The Gaia mission is a space astrometry project operated by the European Space Agency (ESA), designed to create the most precise three-dimensional map of the Milky Way galaxy ever constructed. Launched on 19 December 2013 from the Guiana Space Centre in Kourou, French Guiana, aboard a Soyuz-STB/Fregat rocket, Gaia operated at the Sun-Earth L2 Lagrange point approximately 1.5 million kilometres from Earth. Its primary objective was to measure the positions, distances, and motions of approximately one billion stars with unprecedented accuracy, using trigonometric stellar parallax as its principal distance-measurement technique alongside photometric and spectroscopic observations. Science observations concluded on 15 January 2025, and the spacecraft was decommissioned on 27 March 2025.

Gaia carried two telescopes feeding a single focal plane assembly, which housed the largest digital camera ever deployed in space - a mosaic of 106 charge-coupled devices (CCDs) totalling nearly one billion pixels. The spacecraft continuously rotated, scanning the entire sky over a five-year nominal mission period, observing each source an average of 70 times. From these repeated observations, Gaia derived five astrometric parameters for each star: two angular position coordinates, two components of proper motion across the sky, and parallax - the apparent angular shift in a star's position as Earth (and Gaia) orbit the Sun. Parallax measurements allow direct calculation of stellar distances out to several thousand parsecs, with precision degrading at greater distances where the parallax angle becomes smaller than the measurement uncertainty.

ESA has released Gaia data in successive catalogues. Gaia Data Release 1 (DR1) was published in September 2016, providing positions and parallaxes for approximately two million stars using a combination of Gaia and Hipparcos data. Gaia DR2 followed in April 2018 with full five-parameter solutions for 1.3 billion sources and radial velocities for seven million stars. Gaia Early Data Release 3 (EDR3) was published in December 2020, and the full Gaia DR3 was released in June 2022, adding astrophysical parameters, non-single-star solutions, Solar System object data, and a catalogue of quasars and galaxies. Gaia DR4, covering the first 5.5 years of mission data, is scheduled for release in December 2026. A final data release (DR5), covering the full 10.5 years of observations, is anticipated around the end of the decade.

Gaia data have been applied across a broad range of astrophysical disciplines. In Galactic archaeology, Gaia's kinematic data revealed previously unknown stellar streams and substructures in the Milky Way's halo, including the Gaia-Enceladus (or Gaia Sausage) merger remnant, interpreted as evidence of a major collision between the Milky Way and a dwarf galaxy several billion years ago. The mission has also refined the cosmic distance ladder by providing a large sample of calibrating stars, with implications for measurements of the Hubble constant and ongoing investigations of the Hubble tension. Gaia observations have detected binary star systems, exoplanet candidates through astrometric wobble, and contributed to Solar System science through precise asteroid orbit determinations.

The scientific methods underlying Gaia - trigonometric parallax as a geometric distance indicator and the astrometric principles of the mission - reflect settled techniques in observational astronomy. The stellar parallax consensus page addresses the empirical and methodological basis for these measurements. Specific interpretations derived from Gaia data, including aspects of Milky Way formation history and distance ladder calibrations, remain active research areas with ongoing scholarly discussion.

• Standard astrometric interpretation: The mainstream astronomical community accepts Gaia's parallax measurements and derived catalogues as the current best empirical basis for stellar distances and Galactic kinematics. Standard Astrometry Viewpoint
• Hubble tension implications: Some researchers argue that Gaia's calibration of Cepheid variable stars and other distance-ladder anchors sharpens the discrepancy between locally measured and cosmologically inferred values of the Hubble constant, suggesting new physics beyond the standard cosmological model. Local Distance Ladder Viewpoint
• Alternative cosmological frameworks: A minority of researchers working within non-standard cosmological or plasma cosmology frameworks contest aspects of distance-ladder methodology and the cosmological inferences drawn from Gaia data. Alternative Cosmology Viewpoint

• Gaia Mission - History
• Stellar Parallax
• Stellar Parallax - Astronomy Consensus
• Hubble Tension
• Cosmic Distance Ladder
• Hipparcos Mission
• Milky Way
• Gaia-Enceladus Merger

1. European Space Agency. “Gaia Overview.” ESA Science & Technology. https://sci.esa.int/web/gaia
2. Gaia Collaboration; Prusti, T. et al. “The Gaia mission.” Astronomy & Astrophysics 595 (2016): A1. https://doi.org/10.1051/0004-6361/201629272
3. Gaia Collaboration; Brown, A.G.A. et al. “Gaia Data Release 2: Summary of the contents and survey properties.” Astronomy & Astrophysics 616 (2018): A1. https://doi.org/10.1051/0004-6361/201833051
4. Gaia Collaboration; Brown, A.G.A. et al. “Gaia Early Data Release 3: Summary of the contents and survey properties.” Astronomy & Astrophysics 649 (2021): A1. https://doi.org/10.1051/0004-6361/202039657
5. Helmi, A. et al. “The merger that led to the formation of the Milky Way's inner stellar halo and thick disk.” Nature 563 (2018): 85-88. https://doi.org/10.1038/s41586-018-0625-x
6. Riess, A.G. et al. “A Comprehensive Measurement of the Local Value of the Hubble Constant with 1 km/s/Mpc Uncertainty from the Hubble Space Telescope and the SH0ES Team.” The Astrophysical Journal Letters 934 (2022): L7. https://doi.org/10.3847/2041-8213/ac5c5b
7. European Space Agency. “Farewell, Gaia! Spacecraft operations come to an end.” ESA. 27 March 2025. https://www.esa.int/Enabling_Support/Operations/Farewell_Gaia!_Spacecraft_operations_come_to_an_end
8. ESA Gaia Science Community. “Gaia Data Release 4 planned for 2 December 2026.” COSMOS. https://www.cosmos.esa.int/web/gaia