GAIA is a short-listed candidate for the ESA Cornerstone mission C5, meeting the ESA Survey Committee requirement for an observatory mission, dedicated to astrometry, providing 10 micro-arcsecond accuracy at 15th magnitude. The GAIA mission concept follows the dramatic success of the ESA HIPPARCOS mission, utilising a continuously scanning spacecraft, accurately measuring 1-D coordinates along great circles, in (at least) two simultaneous fields of view, separated by a known angle. These 1-D relative coordinates are later converted to the five astrometric parameters of position and motions in a global analysis. GAIA will provide precise astrometry and multi-colour photometry for all the one billion stars, quasars, and compact galaxies to I=20 on the sky. GAIA will additionally provide the sixth phase-space parameter, radial velocity, from a slitless spectroscopic survey of most stars brighter than about magnitude 17. The technical challenges are considerable, but achievable. The scientific returns are spectacular, with greatest impact in the study of stellar populations and dynamical structure of the galaxies of our Local Group, and in providing the first complete census of the stars and massive planets in the Solar neighbourhood. GAIA will revolutionise our knowledge of the origin and evolution of our Milky Way Galaxy, and of the distribution of planetary systems around other stars.

Proc SPIE Conference 3350, March 1998 (in press) Astronomical Interferometry (latex, uses spie.sty)