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As I remember, Gaia (wiki, ESA) was planned to flight away from Earth, so it orbit is not LEO, and not GEO. It is located near 1.5 millions kilometers away from Earth, orbiting thousands kilometers away from L2 Lagrangian point of Sun-Earth system.

What was main reasons to put it so far?

How far is it from the L2 point?

Is sun shadowed there?

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    $\begingroup$ It is not in shadow, it is powered by solar panels. I think they want constant irradiation from the Sun to keep the temperatures of the instruments very stable. In LEO you would enter Earth's shadow every few hours, and Earth would block parts of the sky. Much further away from the Sun it would get larger parallax = better precision, but it would also take longer for it to complete an orbit. 11 years at Jupiter compared to 1 year now in L2. $\endgroup$ – LocalFluff Mar 24 '14 at 10:52
  • $\begingroup$ You should make this an answer. $\endgroup$ – Stu Mar 24 '14 at 18:35
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According to ESA, it's target Lissajous orbit around SEL2 is:

263,000 × 707,000 × 370,000 km, 180 day-long orbit around L2

It's the only place that suits Gaia's design constraints, to keep its solar panels pointed towards the Sun at all times, prevent thermal cycling, and keep its antenna pointed towards the parts of the Earth where it can be communicated from.

    enter image description here

    A cutaway of the Gaia space observatory showing its instruments Credit: EADS Astrium

Sun-synchronous -1° retrograde polar orbit (frozen orbit) wouldn't do, since the majority of its ground track is over the middle of nowhere. L1 wouldn't do, since the antenna can't be a part of the Sun shield then, L3 is behind the Sun, L4 & L5 still don't enable its antenna to point towards the Earth (maximum permissible tilt for its Sun shield relative to the Sun is at 15°), and higher or lower heliocentric orbits would put Earth in opposition during its 5 year mission, complicate communications (1 billion pixels creates a large data stream) and reduce total insolation needed to power the space observatory, if in higher altitude heliocentric orbit.

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  • $\begingroup$ "...keep its antenna pointed to... Earth". But Gaia wobbles, mustn't it, to scan the sky? Is the "phased array antenna" in the illustration above aiming its signal strength regardless of the physical orientation of the spacecraft? $\endgroup$ – LocalFluff Mar 25 '14 at 18:40
  • $\begingroup$ @LocalFluff No, it doesn't "wobble to scan the sky", its mirror assembly rotates 360° every 6 hours and it scans the skies not in front of it but lateral and all around. Notice there's no front opening for its optics / mirrors. It gets the "complete picture" (4π) during one Earth's synodic period, and it'll attempt to do that 5 times (5 years long mission). There's many videos showing that, for example this one. Its attitude is maintained to have maximum angle to the Sun-Earth at 15° both for comms and the sunshield. ;) $\endgroup$ – TildalWave Mar 25 '14 at 18:42
  • $\begingroup$ TildalWave, Thanks! Do you know, what is the angle between Main Lobe of Array Antenna (each of its patch-antennas) and the -Xs axis? Also, there are not all 1 billion pixels downloading from Gaia to Earth, because at 1.5 mln km it has only up to 3-8 Mbit/s of bandwidth. $\endgroup$ – osgx Mar 25 '14 at 19:07
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    $\begingroup$ @osgx I'll check on the angle, but I gotta say I really started to hate ESA's way of documenting things. It's a total mess, in the best traditions of the EU (I should know, I'm in it LOL). As for those billion pixels, well yes, the datastream is highly compressed but at approx. 8 hours per day, 365.25 days a year, and 5 years, it should be at about 67 TB at 1 MB/s. Uncompressed, that's roughly 3x as much of data so let's round it to 200 TB. That's still quite something IMO. It'll be a daunting task compiling that into a comprehensible form :) $\endgroup$ – TildalWave Mar 25 '14 at 19:21
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    $\begingroup$ BTW, I'd expect such info in Gaia's factsheet on ESA's pages, but, of course, it doesn't even mention antenna by a single word. That's what I meant with how well organized or useful ESA's pages are. :} $\endgroup$ – TildalWave Mar 26 '14 at 16:43
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The Earth Sun Lagrange point 2 (SEL2) has the following advantages for the Gaia telescope:

  • Gaia is NOT shadowed in (orbit around) the SEL2. The Earth is way too far away to cast any shadow there. This is good for Gaia because it gets its electric power from its solar panels.

  • In SEL2 Gaia has constant solar exposure. In LEO it would, like the Hubble Space Telescope or the ISS, go into and out of Earth shadow every few hours. Gaia has very sensitive instruments which need predictable and constant temperatures. Not necesserily minimal temperature, but rather constant.

  • LEO would also have the Earth cover a large fraction of the sky. The Hubble Space Telescope is designed to focus on a tiny part of the sky at a time and its operation is carefully planed with respect to where it is relative to Earth. But the purpose of Gaia is to systematically over time scan the entire sky. It wouldn't be practical for it to be near any large object.

  • Gaia uses the parallax, the angular differences from which it observes stars as it orbits the Sun. In a larger orbit it would get larger angles and better precision in its measurements. So it would basically be good. But it would have two major drawbacks:

    First, it would take longer to complete the orbits, and it needs in principle to complete at least two orbits to get both distance and movement data (this might be more complex depending on how it operates). If it were out at Jupiter, it would would have about 5 times as large a parallax, but it would take more than 10 years to complete one single orbit around the Sun. It might not have that long a life and scientists might not want to wait that long. (Maybe a follow up will???)

    Second, SEL2 is at a constant distance and direction from Earth. If in any other orbit, it would drift away and eventually be hundreds of millions kilometers away, on the other side of the Sun. This would make telecommunications a challenge and likely require more investments in both onboard equipment and ground control.

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