Image of the Week

Gaia resolving power estimated with Pluto and Charon

       
   
 

Figure 1: Relative position of Pluto and its largest satellite, Charon, during seven Gaia observations when the system was resolved into two independent sources. The apparent orbit (projection of the true 3D orbit) is drawn in June 2015 for comparison.

 

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.

   
 

Figure 2: The blue marks show the predicted positions of Charon relative to Pluto during the six possible observations of the system in 2016. The 'good' observations mean that the scan directions will allow to detect both components in the Gaia data, while the circles are the four observations leading to a very small along-scan projection of the Pluton-Charon line when the system will not be resolved.

 

Credits: ESA/Gaia/DPAC/François Mignard (Observatoire de la Côte d'Azur, Nice)

[Published: 21/01/2016]

 

Image of the Week Archive

2017
24/03: Pulsating stars to study Galactic structures
09/02: Known exoplanetary transits in Gaia data
31/01: Successful second DPAC Consortium Meeting
2016
23/12: Interactive and statistical visualisation of Gaia DR1 with vaex
16/12: Standard uncertainties for the photometric data (in GDR1)
25/11: Signature of the rotation of the galactic bar uncovered
15/11: Successful first DR1 Workshop
27/10: Microlensing Follow-Up
21/10: Asteroid Occultation
16/09: First DR1 results
14/09: Pluto Stellar Occultation
15/06: Happy Birthday, DPAC!
10/06: 1000th run of the Initial Data Treatment system
04/05: Complementing Gaia observations of the densest sky regions
22/04: A window to Gaia - the focal plane
05/04: Hipparcos interactive data access tool
24/03: Gaia spots a sunspot
29/02: Gaia sees exploding stars next door
11/02: A new heart for the Gaia Object Generator
04/02: Searching for solar siblings with Gaia
28/01: Globular cluster colour-magnitude diagrams
21/01: Gaia resolving power estimated with Pluto and Charon
12/01: 100th First-Look Weekly Report
06/01: Gaia intersects a Perseid meteoroid
2015
18/12: Tales of two clusters retold by Gaia
11/11: Lunar transit temperature plots
06/11: Gaia's sensors scan a lunar transit
03/11: Celebrity comet spotted among Gaia's stars
09/10: The SB2 stars as seen by Gaia's RVS
02/10: The colour of Gaia's eyes
24/09: Estimating distances from parallaxes
18/09: Gaia orbit reconstruction
31/07: Asteroids all around
17/07: Gaia satellite and amateur astronomers spot one in a billion star
03/07: Counting stars with Gaia
01/07: Avionics Model test bench arrives at ESOC
28/05: Short period/faint magnitude Cepheids in the Large Magellanic Cloud
19/05: Visualising Gaia Photometric Science Alerts
09/04: Gaia honours Einstein by observing his cross
02/04: 1 April - First Look Scientists play practical joke
05/03: RR Lyrae stars in the Large Magellanic Cloud as seen by Gaia
26/02: First Gaia BP/RP deblended spectra
19/02: 13 months of GBOT Gaia observations
12/02: Added Value Interface Portal for Gaia
04/02: Gaia's potential for the discovery of circumbinary planets
26/01: DIBs in three hot stars as seen by Gaia's RVS
15/01: The Tycho-Gaia Astrometric Solution
06/01: Close encounters of the stellar kind
2014
12/12: Gaia detects microlensing event
05/12: Cat's Eye Nebula as seen by Gaia
01/12: BFOSC observation of Gaia at L2
24/11: Gaia spectra of six stars
13/11: Omega Centauri as seen by Gaia
02/10: RVS Data Processing
12/09: Gaia discovers first supernova
04/08: Gaia flag arrives at ESAC
29/07: Gaia handover
15/07: Eclipsing binaries
03/07: Asteroids at the "photo finish"
19/06: Calibration image III - Messier 51
05/06: First Gaia BP/RP and RVS spectra
02/06: Sky coverage of Gaia during commissioning
03/04: Gaia source detection
21/02: Sky-background false detections in the sky mapper
14/02: Gaia calibration images II
06/02: Gaia calibration image I
28/01: Gaia telescope light path
17/01: First star shines for Gaia
14/01: Radiation Campaign #4
06/01: Asteroid detection by Gaia
2013
17/12: Gaia in the gantry
12/12: The sky in G magnitude
05/12: Pre-launch release of spectrophotometric standard stars
28/11: From one to one billion pixels
21/11: The Hipparcos all-sky map
15/10: Gaia Sunshield Deployment Test
08/10: Initial Gaia Source List
17/09: CU1 Operations Workshop
11/09: Apsis
26/08: Gaia arrival in French Guiana
20/08: Gaia cartoons
11/07: Model Soyuz Fregat video
01/07: Acoustic Testing
21/06: SOVT
03/06: CU4 meeting #15
04/04: DPCC (CNES) 
26/03: Gaia artist impression 
11/02: Gaia payload testing  
04/01: Space flyby with Gaia-like data
2012
10/12: DPAC OR#2. Testing with Planck
05/11: Galaxy detection with Gaia
09/10: Plot of part of the GUMS-10 catalogue
23/07: "Gaia" meets at Gaia
29/06: The Sky as seen by Gaia
31/05: Panorama of BAM clean room
29/03: GREAT school results
12/03: Scanning-law movie
21/02: Astrometric microlensing and Gaia
03/02: BAM with PMTS
12/01: FPA with all the CCDs and WFSs
2011
14/12: Deployable sunshield
10/11: Earth Trojan search
21/10: First Soyuz liftoff from the French Guiana
20/09: Fast 2D image reconstruction algorithm
05/09: RVS OMA
10/08: 3D distribution of the Gaia catalogue
13/07: Dynamical Attitude Model
22/06: Gaia's view of open clusters
27/05: Accuracy of the stellar transverse velocity
13/05: Vibration test of BAM mirrors
18/04: L. Lindegren, Dr. Honoris Causa of the Observatory of Paris
19/01: Detectability of stars close to Jupiter
05/01: Delivery of the WFS flight models
2010
21/12: The 100th member of CU3
17/11: Nano-JASMINE and AGIS
27/10: Eclipsing binary light curves fitted with DPAC code
13/10: Gaia broad band photometry
28/09: Measuring stellar parameters and interstellar extinction
14/09: M1 mirror
 
Please note: Entries from the period 2003-2010 are available in this PDF document.