Pioneer 4 was launched on March 3, 1959 — its mission being to gather data on the Moon as it flew past it on its way into a solar orbit. In a sense, Pioneer 4’s main job was to provide experience in throwing things at the Moon — and at this, it was fabulously successful. The satellite was tracked to a distance of 658,000 km (407,000 miles) from the Earth, a record at the time. It was also the U.S.’ first spacecraft to be placed into orbit around the Sun.
But the spacecraft had some simple instruments onboard as well. A Geiger counter measured radiation on its travels — showing that radiation levels near the Moon were no higher than those in free space. It also carried a photoelectric sensor, but this returned no data as the sensor was designed to trigger when the spacecraft flew within 30,000 km of the Moon (but the spacecraft flew by at about twice this distance).
So, a not-fully-successful mission — but an impressive step forward, especially considering that it launched barely a year after the U.S. put its first satellite into Earth orbit.