Image of the Week

Whitehead Eclipse Avoidance Manoeuvre marks Gaia's start of mission extension


Figure 1: Part of the Gaia team at ESOC after succesfully finishing the first manoeuvre leg. Image credit: ESA/ESOC/@esaoperations


Yesterday, Gaia's biggest operation since launch was successfully completed, moving the Gaia mission from its nominal phase into its first mission extension. The Whitehead Eclipse Avoidance Manoeuvre was started yesterday morning by firing the thrusters of the spacecraft through commands sent up to the spacecraft by the Gaia mission flight control team from the ESA mission operations centre ESOC in Darmstadt, Germany.


GAia's orbit

The second Lagrange point - L2 - is a fabulous place to do science. Moving together with Earth about the Sun at about 1.5 million km from Earth, the spacecraft keeps the Sun, Earth, and Moon all in one direction and can thus observe the rest of the sky freely. The region around L2 is a special area where the spacecraft can be maintained at roughly a constant distance from the Earth at the cost of small and infrequent orbit manoeuvres.

Gaia's launch to orbit showing its location in space with respect to Sun and Earth. The Earth shadow is visible near the end of the video and the Whitehead Eclipse Avoidance Manoeuvre performed on 16 July 2019 is meant to keep Gaia out of this shadow for the coming years in mission extension. Credits: ESA - C. Carreau / ATG medialab.


Avoiding an Earth eclipse

Without any measures taken to change its orbit, the Gaia space observatory would have moved into the Earth's shadow in August and November this year. During these eclipses, part of the sunlight would be blocked from reaching the solar panels causing Gaia power down. Being in the shade would cause a thermal disturbance as well which would impact the scientific data acquisition for weeks. To avoid ending up in these unfavourable conditions, a manoeuvre was undertaken to change Gaia's orbit in such a way that no new Earth eclipses would be encountered until 2026.

Even though Gaia just finished its nominal lifetime, a mission extension was granted. The cold-gas fuel onboard of Gaia, used to keep the spacecraft in a very stable spinning mode for science operations, is expected to keep Gaia spinning till the end of 2024. The Gaia mission extension is expected to improve positions, parallaxes, photometry, and radial velocities by about 40% with respect to the nominal, five-year mission, and improve upon proper motions even more. The mission extension will also have a positive effect on the detection of exoplanets and asteroids as well as on the temporal sampling (window function) of variable stars.


the Whitehead Eclipse Avoidance Manoeuvre

The manoeuvre performed successfully yesterday was called the Whitehead Eclipse Avoidance Manoeuvre to honour Gary Whitehead, who worked fulltime on Gaia since 2007 but sadly passed away last month. Gary was a much valued member of the Gaia Flight Control Team.

Before the start of the manoeuvre, Gaia came out of its normal "fine control mode" in which the spacecraft's cold-gas micro-thrusters produce up to 70 microNewtons of force (equivalent to the weight on Earth of a couple of mosquitos). This small force is used to keep the Gaia spacecraft in its very stable spinning mode. After stopping the science data acquisition and the spin down, a series of thruster burns was started. A combination of the onboard chemical thrusters was used for these events, pushing Gaia in a diagonal direction, away from the shadow, in a special technique called 'thrust vectoring'. Gaia has a total of eight chemical thrusters onboard, four designed specifically for moving the observatory in different directions through space, while the other four can also control the atttitude (so the pointing direction).

Some thorough advance preparation was needed to get ready for this manoeuvre and the availability of ground stations during the full manoeuvre was ensured. The entire maneouvre lasted a full day, so multiple ground stations have been used to keep in contact with the spacecraft over time, when Earth rotates over the course of the day.


Figure 2: The Main Control Room at ESA/ESOC in Darmstadt, Germany, from which the Gaia Whitehead Eclipse Avoidance Manoeuvre was performed yesterday. Image credit: ESA/ESOC/@esaoperations.

Read through some of the tweets from @esaoperations on the thruster burns performed. In between thruster burns, pauses were inserted to allow for the fuel to settle before starting with the next burn.

These nine thruster burns together achieved a total Delta-V of about 14 m/s through the use of about 10 kg of propulsion.


Gaia's extreme sensitivity

The Gaia spacecraft is many times more stable than any other spacecraft in operation today. Maintaining this stability requires certain measures to be taken. Like keeping the spacecraft out of Earth eclipses to maintain its thermal stability. Gaia's payload is operated at a temperature of about -110°C and the sunshield protects against direct illumination of the telescope while the spacecraft rotates around the spin axis at a constant Sun aspect angle of 45°.

Gaia's extreme sensitivity allows to see several effects, from small bubbles moving in the Gaia fuel tanks (as seen from tiny wobbles in the attitude while Gaia is in 'fine control mode') to tiny dips in the temperature of the Sunshield corresponding to Sunspots appearing on the Solar surface. A story on Gaia catching a Sunspot can be found here.


Figure 3: David Milligan, Gaia Spacecraft Operations Manager. Image credit: ESA.


Reverse-precession scanning law

Following the Whitehead Eclipse Avoidance Manoeuvre, Gaia was spinned up again to start scanning the sky. Instead of returning to what is called the "nominal scanning law", Gaia has been brought into a "reverse-precession scanning law" which will be maintained for 1 year. This "reverse-precession scanning law" has been implemented to break a particular calibration degeneracy in the astrometric data processing which will improve the astrometric solution even further.


A story published by ESA Operations on this manoeuvre can be found here.


We'd like to congratulate the Gaia team at ESOC with their great achievement yesterday and hope to celebrate with you the start of Gaia's first mission extension!

Image credit: Background image: ESA/Gaia/DPAC, Spacecraft image: ESA/ATG medialab.


Credits: ESA/Gaia/DPAC, ESA Operations, Gaia team at ESOC

This story has largely been based on the tweets sent out by ESA Operations from the @esaoperations twitter channel.

[Published: 17/07/2019]


Image of the Week Archive

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
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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
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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
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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
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29/02: Gaia sees exploding stars next door
11/02: A new heart for the Gaia Object Generator
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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
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15/01: The Tycho-Gaia Astrometric Solution
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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
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06/01: Asteroid detection by Gaia
17/12: Gaia in the gantry
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05/12: Pre-launch release of spectrophotometric standard stars
28/11: From one to one billion pixels
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15/10: Gaia Sunshield Deployment Test
08/10: Initial Gaia Source List
17/09: CU1 Operations Workshop
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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
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21/10: First Soyuz liftoff from the French Guiana
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Please note: Entries from the period 2003-2010 are available in this PDF document.