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International cooperation and technology transfer
Mussio, Luigi

GAIA: global astrometry from space at 10 pas level
D. Cardio, B. Bucciarelli, M. Gai, M. G. Lattanzi and S. Cesare*
Astronomical Observatory of Torino
Strada Osservatorio, 20 - 10025 Pino T.se (To) - Italy
'Alenia Spazio, C.so Marche, 31 - 10100 Torino - Italy
Keywords: space instrumentation - astrometry - interferometry - data analysis
Based on the success of its previous astrometric space mission HIPPARCOS, the European Space Agency
recommended the study of a cornestone mission named GAIA dedicated to astrometry, with the goal of 10 pas
astrometric accuracy at magnitude V = 15. GAIA’s direct output will be an extensive network of stellar distances, proper
motions and positions throughout a significant fraction of the Galaxy, which would lead to important scientific results in
major astronomy and astrophysics fields. In this paper the mission concept and feasibility study are outlined. Critical
issues concerning instrumental calibrations versus scientific mission requirements are also addressed.
GAIA is a preliminary concept for an astrometric mission
of the European Space Agency (ESA) which should
provide positions, proper motions, and parallaxes of some
5 10 7 objects, down to about V=15 mag, with an accuracy
of better than 10 pas, along with multi-color multi-epoch
photometry of each object (Lindegren & Perrymann, 1996)
GAIA performs global astrometry, i.e. the entire celestial
sphere is observed, and the results refer to a unique and
well-defined reference system. This implies the
determination of absolute parallaxes and of a dense and
undistorted optical reference frame for positions and
proper motions.
From a technical viewpoint, the main characteristics of the
GAIA mission concept can be summarized as follow:
■ geostationary orbit or libration orbit around the
Lagrangian point L2 in the Sun-Earth system;
■ continuous scanning of the celestial sphere along
instantaneous great circles, with simultaneous
observations of two sky regions separated by a wide
basic angle (54°);
■ wide field of view (FOV ~1 deg 2 ) and high angular
resolution in the scanning direction;
■ the spacecraft rotation and precession, along with the
orbit of Earth around the Sun, combine to give
complete sky coverage;
■ each star is observed several times during the
mission allowing a complete determinations of the
astrometric stellar parameters. All the parameters are
in principle determined in a single, global adjustment
The satellite could be launched in 2009, with vectors
Arianne 5 in a dual launch, and the expected nominal
mission lifetime is 5 years.
1.2 General scientific objectives
GAIA will provide an enormous quantity of data of
extremely accurate astrometry and photometry from which
all branches of astrophysics will benefit, in particular in the
field of the physics and evolution of individual stars and of
the whole Galaxy. This process has began already with
the results of the ESA astrometry mission HIPPARCOS
launched in June ’89. However, while HIPPARCOS could
probe less than 0.1 % of the volume of the Galaxy by
direct distance measurement, GAIA will encompass a
large fraction of the Milky Way system within its parallax
horizon, including much of the Galactic halo, and even
touching on the nearest companion galaxies such us the
Magellanic Clouds.
We can summarize the scientific objectives as follow:
■ establishment of an optical reference system by
means of an accurate set of reference directions for
dynamical interpretations of the motion of the Earth
and other planets and of the Milky Way;
■ physics and evolution of individual stars;
■ dynamics of stellar systems;
■ formation and evolution of the Galaxy;
■ distance determination of the nearby Cepheydes and
RR-Lyrae stars, fundamental to define the distance
scale to nearby galaxies and cluster of galaxies;
■ detection of extra-solar planets and brown dwarfs: in
a volume of 200 pc of radius the number of stellar
candidates is about 2-3 hundred thousand; up to this
distance GAIA will be able to detect perturbations of
Jupiter like planets (p=11.2 years, a= 5 AU), with
probability larger then 50%;
■ general relativity: possible detection of gravitational
waves, and accurate ( 10 -6 ) measure of the
parameter y due to the Sun.