The ESA Gaia Spacecraft Has Ended Its Life; A View From Gaia PM Giuseppe Sarri
15th Jan 2025
January has seen the end of Gaia’s operations. Gaia mission has been one of the most important scientific missions of the European Space Agency (ESA).
The spacecraft (ref. Figure 1) was launched on 19 December 2013 on a Soyuz-STB/Fregat-MT launch vehicle from the European Spaceport in Kourou, French Guiana. Its main goal was to study our Milky Way and its stellar content and to provide their highly accurate astrometric and photometric parameters. But the satellite did also survey many other objects, like asteroids and minor bodies of our solar system, exoplanets outside our solar systems and many galaxies. This allowed to chart the precise position and velocity, in three dimensions of almost two billion celestial objects.

After more than 10 years of operations the cold gas needed to keep the spacecraft scanning at a stable pace is over and the mission is considered accomplished. The phase of capturing science observations was ended on 15 January 2025 and will be followed by a series of technology tests to help designing future missions with similar instruments onboard. At the end of March, the spacecraft will leave its current orbit around the Lagrange point L2 (around 1.5 million kilometers from Earth on the anti-Sun side) and it will be sent to its final safe orbit around the Sun where it will be switched off.
An extra life for Gaia: What’s Next?
However, this is not the end of the story. Although the spacecraft’s operations will be over, the Gaia mission‘s science life and data processing will continue for many years working on the immense amount of data collected and be marked by two major data releases during the reminder of this decade. The current data catalogue will be updated with the final data legacy expected after 2030.
Since the start of science operations on 25 July 2014, Gaia has scanned billions of objects several times (multiple scanning of the same object is key to improve the astrometric accuracy). Built by an industrial consortium lead by the French branch of ADS (Airbus Defence and Space), at its heart Gaia contains two optical telescopes (106.5 degrees apart) that work with three science instruments to precisely determine the location of stars and their velocities, and to split their light into spectra for analysis. During its mission, the spacecraft spun slowly, one rotation every 6 hours, sweeping the two telescopes across the entire celestial sphere. The spin axis in turn has a slower precession across the sky. Over the course of the mission, each star is scanned many times at various scan directions, providing interlocking measurements over the full sky. As the detectors repeatedly measure the position of each celestial object, they are detecting the object’s motion through space.
The typical star flow in the focal plane is about 3400 stars per second. Of course, it makes a big difference whether Gaia scans over the Galactic plane, an area bursting with stars, or over the Galactic anticentre, a less crowded area in the sky. In case the area was too crowded with stars, the on-board computer adapted the way of observing to cope with the increased number of objects. When Gaia scanned objects, they transited across the CCDs in the focal plane. The CCD (Charge-Coupled Device) is a light sensitive circuit that captures images by converting photons into electrons, CCDs are used overall in modern cameras, including those of our mobile phones. The Gaia focal plane consists of 106 CCDs of 4500 × 1966 pixels each, for a total of almost one billion pixels. A visual explanation of the principle of the Gaia operations can be seen on YouTube.
To achieve the extremely high accuracy required by the scientific objectives, the spacecraft is kept thermally stable by a large sunshield which was deployed right after launch (ref. Figure 2). The achieved astrometric accuracies depended on the brightness of the observed stars and varied from around 6 micro-arcseconds for star magnitude in the range 3 to 12 to 25 micro-arcseconds for 15th magnitude stars and few hundred micro-arcseconds for 20th-magnitude stars. This was orders of magnitude better that what had been achieved by previous mission or ground observations.

Gaia’s data releases (so far hundreds of terabytes of data) have brought a wealth of data on stars in our Milky Way and beyond. But Gaia adds many other data products on top of its star data. The Gaia catalogues contain information on Solar system objects as well as on galaxies and quasars. The Gaia data releases are prepared by a scientific consortium featuring the collaboration of about 450 people from 25 countries. The data sets released so far have been used heavily by astronomers across the world (ref. Figure 3). Currently, on average, about five Gaia-related papers are published per day. Stories discussing the science results can be found on ESA’s website. There the interested reader is presented with an extensive overview on the advances made with the Gaia mission.

Gaia mission and our understanding of the galaxy and beyond
Our knowledge of our home galaxy has improved a lot. An updated view of the Milky Way top-down was published few months ago based on Gaia data (ref. Figure 4). While detailed images exist of many external galaxies, our own galaxy requires extensive mapping from within to observe the structures that together give its shape. It shall however be noted that such map is based on about 2 billion stars which only covers 1% of the Milky Way.

Using the Gaia data astronomers have created the largest map of quasars: bright and active centres of galaxies powered by supermassive black holes. The map shows the location of about 1.3 million quasars in space and time, with the furthest shining bright when the Universe was only 1.5 billion years old. ESA’s Gaia mission has also helped discover a new kind of black hole closer to Earth than any other black holes that we know of (ref. Figure 5). Three black holes were discovered by studying ultra-precise measurements of stellar positions which showed a strange ‘wobble’ in the movement of two stars on the sky indicating that they are orbiting a very massive object. In all cases, the objects are approximately ten times more massive than our Sun.

The field of astrometry is in a golden era thanks to the massive progress made with the Gaia mission at multiple wavelengths in both the number of objects and precision of its optical astrometry survey. Built on the heritage of the Hipparcos spacecraft (1989-1993) Gaia has achieved order-of-magnitude higher performances and has enabled Europe to remain at the forefront of astronomy. High-accuracy astrometric data is now used across several astronomy disciplines and in solar system science as well as mission-enabling disciplines such as spacecraft tracking. Gaia, which was initially planned for a scientific mission lasting 5 years, has by far exceeded the required lifetime and has collected a wealthy amount of data that will keep astronomers and astrophysicists of the world busy for the next decades. In the meantime, scientist and engineers have already teamed up for a Gaia follow on, may be in the infrared….
Written by Giuseppe Sarri
Giuseppe Sarri holds a Masters Degree in Nuclear Engineering from Politecnico di Torino (I). From 1983 to 1989 he worked at the Microtecnica SpA on several aircraft projects. He joined the European Space Agency in 1989 in the Human Space Flight Directorate (Columbus programme). In 1994 he moved to the Science Directorate. He has been the Payload Manager of Integral (the high energy observatory), the Study Manager of Eddington (the planet finding mission, now called Plato) and he was involved in the development of Planck (the cosmic background observatory). He was the Project Manager of Gaia (the ESA’s cornerstone mission for mapping more than one billion stars of our galaxy). After the launch of Gaia in 2013 he was appointed Project Manager of JUICE, the first European mission to Jupiter and its icy moons which was launched in April 2023 and is now flying toward the Jovian system. Giuseppe Sarri is now retired.
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