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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.

AndyF
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    I think you could get some very precise parallax measurements, if you're prepared to wait 125 years! – Ben Hillier Feb 10 '20 at 13:15
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    I assume you're familiar w/ the planned James Web Telescope? It proposes to do this very thing - sorta. https://www.jwst.nasa.gov/ - while not going beyond Pluto, it will be a 'deep' space telescope, relative to Hubble - beyond the Moon at L2 – NKCampbell Feb 10 '20 at 16:13
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    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. – AndyF Feb 10 '20 at 17:46
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    Well, it would be easier to see Pluto... – RBarryYoung Feb 10 '20 at 18:09
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    @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) – Skyler Feb 10 '20 at 19:12
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    The Hubble is incapable of resolving our previous activity on the moon. It already can't see anything of interest that doesn't take a long exposure to acquire. – Mazura Feb 11 '20 at 01:08
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    @Skyler If we launch one out there, we could launch two 180° apart, and get double the parallax compared to the Hubble/probe pair. The results might well be interesting. – Wayfaring Stranger Feb 11 '20 at 16:45
  • Tis an interesting twist as the cost would I assume ( ? ) not actually double, as I understand it a high percentage of monies spent are to "get stuff there" rather than the hardware (even though a fortune) itself... – AndyF Feb 11 '20 at 18:55
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    @AndyF you'd basically need two rockets though, since the second one would be launching in the opposite direction (which in practice means it would need to launch six months later). – Skyler Feb 12 '20 at 15:03
  • @Skyler that is completely true. I think I overlooked "the blindingly obvious" with my previous comment sorry. :) – AndyF Feb 12 '20 at 15:20

2 Answers2

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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.

James K
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    "Saw something interesting, can't just get hubble to take a look, because there is a five hour light time." What does this mean? If a telescope sees something interesting out at Pluto and sends a signal back to Earth, the delay will still be the time difference between us seeing what the telescope saw and us analyzing and responding to it. – BMF Feb 09 '20 at 16:42
  • @James K Thanks for explaining in a way I can easily comprehend. This is why I put in the tiny bit of text about my limited knowledge. The only part that does not seem too clear is one of low resolution images, granted higher res = larger = longer download times. If say a 100mb data chunk took a day then I do not see a problem in waiting four days for a higher quality 400mb chunk . (Figures just random for purely example purposes only, probably wildly inaccurate) Unless the power factor to process, build and ultimately transmit said data from the source is a concern , that I can see. – AndyF Feb 09 '20 at 19:07
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    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. – James K Feb 09 '20 at 20:19
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    @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. – James K Feb 09 '20 at 20:23
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    Re, "No nice high resolution images,...Low resolution only." Which, of course, completely defeats the purpose of putting an optical telescope in space. You want low-res images? You can get those for way less money with a ground-based instrument. – Solomon Slow Feb 09 '20 at 22:16
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    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. – Tod R. Lauer Feb 10 '20 at 00:39
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    While no panacea, optical communications will help at least with bandwidth by a factor of ~1000. See answers to How is long-distance optical communications coming along in space? as well as Quantitatively, why will optical communication be better than X-band for deep-space communications? and also Are there plans or a program for an optical relay pathfinder for deep space? – uhoh Feb 10 '20 at 01:52
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    @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. – uhoh Feb 10 '20 at 01:55
  • @uhoh Optical communications on that distance is both quite experimental and seems to take a massive amount of energy. How is a probe at that distance (so no solar-power) going to stay operational with such a power-drain? – Mast Feb 10 '20 at 10:04
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    @Mast answers to the first (and third) question suggest that thought it's not here yet, deep space optical is coming along quite well. In a decade or two (when such a telescope might be launched) it could be ready. This answer to the second question is based on a 4 Watt laser. Right now the Voyagers are using 10 to 20 Watts even with their "ancient" RTGs. I think it's all quite reasonable, but if you can make a quantitative argument to the contrary please let me know! – uhoh Feb 10 '20 at 10:26
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    @Mast for more about the Voyagers' transmit power see answers to How much power does the Voyager radio dish draw? and also Are the Voyager probes radio output power scalable? – uhoh Feb 10 '20 at 10:48
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    @uhoh Hmmm, I don't know what the steering, camera, sensors and other systems would take in total, but that seems doable (considering modern RTG) indeed. – Mast Feb 10 '20 at 10:52
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    @uhoh, Interesting point. So,... Won't be using the instrument to do any "deep field" surveys, but looking at particular points of interest... O.K., I can buy that. – Solomon Slow Feb 10 '20 at 13:07
  • @SolomonSlow ya it's a stretch I know... but I try to always look on the bright side of life – uhoh Feb 10 '20 at 13:10
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    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. – leftaroundabout Feb 10 '20 at 14:48
  • @leftaroundabout at first I wanted to say that a 2.4 meter ("another Hubble telescope") diameter solar panel would collect just as much light as a smaller one at the focus of a 2.4 meter telescope, so as long as you can have a panel the same size as the payload there's no benefit. But out at Pluto the Sun is so dim that most PV cells might be really inefficient. I think at very low light levels their efficiency drops dramatically, though low temperature can help. So in that sense a concentrator could definitely boost efficiency back to normal. – uhoh Feb 11 '20 at 00:38
  • @leftaroundabout See What spacecraft relied on solar electrical power farthest from the Sun? (roughly 70 square meters!) and also concentrating starlight with mirrors for photovoltaics in deep space – uhoh Feb 11 '20 at 00:40
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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.

Tod R. Lauer
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    I'm sure this is correct, but where does "nearly 100x darker" come from? Can you add a link or cite a source? Thanks! – uhoh Feb 10 '20 at 01:43
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    @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? – Oscar Bravo Feb 10 '20 at 13:57
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    @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! – uhoh Feb 10 '20 at 14:03
  • 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. – AndyF Feb 10 '20 at 17:52
  • 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: https://en.wikipedia.org/wiki/Interplanetary_dust_cloud – Wayfaring Stranger Feb 11 '20 at 16:50