Astrometry, the first scientific discipline, is the measurement of the positions and motions of planets and stars. 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 distortions of the telescopes, and the difficulties of stitching together data from telescopes in different continents.

The concepts of precision space astrometry date back to the 1960s, and these were first realized with the launch of the European Space Agency’s Hipparcos mission in 1989. Hipparcos measured the positions and motions of over 100,000 stars with accuracies 100 times better than ground-based observatories. By measuring small variations in stellar positions as the Earth traveled around its orbit (parallax), Hipparcos also determined distances to over 20,000 stars with uncertainties of less than 10%.

The successor to Hipparcos was the Gaia mission, launched in 2013 and planned to operate at least until 2025. Gaia is based on the same design principles as Hipparcos but has vastly greater capabilities. It has measured 10,000 times as many stars as the Hipparcos catalog, and the positions and motions of these stars are measured 100 times more accurately. Gaia can measure changes in the position of stars on the sky as small as the width of a human hair in Beijing as viewed from Hong Kong. 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.

Accurate astrometry provides fundamental data ― positions, velocities, and distances ― that underpin almost every aspect of modern astronomy and astrophysics. Parallax-based distances are used to calibrate all distances in astronomy and thus are the foundation for measuring the size of the Universe. Accurate distances to stars allow us to measure their intrinsic luminosities, and this in turn is a sensitive probe of their internal physical processes, such as crystallization in the interior of degenerate stars. Measurements of the velocities of stars allow us to infer their Galactic orbits, which in turn provide insight into the formation history of the Milky Way and the distribution of the mysterious dark matter within it. Gaia has measured irregularities in the distribution of stars in the Galactic disk that may reflect recent disturbances from surviving satellite galaxies or unseen clumps of dark matter. Gaia measurements have allowed us to determine for the first time the orbits of hundreds of distant star clusters and dwarf galaxies.

Gaia will eventually provide a rich harvest of ancillary astronomical results, including an all-sky survey of the brightnesses and colours of a billion stars; Doppler-shift based velocities of many millions of stars; light curves for hundreds of thousands of variable stars; thousands of newly discovered extrasolar planets; a survey of asteroids and other small Solar System bodies with unprecedented detail; a uniform catalogue of hundreds of thousands of distant quasars; and stringent new tests of Einstein’s theory of gravity.

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, and even richer, larger and more accurate catalogues will be produced before the mission is completed in a few years. Gaia is providing a survey of our Galaxy that will not be surpassed in quantity or quality for decades to come.

Hipparcos and Gaia succeeded because of the sustained collective effort of hundreds of researchers, managers and engineers over the past half century. The 2022 Shaw Prize in Astronomy recognizes two of these individuals for their lifetime contributions to space astrometry, and in particular for their key scientific contributions to these two missions. Lennart Lindegren originated many of the concepts of the Hipparcos mission design and led 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.

29 September 2022 Hong Kong