Space Astrometry Missions

Abstract

The successful hipparcos missionhas brought, and proved the validity of, a new observationalconcept to achieve absolute measurements of star positions inspace. Only five years after the final publication of the resultsthere are now four astrometry missions scheduled for the nextdecade, three of which appearing as the natural heirs of thispioneering mission. Space astrometry missions share a certainnumber of common features imposed by their observationalprinciples and their scientific goals. The first part of thispaper attempts to show how the objectives put severe constraintson the design and that this can be investigated with a fairlygeneral approach. The German mission diva is then detailed as anillustration (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 theirscientific returns.