The satellite is equipped with a billion-pixel camera, the largest ever in space. Just over half-way through its five-year mission, on Wednesday the Gaia team will release the positions of roughly one billion stars (That's still only one percent of the Milky Way's estimated stellar population, scattered over an area 100,000 light years in diameter.) and the distances and proper (or sideways) motions of the two million brightest stars in the sky (with the exception of a few). Although those two million stars are not necessarily the closest, they will all be located somewhere in our galactic neighborhood. And with each subsequent data release (planned once a year until 2019, with a final release in 2022) astronomers will chart stars at greater and greater distances within the galaxy, creating successive maps that radiate outward from the sun like ripples on a pond.

After its five-year run Gaia will release the three-dimensional positions and two-dimensional velocities for a full one billion stars, stretching out to the halo of the Milky Way—a large sphere of stars that enshrouds the galaxy’s disk and spiral arms.

In addition to stars, the tiny satellite is expected to detect thousands of exoplanets and may even map the invisible dark matter that inhabits the Milky Way.