File:VST Snaps Gaia en Route to a Billion Stars.jpg

English: These new images from ESO's Very Large Telescope Survey Telescope (VST) show ESA's Gaia spacecraft in its position some 1.5 million kilometres beyond Earth's orbit.

Launched on the morning of Thursday, 19 December 2013, the satellite is on a quest to build a 3D map of our galaxy over the next five years. Mapping the sky has been one of humanity's quests since the dawn of time, and Gaia will take our understanding of our stellar neighbourhood to a whole new level. It will measure very precisely the positions and motions of about one billion stars in our galaxy, to explore the Milky Way's composition, formation and evolution.

These new observations are the result of a close collaboration between ESA and ESO to monitor the spacecraft from the ground. Gaia is the most accurate astrometric device ever built, but in order for its observations to be useful it needs to know exactly where it is in the Universe. The only way to know the velocity and position of the spacecraft with very high precision is to observe it on a daily basis from the ground — using telescopes including ESO's VST in a campaign known as Ground-Based Optical Tracking, or GBOT.

The VST is a state-of-the-art 2.6-metre telescope equipped with OmegaCAM, a monster 268-megapixel CCD camera with a field of view four times the area of the full Moon. The VST captured these images using OmegaCAM on 23 January 2014, taken about 6.5 minutes apart. Gaia is clearly visible as a small spot moving against a background of stars. Its location is circled in red. In these images, the spacecraft is about a million times fainter than is detectable by the naked eye.

Gaia was previously observed in December 2013 by the VST, very soon after its launch — it is one of the closest objects ever observed by the VST. It appeared in precisely the location expected, highlighting a successful collaboration between ground- and space-based astronomy!