1. The Historical Context of the GAIA Mission

The origin and development of every momentous scientific undertaking is profoundly influenced by the historical context in which it arises. Following this general rule, the GAIA mission has its roots in the recent dramatic development of space astronomy, and specifically in the renaissance experienced by astrometry thanks to space techniques. Although this study deals with the possibility of performing galaxy observations with the GAIA satellite, an outstanding astrometric accuracy is an essential prerequisite to implement the galaxy observation strategy that is here proposed. More in general, most characteristics of the present mission design are best understood when considering that GAIA is mainly an astrometric satellite. It will therefore be useful to briefly review the advantages offered by space astronomy over ground-based astronomy as well as to describe in some detail the characteristics and results of Hipparcos, the ESA astrometric satellite which can be regarded as GAIA ``ancestor'' and to which GAIA design owes much^1.1.