Global Dynamics and Ephemerides

When speaking of Gaia it is instructive to remember the first ESA mission dedicated to high precision astronometry HIPARRCOS launched in August 1989. HIPPARCOS can be considered as Gaia’s precursor, indeed the observing strategy aspects are similar and both missions were designed specifically for high precision astronmetry. After three and a half years of observations HIPPARCOS produced three catalogues. In 1997 the HIPARCOS catalogue of∼ 120000 stars and the first Tycho catalogue containing about one million stars. In 2000 the consolidated catalogue Tycho-2 was released. It contains 99% of all stars down to magnitude 11, approximately ∼ 2.5 million objects. For small solar system bodies (SSSB), HIPPARCOS observed only 48 asteroids, 5 satellites orbiting Jupiter and Saturn, and 2 planets. The accuracy for the HIPPARCOS astrometry was σ ∼ 10mas and for the photometry σ ∼ 0.05magnitude [1] [2]. These results are certainly spectacular but Gaia is projected to achieve much more. The expected performances for Gaia compared to HIPPARCOSS are listed in Table 1. It is clear that the performances of Gaia will greatly excede those of HIPPARCOS.