Gaia is measuring the positions, distances, motions and many physical characteristics of some one billion stars in our Galaxy and beyond.

This data will be used to study stellar evolution and star formation, as well as the origin and formation history of our Galaxy.  With the knowledge of how everything is moving we will be able to infer where the dark matter is and to study the dynamical behaviour within the Local Group of galaxies.

If you look, you will find. Gaia will also find large numbers of previously rare objects (many thousands of extra-solar planets; tens of thousands of brown dwarfs and white dwarfs; thousands of extragalactic supernovae; hundreds of thousands of objects in our Solar System).

Gaia will follow the bending of star light by the Sun and major planets over the entire celestial sphere, and therefore directly observe the structure of space-time. This will be one of the most precise tests of General Relativity.

Gaia is a space telescope,  or rather, two space telescopes that work together. It has a total of ten mirrors of various sizes and surface shapes to collect, focus and direct light onto the focal plane where it is detected. The image below shows the focal plane assembly fully integrated into the payload.

Gaia was launched by Arianespace, using a Soyuz ST-B rocket with a Fregat-MT upper stage, from the Ensemble de Lancement Soyouz at Kourou in French Guiana. It launched into the dawn of a new day and was beautiful.

There are three components which describe a star's motion, one is its motion towards or away from us (this is called radial velocity), and the other two are the movements of the star across the plane of the sky (this is called proper motion).

Why do we need two in the plane of the Sky? Well we need the component of the motion in the North-South direction and one in the East-West direction.

We can measure the motion towards or away from us (radial velocity) from the blueshift or redshift of the star's spectrum, and the motion on the plane of the sky (proper motion) by measuring the position of the star at lots of different times (and seeing how it moves).

We do this by using the apparent motion of an object, resulting from looking at it from different positions. Over the course of the year as the Earth moves round the Sun, nearby stars appear to move with respect to more distant stars (these still move but this movement is much smaller - Gaia uses Quasars (the extremely bright centres found in some galaxies) to define what is not moving). The angle through which the star appears to move is related to how far away it is.  

As the stars are a long way away these angles are really small. The get an idea of the size of these angles the full moon, covers about half a degree. If we take a degree and divide it into 60 pieces, 1 of those pieces would be 1 arc minute, and this would be the angle covered by a regular (22cm diameter) football if it was 756 metres away. If you were to take 1 arc minutes and divide it into 60 pieces, 1 of those pieces would be 1 arc second , this would be the angle covered by a football 45.4 kilometres away.

If we measure the parallax of a star and found that is was 1 arc second we would known that the star was 1 parsec away. (1 parsec is 3.26 light years or 30,856,778,570,831 kilometres) However, the closest stars are further away than this so we need to be able to measure really small angles. Gaia can do this, and by the end of the mission will be able to find the distances to stars up to half-a-millon light years away.

To cover the whole sky, Gaia spins slowly, changing where it's two telescopes are looking on the sky. On each complete rotation of the satellite we have observed a circle on the sky. The axis of rotation (spin axis) is then changed slightly, so that the next circle is in a slightly different position on the sky.  Gaia keeps repositioning its spin axis to observe another circle on the sky and eventually we have observed the whole sky.

We call the information which describes how Gaia repositions its spin axis, and hence where Gaia is looking when the scanning law. The are many different ways of mapping the sky and hence many different scanning laws. There are many different factors which must be considered when choosing the best scanning law for Gaia. One of the more important ones is keeping the angle between the spin axis and the Sun constant.

If we know where all the stars are we can make a map, and first map (in colour) made from the Gaia data is shown below. (The colour information comes from the spectra of light from the stars that Gaia also measures).

If a moon or planet passes in front of a star in the direction in which we are looking at it (our line-of-sight) it will block the star's light from reaching us as it passes in front (this is known as a stellar occultation). In a stellar occultation we should see the star disappear when the object passes in front. If it has an atmosphere we should see the star dim first as some of the light is first blocked by the atmosphere. If we can observe stellar occultations we can learn about atmospheres on other worlds. To observe them we need very precise positions for both objects in order to know where and when to look. This is where Gaia can help.