This is the Gaia spacecraft.
It’s purpose is to 3-D map the Milky Way Galaxy. Cataloging approximately 1 billion astronomical objects.
"Gaia will create a precise three-dimensional map of stars throughout the Milky Way galaxy and map their motions, which encode the origin and subsequent evolution of the Milky Way. The spectrophotometric measurements will provide the detailed physical properties of each star observed, characterising their luminosity, effective temperature, gravity and elemental composition. This massive stellar census will provide the basic observational data to tackle a wide range of important questions related to the origin, structure and evolutionary history of our galaxy. Large numbers of quasars, galaxies, extrasolar planets and Solar System bodies will be measured at the same time.”
It will reach its designated orbit around the SEL2 point about three weeks after launch Dec 19th, 2013.