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Gaia dances the night away: Farewell to a fading Gaia!

Figure 1. Movie clip of Gaia just after the passivation on 27 March 2025. Credits: Liverpool telescope, J. Marchant - CC BY-SA 3.0 IGO.

"Dancing away in space, a star among the stars orbiting around humanity’s star. The show is far from over and Gaia’s data dances on!", says Peter Collins, Gaia Flight Operations Manager.

On 27 March 2025, ESA's Gaia spacecraft was deactivated and placed into a retirement orbit during a moving ceremony at the ESOC premises in Darmstadt, Germany. This means that there is no more communication with the spacecraft, and the system that kept the spacecraft pointing in a stable direction, is no longer active. Gaia is hence purely subject to the laws of gravity and other influences, like the pressure from the solar radiation. In its new retirement orbit, Gaia is moving away from its nominal position near the Sun-Earth Lagrange 2 point, and the probe's orientation (attitude) is no longer under any active control.

While direct communication with the now inert spacecraft is impossible, there is still one way we can keep tabs on what Gaia is doing. This is by observing Gaia with telescopes on Earth, like the GBOT team has been doing for the last 11 years. GBOT stands for Gaia Ground-Based Optical Tracking, which was necessary to ensure that the full accuracy of the ultra-precise Gaia astrometry could be reached.

Figure 2. Brightness of Gaia after the insertion into the retirement orbit on 27 March 2025. The circles and triangles denote data from the Liverpool telescope, the crosses Calar Alto data. Note that generally, red passband filters have been used, except for the data taken on March 27, which covers the g'r'i'z' suite of filters. Credits: ESA/Gaia/DPAC - CC BY-SA 3.0 IGO. Acknowledgements: created by M. Altmann and S. Bouquillon from the Gaia GBOT team.

Starting on the very night after the passivation until the end of May, when Gaia started to become too faint to be observed, the spacecraft was observed from Earth using mainly the trusted Liverpool telescope located on La Palma. Also some data was received from the Schmidt telescope on Calar Alto (Spain). As expected, the spacecraft slowly faded against the dark sky as the distance to Earth grew larger (see Figure 2). There was also a strong magnitude oscillation which became apparent even in the first observations on 27 March (see Figure 1).

Gaia's brightness varies from below the detection threshold to about 16.8 magnitude. The post-passivation peak brightness is thus far brighter than it was in nominal operational conditions, when it had magnitudes of 21 and below. Thus, Gaia has picked up a habit of dancing as it rotates during its journey through space following its new orbit.

Figure 3. The photometric data taken between 4 and 6 April 2025 superimposed. The data from 6 April has been shifted twice to match the secondary peaks seen in the data of the previous two nights. The vertical lines show the observing times of all observations, i.e. those which do not coincide with a datapoint yielded no detection. This means that Gaia was too faint to be detected during those exposures. Credits: ESA/Gaia/DPAC - CC BY-SA 3.0 IGO. Acknowledgements: created by M. Altmann and S. Bouquillon from the Gaia GBOT team.

Unfortunately, because of the limitations of our data, relatively little can be said about the rotational period. The visible peaks have a duration of approximately 20 minutes and seem to be spaced another 20 minutes apart as can be seen in Figure 3. But the bright phase does not have to come from the same reflective surface. Depending on how Gaia is rotating through space, there might be up to four surfaces involved, i.e. the sun-shield seen from below (as in the operational phase), above and the sides of the spacecraft's hull.

Looking at the peaks, there are some indications of mirrored time brightness relations as seen in Figure 3. But it is also good to realize that the geometric layout is evolving between the receding Gaia, Sun and Earth and other intrinsic changes may play a role. Hence, for various reasons, a rotational period cannot easily be concluded from these data.

On 29 May 2025 between 00:15-00:20 UTC, Gaia was observed for the last time, again with the Liverpool telescope (though there was another very faint detection about 20 minutes later, that one is far less sure to be a true detection of Gaia). The distance from Earth was approximately 3.15 million km, roughly twice the nominal distance during the operational phase, and the magnitude was 20.7 magnitude as can be seen in Figure 4.

Figure 4. The last recorded image of Gaia. The green cross indicate where the telescope was looking for Gaia. The green circle shows where Gaia was found. The offset between the cross and the circle is about 20 to 30 arcseconds. Credits: Liverpool telescope, J. Marchant - CC BY-SA 3.0 IGO.

As Gaia is now receding from Earth faster and faster as illustrated in Figure 5, it will be beyond the reach of even large telescopes for the next 14 years. Around 2039 it will pay a visit to Earth again, at a safe distance of about 10 million km. This means that, if Gaia is still rotating like it is now, with the same "absolute magnitude", its peak brightness will be around 23-23.5 mag. This long distance rendevouz will then take place every 14 years.

Figure 5. Gaia's trajectory relative to Earth from 1st March through June 2025. Part of original L2 orbit visible for the period 1 to 27 March 2025,
followed by acceleration away from Earth after the orbital manoeuvre on 27 March. Based on data from JPL Horizons. Credits: J. Marchant - CC BY-SA 3.0 IGO.

While the Gaia spacecraft itself dances the night away and fades away against the dark sky, the mission itself is far from over. Less than one third of the data obtained by Gaia has been released in the form of the Gaia Data Release 3 (Gaia DR3) catalogue. A huge catalogue with diverse data products is being prepared by the Gaia Data Processing and Analysis Consortium. Stay tuned for the upcoming Gaia DR4 and DR5 releases, which will contain much more of the fascinating dataset gathered by Gaia. Gaia has waved farewell, but its legacy (and the work) continues.

“Dancing away in space after serving us well for 11 years, Gaia now leaves the stage – but the performance of its data has only begun!” says Uwe Lammers, Gaia mission manager

Gaia's orbital information has been passed on to various ephemeris services and has become available from the ESOC Flight Dynamics Data Tool, ESA SPICE, JPL Horizons as well as Stellarium mobile, and was passed on to IMCCE as well.

Gaia retirement orbit, as published here on 27 March 2025, the day the Gaia spacecraft was passivated. Credits: ESA/Gaia/DPAC - CC BY-SA 3.0 IGO.

The Gaia space telescope was launched on 19 December 2013 and inserted into its retirement orbit on 27 March 2025. While the Gaia spacecraft is now passivated, the Gaia mission is hard at work preparing the biggest data releases to date, Gaia Data Release 4 expected in December 2026 and Gaia Data Release 5 expected not before end of 2030. Gaia moved into the ESA post-operations phase on the 1st of July 2025.

The Liverpool Telescope is operated on the island of La Palma by Liverpool John Moores University in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofisica de Canarias with financial support from the UK Science and Technology Facilities Council.

Credits: ESA/Gaia/DPAC

Story written by Martin Altmann, Jon Marchant and Sebastien Bouquillon

[Published: 29/09/2025]