1950s-70s USA and Russia philatelic covers that are related to the early satellites development, manned and unmanned spaceflights to the moon.
1958-01-31 USA Explorer 1
Explorer 1 was the first Earth satellite of the United States, launched on February 1, 1958 as part of the United States program for the International Geophysical Year and in response to the launch of the Soviet satellite Sputnik 1. Besides being the first US satellite, it was known for discovering the Van Allen radiation belt.
Van Allen belts refers specifically to the radiation belts surrounding Earth. The existence of the belt was confirmed by the Explorer 1 and Explorer 3 missions in early 1958, under Dr James Van Allen at the University of Iowa.