Abstract
GAIA is a preliminary concept for an astrometric mission, recently recommended within the context of ESA's `Horizon 2000 Plus' long-term scientific programme. In its present form, the experiment is estimated to lead to positions, proper motions, and parallaxes of some 50 million objects, down to about mag, with an accuracy better than 10 microarcsec, along with multi-colour multi-epoch photometry of each object. The scientific case for such a mission is dramatic: distances and kinematical motions for tens of millions of objects, throughout our Galaxy, would be obtained—the expected accuracy is such that direct (trigonometric) distance estimates to the galactic centre would be accurate to 10%, with transverse motions accurate to about 1 km s-1 at 20 kpc. As `by-products', the global measurements would yield unprecedented information on the space-time metric (γ to a precision of about 1 part in 106 or better, close to values which might distinguish currently competing theories of gravity), angular diameters of hundreds of stars, and a vast body of information on double and multiple systems. Perhaps the most dramatic of these subsidiary goals would be the possibility of screening some stars within 100 pc for periodic photocentric motions, which would provide the most powerful and systematic method of detecting possible planetary companions proposed to date.
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