A couple of weeks old now - (19th Dec)
The 740m-euro (£620m) observatory lifted off from the Sinnamary complex in French Guiana at 06:12 local time (09:12 GMT).
Gaia is going to map the precise positions and distances to more than a billion stars.
This should give us the first realistic picture of how our Milky Way galaxy is constructed.
More: http://www.bbc.co.uk/news/science-environment-25426424 (http://www.bbc.co.uk/news/science-environment-25426424)