The successful hipparcos mission has brought, and proved the validity of, a new observational concept to achieve absolute measurements of star positions in space. Only five years after the final publication of the results there are now four astrometry missions scheduled for the next decade, three of which appearing as the natural heirs of this pioneering mission. Space astrometry missions share a certain number of common features imposed by their observational principles and their scientific goals. The first part of this paper attempts to show how the objectives put severe constraints on the design and that this can be investigated with a fairly general approach. The German mission diva is then detailed as an illustration (gaia being considered elsewhere in this volume). In the last section few words are added about fame and sim followed by global comparisons of these missions and their scientific returns.