The Gaia spacecraft and payload

Gaiais a tremendously precise and advanced instrument, built under the leadership of Airbus Defense and Space in Toulouse (France). With a total weight of about 2 tonnes, it consists in:

• theservice module, which groups all the mechanical, structural and thermal elements sup-porting the instruments and the spacecraft electronics, namely the propulsion system, the data processing and data handling units, the payload thermal tent, the deployable sunshield assembly and the associated solar-array panels.

• thepayload module, built around a quasi-octogonal optical bench of∼3m diameter, sup-porting two identical telescopes, as well as the single integrated focal plane assembly which comprises the three scientific instruments and some optical and metrology sensors.

Figure 2.3 shows a schematic view of theGaiaspacecraft.

Note that, with a total diameter of about10m, the ensemble formed by the deployable sun-shield and the solar panels has two roles: first to sunshade the telescopes and ensure the thermal stability of the satellite for proper working (maintaining a temperature down to−100^◦C), and then to generate electricity contributing to power the spacecraft.

1More information on theGaiascience objectives can be found athttps://www.cosmos.esa.int/web/gaia/

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Figure 2.3: Gaiaexploded view. Credits to ESA and ATG-medialab. From top to bottom: thermal tent, payload module, service module main structure, propellant tanks and pressurant tanks, an-tenna support panel and phased-array anan-tenna, deployable sunshield assembly, and solar panels.

Figure 2.4: Gaiaschematic view of the payload module. Credits to ESA.

It is important to say that, thoughGaiaintegrates three different instruments with three differ-ent tasks, the differ-entire payload consists in a single complex assembly, given the fact that most of the equipments are shared by all the instruments, the main shared components being the telescopes and the focal plane.

As mentioned previously, Gaiacomprises two identical telescopes, with apertures of1.45 x 0.50m and focal length of 35m, pointing towards two different directions, separated by a basic angle of 106.5^◦. The associated lines-of-sight are oftenly referred to as “preceding” and “follow-ing”. In operation, both telescopes illuminate the unique focal plane of the satellite. Actually, the astrometric measurement principle used in Gaiais derived from the strategy used for its astro-metric predecessorHipparcos, and relies on the transformation of transit time differences between the stars observed in both telescopes into angular measurements. The astrometric solution is then deduced from the angular separation and the parallactic displacements, for different sources and at different observation times. This method requires simultaneous measurements in two different pointing directions, justifying the presence of two telescopes instead of one. Moreover, this global astrometry principle necessitates a precise knowledge of the basic angle value at every moment.

Figure 2.4 represents a schematic view of the payload module. For each telescope, the light from a distance source first enters a primary 1.45 x0.50m mirror (M1 or M1’ depending on the telescope), and is folded twice between the M2/M2’ and M3/M3’ (0.65 x 0.275m) to accommo-date the 35m focal length. Then, M4/M4’ mirrors act as flat beam combiners, merging the two light beams into a common path. Finally, this common beam is reflected onto the focal place as-sembly via M5 and M6 folding mirrors (0.54x0.36m). Note that, to achieve the required optical quality necessary to reachGaiahigh performances, the M2/M2’ mirrors are equipped with orient-ing actuators to adjust their alignement and focusorient-ing.

Thefocal plane assemblyis depicted in Figure 2.5. It consists in 106 CCD detectors, arranged in a mosaic of 7 rows and 17 strips, of 4500 x 1966 pixels each for a total of 938 million pixels, working at about163K to reduce the dark current. Pixel size is 10 x 30µm, which corresponds to 58.9 x 176.8 mas area on the sky. The CCDs are operated in Time-Delayed Integration mode (TDI),

2.2. TheGaiaspacecraft and payload

Figure 2.5: Schematic view of theGaiafocal plane. Credits to ESA and R. Kohley.

to allow the collection of charges while the object image moves over the CCD and transits the focal plane, as a result of the spacecraft spin. The integration time per-CCD is of4.42s, corresponding to 4500 TDI lines in the along-scan direction. In order to limit image area saturation when observing bright objects, for each CCD there is the possibility to apply some gating, i.e. to inhibit charge transfer for some of the TDI lines, reducing the effective integration time. The 12 different gates can be used, depending on the magnitude of the star.

The 106 focal plane detectors are divided in five groups, depending on their function:

• 2 CCDs are allocated to the Wave Front Sensor (WFS), whose role is to monitor the optical performances of the two telescopes, to refocus and / or realign the M2/M2’ mirrors accord-ingly.

• 2 CCDs are dedicated to Basic Angle Monitoring (BAM), measuring accurately the fluctua-tions in the basic angle value, at theµas level with a sampling of about23s, which is manda-tory to ensure the astrometric data reduction accuracy, as mentioned previously.

• 14 CCDs, distributed in two strips, constitute the Sky Mappers of the two telescopes (SM1 and SM2), in charge of the detection of real-time sources in the FoVs.

• 62 CCDs, distributed in 9 strips, form the Astrometric Field (AF), which is the main element of the astrometric instrument, collecting information to measure accurately the position and brightness of each transiting star. The wavelength coverage of those CCDs goes from330to 1050nm, defining the white-light photometricGband ofGaia.

• 14 CCDs, distributed in two strips, correspond to the twoGaiaspectrophotometers, with one strip dedicated to the Blue PhotometerBP and the other to the Red Photometer RP. They receive the light from the combined beam of the 2 FoVs after it has been dispersed by two fused-silica prisms, one operating in the range330-680nm (BP), and the other in the range 630-1050nm (RP). The associated magnitudes are notedGBP andGRP respectively.

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• 12 CCDs are devoted to the high resolution spectrograph, theRV S, operating in the wave-length range 847-874nm, with associated magnitudeGRV S, and whose aim is to provide spectra for the bright end of theGaiasample, i.e.G_{RV S}. 16mag.

The payload module also comprises thedata handling system of Gaia, the whole communi-cation system developed for this mission being one of the most powerful computing system on-board a scientific satellite. Each of the CCD rows in the focal plane is associated to its own Video Processing Unit (VPU), a computer in charge of commanding the CCDs in the row, collecting sci-ence data acquired through the focal plane, and transfering it to the on-board data storage structure (or Payload Data Handling Unit, hereafter PDHU) whose storage capacity is around120GB in to-tal. Gaiais not in permanent ground-station contact, hence the collected data is not transmitted to Earth continuously, but rather by bunches over a few hours per day. For downlinking this data, Gaiais equipped with a specially designed on-board Phased-Array Antenna, instead of a conven-tional parabolic one, for quality reasons: actually, with such classical antenna, Gaiamoving parts would cause unacceptable degradation of the image quality through micro-vibrations. The recep-tion of the high volume of data sent byGaia, as well as the control of the spacecraft, is ensured by three ESA’s35m deep-space antennas, respectively in Cebreros (Spain), New Norcia (Australia) and Malargüe (Argentina), which make possible to start the connexion to theGaiaspececraft at any moment of the day. The actual moment of connexion depends on the conflicts with the other ESA missions making use of those antennas, the data transfer planning for all the concerned missions being fixed years in advance.