, and the International Space Station is a collaborative effort initiated many years after Explorer 1's launch.