Gaia is the ESA spacecraft that maps the Milky Way using stellar parallax.
Launched in 2013, Gaia observes from a stable orbit around the Sun near the second Sun–Earth Lagrange point. It repeatedly measures stars’ positions, apparent motions and brightnesses. As Earth and Gaia travel around the Sun, nearby stars appear to shift slightly against distant background stars. That annual shift, called parallax, provides a direct geometric distance measurement.
Gaia’s catalogue goes far beyond a simple star count. Its astrometry, photometry and spectroscopy reveal stellar motions, colours, chemical clues and three-dimensional Galactic structure. Those measurements help astronomers reconstruct the Milky Way’s history, including mergers such as Gaia-Enceladus.
A common mix-up is Hipparcos, Gaia’s ESA predecessor, which pioneered space-based precision astrometry but mapped far fewer stars. Herschel studied infrared astronomy, while Rosetta explored comet 67P—not the Galaxy’s stellar population.