Abstract: Less than two years from now we will make history by dropping the first instrumented probe into the extended atmosphere of the Sun to directly observe the extreme environment responsible for superheating the solar corona and accelerating the solar wind. For centuries solar eclipses have provided brief glimpses of the solar corona, the remarkably structured atmosphere that surrounds the Sun and spreads through interplanetary space as the solar wind. Today, the Sun and the corona are tracked continuously by observatories on Earth and in space. We know much more about solar activity and the impact space weather can have on society than ever before, but we have not been able to answer fundamental questions about the Sun. Why is the corona millions of degrees hotter than the visible surface of the Sun? How does the corona drive a supersonic solar wind? How are solar flares and eruptions able to produce storms of radiation? It has long been recognized that the only way to unambiguously answer these questions is to send an instrumented probe close to the Sun. In 2018 we will finally embark on this journey with the Solar Probe Plus spacecraft, a NASA mission that will repeatedly plunge through the corona to obtain the first direct samples of the Sun. The mission will be reviewed, with a focus on the physics of the solar corona and the design of plasma instruments capable of both making the necessary measurements and of surviving the solar encounters.
