Final farewell to Gaia, the mission that has turned the knowledge of the Milky Way upside down

She hadn't even given birth for a month, but she knew she didn't want to miss it in any way. So she took her baby and went to the Faculty of Physics at the University of Barcelona. From there, with her research group, she was able to follow the launch into space live on large screens. from the Gaia space telescope, the eyes with which the European Space Agency (ESA) has swept the sky for 11 years and revolutionised the vision we had of our galaxy and our cosmic neighbourhood. And where Catalan participation has been decisive.

"It was a very exciting moment," explains Mercè Romero, associate professor at the UB and principal investigator of the mission. "I remember being there with my colleagues with my heart in my mouth following the take-off, a moment that is always delicate, with my baby in my arms," she adds.

This mathematician was part of the team of astronomers and engineers, led by the professor and director of the Institute of Cosmos Sciences of the UB (ICCUB), Xavier Luri, who had spent nearly 20 years thinking and designing what that mission that would map in three dimensions should be like.

"I even thought about calling my daughter Gaia," she confesses, although she ultimately backed out because "I was afraid she wouldn't forgive me in the future for calling it my life's mission."

On December 19, 2013, the probe, in the bowels of a Soyuz rocket, took off from French Guiana; it unfurled a sort of parasol covered with 10-meter-diameter solar panels and began a 1.5 million-kilometer journey to settle at the Lagrange point 2, an area in the Terra-Sol environment. From there, and with two telescopes, during this time it has been tirelessly scanning 2 billion stars and other objects in the visible sky, accurately measuring their positions, distances, movements and changes in brightness.

Thanks to all this data and information, we now have the most complete three-dimensional map of our galaxy we have ever had. The map that Luke Skywalker would need to be able to make interstellar trips between worlds of the Alliance located in the Milky Way and that will guide future human missions to space.

And now, 11 years later, having survived micrometeorite impacts and solar storms, the spacecraft that has circled the galaxy’s astronomy is about to close its eyes for good. Its fuel is nearly exhausted, and on 27 March Gaia will move into its retirement orbit and shut down.

“This incredible mission has exceeded all expectations, lasting almost twice its originally planned lifetime,” said Carole Mundell, ESA’s Director of Science. “The precious dataset it has collected has provided us with unique insights into the origin and evolution of our Milky Way galaxy, and has also transformed astrophysics and solar system science in ways we have not yet fully understood.”

From Catalan 'fathers'

As with all space missions, the history of Gaia is also a long one. It began decades earlier with Hipparcos, a satellite that was launched into space in 1989 and in which there was already significant involvement of UB researchers. "Jordi Torra and other physicists from the faculty did something very innovative at a time when the country was emerging from Franco's regime and investment in science was very scarce: they went to Paris, to an ESA meeting, because they wanted to be part of the mission. And they got fully involved," recalls Luri.

And although the technology at the time was much less sophisticated, this ESA satellite already achieved a great feat: mapping 118,000 stars in the Milky Way. The Hipparcos mission lasted 8 years, and before it came to an end, and with the knowledge acquired, the scientific community already began to think about its replacement. The parents Hipparcos scientists and engineers, including Catalan scientists and engineers from the then Department of Astronomy and Meteorology, also became part of Gaia, ESA's most ambitious project to study the history and structure of the Milky Way. Luri joined the project in 1998, planning and thinking about what the successor to the first telescope that had pioneered the creation of a small star catalogue should be like.

"I was involved from the start, from defining the scientific case, i.e. what Gaia was to study, to the industrial design of the satellite and the data processing and scientific exploitation," says this proud astronomer who is also a member of the executive committee of the consortium that processes the hundred or so terabytes of telemetry sent by the satellite and converts it into science. The Institute for Space Studies of Catalonia (IEEC) is also involved in the mission.

"The leap that Gaia has made compared to Hipparcos is exponential," says Luri, who highlights the fact that it has gone from just over 100,000 stars to 2 billion, which represents a sampling of between 1 and 2% of the stellar population of the galaxy. "[Gaia's] is the first significant census and the data obtained have shown us that the galaxy is much more complex than we expected," he added.

The machine of discoveries

The data sent by Gaia – a challenge given the hundreds of terabytes in stock – have been received by three antennas located in Madrid, Australia and Chile. Once downloaded, they are first processed at the ESA centre in Madrid, ESAC, using programs developed by UB scientists, and then processed again in Barcelona, ​​​​with the help of Marenostrum, the supercomputer at the Barcelona Supercomputing Center.

"We analyse and validate them, and then we cut them up and send them to the rest of the groups in packages," says Romero. In total, there are seven data processing centres throughout Europe, each specialised in a type of information: from astrometry, which is the largest package, to exoplanets and black holes, among others. Once validated, the data are deposited in a final open and public access archive, available to the entire scientific community.

"Gaia gives us individual information about the stars and from there we try to get an image of what our galaxy is like," says Romero, who compares it to "putting together a puzzle." Each of the pieces on its own does not give us information about what the final image will be, which can only be obtained when they all fit together and the puzzle is solved.

And from these small pieces, going up, we begin to get clues about how the Milky Way was formed, what interaction it has had over the more than 10,000 years it has been alive with other galaxies, what its history has been until now. We can also establish its shape more precisely: the center of the Milky Village is like a rugby ball or a bar and has four spiral arms, two with stars and two with gas.

Using Gaia's trail of ancient stars, astronomers have discovered that the Milky Way merged with another galaxy in its early history and is now heading toward a collision with the Sagittarius Dwarf Galaxy, a discovery that has been widely reported. "We were validating some data when some of it surprised us. At first we thought it was wrong," Romero admits. But they began to study them and to carry out simulations, until they found that there was a spiral in the vertical component of the disk and that it was produced by the interaction between our galaxy and Sagittarius," explains Romero.

One of the surprises that Gaia has provided has been inside our small planet, in addition to the Sagittarius System, and all small moons orbiting around it. Gaia has allowed us to better draw the orbits and expand the census of objects that populate our cosmic neighborhood, of which it has managed to capture their spectra, which opens the door to studying the surface and the shape of the objects. There is one of 33 solar masses hidden in the constellation of the Eagle, less than 2,000 light years from Earth. It is the first time that a black hole of stellar origin so large has been observed within the Milky Way.

And it has not only provided knowledge about our galaxy, but also about other neighbors, such as the Magellanic Clouds, two galaxies dwarf stars orbiting the Milky Way, visible from the southern hemisphere. Despite being very far away, more than 160,000 light years away, Gaia has captured individual data from 10 million objects there and has opened the door to studying them in detail.

"Until now we only had information on the brightness of the stars there and now we also have movements, colours, chemistry and we are even getting to know the details of the internal movement, as if we were inside the Magellanic Clouds," says Romero.

With the data published so far, more than 13,000 scientific articles have already been generated. But there are still two new packages to be presented that will continue to provide new discoveries. The teams of engineers and scientists are working at full capacity on what will be the fourth data publication, Gaia DR4, scheduled for 2026, for which observations from the first 5.5 years of the mission will be used. With this new package, it is hoped that the catalogue of binary stars and also of exoplanets can be expanded.

"Gaia will allow a whole generation of researchers to do science," says Luri, who reveals that they are already thinking about the next mission, which will take over from Gaia and will have to show the other half of the galaxy, the one Gaia cannot see. "We will have to incorporate infrared to be able to capture the stars hidden by gas and dust," says Romero.

In recent days, before entering the darkness, Gaia has changed its orientation to be several magnitudes brighter. In this way, dressed in light, a small diamond among diamonds, it will be visible from Earth with the help of a small telescope. One last gift before turning off forever.