Short answers, pulled from the story.
Pioneer 6, 7, 8, and 9 were designed to make the first detailed, comprehensive measurements of the solar wind, the solar magnetic field, and cosmic rays from widely separated points in interplanetary space. They also served as the world's first space-based solar weather network, providing practical data on solar storms that affect communications and power systems on Earth.
Pioneer 6 launched on the 16th of December, 1965; Pioneer 7 on the 17th of August, 1966; Pioneer 8 on the 13th of December, 1967; and Pioneer 9 on the 8th of November, 1968. A fifth spacecraft, Pioneer E, was launched on the 27th of August, 1969, but was destroyed after a launch vehicle failure and never received a numerical designation.
Pioneer 6 operated for at least 12,758 days, far exceeding its original design life expectancy of six months. A successful telemetry contact was made on the 8th of December, 2000, marking 35 years of operation since launch. It was the oldest operating space probe until Voyager 2 surpassed it on the 13th of August, 2012.
In early August 1972, Pioneer 9 recorded significant observations of one of the most potent solar storms ever recorded during the Space Age. Scientists identified it as the most hazardous solar storm to human spaceflight during that era.
On the 20th of March, 1986, Pioneer 7 flew within 12.3 million kilometers of Halley's Comet. It discovered helium ions produced by charge exchange between solar wind helium and neutral material from the comet's hydrogen tail.
JPL described the Pioneer 6-9 program as one of the least expensive of all NASA spacecraft programs in terms of scientific results per dollar spent. The shared cylinder design, spin stabilization, and distributed instrument suites kept costs low while maximizing the scientific return from all four spacecraft.