Solar Probe will be a historic mission, flying into one of the last unexplored regions of the solar system, the Sun’s atmosphere or corona, for the first time. Approaching as close as 3 RS above the Sun’s surface, Solar Probe will employ a combination of in-situ measurements and imaging to achieve the mission’s primary scientific goal: to understand how the Sun’s corona is heated and how the solar wind is accelerated. Solar Probe will revolutionize our knowledge of the physics of the origin and evolution of the solar wind. Moreover, by making the only direct, in-situ measurements of the region where some of the deadliest solar energetic particles are energized, Solar Probe will make unique and fundamental contributions to our ability to characterize and forecast the radiation environment in which future space explorers will work and live. Solar Probe is currently under study as part of the Sun-Solar System Connection within NASA's Science Mission Directorate.

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 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 ES-1 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-and fields 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 Solar Probe has remained at the top of various National Academy and NASA science priority lists. Most recently, the National Research Council’s “decadal survey” in solar and space physics recommended implementation of a Solar Probe mission “as soon as possible” (NRC, 2003), while NASA’s Sun-Solar System Connection Roadmap identifies Solar Probe as a “Flagship” mission that “is ready to fly and is our highest priority for new resources” (NASA, 2005).

To date, however, nearly 50 years after the Simpson Committee report and despite strong and repeated endorsements of a Solar Probe by the National Academy, NASA, and the solar and space physics community, the closest any spacecraft has come to the Sun is 65 RS, far outside the region where the acceleration of the solar wind occurs. Thus the need for a Solar Probe remains. Solar Probe will be the first spacecraft to venture into the unexplored inner reaches of the heliosphere where the solar wind is born. Through high-cadence in-situ measurements of the solar wind plasma, energetic particles, and fields as close to the Sun as 3 RS, supplemented by coronal and photospheric imaging, Solar Probe will provide the data needed to solve, finally, the twin mysteries of coronal heating and solar wind acceleration. This historic mission will transform our understanding both of our Sun and of other stars with hot, x-ray-emitting coronas and supersonic winds as well.