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This answer to the question why we didn't send Gaia to Neptune's orbit raises the question of why we don't send a Gaia-like mission to the orbit of Mars. It seems like it doesn't have the problems mentioned in the answers, and it would make it possible to measure parallax for stars 50% further.

  • orbital speed: waiting 6 extra months to get better precision seems like a better trade off than waiting 9 years to get the same precision as Gaia;
  • partial coverage: after two years, the entire sky could be covered. Far from the 168 years for Neptune;
  • telemetry: sending back data from a Gaia-like mission can't be that much harder than sending back data from any other of the tens of missions to Mars;
  • power: the solar panels don't need to be 900 times bigger;
  • radiative environment: contrary to Jupiter, Mars doesn't have a magnetosphere, so the spacecraft wouldn't be disturbed by it.
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    $\begingroup$ I'd guess its just a cost-benefit trade-off. You'd need a bigger launcher adding to costs, and to run the mission a couple of times longer, also adding to costs. You could maybe do more valuable science by spending that money of a different mission. $\endgroup$
    – Steve Linton
    Nov 25, 2019 at 16:35
  • $\begingroup$ Telemetry is actually one of the key issues, as pointed out in the thread you've linked. Gaia sends huge loads of data back to Earth, and the downlink would have to be reduced to the minimum possible, corresponding to the maximum Earth-Mars distance, otherwise you'd have regions of the sky with heavily biased amounts of sampling. $\endgroup$
    – AtmosphericPrisonEscape
    Nov 25, 2019 at 17:57
  • $\begingroup$ Venus would be even closer to the sun, wouldn't it? This matters because the telemetry bandwidth is probably power-limited. The signal-to-noise ratio improves if you can produce more signal. $\endgroup$
    – MSalters
    Nov 26, 2019 at 12:44
  • $\begingroup$ @MSalters But what would be the point of sending a Gaia-like probe towards the orbit of Venus? $\endgroup$
    – usernumber
    Nov 26, 2019 at 12:49
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    $\begingroup$ @usernumber: An independent set of measurements? Whether that's useful probably depends on the relative sizes of the various measurement errors. $\endgroup$
    – MSalters
    Nov 26, 2019 at 12:54

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The panels need to be about 2.3 times bigger to generate the same power.

The telemetry requirements are limited by distance to some power, assuming that the communications beam cannot be infinitely narrow. Mars is much further away than the L2 point - somewhere between a factor of 50 and 250.

It wouldn't necessarily make it possible to measure parallaxes 50% further. Gaia works by taking approximately 70 (on average) measurements of a stellar position over 5-6 years. This amounts to sampling the 2 au baseline about 10 times, thus reduing the error bar on a single baseline parallax by about $\sqrt{10}$ and making the "equivalent baseline" about 7 au.

To get similar precision with Mars you would need to be able to complete the 3 au baseline about 5 times, which would still take 5 years. So you don't obviously win in precision terms.

Overall I think it is the telemetry problem that is the real killer. Gaia is currently telemetry limited - that is why there is a limit on the number of stars it will measure and on the number of stars it will get spectroscopy for.

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bigger problem - if they take the L2 point of Mars .. that's between Mars and Jupiter partially in the asteroid belt ... not the best place if you want to make precise measurements and have to be constantly on-guard / circumventing stones of which most of the very small ones aren't even known, but can still easily destroy your billion $ satellite or break it just enough to be a costly wreckage. All just to keep your satellite "alive". Smaller problems like much increased delay (up to 24 minutes signal run-time) thus lower data-rate and of course bigger solar panels - which increase the threat by those stones again...

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    $\begingroup$ Mars L2 isn't in the asteroid belt, and no spacecraft we've ever sent to the asteroid belt has ever made any attempt to avoid unknown asteroids. The asteroid belt is far too sparse to need to worry about that sort of thing. $\endgroup$
    – notovny
    Nov 26, 2019 at 21:55
  • $\begingroup$ Indeed the L2 point is only about 1 million km from Mars. The time delay is of no consequence. $\endgroup$
    – ProfRob
    Feb 16, 2020 at 19:39

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