In Greek mythology, Gaia was the "great mother of all... creator and giver of birth to the Earth and all the Universe". An apt name, then, for the craft whose mission is to map our home galaxy to unprecedented levels of precision. It will pinpoint approximately a billion stars in our galaxy, precisely measuring their distances from us by using the parallax method, whereby the apparent movement of a nearby star with relation to distant background stars is measured as an angle, allowing the distance to be found by trigonometry. Simple.
The accuracy with which Gaia will place stars is comparable to measuring the diameter of a human hair at a distance of 1000 km. It is able to do this because of its array of 106 CCDs (charge coupling devices). These turn the light reaching them into electrical signals, which can then be transmitted back to a computer. CCDs are the same tools that your digital camera or smartphone uses. The difference here is that there are a lot more of them; rather than one square centimetre, Gaia's array will form a focal plane of half a square metre. This gives it the capacity to collect a lot of light. It is almost a billion pixels; the Kepler mission, sent out by NASA to look at our neighbouring planets, previously held the most pixels, at 95million - about 9 or 10 times more than a decent digital camera.
This isn't all; as well as measuring the ages, positions, distances, movements, and changes in brightness of its target stars, Gaia will log anything that crosses its field of view - comets, asteroids, supernovae. Scientists hope that this will generate a whole host of new discoveries, and more detailed data about objects that have previously been observed.
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As Gaia observes the stars it tracks at distances of about 500 lightyears, it will watch for tiny fluctuations in the position of the central brightness, as the star is pulled by the gravitational forces of orbiting planets; this could find many thousands of more Jupiter-sized exoplanets (planets outside of our Solar system).
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It will also be able to measure effects in light due to the gravity of very large objects; this is known as gravitational lensing. Gravity warps the fabric of space-time in such a way that as light from very distant galaxies and clusters travels past slightly closer objects, it is bent. This can help us to test key parts of Einstein's Theory of General Relativity to greater levels of precision than ever before.
When astronomers put forward their proposal for the mission around 20 years ago, they thought they were requesting the impossible. It may not be Mission Impossible, but there's still a way to go. Keep posted on Gaia's activities on the blog from the European Space Agency: http://blogs.esa.int/gaia/
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