Gaia (satellite)

Satellite

Gaia must be launched by a launcher Soyuz and join the Point Lagrange L2 located at approximately 1,5 million kilometers of the Ground of which one of the advantages is to get an extremely stable thermal environment. There, it will describe an orbit of the type Lissajous to avoid the eclipses Sun by the Ground, in order to be able to feed its solar panels.

Principles of measurement

Just like its predecessor Hipparcos, pioneer of the space Astrometry, Gaia will observe simultaneously two directions of aiming while turning continuously with light a Précession, and while preserving the same angle at the Sun. By precisely measuring the relative positions of the objects of the two directions of aiming separated by a great angle, a great rigidity of the frame of reference is obtained.

Each object will be observed on average 70 times approximately during the mission, which must last 5 years. These measurements will allow the determination of the astrometrical parameters of stars: 2 for the angular position on the sky, 2 for their derivative compared to time (own Movement), as well as the annual Parallax.

It misses nevertheless a sixth parameter for all to know position and speed of the objects in space. This parameter, the radial Speed, is obtained by Effect Doppler thanks to a Spectromètre also on board Gaia.

Characteristics

The Payload of Gaia consists of:

a mirror of 1,4 × 0,5 m ² for each direction of aiming
a focal plan of 1,0 X 0,5 m on which are projected the two directions of aiming, made up of 106 CCC S of 4500×1966 Pixel S of which some are equipped with filters.

It implements 3 distinct instruments:

the astrometrical instrument (ASTRO) dedicated to the angular measure of location of stars magnitude 5.7 to 20
the photometric instrument, allowing the acquisition of star spectra in the spectral band 320-1000 Nm in the same range magnitude
the spectrometer high-resolution allowing to measure the radial speed of stars by the acquisition of spectra of high-resolution in the spectral band 847-874 Nm (field of the lines of the Calcium ionized) for objects until the magnitude 17

The telemetric bond with the satellite being from approximately 1 on average, whereas the contents of the focal plan represent several, obliges to descend only a few tens of pixels around each object. Consequently, the detection and the follow-up of the objects on board are obligatory, and represent edge a complex treatment of the satellite in the stellar fields densest.

Mission

The mission was adopted by the ESA like Cornerstone mission number 6 on October 13rd, 2000, and the starting of the B2 phase of the project under the Maîtrise of work of EADS Astrium Satellites was authorized on February 9th, 2006, by aiming at a launching for December 2011. The total costs of the mission are approximately 450 million euros, including/understanding manufacture, launching and the operations on the ground.

The multiplicity of the instruments (Astrometry, photometry, Spectroscopy) made of Gaia the analog of a complete observatory orbits and implies an important diversity of data of it. The quantity will not be in remainder: 5 years of mission with a flow of compressed data of 1 to approximately 60 TB corresponds.

The scientific treatment the ground of these data, with the load of the Member States, will appear extremely complex besides, taking into account the diversity of the sky (variable stars, double stars, etc): to devote would not be this that 1 S of processing time per object would require 30 years of calculation in all.

Scientific objectives

The justification of the Gaia space mission comes from several reports:

the precise intrinsic luminosity of stars requires to know (directly or indirectly) their distance. One in the only manners of obtaining it without physical assumptions is via the annual Parallaxe. The observation starting from the ground would not make it possible to obtain these Parallaxe S with sufficient precision, because of the effects of the atmosphere and the instrumental systematic errors.
it is necessary to observe the weakest objects to have a complete vision of the stellar function of luminosity, and in addition it is necessary to observe all the objects until some magnitude so as to have not skewed samples.
to know the fastest phases of stellar evolution, and to thus force the models of evolution, it is necessary to observe sufficient objects. A big number of objects is also necessary to know our Galaxy: a billion stars accounts for 1% of its contents roughly.
a very good precision astrometrical and kinematics is necessary correctly to know the various stellar populations, in particular most remote, to reconstitute the stellar orbits, etc

Design of Gaia from of deduced, technical constraints not allowing in addition to observe more objects, objects weaker, or with better precise details. The predicted performances are the following ones:

all the objects (more than one billion) until the magnitude V=20
an accuracy of 7 millionth of Second of arc (μas) to the magnitude V=10 (precision equivalent to the measurement of the diameter of a hair to 1000 km), between 12 and 25 μas with V=15, between 100 and 300 μas with V=20, this depend on the color of the star
is approximately 20 million stars with a precision in distance better than 1% and 40 million with a precision of tangential speed better than 0,5 km/s

On the basis of nominal performance and model, Gaia should contribute significantly in the following sets of themes:

Galactic physics

space and kinematic Structure of all the star populations, in all the parts of our Milky Way: thin disc, thick disc, spiral arms, bulb and bar, halation, globular clusters, zones of star formation, open clusters.
Determination of the age and the metallicity of stars of various populations, age of the oldest objects.
Consequently, Gaia will make it possible to study the formation and the evolution of our Galaxy.
astrometrical Detection of 10 ⁴ planets extrasolaires of mass similar to Jupiter to approximately 200 PC, several thousands of orbits. Detections of several thousands of Transit S of planets extrasolaires in front of their star host.

Physical stellar

Statistical of all the types of stars, including in the phases of fast evolution of the Diagram of Hertzsprung-Russell.
Determination of the fundamental parameters (mass, ray, luminosity, temperature and chemical composition).
systematic Detection (though nonexhaustive) of the binarity and the variability.
Forced for the models of structure and stellar evolution.

Solar system

Observations of 10⁶ objects.
Determination of masses, orbits improved by a taxonomic factor 30.
Classification (connected to the mineralogical composition of surface) starting from the photometry.

Galaxies and frame of reference

Determination of the direct distances in the Clouds of Magellan for the most brilliant stars.
Distance of the Céphéide S and RR Lyrae allowing recalibrer the distance scale in the universe.
Dynamic and parallax of rotation of the galaxies of the local Group.
Photometry of more than one million Galaxy S.
Detection of more than 10 ⁵ Supernovas.
Realization of a frame of reference starting from 5×10 ⁵ Quasar S.

Fundamental physics

Because of the presence of the mass of the sun (and another planets of the Solar system), one expects a deflection of the luminous rays of each star.

In a post-Newtonian formalism, this deflection is proportional to (1+γ) /2 where the parameter γ is worth 1 within the framework of the General relativity: Gaia should obtain a precision about 5×10^-7, thus providing one of general relativity. Other contributions will be possible with the objects of the Solar system (for example, Périhélie advances).

Lastly, thanks to the observations of the luminosity of the white dwarf , Gaia should provide a constraint as for a variation (hypothetical) of the gravitational Constante.

See too

the satellite Hipparcos

External bonds

Gaia Site at the European space agency
Gaia Site at the Observatory of Paris

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