The earth is immersed in a hot, rarefied, energetic flow of particles and electromagnetic fields originating from the sun and engulfing the entire solar system. The solar wind controls interplanetary space and creates the heliosphere. The existence of this flow has been established for over forty years now, and abundant data has accumulated concerning its average properties, as well as intermittent energetic manifestations which often impact the earth's magnetosphere (causing geomagnetic storms and aurorae). The fundamental mystery of how the solar corona is heated and the solar wind is generated remains unsolved, however, because of the large gap in our knowledge of the inner region of the heliosphere, inside the orbit of mercury. A Solar Probe mission, with a perihelion well inside 20 Rs, by accessing the regions where energy in the coronal plasma is channeled from internal, magnetic and turbulent energy into bulk energy of the solar wind flow will solve the question of why the sun has a hot corona and produces a solar wind. Solar Probe measurements of the magnetic field structure close to the sun will also allow a more comprehensive understanding of our space environment, providing the measurements required for the understanding of processes such as solar energetic particle origin as well as galactic cosmic ray scattering by the solar magnetic field.