In October 2000, the GAIA astrometric mission was approved as one of the next two "cornerstones" of ESA's science programme, with a launch date target of 2010-12. GAIA will provide positional and radial velocity measurements with the accuracies needed to produce a stereoscopic and kinematic census of about one billion stars throughout our Galaxy (and into the Local Group), amounting to about 1 per cent of the Galactic stellar population. GAIA's main scientific goal is to clarify the origin and history of our Galaxy, from a quantitative census of the stellar populations. It will advance questions such as when the stars in our Galaxy formed, when and how it was assembled, and its distribution of dark matter. The survey aims for completeness to V=20 mag, with accuracies of 10 μas at 15 mag.

Three topics within GAIA photometry have been selected for discussion here. (1) The sampling scheme of the CCDs has been tuned to serve the various purposes: detection of stars, astrometry, broad- and medium-band photometry, and imaging of a small area around each detected star, but the tuning may not yet be optimal. (2) The calibration of CCD sensitivity is critical since we aim for millimagnitude accuracy, the spikes of the diffraction image are suited for an accurate calibration of the magnitude scale, and for extending the range of astrometry and photometry to very bright stars. (3) Finally the various photometric filter systems are illustrated.

GAIA spectroscopic and radial velocity performances are reviewed on the base of ground-based test observations and simulations. The prospects for accurate analysis of stellar atmospheres (temperature, gravity, chemical abundances, rotation, peculiarities) and precise radial velocities (single stars, binaries, pulsating stars) are colourful provided the spectral dispersion is high enough. A higher dispersions also favors a given precision of radial velocities to be reached at fainter magnitudes: for example, with current parameters for GAIA spectrograph, a 1 km s-1 accuracy on individual epoch RVs of a K0 star is reached at V~13.0 mag with 0.25 Å/pix dispersion spectra, at V~10.3 mag for 0.5 Å/pix, and V~6.7 mag for 1 Å/pix. GAIA radial velocities for single stars can match the ~0.5km s-1 mean accuracy of tangential motions at V=15 mag, provided the observations are performed at a dispersion not less than 0.5 Å/pix.

GAIA will obtain, in a five-year mission, astrometric, photometric and spectroscopic data for more than a billion stars. The treatment of this huge amount of data is a challenge for the astronomical community, not only due to the database size, but also to the complex relationships existing between the different sets of data and with the instrumental and orbital parameters. The GAIA Data Analysis Study (GDAAS), under ESA's contract, aims to prove the feasibility of the data reduction scheme and to implement and test a prototype of such a large database. We present the main aspects that GDAAS addresses and review its present status.

The successful hipparcos mission has brought, and proved the validity of, a new observational concept to achieve absolute measurements of star positions in space. Only five years after the final publication of the results there are now four astrometry missions scheduled for the next decade, three of which appearing as the natural heirs of this pioneering mission. Space astrometry missions share a certain number of common features imposed by their observational principles and their scientific goals. The first part of this paper attempts to show how the objectives put severe constraints on the design and that this can be investigated with a fairly general approach. The German mission diva is then detailed as an illustration (gaia being considered elsewhere in this volume). In the last section few words are added about fame and sim followed by global comparisons of these missions and their scientific returns.

The problem of relativistic modelling of GAIA observations is reviewed. Basic principles of the relativistic model and its overall structure are described in detail. All relativistic effects which may amount to 1 μas are revealed and discussed. It is argued that relativistic definitions of astrometric parameters (position, parallax, proper motion, radial velocity, etc.) consistent with an accuracy of 1 μas should be considered only within a well-defined algorithm of relativistic reduction of observational data.

The baseline of GAIA is the study of the formation and evolution of the Milky Way from astrometric and photometric measurements. The mission displays a remarkable versatility in its applications, including a significant impact in fundamental physics. In the following I will try to define the relationship between astrometry and fundamental physics, before considering in more detail the determination of the space curvature and the study of the non linearity of gravity from the astrometric measurements. It is shown that GAIA is perfectly at home to sense the bending of light-rays in the solar gravitational field and should be able to determine the PPN parameter γ with a precision between 10-6 and 10-7, much better than any other determination expected by 2015. A favorable combination of distances and eccentricities on a handful of minor planets will permit to search for the relativistic perihelion precession. The parameter β should be ascertained to 10-3-10-4 provided the solar quadrupole moment is not solved simultaneously and constrained from other sources. This achievable accuracy rests also on assumptions regarding the reconstruction of orbits of minor planets from GAIA-only observations.

In this paper we examine the effects introduced in the luminosity function of white dwarf stars by a change of the gravitational constant during the life of the Universe. A decrease of G induces the expansion of the star and accelerates its cooling. This effect has noticeable effects on the luminosity function and can be used to set upper bounds on the rate of change of G. Preliminary results indicate that Ġ/G ≤ 10-13 yr-1. These values are the best ever obtained and will improve as the white dwarf luminosity function will improve.

Understanding the stellar interiors is an important issue for astrophysics, but also for fundamental physics. The physical description of the stellar material implies statistical physics as well as hydrodynamics. We give examples of unsolved physical problems and their consequences for predictions of stellar properties and evolution. It is shown that the accumulation of high-quality data like distances and photometry on a large variety of objects, as proposed by GAIA, will be very useful. During the same period, asteroseismology will develop both from the ground in velocimetry, and from space in photometry, providing for the closest stars, a direct insight into their interiors. This ensemble of new data will constitute the basis of the general physical description of stellar interiors.

We show that there is a wide range of problems, where high accuracy astrometry can bring new information in stellar evolution, not limited to the classical applications in the HR diagram, such as the determinations of the helium and metal contents and of the size of the convective core. In particular, we show some new applications to the study of star formation. The accurate measurement of the luminosity of the observed birthline in the HR diagram is a key information very sensitive to the mass accretion rates. These rates determine all the properties of the pre-MS models. For the evolution of massive stars with mass loss and rotational mixing, strong constraints can be obtained from the combination of astrometric data and abundance determinations obtained by high resolution spectroscopy. We also emphasize that a better knowledge of the luminosity distribution of WR stars is strongly constraining the final masses at the time of supernova explosions as well as the nucleosynthetic yields.

The study of the oldest stellar populations in the Galaxy provides important cosmological information (e.g. a lower limit to the age of the Universe) and helps constraining models of galaxy formation. The most commonly used stellar age indicators are reviewed and discussed, and the impact of GAIA on these topics is estimated.

Stellar candles are the basic tools to determine the extragalactic distance scale. Its use relies on some given fundamental relation that allows to estimate the absolute magnitude of an object from observable parameters and from that its distance. The Hipparcos mission greatly contributed to the refinement of these fundamental relations, but even with its milliarcsecond-precision data, the uncertainties in the calibration of Stellar Candles are still high. To improve these results larger samples and higher astrometric precision are needed. GAIA, with its microarcsecond, one-billion objects catalogue, will provide all the necessary data for this task.

We present, in a first part, a short review of existing and planned large catalogues and surveys. In a second part, we present the current plans for building the Virtual Observatories of the future. Finally we give a few examples of emerging methodologies for the analysis of huge catalogues.

Important progress has been made within the past few years regarding the theory of low mass stars (m < 1 M⊙) and brown dwarfs (m < 0.075 M⊙). The main improvements concern the equation of state of dense plasmas and the modelling of cool and dense atmospheres, necessary for a correct description of such objects. These theoretical efforts now yield a better understanding of these objects and good agreement with observations regarding colour-magnitude diagrams of globular clusters, mass-magnitude relationships and near-IR colour-magnitude diagrams for open clusters. However uncertainties still remain regarding synthetic optical colours and the complex problem of dust formation in the coolest atmosphere models. We will present a review of the current success and uncertainties of the theory of these objects, which may guide forthcoming projects with GAIA.

Because of their very red colours and intrinsic faintness, field brown dwarfs will represent a small but valuable subset of the GAIA catalogue. The return of the astrometric satellite is expected to be important because of the inherent difficulty of obtaining good parallaxes in general and for this class of objects in particular. Our first estimates show that, due to the photometric sensitivity of the astrometric CCD (ASM1) towards relatively blue objects, GAIA is unlikely to detect field brown dwarfs that have not been already seen is previous near-IR surveys, to the notable exception of the galactic plane region. The real advantage of GAIA over ground-based surveys will be the very accurate (to within a few percents) astrometric data for a few thousands brown dwarfs. These data should permit a detailed mapping of the transition region between stellar and substellar regimes, together with the kinematical and density patterns of the youngest brown dwarfs in our neighbourhood.

The ESA Cornerstone Mission GAIA, to be launched prior to 2012 and with a nominal lifetime of 5 years, will improve the accuracy of Hipparcos astrometry by more than two orders of magnitude. GAIA high-precision global astrometric measurements will provide deep insights on the science of extra-solar planets. The GAIA contribution is primarily understood in terms of the number and spectral type of targets available for investigation, and characteristics of the planets to be searched for. Several hundreds of thousands of solar-type stars (F-G-K) within a sphere of ~200 pc centered on our Sun will be observed. GAIA will be particularly sensitive to giant planets (MP ~ MJ) on wide orbits, up to periods twice as large as the mission duration, the potential signposts of the existence of rocky planets in the Habitable Zone. Thousands of new planets might be discovered, and a significant fraction of those which will be detected will have orbital parameters measured to better than 30% accuracy. By measuring to a few degrees the relative inclinations of planets in multiple systems with favorable configurations, GAIA will also make measurements of unique value towards a better understanding of the formation and evolution processes of planetary systems.