The Gaia satellite is 1.5 million km away and is orbiting a spot in space known as L2. The spot, L2 is a Lagrange point, think of it as a gravity balance point and makes a nice parking spot. ESA has a more in depth explanation of Lagrange points..
ESA can actually keep tabs on Gaia visually. I think this is just amazing. Using the Very Large Telescope at the European Southern Observatory in Chile Gaia actually can be seen. It’s a very small satellite very far away, over a million times fainter than can be see with the human eye.
From the ESA caption:
To measure Gaia’s position in the sky, a network of small and medium telescopes are monitoring the spacecraft on a daily basis. This information is being fed into the orbit reconstruction being performed at ESA’s Space Operations Centre, yielding an accuracy of 150 m on Gaia’s position and of 2.5 mm/s on its motion.
These two images, taken about 6.5 minutes apart on 23 January, are the result of a close collaboration between ESA and the European Southern Observatory to observe Gaia.