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%.