Image of the Week


Gaia's first Photo shooting of James Webb



Figure 1: The orbits of Gaia and the James Webb Space Telescope. Left: The distance in kilometers between the two spacecraft as function of time during the first two years of Webb’s stay at L2. The red line indicates the event of February 18. Center: The relative sizes and locations of the Gaia orbit (yellow) and the James Webb orbit (white). In this view the Earth is located to the left, not far outside the frame. Gaia’s Lissajous loops have the Lagrange point L2 right in their center, while the James Webb’s halo orbit loops on average are closer to Earth by about 100 000 km. Right: The observation geometry on February 18. The red line indicates the annual path of L2 around the Earth. The sizes and orientations of the two spacecraft icons are not realistic. Image credits: ESA/Gaia/DPAC, CC BY-SA 3.0 IGO.


In early 2014, ESA’s space astrometry spacecraft Gaia arrived at its working place near the Earth-Sun Lagrange point L2, about 1.5 million km from the Earth and always in the direction opposite to the Sun. Almost exactly eight years later, in late January 2022, the James Webb Space Telescope took its station at L2, where the combined gravitational attraction of Earth and Sun allow spacecraft to follow their home planet Earth around the sun, for several years and with almost no fuel expenses.

Already a few weeks before that arrival, some Gaia experts realised that during Gaia’s continuous scanning of the entire sky, its new neighbour at L2 should occasionally cross the fields of view of Gaia's telescopes. A quick back-of-the-envelope calculation showed that it might be bright enough – as seen from Gaia – to be automatically detected by the on-board measuring algorithms. Every six hours, Gaia scans a narrow 360-degree strip around the entire celestial sphere. The successive strips are slightly tilted with respect to each other, so that every few months the entire sky is covered by them – touching everything that’s there and that’s bright enough to be seen by Gaia, including about two billion stars, and hopefully also the James Webb.


Animation illustrating the Gaia's scanning law. Credit: ESA - C. Carreau


As soon as the operational orbit of Webb became sufficiently well known, specific predictions could be calculated. It turned out that the two spacecraft would always be quite far from each other – between 400,000 and 1,100,000 km apart (see Figure 1) – but that dozens of such photo shooting opportunities would arise over the remaining 3 to 4 years lifetime of Gaia. Due to the permanently large separation between the two cousin observatories, it was clear from the beginning that the Webb spacecraft would invariably just be a tiny, faint speck of light in Gaia’s telescopes without any details visible. But wouldn’t it be nice anyway? A real first in the history of scientific spaceflight: One great observatory capturing a completely unrelated second one, both in their operational locations, both far out in space, both not near any planetary surface.


Visualisation of the orbits of the space telescopes Gaia and James Webb. Visualisation credits: ESA/Gaia/DPAC - CC BY-SA 3.0 IGO. Acknowledgements: Video created with Gaia Sky (developed by Toni Sagristà), edited by Stefan Jordan and Ulrich Bastian.


The first such event was predicted to occur on 18 February 2022 already, and it was foreseen to be an unfavourable one: The separation turned out to be 1.02 million kilometers - almost the maximum possible - and the viewing angle towards Webb’s huge sunshield to be almost edge-on. Very little reflected sunlight was expected at Gaia. The scientifically bright Webb observatory for once would look very dim.

Furthermore, Gaia is not designed to take real pictures of the two billion celestial objects that cross its fields of view. Instead, it is supposed to collect very precise measurements of positions, motions, distances, colours, etc. – and to send the minimum amount of data to the ground that just allow these measurements to be extracted. However, one part of the instruments on board actually takes a sort of sky images. It is the “finder scope” of Gaia, technically called the Sky Mapper. At a millisecond cadence, it successively records slices of the narrow 360-degree strip around the celestial sphere that are scanned by Gaia every six hours. Within seconds, these slices are automatically scrutinised for star images, the positions of which are then used to predict when and where those stars could be recorded in Gaia’s main scientific instruments. Afterwards, almost all of those image data are immediately deleted while still in the onboard computer. Only tiny “windows” (around the detected stars) are kept and transmitted to the ground stations. But the computer can be requested to exceptionally keep a stretch of the image data. This facility was originally planned for technical servicing purposes mainly. But during the mission it has also found some scientific uses. Why not use it for a snapshot of Webb?


Figure 2: Images of the James Webb Space Telescope taken by ESA’s Gaia observatory on February 18. Background frame: Cutout of the specially recorded image from Gaia’s Sky Mapper instrument at the first of the two observations. The reddish colour is artificial, chosen just for illustrative reasons. The frame shows a few relatively bright stars, a number of faint stars, a few disturbances – and a spacecraft! It is marked by the green circle. Large grey inset: Zoom into the frame showing the James Webb image at full Sky Mapper resolution. It is the slightly extended speck of light in the centre. The other three bright dots are traces of energetic cosmic-ray particles which hit the CCD chip during the 2.5 seconds of exposure. The on-board software is capable of autonomously and reliably distinguishing the vast majority of these from star images. The small rest is subsequently eliminated in the ground processing. Small grey inset: The second “photo” of James Webb, taken in the second field of view of Gaia’s telescopes about 106.5 minutes after the first one. Each of the two images was created by just under 1000 sunlight photons arriving from the James Webb spacecraft. More images: here. Image credits: ESA/Gaia/DPAC, CC BY-SA 3.0 IGO.


The proposal was quickly implemented by the Gaia Science Operations Centre, and resulted in the images shown in Figure 2. They have been recorded by the two telescopes of Gaia, with a separation of 106.5 minutes in time. Watching closely this image, an obvious question arises : How can we be sure that the tiny bright specks actually are the great James Webb Space Telescope?

This is explained by Figure 3. This is not an image, but a sky map of the area around the predicted locations of James Webb on the Gaia sky during the two passages of 18 February 2022. All the spots in this map denote the celestial positions of star images automatically detected and measured by Gaia while scanning that sky area on that day. Their size approximately indicates their recorded brightness. The centers of the two small circles denote the predicted location of James Webb at the two instants of observation.

  • In both circles, there is a star-like image of a faint object!
  • In all its almost eight years of observations, Gaia has never seen a star in these two positions in the sky. And, importantly, on 18 February in either position the object was recorded just once, not twice.
  • None among the 1 million known minor planets was expected at these places.
  • The images in both circles are slightly offset from the center, but in the same direction and by the same amount. That offset amounts to about 40 km at the distance of 1 million km. It is due to the expected uncertainty of the predicted Webb orbit (the Gaia orbit is known much more precisely).
  • The motion of the object during the 106.5 minutes interval between the two observations is exactly as predicted from the known orbits of Gaia and Webb.
  • Even within each of the two field-of-view crossings, the motion of the object (of order 5 milli-arcsec per second) was very precisely measured - and found in complete agreement with the expectation. This measurement is made possible because any star (or spacecraft, or whatever) at every field transit successively gets its position measured on at least nine CCDs, each separated by 4.85 seconds in time.

So, ESA’s Gaia observatory indeed captured its joint NASA/ESA/CSA cousin, the James Webb Space Telescope, at the very first such occasion. And it will do so again and again. But these other occasions will never rival the special flair of this first one – as likewise every true first in life has a special meaning.




Figure 3: Left: Reference map of a small sky area surrounding the expected positions on 18 February 2022 of the James Webb Space Telescope, marking the positions of the stars as available in Gaia's Early Data Release 3. The centres of the two circles indicate the computed positions where James Webb should be seen with Gaia. The size of the field shown is 0.4 degrees, just a bit smaller than the apparent diameter of the full moon as seen from Earth. The size of the disks is indicative of the sources brightness.
Center: Map of the same sky area captured by Gaia on 18 February 2022, once in each of its two fields of view. The images of the James Webb appear within the predicted areas as a 19.3 magnitude star-like source. This map was produced from Gaia's routine, fully autonomous on-board object detection scheme; it does not make use of the special images recorded for Figure 2. It contains all the on-board detections, including moving objects and a significant fraction (~10%) of spurious detections, not associated to a source in the sky, but due to cosmic hits or other electronic glitches and CCD leaks. All these are filtered out later in the processing. The curious chain of white dots at the left for example is due to the straylight from the star Xi Tauri which is slightly west of the shown field of view here.
Right: Both images combined in a GIF. Image credits: ESA/Gaia/DPAC, CC BY-SA 3.0 IGO.


This animation illustrates the orbits of the Gaia space telescope and the James Webb Space Telescope. Since 24 January 2022, the James Webb Space Telescope is in a halo orbit around the Lagrange point L2, about 1.5 million kilometres from Earth. In the same region of space, the Gaia space telescope has performed its measurements since July 2014, orbiting L2 in a Lissajous orbit. On Friday 18 February 2022 at 08:53 and 10:38 CET, the James Webb observatory has been seen through Gaia's telescopes for the first time. Due to Gaia's continuous scanning of the sky this will happen again in the future - several dozen times over the next few years. A version of this animation with an artificial voice added is available here. Visualisation credits: ESA/Gaia/DPAC - CC BY-SA 3.0 IGO. Acknowledgements: François Mignard, Toni Sagristà Sellés, Stefan Jordan, Ulrich Bastian, José Hernandez, Uwe Lammers. Video created with Gaia Sky (developed by Toni Sagristà), edited by Stefan Jordan.


Figure 4: Two cross-eyed images of the orbits of the space telescopes Gaia and James Webb. Image credit: ESA/Gaia/DPAC, CC BY-SA 3.0 IGO. Acknowledgements: created with Gaia Sky by Stefan Jordan.


Figure 5: Greetings from L2 - love Webb and Gaia. Image credit: ESA, ESA/Webb, Science Wave. Background and foreground image: ESA/Gaia/DPAC, CC BY-SA 3.0 IGO.



More information about Gaia's scanning law?


Further reading on Find the article here.


ESA/Gaia/DPAC, Ulrich Bastian, Francois Mignard, Stefan Jordan, Juanma Martin-Fleitas, Toni Sagristá, the Gaia Mission Operations Team at Darmstadt, the Gaia Science Operations Centre at Villafranca and the Europe-wide Gaia DPAC CU3 team.


[Published: 16/03/2022]

Image of the Week Archive


28/05: Did Gaia find its first neutron star?

26/04: A textbook solar eruption

22/04: Gaia's contribution to discovering distant worlds

16/04: Gaia spots Milky Way's most massive black hole of stellar origin

02/04: The Gaia Cataclysmic Variable hook


19/12: 10 Science topics to celebrate Gaia's 10 years in space

31/10: Gaia observes cosmic clock inside a heavenly jewel

10/10: Gaia Focused Product Release stories

27/09: Does the Milky Way contain less dark matter than previously thought?

22/09: Mass-luminosity relation from Gaia's binary stars

13/09: Gaia DPAC CU8 seminars

13/06: Gaia's multi-dimensional Milky Way

18/05: Mapping the Milky Way

15/05: Goonhilly station steps in to save Gaia science data

25/04: The Gaia ESA Archive

05/04: Dual quasar found to be hosted by an ongoing galaxy merger at redshift 2.17

21/03: GaiaVari: a citizen science project to help Gaia variability classificaton

09/02: Missing mass in Albireo Ac: massive star or black hole?

31/01: Gaia reaches to the clouds – 3D kinematics of the LMC

25/01: Meet your neighbours: CNS5 - the fifth catalogue of nearby stars

18/01: A single-object visualisation tool for Gaia objects


25/11: 100 months of Gaia data

23/11: The astonishment

09/11: Gamma-Ray Burst detection from Lagrange 2 point by Gaia

04/11: Gaia's first black hole discovery: Gaia BH1

26/10: Are Newton and Einstein in error after all?

21/10: Gaia ESA Archive goes live with third data release

06/10: Mapping the interstellar medium using the Gaia RVS spectra

26/09: Gaia on the hunt for dual quasars and gravitational lenses

23/09: Gaia's observation of relativistic deflection of light close to Jupiter

13/06: Gaia Data Release 3

10/06: MK classification of stars from BP/RP spectrophotometry across the Hertzsprung-Russell diagram

09/06: BP/RP low-resolution spectroscopy across the Hertzsprung-Russell diagram

27/05: Cepheids and their radial velocity curves

23/05: The Galaxy in your preferred colours

19/05: GaiaXPy 1.0.0 released, a tool for Gaia's BP/RP spectra users

11/05: Systemic proper motions of 73 galaxies in the Local group

28/03: Gaia query statistics

16/03: Gaia's first photo shooting of the James Webb Space Telescope

08/03: Gaia's women in science - coordination unit 8

25/02: Not only distances: what Gaia DR3 RR Lyrae stars will tell us about our Galaxy and beyond

11/02: Gaia's women in science

31/01: Astrometric orbit of the exoplanet-host star HD81040

12/01: The Local Bubble - source of our nearby stars

05/01: A Milky-Way relic of the formation of the Universe


23/12: Signal-to-Noise ratio for Gaia DR3 BP/RP mean spectra

22/12: The 7 October 2021 stellar occultation by the Neptunian system

01/12: Observation of a long-predicted new type of binary star

24/09: Astrometric microlensing effect in the Gaia16aye event

22/09: the power of the third dimension - the discovery of a gigantic cavity in space

16/09: An alternative Gaia sky chart

25/08: Gaia Photometric Science Alerts and Gravitational Wave Triggers

09/07: How Gaia unveils what stars are made of

23/06: Interviews with CU3

27/04: HIP 70674 Orbital solution resulting from Gaia DR3 processing

30/03: First transiting exoplanet by Gaia

26/03: Apophis' Yarkovsky acceleration improved through stellar occultation

26/02: Matching observations to sources for Gaia DR4


22/12: QSO emission lines in low-resolution BP/RP spectra

03/12: Gaia Early Data Release 3

29/10: Gaia EDR3 passbands

15/10: Star clusters are only the tip of the iceberg

04/09: Discovery of a year long superoutburst in a white dwarf binary

12/08: First calibrated XP spectra

22/07: Gaia and the size of the Solar System

16/07: Testing CDM and geometry-driven Milky Way rotation Curve Models

30/06: Gaia's impact on Solar system science

14/05: Machine-learning techniques reveal hundreds of open clusters in Gaia data

20/03: The chemical trace of Galactic stellar populations as seen by Gaia

09/01: Discovery of a new star cluster: Price-Whelan1

08/01: Largest ever seen gaseous structure in our Galaxy


20/12: The lost stars of the Hyades

06/12: Do we see a dark-matter like effect in globular clusters?

12/11: Hypervelocity star ejected from a supermassive black hole

17/09: Instrument Development Award

08/08: 30th anniversary of Hipparcos

17/07: Whitehead Eclipse Avoidance Manoeuvre

28/06: Following up on Gaia Solar System Objects

19/06: News from the Gaia Archive

29/05: Spectroscopic variability of emission lines stars with Gaia

24/05: Evidence of new magnetic transitions in late-type stars

03/05: Atmospheric dynamics of AGB stars revealed by Gaia

25/04: Geographic contributions to DPAC

22/04: omega Centauri's lost stars

18/04: 53rd ESLAB symposium "the Gaia universe"

18/02: A river of stars

21/12: Sonification of Gaia data
18/12: Gaia captures a rare FU Ori outburst
12/12: Changes in the DPAC Executive
26/11:New Very Low Mass dwarfs in Gaia data
19/11: Hypervelocity White Dwarfs in Gaia data
15/11: Hunting evolved carbon stars with Gaia RP spectra
13/11: Gaia catches the movement of the tiny galaxies surrounding the Milky Way
06/11: Secrets of the "wild duck" cluster revealed
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
04/10: 1:1 Gaia model arrives at ESAC
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
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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.