Pioneer 3 was a spin-stabilized spacecraft launched at 05:45:12 GMT on 6 December 1958 by the U.S. Army Ballistic Missile Agency in conjunction with NASA, using a Juno II rocket. This spacecraft was intended as a lunar probe, but failed to go past the Moon and into a heliocentric orbit as planned. It did however reach an altitude of 102,360 km before falling back to Earth. The revised spacecraft objectives were to measure radiation in the outer Van Allen radiation belt using two Geiger-Müller tubes and to test the trigger mechanism for a lunar photographic experiment.

Juno II was an American space launch vehicle used during the late 1950s and early 1960s. It was derived from the Jupiter missile, which was used as the first stage.