Image 1 Caption: A not-to-scale diagram of the trajectory of the Gaia spacecraft (mission 2013--2025?).

Features:

1. The European Space Agency (ESA) Gaia was launched 2013 Dec19.

2. Gaia has obtained stellar parallaxes to large regions of the Milky Way and beyond.

3. Stellar parallaxes (and therefore distances) of 1 % accuracy are expected out to 10 kpc which is ∼ 1/3 of the diameter of Milky Way galactic disk.

4. Gaia is currently planned to operate to 2025 (see Wikipedia: Gaia: Mission progress).

5. Gaia is orbiting Lagrange L2 point of Earth-Sun gravitationally bound 2-body system in a Lissajous orbit.

6. For more information on the Lagrange L2 point and all 5 Lagrange points (L-points), see Orbit file: lagrange_points.html.

7. Why wasn't Gaia put in orbit around the Earth?

   Yours truly is too bored with this caption at this point to do more than guess.

   The mission planners probably wanted to keep Gaia far from the Earth since the Earth would block some of its field of view (FOV).

   The L2 point is quite far (0.010 AU ≅ 1.5*10**6 km) from the Earth, and so the solid angle taken up by the Earth is small.

   But why NOT just an orbit about the Earth with a large mean orbital radius?

   Maybe such an orbit is tricky maintain somehow.

   Why NOT put Gaia even farther from the Earth. Yours truly guesses that the mission planners wanted to keep the signal travel time to Gaia relatively short so that communications are relatively prompt in cases of emergency. Remember radio signals travel at the vacuum light speed.