The Shaw Prize in Astronomy 2022 is awarded in equal shares to Lennart Lindegren, Professor Emeritus, Department of Astronomy and Theoretical Physics, Lund Observatory, Lund University, Sweden and Michael Perryman, Adjunct Professor, School of Physics, University College Dublin, Ireland for their lifetime contributions to space astrometry. In particular, the award recognises their role in the conception and design of the European Space Agency’s Hipparcos and Gaia missions. This prestigious annual prize is one way in which the Shaw Prize Foundation and the IAU are collaborating on their shared goal of promoting astronomy.

Hipparcos, launched in 1989, measured the positions and motions of over 100 000 stars with an accuracy two orders of magnitude better than ground-based observatories. Gaia, launched in 2013 and still operating, has measured the positions and motions of billions of stars, quasars and Solar System objects with even higher accuracy. The results from these missions offer an exquisitely detailed portrait of the distribution and properties of the stars in our Galaxy, as well as unique insights into its formation and history, and they impact almost every branch of astronomy and astrophysics. This award is also intended to honour the much larger community of astronomers and engineers who made Hipparcos and Gaia possible.

The measurement of the positions, distances and motions of planets and stars has been central to astronomy since prehistoric times. The early naked-eye star catalogues of Ptolemy (ca. 100–170 CE), Ulugh Beg (1394–1449) and Tycho Brahe (1546–1601) were supplanted in the last two centuries by telescopic catalogues of ever-increasing size and accuracy. However, by the late twentieth century, astrometry from ground-based optical telescopes encountered insurmountable barriers to further improvements, arising from atmospheric distortions, thermal and gravitational forces on the telescopes, and the difficulties of stitching together data from different telescopes.

The era of precision space astrometry began with the European Space Agency’s Hipparcos mission (1989–1993). Hipparcos catalogued over 100 000 bright stars. It measured annual changes in the apparent position of these stars on the sky as small as the width of a human thumb in Beijing as viewed from Hong Kong. By measuring small variations in stellar positions as the Earth travelled around its orbit (parallax), Hipparcos determined distances to over 20 000 stars with uncertainties of less than 10%.

ESA’s Gaia mission, launched in December 2013, is based on the same design principles as Hipparcos, but has vastly greater capabilities. Gaia has measured the positions of 10 000 times as many stars as Hipparcos with an accuracy 100 times greater. Gaia has catalogued almost one per cent of all the stars in the Milky Way, and so far has measured parallax-based distances to over 50 million stars with uncertainties of less than 10%. Such parallaxes are the foundation of all distances in astronomy and thus are the firmest foundation we have for measuring the scale of the Universe.

The study of the preliminary catalogues released by the Gaia project, all of which are in the public domain, has already transformed many areas of astronomical understanding. Even richer, larger and more accurate catalogues will be produced before the mission is completed in 2025 or later. Gaia is providing a survey of our Galaxy that will not be surpassed in quantity or quality for decades to come.

Gaia can measure changes in the positions of stars on the sky as small as the width of a human hair in Beijing as viewed from Hong Kong, and motions on the sky smaller than the apparent rate of growth of a hair belonging to an astronaut on the Moon, as seen from Earth. This remarkable performance is achieved by a unique architecture consisting of two telescopes pointing in very different directions, whose images are combined on a single detector. The telescope spins once every six hours, and sends back to Earth precise measurements of the times at which the stars cross a fixed point on the detector.

Why is accurate astrometry so important? The answer is that it provides fundamental data — positions, velocities, and distances — that underpin almost every aspect of modern astronomy and astrophysics. Accurate distances to stars allow us to measure their intrinsic luminosities, and this in turn is a sensitive measure of their internal physical processes, such as crystallisation in the interior of degenerate stars. Small-scale inhomogeneities in the spatial distribution of stars provide a glimpse of disrupted clusters of stars, perhaps similar to the one in which the Sun was born. Measurements of the velocities of stars allow us to infer their Galactic orbits, which in turn provide clues to the formation history of the Milky Way and the distribution of the mysterious dark matter within it.

Gaia is detecting debris from small satellite galaxies that were disrupted long ago by the Milky Way, as well as irregularities in the distribution of stars in the Galactic disc that may reflect recent disturbances from surviving satellite galaxies or unseen clumps of dark matter. Gaia measurements have for the first time allowed us to determine the orbits of distant star clusters and dwarf galaxies. Gaia will provide a rich harvest of ancillary astronomical results, including an all-sky multi-colour photometric survey of a billion stars; radial velocities of many millions of stars; light curves for hundreds of thousands of variable stars; the discovery and measurement of thousands of extrasolar planets; a survey of asteroids and other small Solar System bodies in unprecedented detail; a uniform catalogue of hundreds of thousands of distant quasars; and stringent new tests of Einstein’s theory of gravity.

Hipparcos and Gaia succeeded because of the collective efforts of many people lasting over half a century. The Shaw Prize recognises two of these individuals who have made sustained key scientific contributions to the two missions. Lennart Lindegren originated many of the concepts of the Hipparcos mission design and was leader of one of the two independent teams that carried out the data analysis for Hipparcos. He was a member of the Hipparcos science team for two decades and the Gaia science team for two decades after that. Michael Perryman was Project Scientist for Hipparcos from 1981 to 1997, Chair of the Hipparcos Science Team for the same period, and lead author on the 1997 paper describing the Hipparcos catalogue. Perryman was also Project Scientist for the Gaia mission from 1995 to 2008, Chair of the Gaia Science Advisory Group from 1995 to 2000, and Chair of the Gaia Science Team from 2001 to 2008. Lindegren and Perryman proposed the concept for Gaia in the 1990s and were instrumental in its scientific and technical design.

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The IAU is the international astronomical organisation that brings together more than 12 000 active professional astronomers from more than 100 countries worldwide. Its mission is to promote and safeguard astronomy in all its aspects, including research, communication, education and development, through international cooperation. The IAU also serves as the internationally recognised authority for assigning designations to celestial bodies and the surface features on them. Founded in 1919, the IAU is the world's largest professional body for astronomers.

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