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

Question

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.

Answer 1

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.

Answer 2

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.

Lastly, it's much harder/more costly to send the same spacecraft to Neptune rather than the L2 point.

Answer 3

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.

Answer 4

One could certainly send a Gaia-like spacecraft into deep space, and take parallax measures at all times along its orbit. This is unattractive for several reasons, however. In short the large baseline may get you only a factor of 10 accuracy at the cost of several expensive modifications. The money would be better spent making a more powerful telescope for use in the Earth environment. Some issues:

1. Deep space missions require an RTG power supply, careful thermal control, and so on. Solar power is much simpler and the thermal control simpler.

2. Telemetry, data transfer, commanding, etc. become much, much more difficult with increasing distances, requiring large dish antennae, powerful transmitters, complex downlink schedules, and so on. The squared-distance ratio between a even a few AU and local Earth space is immense.

3. Injection of the spacecraft into is trajectory is considerably more expensive.

4. The angular/temporal profile of when what parallax accuracy is available is extremely anisotropic. Stars along the trajectory will still show small parallaxes. Good parallaxes to stars perpendicular to the trajectory will only be available after several years.

Answer 5

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.