Space astrometry

AbstractAstrometry, 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.Keywords: Astrometrystar positionsstar distancesstellar structuregalactic dynamicsHipparcosGaia AcknowledgmentsMy overview of the history of astrometry is based on my more extensive review from 2012 [Citation1], which in turn drew heavily on the researches of Allan Chapman [Citation2], David Goodman & Colin Russell [Citation3] and Alan Hirshfeld [Citation4]. The early history of the Hipparcos project is given in greater detail in my popular account of the mission, The Making of History's Greatest Star Map [Citation5]. From my retirement armchair, I express my appreciation to ESA and its advisory bodies for taking on these pioneering missions. I express my deep admiration for the engineering and management capabilities of European industry, with whom I worked closely for almost 30 years, exemplified by Matra Marconi Space (Toulouse, subsequently subsumed into Airbus Defence & Space), as industrial prime contractor for Hipparcos. Airbus Defence & Space (Toulouse) was also the prime contractor for Gaia. Finally, I express my thanks and admiration to the Gaia DPAC members, some 450 scientists across Europe, who are working together, and often under very great schedule pressures, to deliver this remarkable twenty-first century view of our Galaxy.Disclosure statementNo potential conflict of interest was reported by the author.Additional informationNotes on contributorsMichael PerrymanMichael Perryman joined the European Space Agency as a postdoctoral research fellow in 1980, after an undergraduate degree in theoretical physics at Cambridge, and a PhD in radio astronomy at the Cavendish Laboratory. He was appointed as ESA's project scientist for Hipparcos in 1981, and led the project until its completion in 1997, including the role of project manager after launch. He was one of the originators of the Gaia mission, and was its project scientist until his retirement from ESA in 2009. He was Professor of Astronomy at Leiden University (NL, 1993-2009), the Bohdan Paczynski Visiting Professor, Princeton University (2013), and has been Adjunct Professor in the School of Physics, University College Dublin, since 2013. Amongst recognition for his pioneering contributions to space astrometry, he has received the Prix Jules Janssen of the French Astronomical Society, the Academy Medal of the Royal Netherlands Academy of Arts and Sciences, an Honorary Doctorate from Lund University in Sweden, the Tycho Brahe Prize of the European Astronomical Society, and the 2022 Shaw Prize for Astronomy.