A discovery machine
Gaia is one of the most precise and technologically advanced instruments in space ever built. Its two telescopes are mounted on a resistant, and at the same time light, silicon carbide toroidal structure. Its solar panels provide the energy needed by the satellite (little more than what is needed for a dishwasher). The installation of every element in the satellite must be extremely precise in order to ensure successful operations during the mission.
Scanning the sky
The basic measuring principle of Gaia is the same as used in the Hipparcos mission (ESA, 1989-1993). It consists on two telescopes observing two very separated viewing directions in the sky. Their angular separation (106 degrees) has to be very precisely known at any moment. While Gaia spins on itself (once every 6 hours), it collects more and more observations of stars, covering all the sky in 6 months. Comparing the position of the stars in different parts of the sky during its 5 years mission, a consistent and precise global map can be built. At the end of the mission each star will be observed from 40 to 220 times (70 times in average).
The eyes of Gaia
106 CCD cameras (with a total of almost 1000 million pixels) are the responsible to detect the light reaching both telescopes from the stars. All these CCDs are mounted in the largest focal plane ever built for a space mission (104 x 42 cm). Three different types of CCD are used in order to obtain the astrometry, the blue and red spectrophotometry and the high resolution spectroscopy of each observed object.
Gaia in numbers
|Sun-shield diameter:||11 meters|
|Focal length:||35 meters|
|Primary mirrors:||1.45 x 0.5 meters|
|Number of mirrors:||10|
|Number of CCDs:||106|
|Number of pixels per CCDs:||4500 x 1966 pixels|
|Total number of pixels:||938 Megapixels|
|Pixel size:||10 x 30 micrometers|
|Pixel angular size:||59 x 177 milliarcsec|
|Power consumption:||1720 W|
|Cost:||700 million euros|
After 2 strips of CCDs devoted to the detection of the sources (Sky Mapper), 9 CCD strips in the Gaia focal plane are devoted to measure the position and brightness of the stars in the sky very accurately. The Gaia spinning movement makes the image of the star transit through the different CCD strips, collecting more and more light.
Two columns of CCDs in the focal plane are devoted to detect the light of the stars after passing through two different prisms (one producing a spectra in the blue range and the other in the red range). These low resolution spectra will be used to derive the astrophysical parameters (temperature, surface gravity, composition, …) of the objects observed by Gaia.
The light of the stars, when they enter into the prisms, is split in different colours, producing a kind of rainbow with many colours, called spectra. Depending on the physical parameters of the stars (as their temperature, gravity and metallicity), these spectra will be brighter in some given colours. This will help us, then, to recognise the physical properties of the observations.
Radial Velocity Spectrograph (RVS)
A high resolution spectrograph is also included in Gaia to measure the velocity of the observed object getting away or closer to us (radial velocity). This can be done by using the effect of this radial movement in the spectral lines (Doppler effect). The light from a star contains some dark lines due to the presence of some elements present in their atmosphere. The positions in the spectra of these lines will change if the star is moving with respect to Gaia. If the line is shifted towards red, it is called redshift and indicates that the astronomical object is moving away from us. On the contrary, if the object is moving toward us, the spectral lines will suffer a shift towards blue (blueshift).
Payload data handling and communications
There are 7 Video Processing Units (VPU) on-board Gaia, each containing 3 PowerPC processors (for triple redundancy), overall placing the Gaia payload amongst the most powerful computing systems onboard a scientific satellite. They will detect, process and compress the images of the objects seen through the Gaia CCDs, generating over 5Mbps on average.
The on-board storage (about 100GB) will store this data until contact with the ground station is established, which will happen during 8 hours every day on average. The stability requirements on-board Gaia are so strict that a normal parabolic antenna would lead to unacceptable vibrations, and thus a special “phased array antenna” was required.