![]() While both options have the potential to bring us in a new era of discovery, they have to face enormous issues. The next big advance in space astrometry would be either to improve the astrometric accuracy of one order of magnitude, or to move to a different wavelength domain. We briefly outline the properties of Gaia first and second data release, and the accuracies expected end-of-mission. ESA space missions Hipparcos first and then Gaia point out the outstanding contribution that space astrometry can provide to our knowledge in many fields of astrophysics, going from the Milky Way formation and evolution, to stellar astrophysics, extra-galactic astrophysics, and fundamental physics. Nowadays a major breakthrough is on the way due to astrometric sky surveys from space. Over the centuries astrometry has greatly contributed to the advance of the knowledge of the Universe. This contribution focuses on the importance of astrometry and on its future developments. * The longer term promises exciting new opportunities for microarcsecond astrometry and beyond, including the plans for an infrared version of Gaia which would offer the dense sampling of phase space deep into the Milky Way's nuclear regions. * In the coming decade, we can look forward to more accurate and richer Gaia data releases, and new photometric and spectroscopic surveys coming online that will provide essential complementary data. * Gaia DR2 provides for the first time a dense sampling of Galactic phase space with high precision astrometry, photometry, and radial velocities, allowing to uncover subtle features in phase space and the observational HR diagram. * A selected set of results from Gaia highlight the breadth of exciting science and unexpected results, from the solar system to the distant universe, to creative uses of the data. A good understanding of the Hipparcos/Gaia astrometry concept, and of the data collection and processing, provides insights into the origins of the systematic errors and how to mitigate their effects. * Gaia provides splendid astrometry but at the limits of the data small systematic errors are present. In particular the publication of the second data release from the Gaia mission made it possible for every astronomer to work with easily accessible, high-precision astrometry for 1.7 billion sources to 21st magnitude over the full sky. Each chapter ends with comprehensive references to relevant literature.Īccess to microarcsecond astrometry is now routine in the radio, infrared, and optical domains. It contains a detailed overview of the Hipparcos and Tycho Catalogues, their annexes and their updates. It reviews the applications of the data in different areas, describing the subject and the state of the art before Hipparcos, and summarizing all major contributions to the topic made by Hipparcos. This authoritative account of the Hipparcos contributions over the last decade is an outstanding reference for astronomers, astrophysicists and cosmologists. Amongst the key achievements of its measurements are refining the cosmic distance scale, characterizing the large-scale kinematic motions in the Solar neighborhood, providing precise luminosities for stellar modelling, and confirming Einstein’s prediction of the effect of gravity on starlight. ![]() The Hipparcos satellite, developed and launched by the European Space Agency (ESA) in 1989, was the first space mission dedicated to astrometry – the accurate measurement of positions, distances, and proper motions of stars. ![]()
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