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Gaia was primarily designed to chart the stellar content of the Milky Way, observing again and again the stars to pinpoint their positions and distances. Yet, during its continuous scanning, many nearby sources from the solar system are systematically screened with one minor planet captured every ten seconds on average, promising a rich science return at mission completion.
Between September 2014 and December 2015, the dwarf planet Pluto had been predicted to cross one of the Gaia Fields-of-View nine times with a brightness well within the Gaia reach. Mining in the Gaia database, we found that these nine passages were successfully detected by the Sky Mapper with the planet seen as a point source of magnitude G = 14.5. Searching the data collected at theses times in the vicinity of Pluto, we also had the good fortune to see that, on 7 instances, Gaia observed simultaneously Pluto's largest satellite, Charon, just two magnitudes fainter and at a distance of less than 0.6 arcsec from Pluto.
Figure 1 shows the relative position of the double (dwarf) planet for seven observations when both components were seen as two independent sources. The observed positions are provided by the very crude astrometry of the Initial Data Treatment (IDT), relying on the first on-ground attitude solution (the so called OGA1) to derive the celestial position from the image coordinates seen on the Gaia detector. Based on ICRF sources, the accuracy is about 70 mas in each coordinate, an already remarkable feat for the Gaia 'finder'.
The apparent orbit at the median observation time is also plotted for comparison, and its uncertainty, before the New Horizon flyby, is around 30 mas, not much better than the Gaia raw data. The orbit of Charon is circular and lies in the equatorial plane of Pluto. However, from the Earth or from Gaia one sees a projection of this circle onto the tangent plane, which is normal to the line of sight, which results into the elliptical curve shown in the plot. Since both the Earth and Pluto have moved between the first observation on 28 February 2015 and the last one on 29 October 2015, the perspective has changed slightly and the projection of the real path spirals between the two extreme elliptical projections. The median apparent orbit is a good compromise to find out how well the Gaia measurements match the orbit. When the astrometric solution is completed for the solar system objects, the uncertainty for each data point will be much smaller than the size of the red triangles.
The ability of Gaia to record two sources with a small angular separation is determined by the magnitude of the projected separation along the scan direction, where the highest spatial resolution is available. One sees with the projected orbit that the true separation between Pluto and Charon is about 0.6 arcsec on the sky, which is the maximum one can have in the along-scan direction. However, if the line between the two bodies is perfectly aligned along the across-scan direction, Gaia will see a single source more or less located at the system's photocentre. During the 7 passes with Charon detected, the smallest along-scan separation, 0.36 arcsec, can be computed with the Gaia attitude. But during the two passages when Gaia was not able to see the two components of the Pluto-Charon system, the computed along-scan separations were 0.17 and 0.23 arcsec, respectively. So, at least we have learnt with these along-scan distances that the resolving power of Gaia falls somewhere between 0.23 and 0.36 arcsec. However, even when a single image of a source is recorded on-board, a detailed analysis of the image should allow to see extended sources or to resolve two point sources down to about 0.1 arsec. One should keep in mind that Gaia's resolving power is not similar to the usual resolving power of a telescope: either the on-board system allocates two observing windows or not. This is a Yes or No process, and not a gradual degradation when one approaches the limit. When we have two windows, even overlapping, we have two sources and two independent astrometric and photometric solutions.
What will 2016 bring? Could we expect more observations with good projected separations? Clearly, 2016 will not be a great 'Pluto Year' for Gaia as shown in Figure 2. Here the observations of 2015 (red triangles) are plotted along with the predicted ones for 2016 (blue marks), assuming that the scanning law remains as it is today. The computed blue marks of 2016 do not exactly lie on the projected orbital path, since the latter is drawn for the conditions of mid-2015.
Only six new observations will be possible in 2016, between 10 March and 15 October. There are only two with an along-scan separation larger than 0.36 arcsec, and in fact much larger, leaving no doubt about the fact that the two bodies will be seen again twice. Among the four other observations, only one, on 4 April, could be valuable. With an along-scan separation of 0.28 arcsec, it will narrow the bounds of the resolving power: lowering the upper limit if one sees Pluton and Charon, or raising the lower limit otherwise. We will have an answer in a few months time.
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