[en] The detection of terrestrial exoplanets is at its dawn with the detection of the first very low mass planets by microlensing and radial velocimetry. The transit method from space should help getting statistical information about these objects, their abundances, sizes, masses and distance distribution. This goal should be reached by CoRoT and Kepler allowing the next step: the spectral characterization of the potential atmosphere of such planets by the DARWIN/Terrestrial Planet Finder (TPF) mission or similar concepts. In this paper, we describe the key parameters of space missions which will provide in the next decade, their limits, and what can realistically be expected from the next generation of space observatories