The Gaia mission is one of the most ambitious space experiments in the European Space Agency's Science Programme. Designed after the remarkable success of the European mission Hipparcos, the first ever space astrometry mission, Gaia will provide unprecedented positional and radial velocity measurements with the accuracies needed to produce a stereoscopic and kinematic census of about one billion stars in our Galaxy and throughout the Local Group, about 1 percent of the Galactic stellar population. The survey aims at completeness up to visual magnitude of V = 20 mag, with astrometric accuracies of 10 microarcsec at 15 mag.
Combined with astrophysical information for each star, provided by on-board multi-colour photometry and spectroscopic measurements, these data will have the precision and depth necessary to clarify the origin and history of our Galaxy from a quantitative census and multi-parameter characterisation of the stellar populations. Additional products include detection and characterisation of tens of thousands of extra-solar planetary systems, a comprehensive survey of some 100,000 to 1 million minor bodies in our Solar System (including near-Earth objects), observation of galaxies in the nearby Universe and of some 500 000 quasars, and stringent new tests of general relativity and cosmology.
Gaia has been selected as a Cornerstone of the ESA Science Programme in October 2000, and confirmed as a flagship of the new ESA Science Programme “Cosmic Vision 2020” in May 2002. From that date, in-depth studies have been progressing in many of the scientific areas of relevance to the system design, as well as the definition of the radial velocity spectrometer and in the development of a prototype data analysis system. In parallel, technical studies have been initiated, aiming to reach, by the end of 2004, full confidence in the feasibility of the Gaia project, and detailed understanding and control of all critical technologies required to build and operate the mission, and analyse its data.
October 2004 is a timely opportunity to fully review the progress that will have been made, following the completion of these multifaceted studies. The goal of the meeting is to present the design chosen for the Gaia mission and its performances, to update the science case, and to organise the next stages for the scientific preparation of the mission.