III.

Solar Studies

In May 1960 NASA began studying probes for use in exploring the influence of the Sun on inner solar-system space. A new Pioneer spacecraft series was designed to explore interplanetary space using radiation and particle detectors but no photographic or imaging sensors. These craft, like the earlier series, were equipped with solar panels to generate electrical power. This series was complementary to the early Mariner series of probes launched in the 1960s that concentrated on observations and images of individual planets. Planning and design of this new Pioneer series continued through 1965.

Between 1965 and 1968, Pioneers 6 through 9, weighing 62.1 kg (137 lb), 62.8 kg (138 lb), 65.4 kg (144 lb), and 65.4 kg, respectively, were all launched by Thor-Delta rockets into orbits around the Sun slightly inside or outside of the orbit of Earth, allowing the spacecraft to explore the interaction of the interplanetary environment with that of Earth. It was known that the Sun shed from its surface a plasma (its own hydrogen broken down into charged components) that traveled rapidly through the solar system, carrying with it the solar magnetic field. As the Sun slowly rotated, this plasma and the magnetic field were twisted into an expanding spiral. Pioneers 6 through 9 carried their magnetic-field and particle detectors through this spiral, mapping the gross characteristics, exploring the fine structure, and observing the dynamic changes as solar flares sent more energetic bursts of material toward Earth. In addition, these spacecraft provided advance warning of material and radiation ejected from solar flares, which would affect radio communications and electric utility lines and were potentially hazardous to astronauts traveling beyond the shielding of the radiation belts, regions of radiation surrounding Earth. Pioneers 6 through 9 all continued to operate into the 1980s; Pioneer 6, 7, and 8 are still functioning, though NASA only makes contact with the spacecraft occasionally.