Solar Probe Plus will be an extraordinary and historic mission, exploring what is arguably the last region of the solar system to be visited by a spacecraft, the Sun’s outer atmosphere or corona as it extends out into space. Solar Probe Plus will repeatedly sample the near-Sun environment, revolutionizing our knowledge and understanding of coronal heating and of the origin and evolution of the solar wind and answering critical questions in heliophysics that have been ranked as top priorities for decades. Moreover, by making direct, in-situ measurements of the region where some of the most hazardous solar energetic particles are energized, Solar Probe Plus will make a fundamental contribution to our ability to characterize and forecast the radiation environment in which future space explorers will work and live.

Our First Visit to a Star

Two of the transformative advances in our understanding of the Sun and its influence on the solar system were the discovery that the corona is several hundreds of times hotter than the visible solar surface (the photosphere) and the development—and observational confirmation—of the theory of the corona’s supersonic expansion into interplanetary space as a “solar wind.”

In the decades that have followed these important milestones in solar and space physics, the composition, properties, and structure of the solar wind have been extensively measured, at high heliolatitudes as well as in the ecliptic and at distances far beyond the orbit of Pluto. The corona and the transition region above the photosphere have been imaged with unprecedentedly high resolution, revealing a complex architecture of loops and arcades, while photospheric magnetography has uncovered the “magnetic carpet” of fine-scale flux bundles that underlies the corona. Observational advances have been accompanied by advances in theory and modeling, with a broad range of models offering plausible and competing scenarios to explain coronal heating and solar wind acceleration.

We now know more about the corona and the solar wind than ever before. And yet the two fundamental questions, raised in the 1940s by the discovery of the corona’s million-degree temperature and in the early 1960s by the proof of the supersonic solar wind’s existence, remain unanswered: Why is the solar corona so much hotter than the photosphere? And how is the solar wind accelerated?

The answers to these questions can be obtained only through in-situ measurements of the solar wind down in the corona. A mission to provide these measurements, to probe the near-Sun particles-andfields environment, was first recommended in 1958, at the dawn of the space age, by the National Academy of Science’s “Simpson Committee.” Since then, NASA has conducted several studies of possible implementations of a Solar Probe mission, and a Solar Probe has remained at the top of various National Academy and NASA science priority lists.