Introduction

As discussed in detail in Chapter 2, two emblematic (space-based) astrometric missions to be launched in the near future, namely, Gaia and SIM Lite, will have a profound impact in many branches of astrophysics, including Galactic structure and stellar populations. It is therefore interesting to mention, first, some of the expected capabilities of these astrometric missions. In a nutshell, SIM Lite (recently deferred) will be an optical interferometer operating in an Earth-trailing solar orbit. The goal is to measure stellar positions, trigonometric parallaxes, and proper motions down to magnitude 20 with an accuracy of 4 microarcsec-onds (Has). This breakthrough in capabilities will be possible because SIM will use optical interferometry. On the other hand, the primary goal of Gaia is to investigate the origin and subsequent evolution of our Galaxy through a census of more than 109 stars (both in our Galaxy and in nearby members of the Local Group), measuring their positions with a best accuracy of 7 üas, 24 üas at magnitude 15 with decreasing accuracy to magnitude 20. Together with stellar distances and motions, this mission will allow astronomers to build the most accurate three-dimensional map to date of the stellar constituents of our Galaxy. Gaia will also perform low-spectral-resolution and photometric measurements for all objects. With an approximate launch date on late 2013, the final catalog is expected to be released by 2022.