Space astrometry

Abstract

Astrometry, the measurement of accurate star positions and motions, was first carried out from space by the European Space Agency's Hipparcos mission in the 1990s. Today, it is undergoing a particularly dramatic advance with ESA's ongoing Gaia mission, launched in 2013. I explain why star positions are of such importance in astronomy, and outline the 2000-year history of the field. This illustrates the profound scientific insights that have been gained over the past centuries as measurement accuracies have advanced, as well as the technical and computational challenges involved. I explain the reasons why measurements from space became necessary in order to advance the field, and outline the measurement principles underpinning these two space mission. I conclude with a summary of the contents of the latest Gaia catalogue release, list some of the scientific highlights that have been revealed by Gaia so far and, in the process, demonstrate how these measurements are revolutionising our understanding of the origin, structure, and evolution of our Galaxy.