I want to pose a question from someone who's astronomy knowledge would fit neatly onto a single piece of paper so please bear this in mind with any comments or answers. I am interested and want to learn. I could not find a sane answer to this via a search engine.

The question: If in theory we placed another Hubble telescope (or similar instrument of at least equal or ideally more power) either on or very near the dwarf Pluto, as in in its orbit if we did not consider a ground mounted instrument, what if anything would or could we gain in astronomical knowledge ?

I do realise data transmission from such a distance would take time, power could be a concern and the fact if anything went wrong it would be beyond any sensible help, not to mention logistics in getting it there in the first place.

I also realise this is a completely hypothetical question too but I am genuinely curious if we would "see more" (so to speak) with such an instrument so far out.

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    $\begingroup$ I think you could get some very precise parallax measurements, if you're prepared to wait 125 years! $\endgroup$ – Ben Hillier Feb 10 '20 at 13:15
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    $\begingroup$ I assume you're familiar w/ the planned James Web Telescope? It proposes to do this very thing - sorta. jwst.nasa.gov - while not going beyond Pluto, it will be a 'deep' space telescope, relative to Hubble - beyond the Moon at L2 $\endgroup$ – NKCampbell Feb 10 '20 at 16:13
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    $\begingroup$ I did consider saying James Webb instead of Hubble but I also (rightly or wrongly) felt the basic question was slightly better posed with an instrument that was already well proven etc. $\endgroup$ – AndyF Feb 10 '20 at 17:46
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    $\begingroup$ Well, it would be easier to see Pluto... $\endgroup$ – RBarryYoung Feb 10 '20 at 18:09
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    $\begingroup$ @BenHillier you could get some very precise parallax measurements while only waiting a few hours, if you measured the parallax between that telescope and the Hubble (or another Earth-based scope) $\endgroup$ – Skyler Feb 10 '20 at 19:12

The disadvantages would likely outweight the advantages.

  1. It's cold out there. This makes it easier to keep an infra-red telescope cool

  2. The sun's just a super-bright star. This means more of the sky is visible and not in the glare of the sun. However you orbit so slowly that there will be a few objects that you won't be able to image because they are behind the sun for years.

  3. There's no Earth to get in the way. The Earth blocks half the sky for Hubble. Putting a telescope on Pluto would have the same problem.

  4. There's no space debris there (yet)

There may be other advantages, but compared to the immense difficulty of putting a probe into an 250 year orbit (you need to get it out there, with enough fuel left to put it into orbit) And the time it takes (you can't take much advantage of slingshots because you need to be in an orbit not just on an escape trajectory). And all the other disadvantages

  1. Saw something interesting, can't just get hubble to take a look, because there is a five hour light time.

  2. No power, sun is too weak so depend on a box of plutonium or similar.

  3. No nice high resolution images, sorry those take too long to download. Low resolution only.

  4. We've never landed anything as complex and fragile as Hubble on any other body, And the landing has to be fully automatic.

And you don't get to see anything new because space is just as clear from Hubble's orbit as it is from Pluto.

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    $\begingroup$ It's just because you'd have 4 impatient researchers annoyed that they couldn't use the telescope because you were downloading. Remember that New Horizons took about 8 months to download its data. Not a problem when it has nothing better to do, but the telescope would have a queue of people waiting to use it. $\endgroup$ – James K Feb 9 '20 at 20:19
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    $\begingroup$ @BMFForMonica For example, following the detection of gravitational waves from a neutron star merger, many telescopes were used to measure the optical signature of the event. This was time critical as the sooner you can act the better data. It would be frustrating if it took 5 hours to send the message "Stop what you're doing and look at this." Data could be lost in that 5 hour window. $\endgroup$ – James K Feb 9 '20 at 20:23
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    $\begingroup$ Nothing happens quickly with the Hubble except emergency safing due to hardware/software glitches. Quick target-or-opportunity observations can be commanded if they fall within previously established parameters for execution, but this requires a number of checks, qualifications and so on. Light travel time would would not be a large factor compared to the time required for a response anyway. $\endgroup$ – Tod R. Lauer Feb 10 '20 at 0:39
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    $\begingroup$ @SolomonSlow the resolution can be low in pixels but simultaneously very high in angle. Getting outside the atmosphere gives you angular resolution. If you need to image a compact object rather than do a deep field survey, then 10,000 pixels at 0.05 arcsec per pixel may be enough. $\endgroup$ – uhoh Feb 10 '20 at 1:55
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    $\begingroup$ If you have the budget to put a telescope mirror into Pluto orbit, then you'll also be able to do solar power there. Just replace the sensor with a solar array and point the thing at the sun. $\endgroup$ – leftaroundabout Feb 10 '20 at 14:48

There is one excellent reason for putting a space telescope way out from the sun and that's because it's dark out there! No matter where you look, there's always a background sky level that limits the ultimate sensitivity of a telescope. Now even in the near-earth space environment the sky is markedly darker than it is for even the darkest ground-based sites, and this after Hubble's sharp angular resolution contributes heavily to its sensitivity to distant the faintest stars, galaxies in the early Universe, etc.

The Earth, however, orbits in the plane of the solar system, and the Sun lights up the residual dust that it contains. The sky in the outer solar system is nearly 100x darker, and that maps directly into greater sensitivity. For faint compact sources limited by the brightness of the sky, the sensitivity of telescope goes like D^4, where D is the diameter of its mirror. Hubble at the edge of the solar system could detect objects as well as a 7-meter telescope orbiting Earth.

As noted, however, by a previous answer, however, deployment, operations, telemetry of a telescope way out in say the Kuiper Belt has its own issues and expenses. Build a bigger telescope near Earth or send a smaller one way out? The bigger telescope will still clean up on brighter objects, spectroscopy, and so on, in the end being likely more versatile. But the question has been asked in real studies, and if you can crack the operation/telemetry issues, then it might deserve a deeper look.

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    $\begingroup$ I'm sure this is correct, but where does "nearly 100x darker" come from? Can you add a link or cite a source? Thanks! $\endgroup$ – uhoh Feb 10 '20 at 1:43
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    $\begingroup$ @uhoh Tod raises the concern of light pollution from interplanetary dust being lit up by the Sun (is that Zodiacal Light?). Assuming a constant dust density (maybe a big assumption..), you'd expect the light pollution intensity to just go with the Sun's brightness. Since Pluto is 40 times further out than Earth, that should mean it's 1600 times darker. Or am I missing something? $\endgroup$ – Oscar Bravo Feb 10 '20 at 13:57
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    $\begingroup$ @OscarBravo good question for the author. If "outer solar system" is Saturn rather than Pluto then it's 10 AU so 100x dimmer dust, but that's only if the dust density is constant. I don't know why "the sensitivity of telescope goes like D^4", there may be some assumptions like arbitrary exposure time. One can read about dust models in answers to What is the origin of the dust near the sun? I assume that it's complicated because there may be inflow and outflow and in-plane and out-of-plane fluxes, sounds like an excellent new question to me! $\endgroup$ – uhoh Feb 10 '20 at 14:03
  • $\begingroup$ At one point when composing the question I actually gave a moments thought to (purely in theory due to debris I suspect) to having said instrument "attached" to Halley's Comet. I should say only a moments thought. But I did feel an erm rounder body such as Pluto would be a more saner choice if it was "surface ground mounted" . I thought a bit too about some of the other distant bodies (such as some of the outer planets moons) but plumped for Pluto given its distance. It seemed the best way to pose a sensible question. $\endgroup$ – AndyF Feb 10 '20 at 17:52
  • $\begingroup$ Getting up off the ecliptic would be helpful even nearer in. Should cut down on that nasty zodiacal light. The dust that creates that straddles the ecliptic: en.wikipedia.org/wiki/Interplanetary_dust_cloud $\endgroup$ – Wayfaring Stranger Feb 11 '20 at 16:50

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