Gaia

The GAIA (Global Astrometric Interferometer1 for Astrophysics, Fig. 8.4) is an astrometric mission planned by ESA for 2011, which has numerous astrophysical objectives, primarily concerned with understanding the structure and dynamics of the Galaxy (the Milky Way). In particular, GAIA is expected to measure the positions and motions of several tens of millions of stars to a high degree of accuracy.

Possessing from the very start both a catalogue of about 1000 million stars, with 340 000 down to V magnitude 10, and 26 million to V magnitude 15, and with an astrometric accuracy of 4 |as at V magnitude 10, and 11 |as at V magnitude 15, GAIA is an instrument that is perfectly suitable for searching for giant planets. Its sensitivity and its accuracy should enable it to detect a large number of objects down to the mass of Uranus. In contrast to the radial-velocity method, which is most sensitive to massive objects close to the parent star, the astrometric method is all the more sensitive, the more massive the object and the farther from the star (cf. Chap. 2). However, the one major constraint is that the observations must be carried out during a significant portion of the planet's orbit for the astrometric variations to be detected. In practice, and considering the duration of the mission (5 years minimum, of which 4 will be devoted to observation, and with the possibility of the mission being extended for 6 more years), the mission should detect objects with periods less than 30 years (one-third of the period being observed during the

1 As initially conceived, the GAIA mission was a Fizeau interferometer and was included in ESA's Horizon 2000+ programme, under the category of 'space interferometer'. A revision of the mission's objectives meant that it was no longer an interferometer, because the desired aims could be achieved with a classical form of telescope. Nevertheless, the name of the mission was retained.

Fig. 8.4 Artist's impression of the GAIA mission (image credit: courtesy ESA)

satellite's lifetime). Relative to its predecessor, the Hipparcos mission, GAIA should improve the accuracy of measurements by a factor of more than 100.

From the instrumental point of view (Fig. 8.5), GAIA consists of two identical, astrometric telescopes pointing in two directions, separated by 106°. Each telescope

Spectrometric instrument (primary and tertiary mirrors)

ASTRO-1 secondary mirror

ASTRO-1 focal plane

Basic angle monitoring device

ASTRO-1 focal plane

Basic angle monitoring device

Common optical bench

ASTRO-2 primary mirror

ASTRO-1 primary mirror

Spectrometric instrument (Secondary Reflector & Focal Plane)

Fig. 8.5 Schematic diagram of the instrumentation on board GAIA (ESA)

Common optical bench

ASTRO-2 primary mirror w Platform interface

ASTRO-1 primary mirror

Spectrometric instrument (Secondary Reflector & Focal Plane)

Fig. 8.5 Schematic diagram of the instrumentation on board GAIA (ESA)

has a focal length of 50 m, a field of 0.32 square degrees, and observes in the spectral range of 300-1000 nm. GAIA is also equipped with a spectrometer having a field of view of 4 square degrees and a focal length of 4.17 m. It provides radial-velocity measurements with an accuracy of about 1-10 km.s-1 of objects of V magnitude 16-17, and also spectrophotometry.

The instrument functions in spin mode (rotation around a satellite axis with a rotation rate of 120 arcsec/s), the axis of which undergoes precession (the precession period is 76 days), thus allowing gradual coverage of the whole sky during the mission.

GAIA will be placed in a Lissajous orbit around the L2 Lagrangian point by an Ariane 5 launcher.

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