Exploded diagram of the GAIA satellite design. In the payload module, the optical bench carries the scientific instruments and telescopes.(Image credit: ESA / Matra Marconi Space.)

In 2010, the GAIA mission will begin to produce a three-dimensional map of the stars in our Milky Way galaxy in unprecedented detail.

From 1989 to 1993, despite being placed in the wrong orbit, the Hipparcos astrometry mission measured the positions of and distances to 2.5 million stars. The GAIA mission will build on that success and provide results that improve on the accuracy of those from Hipparcos by a factor of 100.

GAIA is named after the Greek Earth Goddess worshipped as the universal mother who had created the Universe. It has been devised to give precise and detailed information about the billion brightest objects in the sky. This involves measuring the positions and velocities of a large number stars far more accurately than ever before. As a result of this scientists will be able to determine the Galaxy's three-dimensional structure. The mission's future impact on astronomy has been compared to the introduction of weather satellites to meteorology and genome projects to genetics.

The spacecraft will be positioned at Lagrange Point No. 2 or L2, where it will be held in place by the balanced gravitational pull of the Earth and Sun on the night side of the Earth. Therefore it will be shielded from glare from the Earth, Sun and Moon that would otherwise interfere with the stellar observations.

Angular positions can be used to determine the distances to nearby stars and to study the motions of stars in our Galaxy. Distances are of fundamental importance in astrophysics, and the only direct method of measuring them is by triangulation. As the Earth moves around the Sun, a minute steady change in the apparent position of a nearby star against the background of more distant stars may be detected. This annual change is called parallax.

The GAIA observatory will be comprised of two main, identical telescopes of 1.7-m aperture, which will carry out a form of parallax detection on stars out to a distance of about 10 kiloparsecs, where one kiloparsec is 1000 parsecs and 1 parsec is 3.26 light years. This is greater than the distance from the solar system to the galactic centre, so for the first time we will have an accurate map of a large fraction of our Galaxy. Separated by an angle of 106 degrees, the telescopes will look simultaneously at stars in two directions in the sky. This will allow an accurate measurement of the relative separations of stars. Each telescope will also detect the brightness of each star at four different wavelengths of light.

Another telescope with a diameter of 75cm will be placed between the two main telescopes and observe the same stars. However it will measure their brightness in fourteen narrow wavelength bands, as well as their Doppler shifts, which indicate whether an object is approaching or receding.

GAIA will also view all kinds of transient objects like supernovae, flaring stars and asteroids. It will play a major role in Near Earth Object (NEO) and even Potentially Hazardous Object (PHO) detection, being able to detect Atens – asteroids that spend most of their time inside Earth's orbit.

The mission is due to launch in 2010 and is expected to run for about five years. It has been predicted that the average daily discovery count will include about 100 asteroids, 30 stars possessing planets, 50 stars exploding in other galaxies and 300 distant quasars. GAIA may ultimately identify half a million new quasars. It should also produce 3D views of distant star systems.

Observing the motions of stars should allow astronomers to deduce where bursts of star formation took place, where dark matter is present and where the stars which formed in the vicinity of our Sun 4.6 billion years ago are now located. Ultimately, GAIA will help scientists to develop a greater understanding of the origin of our Galaxy and its subsequent evolution.