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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.

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    $\begingroup$ My question is very similar, although it doesn't have a good answer to it yet. astronomy.stackexchange.com/questions/30285/… $\endgroup$ – Ingolifs Nov 25 at 22:45
  • $\begingroup$ It's a pity that the community cannot overrule the decision which answer to accept. $\endgroup$ – Walter Nov 29 at 11:08
  • $\begingroup$ @Walter what's wrong with the accepted answer? I don't consider it worse than Rob Jeffries' answer - different, but not worse. $\endgroup$ – Allure Nov 29 at 11:13
  • $\begingroup$ Apparently, we disagree on that last point. $\endgroup$ – Walter Nov 29 at 11:14
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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.

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    $\begingroup$ Also, good luck getting power from Gaia's solar panels out by Neptune. $\endgroup$ – notovny Nov 25 at 2:41
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    $\begingroup$ @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. $\endgroup$ – Luaan Nov 25 at 9:54
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    $\begingroup$ It isn't wrong, so much as not telling the whole story. $\endgroup$ – Rob Jeffries Nov 25 at 9:58
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    $\begingroup$ @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. $\endgroup$ – Luaan Nov 25 at 14:28
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    $\begingroup$ @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. $\endgroup$ – HiddenWindshield Nov 25 at 16:09
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I think it is to do with (a) orbital speed and (b) telemetry and (c) power.

In order to measure Parallax you need to measure the position of the star from different locations in the solar system. The Parallax becomes more precise the greater the separation between those positions.

At Earth-Sun L2 you get a difference of about 2 au in 6 months. i.e. the spacecraft has a baseline that changes at 4 au/yr. In a 5-year mission, you essentially get 10 samples of the full baseline, that enables you to beat down the errors by $\sqrt{10}$, equivalent to an effective baseline of 6.3 au. At the same time, because the spacecraft has executed complete orbits, all of the sky has been sampled with a similar baseline (imagine viewing a line tracing out the spacecraft orbit from a distance - it will have a similar length when viewed from any direction).

If you calculate how long it takes a satellite in orbit at Neptune to make a baseline (defined by the chord of a circular orbit) of 6.3 au, it is only 5.5 years.

However, that would only be for part of the sky - the part at right angles to the spacecraft motion. Large portions of the sky would have barely any baseline at all because the spacecraft motion is essentially straight towards it. Solving for Parallax and proper motion (the relative tangential velocity of the stars) would also be difficult if the proper motion was then parallel to the satellite motion. At Earth-Sun L2, this problem goes away because every 6 months the Parallax motion reverses, but the proper motion doesn't. Around Neptune you would have to wait 84 years for that to happen.

Of course you would also get the observational baseline between where the spacecraft journey began (the Earth) and Neptune, which is potentially 30 au. However, this doesn't solve the problem of all-sky coverage and also wouldn't solve the issues discussed below.

The other issues are practical and I suppose potentially soluble if you throw enough money at them.

Gaia has a limited telemetry bandwidth. At the moment there is significant autonomous decision making and processing before a subset of the data is sent back to Earth. These problems become many orders of magnitude harder when you are 30 au, rather than at the Earth-Sun L2 point which is a mere 1.5 million km away.

Gaia also needs power and it uses solar panels. You get about 900 times less power per unit area at Neptune, meaning 900 times bigger solar panels or some alternative (nuclear) power source.

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    $\begingroup$ One could imagine a future mission that sent four identical spacecraft out to Jupiter and then used Jupiter's gravity to put them on solar escape trajectories in (as far as possible) four directions spaced tetrahedrally (like the bonds in methane). Over time, they would give a set of steadily increasing baselines in all directions. This would be massively more costly and technically challenging that Gaia though, and that wasn't easy. $\endgroup$ – Steve Linton Nov 25 at 8:56
  • $\begingroup$ @SteveLinton Yes, that would be a way to do it. $\endgroup$ – Rob Jeffries Nov 25 at 9:59
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    $\begingroup$ nicely written and clear, and still with all the numbers! props. $\endgroup$ – Spike0xff Nov 25 at 20:15
  • $\begingroup$ It's worth pointing out that one might propose an alternative power source (EG thermal nuclear), similar to those used in robot rovers. So whilst distance applies with solar panels, it might not be necessarily relevant with an alternative power source. $\endgroup$ – SSight3 Nov 26 at 10:25
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    $\begingroup$ @SSight3 "or some alternative (nuclear) power source" $\endgroup$ – Rob Jeffries Nov 26 at 16:07
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3 problems.

1) Time. As previous answers say, to make use of the larger diameter around the sun at Neptune's L2 point, you need to wait for a full rotation which takes 168+ years.

2) Energy. Solar panels provide significantly less energy, potentially not enough.

3) Distance. Data from a probe around Neptune take a good 4h10min to earth on average, which limits the data rate you can transmit, just like New Horizons from Pluto.

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    $\begingroup$ Nice, I forgot about the data rate issue, which is already nearly critical for Gaia. Missions with higher data rates and/or further away would have serious trouble with that. $\endgroup$ – AtmosphericPrisonEscape Nov 25 at 14:51
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    $\begingroup$ No, you don't have to wait for a full rotation. Same mistake as AtmosphericPrisonEscape's answer. The other two points are already in my answer. $\endgroup$ – Rob Jeffries Nov 25 at 18:19
  • $\begingroup$ @RobJefferies - Sure, for one parallax measurement. But for the whole sky, a full rotation is preferable, as implied in your answer. $\endgroup$ – IronEagle Nov 26 at 1:46
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    $\begingroup$ I am not sure that (3) is sound either. Essentially you are saying that a larger latency implies a lower bandwidth, which is false. Data can still flow at the same speed. $\endgroup$ – Federico Poloni Nov 26 at 20:44
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    $\begingroup$ I'm undecided if I should upvote this or downvote it. On the one hand, you're right that distance means a lower data rate, but on the other, you're wrong about why. $\endgroup$ – Mark Nov 27 at 6:12
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Lots of interesting answers. Lots of unnecessary arguing.

1) Neptune sweeps out a parallax of nine billion km every 84 years, Earth sweeps out a parallax of 300 million km every six months so if you want max parallax, then Neptune is a 30 times better place to be. However, Neptune only sweeps out 53 million km in the same six months so you can get better results there but you will have to wait six times longer before you start to get them.

2) Earths L2 point moves with Earth so only requires getting a satellite to a speed of zero (relative to Earth) in the right place 1.5 million km away. remember that when you launch something away from Earth, the Earths gravity will start slowing it down as soon as the engines stop burning so all you have to do is run out of fuel at exactly the right place. Difficult and expensive to say the least but yes, we can manage that. Neptune is 4.5 billion km away, give or take 300 million km depending on the time of year. The good news is that if we could get a satellite out there, we would just require a stable orbit, we would not need to have a zero velocity (relative to Neptune) at Neptune's L2 point. The bad news is we do not know how to put a satellite into any sort of orbit around Neptune at a price we can afford.

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  • $\begingroup$ Your first point is not the full story. While you are waiting "six times as long" you get a $\sqrt{6}$ improvement in the Parallax precision from Earth L2 by repeated measurement. That is how Gaia works. Secondly, it is no good waiting 6 times as long (say 30 years), because you won't get accurate parallaxes across the whole sky. $\endgroup$ – Rob Jeffries Nov 28 at 7:40
  • $\begingroup$ @RobJeffries - Did you read your own answer? The improvement from repeated measurements on Earth which as you described give a 'effectively' longer baseline, are replaced in Neptune with an actually longer baseline. After 3 years you will have mapped out the same baseline as Earth. After 5 years you will have mapped out a baseline longer then the improvements possible from repeated measurements, after 168 years you will have mapped the whole sky with a baseline 30 times longer then possible from Earth, and 20 times longer then the 'effective' baseline possible with repeated measurements. $\endgroup$ – Paul Smith Nov 28 at 12:32
  • $\begingroup$ And you can see from my answer that you would get the same precision as the 5 yr Gaia mission in about 9 years at Neptune, not 30 years as implied in your answer. You misunderstand how averaging many position measurements increases Parallax precision. Indeed, (diminishing) improvements will continue as Gaia extends its mission. There would be some maximum precision reached asymptotically, but we aren't at, and won't get to, that point. $\endgroup$ – Rob Jeffries Nov 28 at 12:57
  • $\begingroup$ @RobJeffries - I never mentioned 30 years. I don't know who you are arguing with, but I don't think it is me. $\endgroup$ – Paul Smith Nov 28 at 16:46
  • $\begingroup$ Then what do you mean by "you will have to wait six times longer"? The original Gaia mission is 5 years. $\endgroup$ – Rob Jeffries Nov 28 at 17:03

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