1950s-70s USA and Russia philatelic covers that are related to the early satellites development, manned and unmanned spaceflights to the moon.
1961-03-25 USA Explorer 10
Explorer 10 (also known as Explorer X or P14) was an American Earth-orbital satellite that investigated Earth's magnetic fields and nearby plasma. Launched on March 25, 1961, it was an early mission in the Explorer program and was the first satellite to measure the "shock wave" generated by a solar flare. The objective was to investigate the magnetic fields and plasma as the spacecraft passed through Earth's magnetosphere and into cislunar space. Because of the limited life of the spacecraft batteries, the only useful data were transmitted in real time for 52 hours on the ascending portion of the first orbit. All transmission ceased several hours later.