What is Gaia and why is it useful?

Gaia is a space observatory of the European Space Agency (ESA) designed  to construct the largest and most precise 3D space catalog ever made  totalling approximately 1 billion astronomical objects, mainly stars but also planets, comets, asteroids and quasars among others. The spacecraft will monitor each of its target stars about 70 times over a period of five years to study the precise motion of each star relative to theMilky Way galaxy. This will involve approximately 1% of the Milky Way population .

This massive stellar census will provide the  data to tackle a wide range of important questions related to the origin, structure, and evolutionary history of our galaxy.