GAIA and beyond: The perspective from the milliarcseconds of Hubble and Hipparcos

Astrometry with the Hubble space telescope and Hipparcos satellite advanced the optical astrometric accuracies to the milliarcsec level. The global astrometric interferometer for astrophysics (GAIA) satellite and other proposed optical interferometry space missions would advance the optical astrometric accuracy to better than 10 microarcsec. A catalog figure of merit is defined which allows a quantitative comparison to be carried out for astrometric catalogs. Using two specific astrophysical problems, the level of the expected scientific contribution from a GAIA-type mission is assessed. The two problems are: the age of the globular clusters in relation to the age of the Universe, and the initial mass function compared with fluctuations in the star formation rate with time.