The European space astrometry mission Gaia aims to produce a complete sky survey down
to V= 20 with an astrometric accuracy of 25 mu as at V= 15. During its 5-year mission the
satellite will also repeatedly measure the position of∼ 500,000 quasars in a consistent way,
leading to a direct realisation of the primary inertial frame in the visible in the framework the
ICRS concepts. At V= 20 the sky density of the QSOs is about 1000 times smaller than that of
the stars at mid galactic latitude. Given their number and their stellar-like images, this implies�…