From Hipparcos to Gaia

Institute of Astronomy, Cambridge University, Cambridge, UKAbstract.The measurement of the positions, distances, motions and luminosities of stars representsthe foundations of modern astronomical knowledge. Launched at the end of the eighties, theESA Hipparcossatellite was the first space mission dedicated to such measurements. Hipparcosimproved position accuracies by a factor of 100 compared to typical ground-based results andprovided astrometric and photometric multi-epoch observations of 118,000 stars over the entiresky. The impact of Hipparcos on astrophysics has been extremely valuable and diverse. Buildingon this important European success, the ESA Gaiacornerstone mission promises an even moreimpressive advance. Compared to Hipparcos, it will bring a gain of a factor 50 to 100 in positionaccuracy and of a factor of 10,000 in star number, collecting photometric, spectrophotometricand spectroscopic data for one billion celestial objects. Duringits 5-year flight, Gaiawill measureobjects repeatedly, up to a few hundred times, providing an unprecedented database to studythe variability of all types of celestial objects. Gaiawill bring outstanding contributions, directlyor indirectly, to most fields of research in astrophysics, such as the study of our Galaxy and ofits stellar constituents, the search for planets outside the solar system.Keywords.space vehicles, surveys, stars: variables: other, astrometry, techniques: photometric,techniques: spectroscopic