Image of the Week

25 years since the initial Gaia proposal


Figure 1: Each interferometer consists of two off-axis telescopes (as if cut out of a single large telescope), with a common focus. Three mirrors are probably needed to obtain a sufficiently wide field. (From the original GAIA Proposal by L. Lindegren et al.)

It is not easy to set an exact date to the birth of a project. Often, as in the case of Gaia, it is a fusion of several ideas that have been around for some time and which continue to develop over many years. But it can be claimed that the 12 October 1993 was a special day in the history of Gaia...

The ESA Hipparcos mission, launched in 1989, proved the feasibility of an entirely new concept in observational astronomy: global, absolute astrometry from space, using a scanning satellite. By 1993 it was clear that Hipparcos would become a major scientific success, in spite of its troublesome orbit, and astronomers were considering how to exploit the new paradigm in a future mission. Two techniques *not* employed by Hipparcos promised to provide radically improved scope and precision of such a mission: the use of CCD detectors for simultaneous observation of many stars, and interferometry for increased angular resolution.

Already in 1992 a concept for a rotating satellite using CCDs, the Roemer mission, had been proposed by Erik Høg and an improved version was proposed to ESA in 1993 in response to the M3 Call for Mission Ideas. The notion to use optical interferometry in space for high-precision astrometry had been around for some years, but always as pointed missions capable of observing only a small number of carefully selected targets. But it seemed impossible to combine the high throughput and survey character of a scanning mission with the high angular resolution and precision of interferometry.

The GAIA proposal, submitted to ESA on 12 October 1993 as a Cornerstone Mission concept for the post-Horizon 2000 planning, tried to do the impossible: using interferometry to increase the precision of a continuously scanning survey satellite. The name GAIA, an acronym for Global Astrometric Interferometer for Astrophysics, had been coined a month earlier by Michael Perryman, who drafted the proposal together with Lennart Lindegren.

The 1993 GAIA concept consisted of two mechanically connected Fizeau interferometers, each equipped with two 30 centimetre apertures on a 3 metre baseline (Figure 1), and an ingenious modulating grid (Figure 2) to record the motion of the interferometric fringes across the field of view. It was estimated that some 50 million objects to a limiting magnitude around V = 15-16 could be observed, yielding positions, parallaxes, and annual proper motions to an accuray of about 20 micro-arcseconds.

Figure 2: Part of the modulating grid (G) with an array of field lenslets (F) and detector (CCD). The two circular pupils of the interferometer are imaged on the CCD at P1 and P2. Diffracted (and unmodulated) light is masked off (M). The modulating grid may consist of a phase hologram, which doubles the light throughput; in this case P1 and P2 are modulated in anti-phase. (Not drawn to scale). (From the original GAIA proposal by L. Lindegren et al.)

A Fizeau (full-field) interferometer is essentially a big telescope, in which light only enters through two smaller apertures, so that only small sections of the mirrors are needed. Light from the two apertures combine in the focal plane to form stellar images interweaved by interference fringes. In contrast to the more familiar Michelson stellar interferometer, envisaged for pointed space interferometers and which only gives fringes in a very small field, the Fizeau in principle makes it possible to observe many stars in a large field of view at the same time - a prerequistite for a scanning mission.

Although even higher astrometric precision might be achieved by using a longer baseline, the proposed 3 metre was about the maximum size that could be accommodated in an Ariane launcher. The two interferometers in GAIA were set to point in different directions, forming a fixed basic angle as in Hipparcos and the future Gaia. The GAIA proposal was favourably viewed by ESA and technological studies of the concept started a few years later with Perryman as study scientist. Studies by Matra Marconi Space soon indicated that the use of a filled aperture was much superior to the interferometric configuration having two smaller apertures.

Using a sufficiently long focal length and a large focal plane it could be combined with direct imaging on the CCDs, thus dispensing with the modulating grid. This dramatically increased the limiting magnitude to around V = 20 and the potential number of targets to over one billion. The new concept had more features in common with the Roemer mission than with the original GAIA. While the name was retained the acronym had to be dropped, as Gaia is no longer an interferometer. It is now named after the ancient Greek goddess, the mother and spouse of Uranus.

The Gaia mission was approved on 11-12 October 2000 by the ESA Science Programme Committee.


Figure 3: Schematic payload overview without protective tent of the current Gaia Space telescope. Image from Gaia Collaboration et al. 2016


More information on the Gaia Space telescope and its focal plane and optics can be found here:

Telescopes of Gaia

The Gaia focal plane / Payload module: the focal plane

Gaia payload module

Gaia data release documentation on the spacecraft

Gaia telescope light path


Credits: ESA/Gaia/DPAC, L. Lindegren

[Published: 12/10/2018]


Image of the Week Archive

12/10: 25 years since the initial GAIA proposal
09/10: 3rd Gaia DPAC Consortium Meeting
30/09: A new panoramic sky map of the Milky Way's Stellar Streams
25/09: Plausible home stars for interstellar object 'Oumuamua
11/09: Impressions from the IAU General Assembly
30/06: Asteroids in Gaia Data
14/06: Mapping and visualising Gaia DR2

25/04: In-depth stories on Gaia DR2

14/04: Gaia tops one trillion observations
16/03: Gaia DR2 Passbands
27/02: Triton observation campaign
11/02: Gaia Women In Science
29/01: Following-up on Gaia
19/12: 4th launch anniversary
24/11: Gaia-GOSA service
27/10: German Gaia stamp in the making
19/10: Hertzsprung-russell diagram using Gaia DR1
05/10: Updated prediction to the Triton occultation campaign
31/08: Close stellar encounters from the first Gaia data release
16/08: Preliminary view of the Gaia sky in colour
07/07: Chariklo stellar occultation follow-up
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
27/08: Quest for the Sun's siblings
Please note: Entries from the period 2003-2010 are available in this PDF document.