Solar Orbiter is an ESA/NASA mission that will provide an unprecedented opportunity to discover the fundamental connections between the rapidly varying solar atmosphere and the solar wind. The Solar Wind Analyzer (SWA) plasma package shall provide comprehensive in situ measurements of the solar wind. In particular, the Proton-Alpha Sensor (PAS) will determine the high time resolution properties solar wind ion populations through the measurement of the 3D distribution function, density, bulk velocity, temperature and heat Vux. The closest approach of Solar Orbiter to the Sun is 0.28 AU. At this distance the solar wind Vow, solar UV, and solar infrared Vuxes increase by a factor 13 compared to near-Earth space. The PAS instrument will provide high cadence 3D distribution function measurements (up to 13 per second) all the way from closest approach to 1 AU. The extreme UV and thermal conditions at closest approach precluded the use of standard ion optics schemes. We thus developed a new ion sensor optic design combined with conventional robust channeltron detectors. This paper describes the PAS measurement scheme adopted for varying solar wind conditions and our approach to the fast sampling of 3D distribution functions. We also highlight the expected scienti:c return of the instrument.