Launched on December 19, 2013 from Kourou, the European satellite Gaia was stationed at the Lagrange point, 1.5 million km from Earth. Since July 25, 2014, it has been systematically scanning the sky with a rotating motion. As a result, it detects all objects in and around the Milky Way with apparent magnitudes between 3 and 21. This mainly concerns stars, but also planets, satellites and asteroids of the Solar System, quasars, gravitational lenses and, indirectly, exoplanets. Its three instruments - astrometric, photometric and spectroscopic - allow for a complete characterization. To date, Gaia has performed 1,600 billion astrometric measurements, 320 billion spectrophotometric measurements and 32 billion spectra.
These data have already made it possible to deliver a first catalog, DR1, on September 16, 2016, and a second, DR2, on April 25, 2018. Since then, ESA has been recording records of monthly hits on its dedicated database : 5,000 users and 2.5 to 5 million queries. In total, more than 3,800 scientific publications are based on DR2 data.
On December 3, 2020, the third catalog will be launched, or more precisely its first part : the "early" DR3. If it reaches us in a partial form, it is because the total volume of data collected is colossal, and it is appropriate to share with the community those that are already ready.
The complete Gaia DR3 catalog is scheduled for release in the first half of 2022. At this date, it will contain all the analysis of the data obtained by Gaia during the first 34 months of the mission, i.e. over a period between July 25, 2014 and May 28, 2017. This version will offer many new results, of which France is the main contributor. It mobilizes at CNES, in Toulouse, a farm of 6,000 computing cores and a volume of 5 petabytes of data.
The contributions of EDR3
Compared to the previous version DR2, this new catalog is considerably enriched : it now contains the positions, flashes and colors of 1.8 billion stars. For objects whose apparent magnitude is between 12 and 17, the catalog is almost complete. For 1.5 billion of these sources, parallaxes, proper motion and color are also available.
Compared to Gaia DR2, Gaia EDR3 is a major step forward. For astrometric and photometric data, it gains in precision, accuracy and homogeneity. Accuracy on trigonometric parallaxes is improved by about 30% and accuracy on eigenmotions is typically twice as good.
All positions and eigenmovements of objects are linked to the third realization of the Gaia reference frame (Gaia CRF3, Gaia Reference Frame). Materialized by 1,614,173 quasars (three times more than for Gaia DR2), Gaia-CRF3 is aligned to the International Reference Frame (ICRF) at better than 0.01 milliseconds of a degree.
The photometry is also more precise, but above all much more homogeneous over the entire sky and over the entire range of magnitudes and colors, without any systematic error greater than 1%.
Gaia DR3 finally includes the 7 million radial velocities already contained in Gaia DR2, but corrected for the occasion of a small number of erroneous values.
First applications of EDR3 }
The release of Gaia EDR3 is accompanied by four scientific articles published in the journal Astronomy & Astrophysics, demonstrating its scientific potential in four different fields :
- A very complete catalog of the 331,312 stars located at less than 100 parsecs from the Sun, allowing a number of researches on the luminosities, kinematics and orbits of these stars, among which notably the weakest whose census was until now very incomplete ;
- A detailed study of the different populations of stars, of galactic and extragalactic origin, in the direction of the galactic anticenter. By probing the confines of the Milky Way disk, its complex dynamics are revealed ;
- A complete study of the structure, kinematics and properties of the two Magellanic Clouds and their different star populations, as well as of the bridge (the "Magellanic Bridge") that connects them, showing the stream of stars from the Small Magellanic Cloud to the Large Cloud ;
- A study of eigenmotions with respect to Gaia EDR3 quasars which reveals a systematic effect due to the acceleration of the barycenter of the Solar System with respect to the reference frame of distant extragalactic sources. It is the spectacular improvement of the quality of astrometry and the drastic decrease of systematic errors that made this measurement possible.
The largest survey of the Milky Way is far from being completed. It will be completed as the data are processed. Three other appointments are planned, DR3 in its integral version, in 2022, then DR4 and DR5. The complete analysis will take nearly 20 years to complete.
Iconography On line on the ESA website
- Contextual reminders of the Gaia space mission
Measuring the distance to the stars, in the Galaxy and beyond, plays a vital role in astronomy. From our Earth, the stars appear to be distributed over the celestial sphere when in fact they are distributed in an immense volume. If we take the example of the stars in the constellation of Leo, they are actually scattered in the sky at great depths - from less than 10 to more than 7,000 light years. Only distance measurements allow us to properly locate each star in space.
Measure the distances
How to measure the distance to the stars ? A phenomenon allows to apprehend this measurement : from the Earth, in orbit around the Sun, the same star, called "near", will not project itself at the same place on a background of "distant" stars, depending on whether it is observed at two distinct moments (for example, in winter or summer). However, measurements of these tiny positional movements are extremely difficult to make from the ground. The first of them were made 170 years ago. But it was only from space that their accuracy could be improved considerably, when they were able to overcome the effects of atmospheric turbulence.
A French idea
As early as 1965, it was the very original idea of Pierre Lacroute, then director of the Strasbourg Observatory, which led to the conceptualization of Hipparcos, the first astrometric satellite of the European Space Agency. Operational from 1989 to 1993, Hipparcos measured the distances and movements of 118,218 stars with an accuracy 50 times greater than on the ground. The Hipparcos catalog published in 1997 remained the only reference in this field until the first Gaia catalog.
The genesis of Gaia
On the strength of this success, European astronomers proposed a new project to the ESA in 1992 with the ambition of measuring a billion objects, with an accuracy 50 times greater than that of Hipparcos. "By imagining a satellite with two telescopes with mirrors much larger in size than the Hipparcos one, and detectors of a much more advanced technology, we wanted to take up the challenge of very high precision and reach the arc microsecond", recalls Catherine Turon, an astronomer emeritus from Paris Observatory, a pioneer in space astrometry. Gaia is revolutionizing the field of celestial cartography at the beginning of the 21st century by introducing 3D on a large scale, and will make it possible to combine astrometric, photometric and spectroscopic data.
Data processing
The volume of data to be processed is unparalleled in the astronomical field today. The processing of all the scientific data has been entrusted to an international consortium, following a call for proposals published by ESA in 2006, and answered by a group of European scientists within a DPAC (Data Processing and Analysis Consortium). The DPAC now brings together 422 European scientists and 10 others from the rest of the world, who are responsible for the design and development of data processing methods.
A leading role for French research
The French astronomical community plays a leading role in this mission. With about a hundred scientists involved, France is the leading contributor in Europe, followed by Italy and Great Britain.
The French aerospace group Airbus Defence & Space (formerly EADS/Astrium) in Toulouse is the prime contractor for the construction of the satellite. CNES, the French space agency, is making a major contribution to the Gaia mission by developing, hosting and operating the French Data Processing Center (DPCC), a fundamental tool in the information processing chain.
