Optimising GAIA: How do we meet the science challenge?

GAIA has been approved to provide the data needed to quantify the formation and evolution of the Milky Way Galaxy, and its near neighbours. That requires study of all four key Galactic stellar populations: Bulge, Halo, Thick Disk, Thin Disk. The complex analysis methodologies required to model GAIA kinematic data are being developed, and in the interim applied to the relatively simple cases of the satellite dSph galaxies. These methodologies, illustrated here, show that we will be able to interpret the GAIA data. They also quantify what data GAIA must provide. It is very unlikely in the present design that GAIA will be able to provide either radial velocities or worthwhile photometry for study of two of the key science goals: the Galactic Bulge and the (inner) Galactic old disk. The implication is that the radial velocity spectrometer and the medium band photometer should be optimised for study of low density fields -- suitable for their low spatial resolution -- and the broad band photometry must be optimised for inner galaxy astrophysical studies.