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Summary
Celestial reference frames are the fixed, imaginary grids against which everything else moves, akin to lines of latitude and longitude on Earth. For the past few decades, astronomers have based them on radio observations of several thousand quasars, cosmic lighthouses generated by black holes in distant galactic centers. These radio beacons not only guide the pointing of telescopes, but they are also the bedrock of the reference frame for spinning, bucking Earth. Without them, GPS devices would lose their accuracy and many ultraprecise studies of processes such as plate tectonics and climate change would be impossible. But observations of these beacons are costly and rely on radio telescopes. In 2018, the European Space Agency's star-mapping satellite produced its own version of a reference frame, based on half a million quasars seen in the visible wavelengths most astronomers use, not radio. And next week, on 3 December, Gaia will release, along with the latest data about billions of Milky Way stars, its newest reference frame, built from 1.6 million quasars scattered across the sky. It promises a better reference frame that could improve the navigation and pointing of interplanetary spacecraft.
