Source: European Space Agency Publications
Gaia is creating an extraordinarily precise three-dimensional map of more than a thousand million stars throughout our Milky Way galaxy and beyond, mapping their motions, luminosity, temperature and composition. This huge stellar census will provide the data needed to tackle an enormous range of important questions related to the origin, structure and evolutionary history of our galaxy.
The Gaia space observatory was launched on 19 December 2013 and is expected to operate until 2025. The spacecraft is 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 main goal of the Gaia mission is to make the largest and most precise three-dimensional map of our galaxy by surveying an unprecedented one per cent of the galaxy’s population of 100 billion stars.
The Gaia space mission has the following objectives:
To determine the intrinsic luminosity of a star, and this 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.
To observe some of the faintest objects and 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 (such as the classification, frequency, correlations and directly observed attributes of rare fundamental changes and of cyclical changes). 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.
To Measure the astrometric and kinematic properties of a star, which is necessary in order to understand the various stellar populations, especially the most distant.
The Gaia spacecraft is composed of the payload module, the service module and the deployable sun-shield. It had a launch mass of around 2 tonnes. The payload module is built around a toroidal-shaped optical bench (about 3 m in diameter) which provides the structural support for the single integrated instrument that performs three functions: astrometry, photometry and spectrometry. The payload module also contains all the necessary electronics for managing the instrument operation and processing the raw data.
The service module comprises all the mechanical, structural and thermal elements supporting the instrument and the spacecraft electronics. It includes the micro-propulsion system, deployable sun-shield, payload thermal tent, solar arrays, and harness. The service module offers support functions to the Gaia payload and spacecraft for pointing, electrical power control and distribution, central data management and radio communications with Earth.
The L2 orbit
Gaia is placed in an orbit around the Sun, at the second Lagrange point L2, which is named after its discoverer, Joseph Louis Lagrange (1736-1813). For the Sun-Earth system, the L2 point lies at a distance of 1.5 million kilometres from the Earth in the anti-Sun direction and co-rotates with the Earth in it’s 1-year orbit around the Sun. One of the principal advantages of an L2 orbit is that it offers uninterrupted eclipse-free observations. From L2 the entire celestial sphere can be observed during the course of one year. To ensure Gaia stays at L2, the spacecraft must perform small manoeuvres every month.
The Gaia payload consists of three main instruments:
The astrometry instrument (Astro) 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 five-year 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.
The photometric instrument (BP/RP) 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 (BP/RP) 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 (BP) operates in the wavelength range 330–680 nm; the Red Photometer (RP) covers the wavelength range 640–1050 nm.
The Radial-Velocity Spectrometer (RVS) 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 (field lines of calcium ion) for objects up to magnitude 17. Radial velocities are measured with a precision between 1 km/s (V=11.5) and 30 km/s (V=17.5). 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. About three weeks after Gaia’s launch, on 8 January 2014, it reached its designated orbit around the Sun-Earth L2 Lagrange point (SEL2), at about 1.5 million kilometres from Earth.
Optical bench (silicon carbide torus)
- Focal plane cooling radiator
- Focal plane electronics
- Nitrogen tanks
- Diffraction grating spectroscope
- Liquid propellant tanks
- Star trackers
- Telecommunication panel and batteries
- Main propulsion subsystem
The testing and calibration phase, which started while Gaia was en route to SEL2 point, continued until the end of July 2014, three months behind schedule due to unforeseen issues with stray light entering the detector. After the six-month commissioning period, the satellite started its nominal five-year period of scientific operations on 25 July 2014 using a special scanning mode that intensively scanned the region near the ecliptic poles; on 21 August 2014 Gaia began using its normal scanning mode which provides more uniform coverage.
Although it was originally planned to limit Gaia’s observations to stars fainter than magnitude 5.7, tests carried out during the commissioning phase indicated that Gaia could autonomously identify stars as bright as magnitude 3. When it entered regular scientific operations in July 2014, it was configured to routinely process stars in the magnitude range 3 – 20. Beyond that limit, special procedures are used to download raw scanning data for the remaining 230 stars brighter than magnitude 3; methods to reduce and analyse these data were developed; and it is expected that there will be “complete sky coverage at the bright end” with standard errors of “a few dozen μas”.
On 12 September 2014, Gaia discovered its first supernova in another galaxy. On 3 July 2015, a map of the Milky Way by star density was released, based on data from the spacecraft. As of August 2016, “more than 50 billion focal plane transits, 110 billion photometric observations and 9.4 billion spectroscopic observations have been successfully processed.”
In 2018 the Gaia mission was extended to 2020. In 2020 the Gaia mission was further extended through 2022, with an additional “indicative extension” extending through 2025. The limiting factor to further mission extensions is the supply of nitrogen for the cold gas thrusters of the micro-propulsion system. 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. Without the cold gas the space craft can no longer be pointed on a micro-arcsecond scale.
In March 2023, the Gaia mission was extended through the second quarter of 2025, when it is expected that the spacecraft will run out of cold gas propellant. It will then enter a post-operations phase that is expected to be completed by the end of 2030
Stars and Other Objects in Gaia Early Data Release
Gaia’s Early Data Release 3 was made public on 3 December 2020. It contains detailed information on more than 1.8 billion sources, as measured by the Gaia spacecraft.
Towards the Final Gaia Catalogue
In the 2020s, the final Gaia catalogue will be published. This will be the definitive stellar catalogue for the foreseeable future, playing a central role in many and varied fields of astronomy. Producing this final catalogue is a complicated endeavour that requires the entire mission dataset and a complex processing chain devised and tested by hundreds of scientists and software experts in the Gaia Data Processing and Analysis Consortium (DPAC). Even before the first data release special subsets of Gaia data were made public to facilitate timely follow-up observations by the wider astronomical community. For example, Science Alerts – announcements to the scientific community about detected transient events such as supernovae and out-bursting stars – were, and continue to be, regularly issued.
Following the first data release in September 2016 based on Gaia’s first 14 months of observations, new datasets are released based on longer time intervals and with additional data products. The second data release was in April 2018, based on 22 months of observations. The third data release is based on 34 months of observations and will be split in two parts: an early data release in December 2020 and the full data release in 2022. These subsequent data sets are characterised by increasingly improving precision and additional parameters for the surveyed stars, as well as for other celestial objects – from Solar System bodies to galaxies beyond our Milky Way.
Eventually, the final catalogue will contain full astrometric (position, distance and motion) and photometric (brightness and colour) parameters for over one billion stars as well as extensive additional information including a classification of the sources and lists of variable stars, multiple stellar systems and exoplanet-hosting stars. For more than 150 million stars there will be measurements of the radial-velocity – the speed at which the star is travelling towards or away from us. The final Gaia catalogue will be a census of our Galaxy of such superb precision and detail that it will redefine the fundamental reference frame used for all astronomical coordinate systems. This will truly mark a new era of astrometry.