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100 months of Gaia data

Figure 1. The 103 resolved detections of Pluto and Charon over the 100 months of Gaia science operations. The plot gives the position of Charon at every epoch on the plane of the sky with reference to Pluto. The positions are derived from the data of the Gaia source finder and a first improvement on the on-board attitude, resulting in a position accuracy of 60 mas in both coordinates. Not too bad compared to ground based standards, but no match for Gaia final astrometry that will be obtained years later after the global iterative processing. Credits: ESA/Gaia/DPAC - CC BY-SA 3.0 IGO.

The Gaia science operations started on the 25th of July 2014, when the spacecraft was turned on in science mode, following the completion of the in-flight commissioning.  This 25 November 2022 marks the 100 months of data collection, with a regular daily flow of an average of 70 million new on-board detections and a single significant interruption lasting several days on February 2018 due to a transponder failure.

The cyclic processing, ending with a Data Release, takes time and imposes a big gap between the data content in a release and the actual data acquired by the spacecraft at the release time: the recent Gaia DR3 in December 2020 and June 2022 dealt with 34 months of data from July 2014 to May 2017. However the data keeps flowing and is checked every day to monitor the spacecraft health, producing big reports from automated analysis with alarms triggering if a problem is spotted.

Among the sources followed in this routine activity, the DPAC Coordination Unit 4 tracks the close pair Pluto and its largest satellite Charon, which were used initially to estimate the rough spatial resolution of Gaia, just by recording when two sources are assigned independent detections by the on-board system. A better resolution is obtained later from the detailed analysis of the signal produced in a single window by two close sources.

The on-board detection with the Gaia source finder is readily processed on ground with a very preliminary attitude, enough to get a position with an accuracy of about 0.06 arcsec in right ascension and declination. Nothing to do with the Gaia exquisite astrometry that will come months or years later from the complex cyclic processing, but not so bad compared to standard ground based astrometry.

Over the 100 months of operations, Gaia has pointed its telescopes to Pluto 119 times, recording two detections 103 times and seeing an unresolved system 16 times. Although the angular separation on the sky is always larger than 0.65 arcsec, the orientation of the line Pluto to Charon matters: Gaia has a good resolution in the scan direction and much poorer in the perpendicular direction, which accounts for the inability to detect both sources at every passage.

The 103 successful detections appear in the animation, dropping each observation as a function of time as it happened. The points nicely outline the orbit of Charon projected on the plane of the sky, whose change of orientation is small over the 8 years, given the 248 years orbital period of Pluto. The scatter is a combination of the 60 mas random uncertainty in position and the slight change of perspective over the 100 months that translates into a maximum 70 mas shift of Charon between the first and last epoch. The data presented in this plot will enter the processing scheduled for the Gaia DR5, when the positions of Pluto and Charon will be provided with a sub-mas accuracy, at least when the position angle is not too far from the scan direction. Therefore the scatter is not at all representative of the Gaia capabilities, but just what is available in real time, so to speak, in the data header!

To rate the performance of the detection system on board Gaia, suffice it to say that Charon presence was only hinted in 1978 from a slight and periodic elongation in the enlarged image of a photographic plate and doubts about its mere existence remain until a fortunate period of 5 years of eclipses and transits started in 1985.

Animation of the 103 resolved detections of Pluto and Charon over the 100 months of Gaia science operations. The plot gives the position of Charon at every epoch on the plane of the sky with reference to Pluto. The positions are derived from the data of the Gaia source finder and a first improvement on the on-board attitude, resulting in a position accuracy of 60 mas in both coordinates. Not too bad compared to ground based standards, but no match for Gaia final astrometry that will be obtained years later after the global iterative processing. Credits: ESA/Gaia/DPAC - CC BY-SA 3.0 IGO.

Credits: Story written by F. Mignard, DPAC and Observatoire de la Côte d'Azur, Nice (France).

[Published: 25/11/2022]