Final farewell to Gaia, the mission that has turned our understanding of the Milky Way upside down.

She hadn't even given birth a month before, but she knew she didn't want to miss it at all. So she grabbed her baby and headed to the University of Barcelona's Faculty of Physics. 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 scanned the sky for 11 years, revolutionizing our view of our galaxy and our cosmic neighborhood. 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 takeoff, 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 Cosmological Sciences of the UB (ICCUB), Xavier Luri, who had spent nearly 20 years thinking and designing what the mission that would map in three dimensions should be like.

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

On December 19, 2013, the probe, in the bowels of a Soyuz rocket, took off from French Guiana; unfurled a sort of umbrella covered with 10-meter diameter solar panels, and began a 1.5 million kilometer journey to reach the Lagrange 2 point, an area in the Terra-Sol vicinity. From there, and with two telescopes, during this time it has been tirelessly scrutinizing 2 billion stars and other objects in the visible sky, measuring with absolute precision 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've ever had. The map that Luke Skywalker would need to be able to make interstellar trips between Alliance worlds located in the Milky Way and that will guide future human missions into space.

And now, 11 years later, after surviving impacts from micrometeorites and solar storms, the spacecraft that has revolutionized galactic astronomy is about to close its eyes forever. Its fuel is almost exhausted, and so on March 27, 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 invaluable data set it has collected has given us 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 'parents'

As with all space missions, Gaia's history is also a long one. It began decades earlier with Hipparcos, a satellite launched into space in 1989, in which UB researchers were already heavily involved. "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 low: they showed up in Paris, at an ESA meeting, because they wanted to be part of the mission. And they got fully involved," Luri recalls.

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

"I was involved from the very beginning, from defining the scientific case, that is, what Gaia was to study, to the industrial design of the satellite, 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 galaxy's stellar population. "[Gaia's] is the first significant census, and the data obtained has shown us that the galaxy is much more complex than we expected," he adds.

The discovery machine

The data that Gaia has been sending—a challenge given the hundreds of terabytes available—has been received by three antennas located in Madrid, Australia, and Chile. Once downloaded, they are first processed at ESA's center 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 analyze and validate them, and then we slice and dice them and send them to the rest of the groups in packages," says Romero. In total, there are seven data processing centers across Europe, each specialized in a type of information: from astrometry, which is the largest package, to exoplanets and black holes, among others. Once validated, the data is deposited in a final open and public access archive, available to the entire scientific community.

"Gaia provides individual information about the stars, and from there we try to get a picture of what our galaxy is like," says Romero, who compares it to "composing a puzzle." Each piece on its own provides no 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 formed, what interactions it has had with other galaxies over the more than 13 billion years it has existed, and what its history has been like so far. We can also establish its shape more precisely: the center of the Milky Way is like a rugby ball or a barbell and has four spiral arms, two with stars and two with gas.

Using the trail of ancient stars captured by Gaia, astronomers have discovered that the Milky Way merged with another galaxy in its early history and is now moving towards a collision with the Sagittarius dwarf galaxy, a discovery or . "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 carry out simulations, until they discovered 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 within our small planet, in addition to the Montreux system, and all the small moons orbiting it. Gaia has allowed us to better draw the orbits and expand the census of objects that populate our cosmic neighborhood, from which it has managed to capture their spectra, which opens the door to studying the surface and the shape of them. There is one with 33 solar masses hidden in the constellation of Aquila, 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 neighboring ones, such as the Magellanic Clouds, two galaxies dwarfs 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 on 10 million objects there, opening the door to studying them in detail.

"Until now, we only had information on the brightness of the stars there, but now we also have motion, color, chemistry, and we're even getting to know the details of their internal motion, as if we were inside the Magellanic Clouds," Romero says.

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 release, 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 catalog of binary stars and also exoplanets can be expanded.

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

In recent days, before entering darkness, Gaia has changed its orientation because it is 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 shutting down forever.