1950s-70s USA and Russia philatelic covers that are related to the early satellites development, manned and unmanned spaceflights to the moon.
1967-07-19 USA Explorer 35
One spacecraft not so closely tied to the Apollo program was Explorer 35, launched on 10 July 1967, and sent 12 days later into a highly elliptical orbit around the moon. Explorer 35 was also known as a lunar-anchored interplanetary monitoring platform, or IMP. It found the moon had been volcanically active. No evidence of trapped radiation belts around the moon that is similar to the Van Allen belt surrounding the earth.