The most sophisticated space telescope ever built by Europe will help create the most detailed map yet of our galaxy.
The £620m observatory lifted off from the Sinnamary launch complex in French Guiana on time at 9.12am (UK time).
The huge telescope is going to map the precise positions and distances to more than a billion stars.
It is hoped that Gaia will give astronomers the first realistic picture of how the Milky Way galaxy is made up.
The observatory’s exceptional sensitivity may also detect many thousands of previously unseen celestial objects, including new planets and asteroids.
As many as 50,000 planets outside our solar system could be spotted during the satellite’s five-year life.
It will also keep a watch for exploding stars – supernovae – which are rarely observed in real time.