### Why is Gaia operating around Earth orbit? Why not send it to Neptune's orbit?

• Gaia is an astrometry spacecraft that's currently operating around the Sun-Earth L2 Lagrangian point. Question: why here? Why not the Sun-Neptune L2 Lagrangian point? By orbiting the Sun at a larger distance, it should be able to get more accurate parallax measurements.

Only reason I can think of is cost. I'm not familiar with estimating how expensive space probes cost, but Wikipedia says Gaia cost ~\$1 billion and this is comparable to the cost of the Voyager program, which also cost about ~\$1 billion. Of course Gaia's instruments should be more sophisticated than Voyager's, but there were also two Voyager probes, not one.

It's a pity that the community cannot overrule the decision which answer to accept.

@Walter what's wrong with the accepted answer? I don't consider it worse than Rob Jeffries' answer - different, but not worse.

Apparently, we disagree on that last point.

• Well, you thought about the spatial aspect of a parallax measurement, but not about the temporal one.

Gaia's intention is to measure 3D positions as well as 3D velocities. For the distance, you need accurate parallactic measurement, which come in with your orbital period.

For a typical Gaia-star with several measurement per year, you'll get 5 values of the parallax after 5 years of time, which you then average. If you'd send Gaia towards Neptune (besides the fact that no one has ever sent an orbiter, to say nothing of a L2 mission that far out) that has a period of 168 years, then after 5 years you'd get... 5/168 th of one paralactic measurement.

It simply couldn't achieve its science goals if put around the L2 behind Neptune. Also no one on this planet has any experience in putting something into a outer system L2 point. This is different than putting it into Earth's L2, because reaching the L2 around one of the giants has vast and very precise $$\Delta v$$ requirements. This would be a massive technological leap, and things don't work that way in space. Small, incremental technological steps are required in an anyways unfriendly environment, to make sure everything works properly and no millions of dollars have been wasted.

Compare that to Gaia's predecessor, the Hipparcos satellite, which was parked in geostationary orbit.

Now you could still say, why not use Jupiter hypothetically anyways. Well, the orbital period there is still 11 years, and Jupiter's L2 still suffers from the intense radiation environment that is provided by Jupiter's magnetosphere. This would lead to rapid degradation of the CCDs used for scanning across the sky.

Also, good luck getting power from Gaia's solar panels out by Neptune.

There seems to be some confusion here that a Parallax measurement has to combine observations from opposite sides of an orbit. It doesn't.

@RobJeffries It doesn't, but it gains benefit from large orbital velocities. Neptune is moving slower, as simple as that - you need longer to get the same parallax. Of course, that's not the only consideration - the size of the orbit affects the maximal precision you can get from your measurements; measuring parallax from a Moon orbit would be much faster, but also gives you less precision than waiting for the Earth-Moon system as a whole to reach the opposite of its orbit.

It isn't wrong, so much as not telling the whole story.

@Luaan: Where is this wrong? You want a large arc on the sky, and this arc has to be $2\pi$ ideally on the sky. With only a small arc of the full ellipse traced, you get massive fitting errors when determining your parallax. Rob Jeffries answer doesn't contradict me there in any way. It's not the orbital speed, its the size of your parallactic ellipse that you want to have. An arc doesn't help.

@AtmosphericPrisonEscape You're mangling together two things that aren't all that related. Larger distance means more accurate measurements. Less time to reach the same distance means you get the same accuracy in less time. There's very little difference between having the full ellipse and having an arc of roughly the same curvature and distance (as a flat projection relative to the object you're observing).

@Luaan: Well, then remind me what the angle is that I take for the parallax, if I have only a small arc of an ellipse to measure this angle in the first place? Without the full arc I don't have my $\pi$ that I need to compute the distance from.

@AtmosphericPrisonEscape Ah, I was hoping we were just misunderstanding each other, but apparently not. Are you actually really trying to say that parallax measurement is impossible if you don't have measurements from two opposite ends of an ellipse? Surely if you know the shape of an ellipse, you can calculate the "expected" angle for the full ellipse regardless of where the two measurements are - of course the more distant the projected points, the more accurate the parallax measurement.

@Luaan: That's my whole point. The ellipse solutions will be all over the place for a few closely-spaced points, while the same number of points spread apart will give a family of ellipse solutions with very similar parameters, i.e. small errors. If you want to learn more about how this process works, I suggest you check out the graphics on https://en.wikipedia.org/wiki/Hipparcos .

@AtmosphericPrisonEscape You don't need an ellipse **at all** to measure a parallax angle. All you need are two separate points. It doesn't matter how you get from point A to point B, whether its a full 180 degree arc, a small part of an arc, a straight line, or a detour through hyperspace. Triangulation has been used for centuries in construction and engineering, and I guarantee you that those engineers didn't walk in a half-circle just to get to their next measuring point.

@HiddenWindshield: This is not about whether it can be done, but how precisely it can be done. See my previous comment.

In addition to the excellent points here, a probe in the Neptune/Sun L2 point would have a much harder time getting data back to Earth: the antennas' aims need to be more precise, the transmission much stronger (and/or the receiver much more sensitive while still dealing with background noise), and there are a lot more things that would block line-of-sight from Earth to the probe (so, the probe would need more data storage to queue up reports and would need an even more robust re-transmission protocol, plus more data runs the risk of being lost due to limits in the probe's hardware).

Side issue, if you're going to put something in the outer system, why L2 rather than a Trojan point?

I am no expert, but my understanding is that having many measurements from opposite sides of the ellipse, is what allows you to factor out the relative motions of the target star and our sun.

@jwdonahue Indeed it does. Distinguishing proper motion from a partial orbit is difficult unless your parallax baseline makes a significant angle with the proper motion.