ISAS: interferometric stratospheric astrometry for solar system

The Interferometric Stratospheric Astrometry for Solar system (ISAS) project is designed for high precision astrometry on the brightest planets of the Solar System, with reference to many field stars, at the milli-arcsec (mas) level or better. The science goal is the improvement on our knowledge of the dynamics of the Solar System, complementing the Gaia observations of fainter objects. The technical goal is the validation of basic concepts for the proposed Gamma Astrometric Measurement Experiment (GAME) space mission, in particular, combination of Fizeau interferometry and coronagraphic techniques by means of pierced mirrors, intermediate angle dual field astrometry, smart focal plane management for increased dynamic range and pointing correction. We discuss the suitability of the stratospheric environment, close to space conditions, to the astrometric requirements. The instrument concept is a multiple field, multiple aperture Fizeau interferometer, observing simultaneously four fields, in order to improve on the available number of reference stars. Coronagraphic solutions are introduced to allow observation of internal planets (Mercury and Venus), as well as of external planets over a large fraction of their orbit, i.e. also close to conjunction with the Sun. We describe the science motivation, the proposed experiment profile and the expected performance.