Image of the Week

Asteroids all around

  Eight months of Gaia data and a sample of 50,000 asteroids were used to test the detection efficiency of software in the Gaia processing pipeline.  

Gaia observes more than a billion stars on the whole sky, without knowing in advance where they are. However, as each source is observed multiple times, the Initial Data Processing (IDT, a highly sophisticated piece of software running on the data transmitted by the satellite, developed by the University of Barcelona team) has the task of grouping together multiple observations of the same source.

This task, the so-called "cross-matching", involves comparing the positions recorded by Gaia. If two sources are observed, within the uncertainty, at the same position on the sky they are recognized to be – in fact – the same source.

For asteroids, this cannot work, as they are always moving amongst the stars - slowly (typically, an asteroid in the Main Belt can take a couple of days to move a distance of a Moon diameter) but fast enough for Gaia (several pixels during a single transit on the focal plane)! As a result, Gaia never sees an asteroid at the same place, and the cross-matching described above leaves these detections as "orphans" that do not repeat over time.

Such "un-matched" observations are processed by software running at the CNES data processing center (in Toulouse, France), written by several European astronomers under the coordination of P. Tanga (Observatoire de la Côte d'Azur, France). The first task of this software is the identification of known asteroids that can proceed only by comparing the position of the "orphan" detections to the predictions provided by a specific software module. The reference data, in this case, are the known asteroid orbits (a few 100,000 objects), that are used to generate the position of all of them for any Gaia measurement of a moving source.

Such a procedure has been implemented in the processing pipeline by J. Berthier (IMCCE) and collaborators and has been applied independently (by F. Mignard and L. Galluccio, OCA, France) on eight months of Gaia data and a sample of 50,000 asteroids to test its detection efficiency and to get a flavour of the performance of asteroid identification.

The picture above is one of the results of such a test, showing where Gaia has seen asteroids on the sky. The reference frame for the plot is equatorial, implying that the ecliptic (the orbital plane of the Earth and – also – of most other Solar System objects) appears inclined with respect to the celestial equator, and follows the sinusoidal path on which the asteroids cluster. A total of 418,000 observations by Gaia, over that period, have been successfully associated to one of the asteroids of the sample, a number very close to the expectations.

The colour of the dots is related to the accuracy of the identification, in other words to the distance between the position detected by Gaia and the one predicted by the computation of the ephemerides. For most asteroids, the identification works well (the discrepancies are less than 1 arcsec) but for others – whose orbit is poorly known – identification may become tricky and requires further processing steps.

Interestingly, the plot clearly shows a region of mixed colours, very localised, where large errors seem to be more frequent. In fact, in that region, the ecliptic crosses the Galactic plane, not far from its densest parts. The high stellar density complicates the task of IDT cross-matching, and more stars there, remaining un-matched, are incorrectly considered as asteroids. Their positions correspond to some predicted asteroid positions only by chance, so that they show up with larger discrepancies. This is an expected effect of lower efficiency in extremely crowded areas of the sky, which are very limited in size.

In fact, both statistics and analysis of samples of individual objects show that Gaia is very efficient in detecting asteroids, and the whole expected population is being observed, within a few percent of the completeness at the magnitude limit of the mission.  

Credits: ESA/Gaia/DPAC/CU4, L. Galluccio, F. Mignard, P. Tanga (Observatoire de la Côte d'Azur)

[Published: 31/07/2015]


Image of the Week Archive

24/04: Gaia reveals the composition of asteroids
20/04: Extra-galactic observations with Gaia
10/04: How faint are the faintest Gaia stars?
24/03: Pulsating stars to study Galactic structures
09/02: Known exoplanetary transits in Gaia data
31/01: Successful second DPAC Consortium Meeting
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
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
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
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
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
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
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.