[7] The mission aims to construct by far the largest and most precise 3D space catalog ever made, totalling approximately 1 billion astronomical objects, mainly stars, but also planets, comets, asteroids and quasars, among others.

[8] To study the precise position and motion of its target objects, the spacecraft monitored each of them about 70 times[9] over the five years of the nominal mission (2014–2019), and about as many during its extension.

The spectrophotometric measurements provide detailed physical properties of all stars observed, characterizing their luminosity, effective temperature, gravity and elemental composition.

This massive stellar census is providing the basic observational data to analyze a wide range of important questions related to the origin, structure and evolutionary history of the Milky Way galaxy.

The successor to the Hipparcos mission (operational 1989–1993), Gaia is part of ESA's Horizon 2000+ long-term scientific program.

It was adopted by ESA's Science Programme Committee as cornerstone mission number 6 on 13 October 2000, and the B2 phase of the project was authorised on 9 February 2006, with EADS Astrium taking responsibility for the hardware.

While the working method evolved during studies and the acronym is no longer applicable, the name Gaia remained to provide continuity with the project.

The sunshade always maintains a fixed 45 degree angle to the Sun, while precessing to scan the sky, thus keeping all telescope components cool and powering Gaia using solar panels on its surface.

[31] The Gaia payload consists of three main instruments: In order to maintain the fine pointing to focus on stars many light years away, the only moving parts are actuators to align the mirrors and the valves to fire the thrusters.

The spacecraft subsystems are mounted on a rigid silicon carbide[7] frame, which provides a stable structure that will not expand or contract due to temperature.

[35] Similar to its predecessor Hipparcos, but with a precision one hundred times greater, Gaia consists of two telescopes providing two observing directions with a fixed, wide angle of 106.5° between them.

Because of the physical constraints imposed by the Soyuz spacecraft, Gaia's focal arrays could not be equipped with optimal radiation shielding, and ESA expected their performance to suffer somewhat toward the end of the initial five-year mission.

This reference frame serves as a fundamental grid for positioning celestial objects in the sky, aiding astronomers in various research endeavors.

[43] The expected accuracies of the final catalogue data have been calculated following in-orbit testing, taking into account the issues of stray light, degradation of the optics, and the basic angle instability.

[47] About three weeks after launch, on 8 January 2014, it reached its designated orbit around the Sun-Earth L2 Lagrange point (SEL2),[4][48] about 1.5 million kilometers from Earth.

The problem was initially thought to be due to ice deposits reflecting some of the light diffracted around the edges of the sunshield into the telescope apertures and on towards the focal plane.

[55] The testing and calibration phase, which started while Gaia was en route to SEL2 point, continued until the end of July 2014,[56] three months behind schedule due to unforeseen issues with stray light entering the detector.

When Gaia entered regular scientific operations in July 2014, it was configured to routinely process stars in the magnitude range 3 – 20.

[58] On the bright side of that limit, special operational procedures download raw scanning data for the remaining 230 stars brighter than magnitude 3; methods to reduce and analyse these data are being developed; and it is expected that there will be "complete sky coverage at the bright end" with standard errors of "a few dozen μas".

[65] The amount of dinitrogen tetroxide (NTO) and monomethylhydrazine (MMH) for the chemical propulsion subsystem on board might be enough to stabilize the spacecraft at L2 for several decades.

In March 2023, the Gaia mission was extended through the second quarter of 2025, when the spacecraft was expected to run out of cold gas propellant.

[82] The full data release for the five-year nominal mission, DR4, will include full astrometric, photometric and radial-velocity catalogues, variable-star and non-single-star solutions, source classifications plus multiple astrophysical parameters for stars, unresolved binaries, galaxies and quasars, an exo-planet list and epoch and transit data for all sources.

ESA's three 35-metre-diameter radio dishes of the ESTRACK network in Cebreros, Spain, Malargüe, Argentina and New Norcia, Australia, receive the data.

[88] In November 2017, scientists led by Davide Massari of the Kapteyn Astronomical Institute, University of Groningen, Netherlands released a paper[89] describing the characterization of proper motion (3D) within the Sculptor dwarf galaxy, and of that galaxy's trajectory through space and with respect to the Milky Way, using data from Gaia and the Hubble Space Telescope.

Massari said, "With the precision achieved we can measure the yearly motion of a star on the sky which corresponds to less than the size of a pinhead on the Moon as seen from Earth."

More surprisingly, the team found that 13 hypervelocity stars were instead approaching the Milky Way, possibly originating from as-of-yet unknown extragalactic sources.

[92] In late October 2018, the galactic population Gaia-Enceladus, the remains of a major merger with the defunct Enceladus dwarf, was discovered.

[108] In September 2023, radial velocity observations were used to confirm an exoplanet orbiting the star HIP 66074 that was first detected in Gaia DR3 astrometry data.

[110] In March 2024, Gaia discovered two streams of stars, named by researchers Shakti and Shiva, that formed more than 12 billion years ago.

[114] Since then the European Space Agency Science Programme Voyage 2050 has selected the theme of "Galactic Ecosystem with Astrometry in the Near-infrared" as one of two potential L-class missions to be implemented in the coming years thus boosting the chances for GaiaNIR which proposes exactly this.