Gaia really does sound like a very, very good mission. It's one of them low-cost, low-risk but scientifically wealthy robotic missions at which ESA has been so good.
http://www.wikipedia.com/wiki/Gaia_probe
From the Wikipedia article on Gaia:
Gaia will compile a catalogue of approximately one billion stars to magnitude 20. Its objectives comprise (a) astrometric (or positional) measurements, determining the positions, distances, and annual proper motions of stars with an accuracy of about 20 micro-arcsec at 15 mag, and 200 micro-arcsec at 20 mag; (b) photometric measurements, providing multi-colour multi-epoch observations of each detected object; and (c) radial velocity measurements. Gaia will create an extremely precise three-dimensional map of stars throughout our Galaxy and beyond, and map their motions which encode the origin and subsequent evolution of the Galaxy. The photometric measurements will provide the detailed physical properties of each star observed, characterising their luminosity, temperature, gravity, and elemental composition. This massive stellar census will provide the basic observational data to tackle a wide range of important problems related to the origin, structure, and evolutionary history of our Galaxy. Large numbers of quasars, extra-solar planets, and solar system bodies will be measured at the same time.
This really does sound like an achivement. The first stellar census? We could very well end up finding hundreds of not thousands of earthlike planets as a result of this mission. I really hope this programme continues to fruition and is at successful as possible.
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If a man is offered a fact which goes against his instincts, he will scrutinize it closely, and unless the evidence is overwhelming, he will refuse to believe it. If, on the other hand, he is offered something which affords a reason for acting in accordance to his instincts, he will accept it even on the slightest evidence. The origin of myths is explained in this way.
-- Bertrand Russell (1872-1970)
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