Extensive simulations of planetary transits in the epoch photometry of the future space astrometry mission has been performed. Thousands to tens of thousands of transiting planets should be detected with GAIA.

A detailed knowledge about the galactic extinction is mandatory to obtain the astrophysical/kinematical parameters timevariation outside the solar neighbourhood. One may hope that the GAIAphotometry itself will contribute to colour excess determinations formost of the sample otherwise the extinction must be deduced fromexternal sources. 3D galactic models where the dust distribution is anintegral part seems a viable solution.

The GAIA mission will produce an extraordinary database from which weshould be able to deduce not only the Galaxy's current structure, butalso much of its history, and thus cast a powerful light on the way inwhich galaxies in general are made up of components, and of how theseformed. The database can be fully exploited only by fitting to it asophisticated model of the entire Galaxy. Steady-state models are offundamental importance even though the Galaxy cannot be in a steadystate. A very elaborate model of the Galaxy will be required toreproduce the great wealth of detail that GAIA will reveal. Asystematic approach to model-building will be required if such a modelis to be successfully constructed, however. The natural strategy is toproceed through a series of models of ever increasing elaborateness,and to be guided in the specification of the next model by mismatchesbetween the data and the current model.An approach to the dynamics of systems with steady gravitationalpotentials that we call the "torus programme" promises to provide anappropriate framework within which to carry out the proposed modellingprogramme. The basic principles of this approach have been worked outin some detail and are summarized here. Some extensions will berequired before the GAIA database can be successfullyconfronted. Other modelling techniques that might be employed arebriefly examined.

The capabilities of the GAIA satellite for thedetection of microlensing events are analyzed. The all-sky averagedphotometric optical depth is ~7×10-8 and there are~4000 photometric microlensing events during the five yearmission lifetime. The all-sky averaged astrometric microlensingoptical depth is ~5×10-5and ~50 000 sources willhave a significant variation of the centroid shift, together with aclosest approach, during the mission lifetime. We show that GAIA isthe first instrument with the capability to measure the mass locallyin very faint objects like black holes and very cool white and browndwarfs.

GAIA will be of paramount importance to understand the Galacticstructure. Here we focus on the determination of the star formationhistory (SFH) of the Galactic disk and bulge. We discuss whether GAIAwill be able to isolate a sample of stars with the samecharacteristics of Hipparcos data (completeness over a certaincritical magnitude and high precision in the distance) which will castlight on the SFH of the whole disk. We analyze the expected results intwo directions, namely the BW and the Galactic pole. Finally wediscuss the contribution of GAIA to the study of the SFH of the bulge.

Presently data from several ongoing surveys, and in particular fromthe Spaghetti Project Survey have allowed us to map substructure inthe halo associated to multiple passages of the Sagittarius dwarf.Future astrometric missions such as GAIA will be able to recover trulyancient accretion, thus providing us with the formation history of theMilky Way.

The exquisite astrometric data to be delivered by GAIA can only besuccessfully exploited if complemented by accurate astrophysicalinformation on all observed objects. In this contribution I willdiscuss the preparations that are necessary for obtaining thisinformation. The emphasis will be on the task of optimising the GAIAphotometric system and the development of photometric data analysisalgorithms.

GAIA will measure with precision, position, velocity, and colours of tens of thousands of stars in the Magellanic Clouds and thus, will provide an important contribution to our knowledge of these two close dwarf irregular galaxies. We will consider here the impact of GAIA on the determination of the distance modulus of the LMC and the SMC, on our understanding of the global dynamics of the Magellanic system and of its star formation history. The existence of a stellar pressure-supported halo around the Magellanic Clouds can be also tested from the velocities measurements of the old stars. Moreover we show that the first generation of stars in the Magellanic Clouds could be detected from the GAIA spectra, if it could be possible to obtain spectra of stars as faint as, at least, V=17.5. A resolution of R=5000 would be sufficient for this detection.

The large galaxies in the Local Group, as all disk galaxies, havediverse stellar populations. A better understanding of thesedifferences, and a physical understanding of the causes, requires moredetailed study of the older populations. This presents a significantchallenge to GAIA but the scientific returns are also significant.

The GAIA satellite will bring about a revolution in the understandingof the dynamics of the Local Group. Here I discuss some salientproblems that may be solved with the expected GAIA dataset.

The performance expected from a galaxy survey to be carried out withGAIA, the GAIA Galaxy Survey, is outlined. From a statistical modelof galaxy number density, size and surface brightness distribution,and from detailed numerical simulations based on real images, it isconservatively estimated that GAIA would be able to detect and observeabout 3 million galaxies brighter than V ≅ 17 and to providemulti-colour and multi-epoch broad-band photometry of these with anend-of-mission angular resolution of ≅0.35 arcsec and aphotometric accuracy of ≅0.2 mag/arsec2 atμV=20 mag/arsec2. The substantial scientific case for performing such a surveyand the additional efforts required in terms of mission preparation,operations and telemetry are also discussed.

The report discusses the possible multi-colour photometry of thediffuse sky surface with the Astro and Spectro telescopes of GAIA.The ordinary photometry of faint stars will give good results of thebackground for at least 500 million spots on the sky. Measurementfrom about 100 of these spots in a small area could be averaged toobtain a precision of about 0.5 S10 of GAIA medium-bandphotometry in each of the 5 million areas. It is concluded that GAIAmedium-band photometry is suited to tackle a whole range ofastrophysically interesting diffuse sky components. After modellingand subtracting the time-variable Zodiacal Light, GAIA could providemulti-wavelength all sky maps of the low galactic latitude DiffuseGalactic Light and the high latitude Galactic Cirrus Clouds. Possiblyalso the Extragalactic Background Light could be detected.

The astrometric mission GAIA will provide a flux limited census ofquasi stellar objects to V=20 mag. This paper addresses the mainphysical and observational properties of the quasars, the existingsurveys and the relevant statistical distributions of the compilationcatalogue of Véron-Cetty & Véron. Then I consider thedetection and recognition with GAIA which should be based on themulticolour photometry complemented by hints on the parallaxes andproper motions. In the last section I discuss the possibility tomaterialize directly in the optical the quasi-inertial frame to thesub-μas accuracy. It is shown that that with a clean sample of lessthan 10 000 quasars the residual spin of the GAIA reference framecan be determined to 0.5 μas yr-1.

GAIA will likely be a real milestone for asteroid science. Itsunprecedented astrometric accuracy can be exploited to compute orbitsbetter than those obtained by using the whole set of availableastrometric observations carried out from ground during the last 100 years. GAIA will make it possible to measure asteroid masses throughthe measurement of the tiny displacements caused by asteroid mutualencounters. This will be a major breakthrough for physical studies ofminor planets. Moreover, sizes will be also directly measured for abig sample of more than 1000 objects. From masses and sizes, estimatesof the average densities will be also obtained. The spectrophotometriccapability of GAIA will be used for purposes of taxonomicclassification, with important implications for surface mineralogicalcharacterization. Finally, GAIA will be an ideal platform to detectnear-Earth asteroids having orbits interior to that of the Earth.

Trojans, Centaurs and Trans-Neptunian objects are small dark objectsorbiting in the outer Solar System. These objects, still very poorlyknown, can be considered as fossils of our Solar System, and, as such,they can still contain information about some primordial processeswhich governed the evolution of the early Solar System.In the next decade, GAIA with its large sky surveys, can allow the discovery of many new objects belonging to these outer solar system populations. In addition, it will provide very precise orbits and visible broad-band colours for many of them.

The GAIA satellite will observe a large number of solar systemobjects, mainly asteroids, and although most of them shall be alreadyknown in the next decade, it will possibly discover new ones. Themajor scientific outcomes of this mission for the science of the SolarSystem is the survey of the inner-Earth orbits region, thedetermination of asteroids' physical parameters (masses, sizes,taxonomy...) for a large number of objects, detection of binaryasteroids, accurate orbits determination, and tests of generalrelativity. Here we discuss the preparative work that should be donebefore launch regarding the scientific case, the software orinstruments requirements, the catalogues output, and complementaryground-based observations.

A method for inverting the equation of stellar statisticincludingproper motions is tested. The supplementary constraintrequired bydynamical consistency is used in order to break the degeneracy. Theinversion gives access to both the kinematics and the luminosityfunction of each population. Its application to data such as theTycho catalog (and in the near future GAIA) will lead - provided thevertical potential, and/or the asymmetric drift or ω⊙ are known- to a non-parametric determination of the local neighbourhoodluminosity function without any reference to stellar evolutiontracks. It should also yield the proportion of stars for eachkinematic component and a kinematic diagnostic to split the thin diskfrom the thick disk or the halo.