Gaia (spacecraft)


Gaia is a retired space observatory of the European Space Agency that was launched in 2013 and operated until March 2025. The spacecraft was designed for astrometry: measuring the positions, distances and motions of stars with unprecedented precision, and the positions of exoplanets by measuring attributes about the stars they orbit such as their apparent magnitude and color., the mission data processing continues, aiming to construct 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.
To study the precise position and motion of its target objects, the spacecraft monitored each of them about 70 times over the five years of the nominal mission, and about as many during its extension. Due to its detectors degrading more slowly than initially expected, the mission was given an extension, lasting until March 27, 2025, when scientists at the ESA switched off Gaia after more than a decade of service. Gaia targeted objects brighter than magnitude 20 in a broad photometric band that covered the extended visual range between near-UV and near infrared; such objects represent approximately 1% of the Milky Way population. Additionally, Gaia was expected to detect thousands to tens of thousands of Jupiter-sized exoplanets beyond the Solar System by using the astrometry method, 500,000 quasars outside this galaxy and tens of thousands of known and new asteroids and comets within the Solar System.
The Gaia mission continues to create a precise three-dimensional map of astronomical objects throughout the Milky Way and map their motions, which encode the origin and subsequent evolution of the Milky Way. 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, Gaia is part of ESA's Horizon 2000+ long-term scientific program. Gaia was launched on 19 December 2013 by Arianespace using a Soyuz ST-B/Fregat-MT rocket flying from Kourou in French Guiana. The spacecraft currently operates in a Lissajous orbit around the Sun–Earth L2 Lagrangian point. The science observation officially ended on 15 January 2025.

History

The Gaia space telescope had its roots in ESA's Hipparcos mission. Its mission was proposed in October 1993 by Lennart Lindegren and Michael Perryman in response to a call for proposals for ESA's Horizon Plus long-term scientific programme. 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. The name "Gaia" was originally derived as an acronym for Global Astrometric Interferometer for Astrophysics. This reflected the optical technique of interferometry that was originally planned for use on the spacecraft. 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 total cost of the mission is around €740 million, including the manufacture, launch and ground operations. Gaia was completed two years behind schedule and 16% above its initial budget, mostly due to the difficulties encountered in polishing Gaia ten silicon carbide mirrors and assembling and testing the focal plane camera system.

Objectives

The Gaia space mission has the following objectives:
  • To determine the intrinsic luminosity of a star requires knowledge of its distance. One of the few ways to achieve this without physical assumptions is through the star's parallax, but atmospheric effects and instrumental biases degrade the precision of parallax measurements. For instance, Cepheid variables are used as standard candles to measure distances to galaxies, but their own distances are poorly known. Thus, quantities depending on them, such as the speed of expansion of the universe, remain inaccurate.
  • Observations of the faintest objects will provide a more complete view of the stellar luminosity function. Gaia will observe 1 billion stars and other bodies, representing 1% of such bodies in the Milky Way galaxy. All objects up to a certain magnitude must be measured in order to have unbiased samples.
  • To permit a better understanding of the more rapid stages of stellar evolution. This has to be achieved by detailed examination and re-examination of a great number of objects over a long period of operation. Observing a large number of objects in the galaxy is also important to understand the dynamics of this galaxy.
  • Measuring the astrometric and kinematic properties of a star is necessary in order to understand the various stellar populations, especially the most distant.

    Spacecraft

Gaia was launched by Arianespace, using a Soyuz ST-B rocket with a Fregat-MT upper stage, from the Ensemble de Lancement Soyouz at Kourou in French Guiana on 19 December 2013 at 09:12 UTC. The satellite separated from the rocket's upper stage 43 minutes after launch at 09:54 UTC.
The craft headed towards the Sun–Earth Lagrange point L2 located approximately 1.5 million kilometres from Earth, arriving there 8 January 2014. The L2 point provides the spacecraft with a very stable gravitational and thermal environment. There, it uses a Lissajous orbit that avoids blockage of the Sun by the Earth, which would limit the amount of solar energy the satellite could produce through its solar panels, as well as disturb the spacecraft's thermal equilibrium. After launch, a 10-metre-diameter sunshade was deployed. 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. These factors and the materials used in its creation allow Gaia to function in conditions between -170°C and 70°C.

Scientific instruments

The Gaia payload consists of three main instruments:
  1. The astrometry instrument ' precisely determines the positions of all stars brighter than magnitude 20 by measuring their angular position. By combining the measurements of any given star over the duration of the mission, it will be possible to determine its parallax, and therefore its distance, and its proper motion—the velocity of the star projected on the plane of the sky.
  2. The photometric instrument ' allows the acquisition of luminosity measurements of stars over the 320–1000 nm spectral band, of all stars brighter than magnitude 20. The blue and red photometers are used to determine stellar properties such as temperature, mass, age and elemental composition. Multi-colour photometry is provided by two low-resolution fused-silica prisms dispersing all the light entering the field of view in the along-scan direction prior to detection. The Blue Photometer operates in the wavelength range 330–680 nm; the Red Photometer covers the wavelength range 640–1050 nm.
  3. The Radial-Velocity Spectrometer is used to determine the velocity of celestial objects along the line of sight by acquiring high-resolution spectra in the spectral band 847–874 nm for objects up to magnitude 17. Radial velocities are measured with a precision between 1 km/s and 30 km/s. The measurements of radial velocities are important "to correct for perspective acceleration which is induced by the motion along the line of sight". The RVS reveals the velocity of the star along the line of sight of Gaia by measuring the Doppler shift of absorption lines in a high-resolution spectrum.
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. It has no reaction wheels or gyroscopes. The spacecraft subsystems are mounted on a rigid silicon carbide frame, which provides a stable structure that will not expand or contract due to temperature. Attitude control is provided by small cold gas thrusters that can output 1.5 micrograms of nitrogen per second.
The telemetric link with the satellite is about 3 Mbit/s on average, while the total content of the focal plane represents several Gbit/s. Therefore, only a few dozen pixels around each object can be downlinked.

Measurement principles

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. The spacecraft rotates continuously around an axis perpendicular to the two telescopes' lines of sight, with a spin period of 6 hours. Thus, every 6 hours the spacecraft scans a great circle stripe approximately 0.7 degrees wide. The spin axis in turn has a slower precession across the sky: it maintains a fixed 45 degree angle to the Sun, but follows a cone around the Sun every 63 days, giving a cycloid-like path relative to the stars. Over the course of the mission, each star is scanned many times at various scan directions, providing interlocking measurements over the full sky.
The two key telescope properties are:
  • 1.45 × 0.5 m primary mirror for each telescope
  • 1.0 × 0.5 m focal plane array on which light from both telescopes is projected. This in turn consists of 106 CCDs of 4500 × 1966 pixels each, for a total of 937.8 megapixels.
Each celestial object was observed on average about 70 times during the five years of the nominal mission, which has been extended to approximately ten years and will thus obtain twice as many observations. These measurements will help determine the astrometric parameters of stars: two corresponding to the angular position of a given star on the sky, two for the derivatives of the star's position over time and lastly, the star's parallax from which distance can be calculated. The radial velocity of the brighter stars is measured by an integrated spectrometer observing the Doppler effect. Because of the physical constraints imposed by the Soyuz spacecraft, Gaia 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. Ground tests of the CCDs while they were subjected to radiation provided reassurance that the primary mission's objectives can be met.
An atomic clock on board Gaia plays a crucial role in achieving the mission's primary objectives. Gaia rotates with angular velocity of 60"/sec or 0.6 microarcseconds in 10 nanoseconds. Therefore, in order to meet its positioning goals, Gaia must be able to record the exact time of observation to within nanoseconds. Furthermore, no systematic positioning errors over the rotational period of 6 hours should be introduced by the clock performance. For the timing error to be below 10 nanoseconds over each rotational period, the frequency stability of the on-board clock needs to be better than 10−12. The rubidium atomic clock aboard the Gaia spacecraft has a stability reaching ~ 10−13 over each rotational period of 21600 seconds.
Gaia's measurements contribute to the creation and maintenance of a high-precision celestial reference frame, the Barycentric Celestial Reference System, which is essential for both astronomy and navigation. This reference frame serves as a fundamental grid for positioning celestial objects in the sky, aiding astronomers in various research endeavors. All observations, regardless of the actual positioning of the spacecraft, must be expressed in terms of this reference system. As a fully relativistic model, the influence of the gravitational field of the solar-system must be taken into account, including such factors as the gravitational light-bending due to the Sun, the major planets and the Moon.
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. The best accuracies for parallax, position and proper motion are obtained for the brighter observed stars, apparent magnitudes 3–12. The standard deviation for these stars is expected to be 6.7 micro-arcseconds or better. For fainter stars, error levels increase, reaching 26.6 micro-arcseconds error in the parallax for 15th-magnitude stars, and several hundred micro-arcseconds for 20th-magnitude stars. For comparison, the best parallax error levels from the new Hipparcos reduction are no better than 100 micro-arcseconds, with typical levels several times larger.