9 Conclusion

GAIA will create an extraordinarily precise three-dimensional map of about one billion stars throughout our Galaxy and the Local group. It will map their space motions, which encode the origin and subsequent evolution of the Galaxy, and the distribution of dark matter. Through on-board photometry, it will provide the detailed physical properties of each star observed: luminosity, temperature, gravity, and elemental composition, which encode the star formation and chemical enrichment history of the Galaxy. Radial velocity measurements on board will complete the kinematic information for a significant fraction of the objects observed.

Through continuous sky scanning, the satellite will repeatedly measure positions and colours of all objects down to V=20 mag. On-board object detection ensures a complete census, including variable stars and quasars, supernovae, and minor planets. It also circumvents costly pre-launch target selection activities. Final accuracies of 10 microarcsec at 15 mag will provide distances accurate to 10 percent as far as the Galactic Centre. Stellar motions will be measured even in the Andromeda galaxy.

In order to limit failure modes, the instrument design includes only one deployable element (the sun shield/solar array) and only three on-board mechanisms (two secondary mirror correctors, one for each of the astrometric telescopes, and an orientation adjustment for the spectroscopic CCDs). Operation of the focal plane is robust against failure of individual CCDs, or against failure of one or more "rows'' of the main focal plane (whether it be the sky mapper, the main detectors, or the on-board data handling units associated with each such row), which would correspond to no more than a loss of overall observing time. The CCDs for the medium-band photometer and radial velocity spectrograph are designed so that the loss if one CCD leads to the loss of only the upper or lower half of one colour band, amounting to a "graceful degradation'' of the mission's science goals. Loss of throughput, of whatever form, would lead to a decrease in the limiting magnitude, and a corresponding degradation of the astrometric accuracy as a function of magnitude. It is not easy to quantify "break points'' in the scientific case at which the astrometric improvement compared to Hipparcos, occasioned by failures or performance limitations, would cease to have a significant scientific impact, as evidenced by the selection of both the FAME and DIVA missions. Nonetheless, a limiting magnitude of 20 mag, accuracies of 10 $\mu$arcsec at 15 mag, and the provision of in-depth photometric and radial velocity data for each object, remain primary mission goals.

GAIA's main scientific objective is to clarify the origin and history of our Galaxy, from a quantitative census of the stellar populations. It will advance fundamental questions such as when the stars in the Milky Way formed, when and how the Milky Way was assembled, and the distribution of dark matter in our Galaxy. In so doing, it will pinpoint exotic objects in substantial numbers: many thousands of extra-Solar planets will be discovered, and their detailed orbits determined; tens of thousands of brown dwarfs and white dwarfs will be identified; rare stages of stellar evolution will be quantified; some 100000 extragalactic supernovae will be discovered, and details communicated for follow-up ground-based observations; Solar System studies will receive a massive impetus through the detection of many tens of thousands of new minor planets; inner Trojans and even new trans-Neptunian objects, including Plutinos, may be discovered. GAIA will follow the bending of star light by the Sun and major planets over the entire celestial sphere, and therefore directly observe the structure of space-time - the accuracy of its measurement of general relativistic light bending may reveal the long-sought scalar correction to its tensor form. The PPN parameters $\gamma$ and $\beta$, and the Solar quadrupole moment J₂, will be determined with unprecedented precision. New constraints on the rate of change of the gravitational constant, $\dot{G}$, and on gravitational wave energy over a certain frequency range, will be obtained.

GAIA is timely as it complements other major space and ground initiatives. Understanding and exploration of the early Universe, through microwave background studies (Planck) and direct observations of high-redshift galaxies (NGST, FIRST, ALMA), are complemented by theoretical advances in understanding the growth of structure from the early Universe up to galaxy formation. Serious further advances require a detailed understanding of a "typical'' galaxy, to test the physics and assumptions in the models. Our Galaxy, a typical example of those luminous spirals which dominate the luminosity of the Universe, uniquely provides such a template.

Acknowledgements

This summary of the GAIA mission, as presented to and approved by the scientific advisory committees of the European Space Agency in September-October 2000, is based on the GAIA Study Report, which is the result of a large collaboration between ESA, the European scientic community and European industry. The scientific aspects of the study were supervised by the Science Advisory Group Members, comprised by the authors. The work of the Science Advisory Group has been supported by a Science Working Group, chaired by P. T. de Zeeuw and G. Gilmore (18 members) and responsible for quantifying the science case; a Photometry Working Group, chaired by F. Favata (18 members); an Instrument Working Group, chaired by L. Lindegren (16 members); and 52 other European scientists directly supporting the GAIA study. We gratefully acknowledge the many and detailed contributions made by the working group members, as well as guidance on the content of this paper from the referee, Donald J. Hutter.

Technological activities have been led by the ESA Study Manager, O. Pace, supported by M. Hechler (ESOC Study Manager), and ESA-ESTEC engineers. ESA scientific involvement includes contributions from S. Volonté (ESA Paris) and from scientists within the ESA Astrophysics Division (K. S. O'Flaherty, F. Favata, W. O'Mullane, M. Vannier, and A. Colorado McEvoy).

The satellite design study has been under contract to Astrium (formerly Matra Marconi Space, F), under Study Manager P. Mérat, and involving EEV Ltd (UK), and Alcatel Space (F). An Alenia Study Team, under Study Manager S. Cesare, evaluated the performance of an interferometric design, with involvement of the Istituto di Metrologia "G. Colonnetti'', EICAS Automazione, the Osservatorio Astronomico di Torino, Matra Marconi Space (F), and Alcatel Space. Other industrial studies have been carried out by SIRA (UK; CCD CTE), and TNO-TPD (Delft; basic angle monitoring).