“It’s the Swiss army knife of astrophysics. There isn’t a single astronomer who won’t use its data, directly or indirectly,” the astronomer from the Côte d’Observatoire told AFP. ‘Azur, François Mignard, responsible for Gaia for France.

The community of astronomers will be able to draw from Monday, from 10:00 GMT, in the third catalog of data collected by the instrument. A harvest, accompanied by around fifty scientific articles, which lists a host of celestial objects.

From the closest, with more than 150,000 asteroids in our solar system, “whose orbit the instrument has calculated with incomparable precision”, says Mr. Mignard, to new measurements concerning more than 1.8 billion stars of the Milky Way. And beyond this galaxy: populations of other galaxies and distant quasars.

Launched on behalf of the European Space Agency (ESA), the Gaia telescope has been operational since 2013, stationed in a privileged position, called L2, one and a half million kilometers from Earth, opposite the Sun.

– Scan the sky –

“Gaia scans the sky and picks up everything it sees,” sums up astronomer Misha Haywood, at the Paris-PSL Observatory. It detects and observes a very small part (barely 1%) of the stars in our galaxy, whose diameter measures 100,000 light years.

But he establishes much more than a simple map. Its two telescopes are associated with a photographic sensor of a billion pixels, where that of a commercial camera is counted in millions. Three astrometry instruments, photometry and spectroscopy, will interpret the photons, real light signals, thus recovered.

“It provides thanks to this a global observation of the positions of what is moving in the sky. This is the first time”, continues Mr. Haywood. Before Gaia, “we had a really limited view of the galaxy”.

Before Gaia? It was Hipparcos, the satellite that revolutionized observation after its launch by ESA in 1997, cataloging more than 110,000 celestial objects.

With Gaia, astronomers have access not only to the positions and motions of a large number of stars, but also to measurements of their physical and chemical characters, and, just as important, their age.

So much information “which informs us about their past evolution, and therefore about that of the galaxy”, explains astronomer Paola di Matteo, colleague of Misha Haywood at the Paris-PSL Observatory.

– Major discoveries –

This is also “one of the reasons why Gaia was built”, continues the astronomer. “Stars have the particularity of living for billions of years. Their measurement is therefore like that of a fossil which tells us about the state of the galaxy at the time of their formation”.

This overview of the movements of the stars of the Milky Way has already led to major discoveries. With the second catalog, delivered in 2018, astronomers were able to show that our galaxy had “merged” with another ten billion years ago.

The catalog has given rise to thousands of scientific articles since its first edition in 2016. The flood of data requires a dedicated ground processing chain, the DPAC, calling on the supercomputers of six European computing centers, and the mobilization of 450 specialists, explains François Mignard, who was in charge.

“Without this processing group there is no mission”, because Gaia produces 700 million star positions, 150 million photometry measurements and 14 million spectra every day. A torrent of raw data, which “human-driven” algorithms transform into measurements that can be used by astronomers.

It will have taken five years to deliver this third catalog of observations spread from 2014 to 2017. And it will be necessary to wait until 2030 to obtain the final version, when Gaia will have finished scanning space, in 2025.