1.3 The Birth and Development of the GAIA Concept

The dramatic success of the Hipparcos mission and the lessons that had been learned from this first dedicated astrometric satellite made the idea of a more ambitious but conceptually similar astrometric satellite mission very popular within ESA, which had a great interest in maintaining the European leadership in this research area.

During the satellite operations already, the Hipparcos community started to discuss the critical issues involved in the design of such a mission, aimed at fully exploiting the astrophysical significance of extremely accurate astrometric observations performed on a large sample of the Galactic stellar population. As a result of this process, in 1993 two astrometric mission proposals, under the name of ROEMER (later ROEMER+) and GAIA, were made within ESA. Both retained the proven Hipparcos-like concept of a scanning satellite performing multi-epoch and multi-color observations along (at least) two lines of sight, and benefitted from technological advances such as Charge Coupled Devices (CCDs). The main difference between the two proposals was the chosen optical configuration. The ROEMER+ ([Høg 1995]) optical system consisted of two Baker-Schmidt telescopes, each producing a direct image on the focal plane, whereas GAIA's ([Lindegren and Perryman 1995]) consisted of three Fizeau-type interferometers. Although more challenging from a technological point of view, the interferometric option had been recommended by ESA survey committee for the Horizon 2000+ plan, in consideration of interferometry's future prospects for space applications, and it thus seemed to prevail in the community ([Lindegren and Perryman 1996]). However, it was later recognized that a more conservative approach was desirable, and the interferometric option was accordingly discarded in favour of direct imaging^1.9. At the same time, the accommodation of an instrument for radial velocity measurements on the satellite was proposed as well, in order to create a six-dimensional phase-space map of the Galaxy, and thus greatly enhance the scientific case of the mission.

In early 1997 ESA established a Science Advisory Group (SAG) for the GAIA mission, whose function was to coordinate the preparation of a detailed Study Report on the mission, which would have formed the basis of the selection process by ESA. The SAG monitored the Concept and Technology Study (CTS) that was carried out for ESA by Matra Marconi Space from September 1997 to January 1999 in order to establish a mission design that satisfied the scientific requirements under given budgetary constraints and to identify the necessary technological developments and space qualifications. The interferometric option was pursued in a separate study carried out by Alenia Aerospazio as part of ESA technological research programme.

The Study Report on all scientific, technological and financial aspects of the mission is due to ESA in summer 2000 ([ESA 2000]). The selection of the Horizons 2000 Cornerstone 5 mission is now expected by ESA in September 2000. If selected GAIA could be launched in 2009.