Last starlight for the ground-breaking Gaia mission

The Gaia satellite, the European Space Agency‘s (ESA) project to map the Milky Way, has completed the sky-scanning phase in a mission that has gathered over three trillion observations of about two billion stars and other objects over the last decade. This work has transformed our understanding of the galaxy and our cosmic environment. Since the beginning, the mission has involved a team of astronomers and engineers from the Department of Quantum Physics and Astrophysics at the University of Barcelona (UB), the Institute of Cosmos Sciences of the UB (ICCUB), and the Institute of Space Studies of Catalonia (IEEC — Institut d’Estudis Espacials de Catalunya).

Gaia’s fuel, launched on 19 December 2013, is about to run out: it uses about ten grams of gas per day to keep spinning with millimetre precision. But this is far from the end of the mission: technology tests are scheduled for the coming weeks before Gaia moves to its ‘retirement’ orbit, and two data releases are planned for 2026 and later this decade, the so-called DR4 and the final catalogue, DR5.

Carole Mundell, ESA Director of Science, says: “Today marks the end of science observations and we are celebrating this incredible mission that has exceeded all our expectations, lasting for almost twice its originally foreseen lifetime”.

“The data collected by Gaia has given us unique insights into the origin and evolution of our Milky Way, and has also transformed astrophysics and Solar System science in ways we have yet to fully appreciate”, continues Mundell. “Gaia has built on unique European excellence in astrometry and will leave a long-lasting legacy for future generations”, she adds.

Xavier Luri, professor at the Department of Quantum Physics and Astrophysics, director of the ICCUB and member of the IEEC, stresses that “the Gaia team at the UB and the IEEC worked on the mission since its beginnings, around 1997”. “Since then, it has participated in all phases, from the definition of the scientific case and industrial design to data processing and scientific exploitation”, he continues. “Now, although Gaia is finishing its observations, several years of work lie ahead to fully process all the data collected during these years and to publish two additional data deliveries (DR4 and DR5)”.

Gaia delivers the best Milky Way map

Gaia has mapped the positions, distances, motions, changes in brightness, composition and numerous other characteristics of the stars by repeatedly observing them throughout the mission with its three instruments. This has allowed Gaia to achieve its main goal: to make the largest and most accurate map of the Milky Way, showing us our galaxy as no other mission has ever done before.

We now also have the best reconstructed view of how our galaxy might look to an outside observer (see main image). This new artist impression of the Milky Way incorporates Gaia data from a multitude of papers over the past decade.

Stefan Payne-Wardenaar, scientific visualizer at the Max Planck Institute for Astronomy (Germany), says that “it contains major changes compared to previous models because Gaia has changed our perception of the Milky Way; even basic ideas, such as the rotation of the central bar of our galaxy, the warp of the disc, the detailed structure of the spiral arms and the interstellar dust near the Sun, have been revised”.

However, Payne-Wardenaar notes that “the distant parts of the Milky Way remain educated guesses based on incomplete data. With further Gaia data releases, our view of the Milky Way will become even more accurate”.

Discovery machine of the decade

Gaia’s repeated measurements of stellar distances, motions and features are key to performing galactic archaeology of our Milky Way, revealing the missing links in the complex history of our galaxy to help us understand our origins. Gaia is rewriting the history of the Milky Way and making predictions about its future: from detecting ‘ghosts’ of other galaxies and multiple trails of ancient stars that merged with the Milky Way in its early history to finding evidence of a present-day collision with the Sagittarius dwarf galaxy.

In the process of observing the stars in our galaxy, Gaia has also detected other objects, such as asteroids in the backyard of our solar system or galaxies and quasars—the bright, active centres of galaxies powered by supermassive black holes—outside the Milky Way.

For example, Gaia has provided pinpoint precision orbits of more than 150 000 asteroids, and has such high-quality measurements as to uncover possible moons around hundreds of them. It has also created the largest three-dimensional map of about 1.3 million quasars, with the furthest shining bright when the Universe was only 1.5 billion years old.

In addition, Gaia has discovered a new class of black holes, including one with a mass almost 33 times the mass of the Sun, hidden in the constellation Aquila, less than 2,000 light-years from Earth. It is the first time such a large stellar black hole has been observed within the Milky Way.

More ground-breaking science ahead

The Gaia scientific and engineering teams are already working full steam on the preparations for Gaia Data Release 4 (DR4), expected in 2026. The data volume and quality improve with every release and Gaia DR4, with an expected 500 TB of data products, is no exception. Furthermore, it will cover the mission’s first 5.5 years, corresponding to the length of the originally foreseen duration of the mission.

Gaia DR4 is set to expand its binary star catalogue, the largest such catalogue to date. Gaia has a unique ability to tease out the tiny motions of pairs of celestial objects orbiting close to each other, and has already spotted previously hidden companions around bright stars.

In this context, the last directed observation of Gaia on 10 January was of the binary pair 61 Cygni. This iconic star attracted the attention of 19th century astronomers and provided some of the first measurements of proper motion and parallax, techniques used by Gaia on some two billion stars.

The number of exoplanets discovered by Gaia will also increase in future data releases, thanks to the increased number of observations available, which facilitate the detection of wobbling stars, whose motion is slightly affected by the action of orbiting planets.

Rocío Guerra, Gaia Science Operations team leader at ESA’s European Space Astronomy Centre (ESAC) near Madrid, Spain, explains that over the next months, “we will continue to downlink every last drop of data from Gaia, and at the same time the processing teams will ramp up their preparations for the fifth and final major data release at the end of this decade, covering the full 10.5 years of mission data”.

“This will conclude an incredible coordinated effort between hundreds of experts across the science operations centre here at ESAC, the mission operations team flying Gaia from ESA’s European Space Operations Centre in Germany, and the huge consortium of data processing specialists, who have together ensured the smooth running of this beautiful mission for so long”, adds Guerra.

Gaia’s retirement plan

While today marks the end of science observations, a short period of technology testing now begins. The tests have the potential to further improve the Gaia calibrations, learn more about the behaviour of certain technology after ten years in space, and even aid the design of future space missions.

After several weeks of testing, Gaia will leave its current orbit around Lagrange point 2, 1.5 million kilometres away from the Earth in the opposite direction to the Sun, and move into its final heliocentric orbit, far from the Earth’s sphere of influence. The satellite will be switched off on 27 March 2025, to avoid any damage or interference with other satellites.

During the technological tests, Gaia’s orientation will change, and it will temporarily become several magnitudes brighter. This will make it much easier to observe with small telescopes, although it will not be visible with the naked eye. A guide to locate the satellite at that time has been prepared, and amateur astronomers are invited to share their observations.

“Gaia will treat us with this final gift as we bid farewell, shining among the stars ahead of its well-earned retirement”, concludes Uwe Lammers, Gaia Mission Manager.

About the IEEC

The Institute of Space Studies of Catalonia (IEEC — Institut d’Estudis Espacials de Catalunya) promotes and coordinates space research and technology development in Catalonia for the benefit of society. IEEC fosters collaborations both locally and worldwide and is an efficient agent of knowledge, innovation and technology transfer. As a result of more than 25 years of high-quality research, done in collaboration with major international organisations, IEEC ranks among the best international research centres, focusing on areas such as: astrophysics, cosmology, planetary science, and Earth Observation. IEEC’s engineering division develops instrumentation for ground- and space-based projects, and has extensive experience in working with private or public organisations from the aerospace and other innovation sectors.

The IEEC is a non-profit public sector foundation that was established in February 1996. It has a Board of Trustees composed of the Generalitat de Catalunya, Universitat de Barcelona (UB), Universitat Autònoma de Barcelona (UAB), Universitat Politècnica de Catalunya · BarcelonaTech (UPC), and the Spanish Research Council (CSIC). The IEEC is also a CERCA centre.