Why no telescopes on the moon?

[tom aaron]

So many of these issues lose their viability in the real world of limited budgets and resources.

Yes, it might be nice to have a more accurate wrist watch...but not at the cost of 100 thousand dollars. I'd rather keep my car, computer, guitar, etc.

Want a Moon based telescope...forget Investment in anything else in the next 75 years. No space probes, budgets for other telescopes, research into 'anything'. We don't need abig project monopolizing the resources of NASA and the space industry.

Budgets and planning are maxed out for the next 20 years. Want to learn more about Neptune or it's moon, Triton? There are no NASA missions planned to go there in the next 25 years. Zip. Zero. Announce one today and add on another 5 to 10 years. Start planning a boondoggle like a Moon based telescope and no new planetary missions this century. This state of affairs would be repeated for every other part of NASA...a big sponge soaking up the budget and taking away from even the limited dreams of space nuts.

 

[Jon Richfield]

No doubt we could find other uses for He-3 if fusion proved unpractical (say, floating balloons in Deuterium or He-4, or for sinkers in protium).
However, given that the moon is such a doubtful source and so expensive to lift from, current prospects for mining He-3 from lunar dust and rock, while evidence for pay dirt still is so speculative and tenuous, would hardly justify a prospecting party, let alone a gold rush.
As I think I have implied, the logical place to collect He-3 is where the He-3 is, in fact where it occurs in effectively unlimited supply and where it is easy to collect and retrieve, using technology that, if not actually off the shelf, already is at least reasonably understood and practical. And where the more questionable technological aspects justify development in the light of the prospects. Which at our present state of play is a lot more than can be said for handwaving about lunar accelerators and the like.

 

[Quds Akbar]

Placing telescopes on the moon would be more expensive and would make no difference, it would only be harder and it would be much harder to repair, instead of just sending an astronaut in orbit to fix it on a routine mission, you would have to build a Saturn V every time it needs repairs which would be harder and more expensive also considering that a Saturn V is not reusable.

 

[OmCheeto]

I think Marcus convinced me that there's no reason for humans to be involved:

I favor robotic space exploration over human...

Just send a few MacGyver-bots, and tell them to build one.

The moons composition looks ideal for manufacturing.

per wiki: Moon

Compound         Formula    Composition (wt %)
                            Maria    Highlands
silica            SiO2      45.4%    45.5%
alumina          Al2O3      14.9%    24.0%
lime              CaO       11.8%    15.9%
iron(II) oxide    FeO       14.1%     5.9%
magnesia          MgO        9.2%     7.5%
titanium dioxide  TiO2       3.9%     0.6%
sodium oxide      Na2O       0.6%     0.6%
Total                       99.9%    100.0%

First, you need a suitable location for solar power:

From images taken by Clementine in 1994, it appears that four mountainous regions on the rim of Peary Crater at the Moon's north pole may remain illuminated for the entire lunar day, creating peaks of eternal light.

I would first have the bots turn the silicon into string ribbon solar arrays, for additional manufacturing power.
Next, I would have the bots create an aluminum structure for the telescope.
And then they could make the mirrors.

Of course, I have no idea of how to do any of the above. But if I can imagine it, then it is probably possible.

 

[HallsofIvy]

Let me add another point- the moon keeps one face pretty much toward the earth. If we put a telescope on the moon on this side, most of the "visible sky" would be the earth. If we put it on the other side, we would have major problems with communicating with it, getting its images and controlling it.

 

[mfb]

Let's take the price of Constellation - even though it doesn't exist. Let's assume that instead of the 3 lunar missions, you get 10 (maybe by dropping some ISS missions). Let's assume you can bring back, I dunno, 3x what Apollo did per mission. (Already suspect because 3He takes up more space per kilo than rocks, especially with cryogenics) You still miss breakeven by a factor of 75. You want to argue that maybe SpaceX will do better someday, fine. But we're still talking about the future.

Again, taking the costs of a science mission does not give a reasonable estimate for costs of commercial missions with a completely different aim.

SpaceX signed contracts for those prices, and launched stuff for twice this price already. They plan to launch the first Falcon Heavy this year. This is a very near future.

The other issue is that the industrial use today of Helium-3 is tens of millions of dollars - call it a round $100M. I think we can all agree that you can't do a moonshot for that. You can barely get to LEO for that. So any mission is going to bring back years or decades worth of 3He. One might argue, yes, but when we use the stuff for fusion, we'll need more, but I would counter that we don't have a fusion reactor yet. Again, we're talking about future technologies that don't exist.

That is a more problematic issue.

80 ppb (what are ppb/m^2?) doesn't sound good. Tens of tons per cubic kilometer. How do we process regolith in the multi-megaton range?

 

[Jon Richfield]

Let me add another point- the moon keeps one face pretty much toward the earth. If we put a telescope on the moon on this side, most of the "visible sky" would be the earth. If we put it on the other side, we would have major problems with communicating with it, getting its images and controlling it.

Although I reckon the idea of a telescope on the moon is ridiculously unpractical compared to any reasonable space telescope, let's not be unfair; there are more practical options than parking the observatory in the Sinus Medii, or even farside dead centre. Four of the four lunar poles, east, west, north and south all would have merit without full skies and would be easy to contact directly.
However, there are other options. No matter where you park the observatory, it would be possible to contact it via relay satellites at a trivial cost compared to the costs of an observatory on Luna. Also, if you park it a little out of sight of Earth behind a limb, then a few relay towers or even a landline could permit direct communication with Earth.
Then all you would need is your head read for bothering to do that instead of establishing a fleet of Hubbles in space in various carefully chosen orbits. You could do a couple of dozen for the cost of a single industrial strength lunar observatory of power equal to anyone of them, and in a small fraction of the time and with greater redundancy in the face of occasional disaster. Not to mention far, far greater versatility. Unlike the lunar boondoggle, you could show a profit as well.

 

[Jon Richfield]

Let me add another point- the moon keeps one face pretty much toward the earth. If we put a telescope on the moon on this side, most of the "visible sky" would be the earth. If we put it on the other side, we would have major problems with communicating with it, getting its images and controlling it.

Oh, sorry, and I forgot to add; I agree that having Earth in the sky would be a nuisance, and noise from Earth probably would be an even greater nuisance for some programmes, but "most of the visible sky" is a bit of an overstatement. If at a first approximation we assume a circle of 12000 km diameter at a range of 400000 km, that should subtend an angle of less than 2 degrees; let's call it 5 degrees to allow for glow and similar interference.
If only that could be our biggest worry; moondust would be a more serious problem!

 

[Chronos]

All objections appear centered on the impracticality of creating a permanent moon base. While I agree that is a daunting task, we humans dare to envision things beyond beyond our current technological grasp. With the proper knowledge. I consider all these outlandish possibilities viable options. I doubt cave men deemed coal a viable energy source 100,000 years ago, and I feel safe in predicting fossil fuels will be considered a hopelessly naive energy source in another 100,000 years. I suggest we not allow our current level of naivety to cloud our vision of the future.

 

[Jon Richfield]

All objections appear centered on the impracticality of creating a permanent moon base. While I agree that is a daunting task, we humans dare to envision things beyond beyond our current technological grasp. With the proper knowledge. I consider all these outlandish possibilities viable options. I doubt cave men deemed coal a viable energy source 100,000 years ago, and I feel safe in predicting fossil fuels will be considered a hopelessly naive energy source in another 100,000 years. I suggest we not allow our current level of naivety to cloud our vision of the future.

I am not sure where you got the impression that all objections appeared to be centered on the impracticality of creating a permanent moon base; as you rightly suggest, long-term projections from contemporary technological and economic situations are hazardous at best, so I for one am chary of expressions such as "not in a million years", let alone "never".
However IMO, speaking as a life-long space nut, I am persuaded that a permanent moon base would be unrewarding in the foreseeable future, say a century or so, and as things stand at the moment I cannot see why a non-trivial manned moon base or Mars base ever should be a paying proposition, as opposed to a Venus or Mercury base, or possibly some asteroid or dwarf-planet bases. Go where the pay-dirt is, say I, and no one has yet explained why the moon or Mars should pay.
No, the problem is not whether the moon base will never, nor even at least for a long time, be a viable prospect, but that so far it not only is not viable, but shows no foreseeable promise of being viable. (3He forsooth! Why not osmiridium while we are at it? And as for incidental observatories...)
Shackling possibly viable prospects such as space telescopes and developments in space engineering technology to such a deadweight is the kiss of death. At the moment we need space telescopes and at the moment we certainly don't need moon bases or Mars bases. And if we insist on squandering our resources on what we don't need now we might never have what we do need and certainly never will have it in our time.
Whereas we certainly could have the useful and urgent things in our time if we scheduled our priorities to match our resources. And might have enough left over for what at present would be luxuries at best.
"With the proper knowledge"? Do tell. Has someone vouchsafed the proper knowledge of which projects would be rewarding, either materially or emotionally, on the basis of our not knowing how to achieve them at present? Nor why to try to achieve them? How about our first Alpha Centauri visit? You are not about to claim that it couldn't succeed, I hope? Or deny that it might prove far more more rewarding than a moon colony? Thar's gold in those thar alien planets ah tell yer! Gold!
Suppose we did in fact commit our entire combined space effort to establishing a moon colony immediately, until such time as we succeeded in sending men up there for stints ten times as long as on the ISS (the gravity after all is more convenient than in space). And suppose we succeeded after say half a century; now what? Twiddle thumbs? Go out and gather moon dust for a telescope mirror (first baking it to collect the 3He of course)? Or go and explore the regolith for iron and copper to build a catapult launcher?
Did I hear anyone muttering about putting carts before horses? Shame on him!
I too suggest we not allow our current level of naïveté to cloud our vision of the future, and if anyone can think of a more pernicious naïveté than beating our ploughshares into bling ornaments for our kiddie cars, in the hope that the bling will make them go faster, please don't bother to tell me.

 

[tom aaron]

All objections appear centered on the impracticality of creating a permanent moon base. While I agree that is a daunting task, we humans dare to envision things beyond beyond our current technological grasp. With the proper knowledge. I consider all these outlandish possibilities viable options. I doubt cave men deemed coal a viable energy source 100,000 years ago, and I feel safe in predicting fossil fuels will be considered a hopelessly naive energy source in another 100,000 years. I suggest we not allow our current level of naivety to cloud our vision of the future.

Meanwhile it is 2015 and we live in a world of limited resources and choices. It was 1.6 billion for another Shuttle launch instead of space probe to a moon of Uranu. Difficult to hop off the conveyor belt when white elephants get rolling. All of NASA's budget for the next 25 Years 'developing' technology (not even being there) for a Moon base or doing other things?

Sure, we will have a Moon base, Mars base. Just not in the next few decades. Maybe next century. The head of NASA has stated there will not be a man on the Moon again in his lifetime.

As for 'energy'. The technology of putting humans in space, on the Moon, etc. is a lot more than addressing 'energy'. It is extremely precise multiple systems of technology with an infrastructure need to both develop and then support them.

The public has a misconception that if NASA declared a Moon project today that there is some warehouse that would be unlocked, staff all waiting for the green light, etc. It doesn't exist. There is no infrastructure. No engineers standing around doing nothing. A President can no longer just wave his hand like during theManhattan Project or the Apollo years and command the resources of the nation.

 

[mfb]

I am not sure where you got the impression that all objections appeared to be centered on the impracticality of creating a permanent moon base; as you rightly suggest, long-term projections from contemporary technological and economic situations are hazardous at best, so I for one am chary of expressions such as "not in a million years", let alone "never".
However IMO, speaking as a life-long space nut, I am persuaded that a permanent moon base would be unrewarding in the foreseeable future, say a century or so

I doubt a century is a foreseeable future. It hasn't been in 1915 and there is no reason to assume predictions get better with an ever increasing speed of technological and scientific progress. Our rockets in 2100 could be designed and built by AIs.

A rocket will never be able to leave the Earth's atmosphere.

 

[Jon Richfield]

I doubt a century is a foreseeable future. It hasn't been in 1915 and there is no reason to assume predictions get better with an ever increasing speed of technological and scientific progress. Our rockets in 2100 could be designed and built by AIs.

If it comes to that, even a day isn't very foreseeable, or a nanosecond in some connections. Humanity might be only decades from being wiped out by a wandering 100 km rock that got overlooked because we were killing each other and concentrating on vanity projects instead of spending less on rational space projects than on sports corruption or bankers' golden handshakes or smoking.
Pessimistic? Meeeeee?
You got to be joking! Shame on you!
But of course, if AIs are designing (what were those old-fashioned things again? Oh yes! Rockets!) rockets, that will be a game changer, won't it? No more cost to space programmes. After all, by that time AIs would be doing the mining for the necessary resources too, right?
Yeah. Silly of me to overlook that.

 

[tom aaron]

I doubt a century is a foreseeable future. It hasn't been in 1915 and there is no reason to assume predictions get better with an ever increasing speed of technological and scientific progress. Our rockets in 2100 could be designed and built by AIs.

I doubt a century is a foreseeable future. It hasn't been in 1915 and there is no reason to assume predictions get better with an ever increasing speed of technological and scientific progress. Our rockets in 2100 could be designed and built by AIs.

Perhaps...and AIs can go to the Moon. No need for humans.

More seriously...the whole AI thing is a big question mark...a black box. A potential game changer. I'm also a nutcase that thinks accelerating technology may put us is some type of communication with one of what I think are quadrillions of other intelligences in the Universe.

The history of the Man may a division before and after AI...or before or after joining some 'Club Universe' and suddenly having technology of billion year old aliens. All speculation.

Meanwhile in 2015 we're crossing our fingers on the success of a few planetary proves, a working JWST, and glitches removed when downloading a movie from Netflix. Reality is neat but it also sucks.

 

[Jon Richfield]

Perhaps...and AIs can go to the Moon. No need for humans.

More seriously...the whole AI thing is a big question mark...a black box. A potential game changer. I'm also a nutcase that thinks accelerating technology may put us is some type of communication with one of what I think are quadrillions of other intelligences in the Universe.

The history of the Man may a division before and after AI...or before or after joining some 'Club Universe' and suddenly having technology of billion year old aliens. All speculation.

Meanwhile in 2015 we're crossing our fingers on the success of a few planetary proves, a working JWST, and glitches removed when downloading a movie from Netflix. Reality is neat but it also sucks.

I generally agree with that. Unless we stuff things up completely and destroy our basis for technological development, I see Homo sap as just a passing phase in the development of intelligent communities. All it takes to set it off is the radical development of teleological evolution by technological means. Who knows, we might be able to engineer people with no recurrent laryngeal nerve and who can calculate pi, or even e to the i pi, or their own tax returns, without a calculator... The list of exciting possibilities extends beyond most of our imaginations.

 

[tom aaron]

I generally agree with that. Unless we stuff things up completely and destroy our basis for technological development, I see Homo sap as just a passing phase in the development of intelligent communities. All it takes to set it off is the radical development of teleological evolution by technological means. Who knows, we might be able to engineer people with no recurrent laryngeal nerve and who can calculate pi, or even e to the i pi, or their own tax returns, without a calculator... The list of exciting possibilities extends beyond most of our imaginations.

True.

This may be the eve of something big. The issue 'today' is that we just don't know. We make predictions based on what we know today yet at the same time understand that some game changer may happen next decade or next century.

Are we one of the last generations for which death is a certainty? The last generation to not know about some alien intelligences? The last generation not to be aware of some Theory that explains existence?

I think about my father who passed away in the early 1990's. He was very intelligent but never heard the words Internet, Yahoo, Cell phone, Text etc. One has to wonder what technologies are going to take off in the next couple of decades. Virtual something or another...Quantum computing. Things that may be game changers in understanding Cosmology and practical in use in Astronomy.

 

[Jon Richfield]

True.

This may be the eve of something big. The issue 'today' is that we just don't know. We make predictions based on what we know today yet at the same time understand that some game changer may happen next decade or next century.

Are we one of the last generations for which death is a certainty? The last generation to not know about some alien intelligences? The last generation not to be aware of some Theory that explains existence?

I think about my father who passed away in the early 1990's. He was very intelligent but never heard the words Internet, Yahoo, Cell phone, Text etc. One has to wonder what technologies are going to take off in the next couple of decades. Virtual something or another...Quantum computing. Things that may be game changers in understanding Cosmology and practical in use in Astronomy.

There are depths in what you say, Tom. For a long time I have agreed with aspects of John Wyndham's theme in "Trouble with Lichen"; we live lives in which we spend typically say 10% of our time learning and 30% pruning and applying what we have learned and refusing to modify the errors we have learned and 30% decaying (pick alternative thumb suck figures according to personal taste). Not many decades ago I would have said that we need larger, more efficient and more flexible brains and an indefinite lifespan. But I thought in terms of traditional Darwinian and Mendelian biology to attain anything of the type.
Nowadays I would be inclined to think rather in technological directions for achieving something of the kind, and less emphasis on the individual and more in terms of the group. Think of a whole community that could share at least some mental pathways directly. Something like termite colonies, but with the members of the community sharing mental facilities and consciousness rather than blind instincts.
Would politicians love it or hate it? I leave the scenarios open to you!
Good old swarming satellites say I, and down with moon mining! (To show that I am keeping on topic of course. )

 

[selvaarchi]

There is a telescope on the moon. China's lander Chang'e 3 has a 150 mm (5.9 in) Ritchey–Chrétien telescope that will be used to observe galaxies, active galactic nuclei, variable stars, binaries, novae, quasars and blazars in the near-UV band (245-340 nm), and is capable of detecting objects at a brightness as low as magnitude 13. The thin atmosphere and slow rotation of the Moon allow extremely long, uninterrupted observations of a target. The LUT will be the first long term lunar-based astronomical observatory, making continuous observations of important celestial bodies to study their light variation and better improve our current models.(information from Wikipedia)

 

[Jon Richfield]

That is very nice (no irony -- really!) They were doing an exploratory landing and getting their hands into establish a national capability, and why not supply a functional payload while they were at it? Cheaper and more useful than sending humans, for one thing.
I applaud. Genuinely.
But really, irrespective of how functional their 150mm shaving mirror plus comms might be, for the same money they could have flown a very functionally large telescope into LEO or a still respectable observatory into lunar orbit. And such a telescope would have had far more flexibility and longer life than the moon-based jobbie, and a higher probability of success too.
Let us not lose perspective. Putting workstations down on the moon commits us to far smaller and more limited functions than putting them into practical orbits.

 

[rootone]

A telescope on one of the moons of Pluto might be a good idea though.

 

[wabbit]

My favourite is still this one : http://science.nasa.gov/science-news/science-at-nasa/2008/09oct_liquidmirror/

A cryogenic liquid-mirror telescope on the moon to study the early universe

 

[Jon Richfield]

A telescope on one of the moons of Pluto might be a good idea though.

That certainly is true, but why on the moon? Why not in orbit?

Landers on the moons also certainly, but leave the telescopes in the orbits where they can do the most good for the cost.

IMO we should already have suitably purpose-designed observatories around all the major bodies in the solar system and a network of comms relay satellites to serve them.

Our current stone-axe approach not only is a reproach, but may lead to our destruction, and well-deserved too.

 

[Jon Richfield]

My favourite is still this one : http://science.nasa.gov/science-news/science-at-nasa/2008/09oct_liquidmirror/

A cryogenic liquid-mirror telescope on the moon to study the early universe

That undoubtedly is a charming idea, and though I am familiar with the idea of liquid-mirror telescopes, this version certainly is new to me.

But don't ask me to hold my breath for completion of its construction. I still suspect that we could do better with membrane-based telescopes in orbit than trying to create the observatory on the moon.

The least charming aspect of projects like that however, is their all-or-nothing approach: get anything wrong and the whole thing is useless.

Personally I think that we could do far better by exploring the options for telescopes in the L2 regions of various orbiting bodies. All those that I can think of that would be of interest would shelter the observatory from the sun, and observatories that would need sun (perhaps half of them?) could be at L1 points.

Now, I note that the proposed mirror might be as much as 100m diameter. (After all why think small?) It might be a bit challenging to cart up a 100m disk from Earth, so I suppose that the idea is to dig it out of the moon surface and construct the necessary kit in situ. This is not an option in orbit , though we could of course work out a similar concept with film and corrective optics, but I reckon that we could do a lot better by building multi-mirror orbiting telescope arrays with all the tricks of aperture synthesis and possibly adaptive optics etc. (not easy with a rotating fluid mirror!)

And if an entire single telescope died, we still would have a working observatory.

For some purposes we could make 100m look pretty miserly.

And old-fashioned before the moon dish even got made.

And start tomorrow...

 

[selvaarchi]

That is very nice (no irony -- really!) They were doing an exploratory landing and getting their hands into establish a national capability, and why not supply a functional payload while they were at it? Cheaper and more useful than sending humans, for one thing.
I applaud. Genuinely.
But really, irrespective of how functional their 150mm shaving mirror plus comms might be, for the same money they could have flown a very functionally large telescope into LEO or a still respectable observatory into lunar orbit. And such a telescope would have had far more flexibility and longer life than the moon-based jobbie, and a higher probability of success too.
Let us not lose perspective. Putting workstations down on the moon commits us to far smaller and more limited functions than putting them into practical orbits.

What you say is true but in China's case they were going to the moon anyway so this was a bonus. Both China and India already have firm plans to launch space based telescopes separately..

 

[wabbit]

don't ask me to hold my breath for completion of its construction.

Given that it's neither budgeted nor planned as far as I know, this might be wise : )
It's an interesting idea though, with a science case and one for which the moon seems rather uniquely suited. Plus liquid mirrors cost far less than solid ones so the whole thing might even be realistic in terms of cost. I haven't seen more recent publications about this however, so it might indeed remain a nice idea for a long time.

I still suspect that we could do better with membrane-based telescopes in orbit than trying to create the observatory on the moon.

I'm not familiar with these, what are the projects in this area and what kind of specs do they have ?

their all-or-nothing approach: get anything wrong and the whole thing is useless.

How is that different from other space based telescopes or from planetary missions ?

Personally I think that we could do far better by exploring the options for telescopes in the L2 regions of various orbiting bodies. All those that I can think of that would be of interest would shelter the observatory from the sun, and observatories that would need sun (perhaps half of them?) could be at L1 points.

Exploring one option does not preclude others. This one, should it prove workable, may provide a very large mirror more easily than it could be done elsewhere (large in surface area that is - resolution wise, presumably a fleet of spacecraft flying in formation can do a lot better than 100m)

I reckon that we could do a lot better by building multi-mirror orbiting telescope arrays with all the tricks of aperture synthesis

For resolution I completely agree. For collecting area, not so sure.

 

[Jon Richfield]

Given that it's neither budgeted nor planned as far as I know, this might be wise : )
It's an interesting idea though, with a science case and one for which the moon seems rather uniquely suited. Plus liquid mirrors cost far less than solid ones so the whole thing might even be realistic in terms of cost. I haven't seen more recent publications about this however, so it might indeed remain a nice idea for a long time.

Generally agreed. However, in space I suspect that certain classes of telescope could be constructed to be cheaper than liquid.

I'm not familiar with these, what are the projects in this area and what kind of specs do they have ?

None yet. It is my own idea, not yet half-baked. I am still uncertain what the best materials would be, and the best way to shape them. I have ideas on those points, but those are not yet half-baked either. I really mentioned the idea to knock it down mainly, because I have a feeling that to make ANY sort of 100m mirror in space, whether in orbit or on the moon, is a flat invitation to disaster because you will inevitably pick up collisions that would not destroy a crude machine, but would surely mean a short life for a delicate and exposed device like an astronomic mirror. That is why I suggested a modular mirror device instead. More below.

How is that different from other space based telescopes or from planetary missions ?

Not all are different, but large, let alone super-large items like single-mirror telescopes are an extreme example. That is why I dropped the idea of a huge membrane-mirror reflector unbaked. I still remember what happened to Echo,our first successful inflated satellite; it lasted only days or weeks before crumpling. Well, it was just a prototype proof-of-concept device, so that was OK at the time.

Modular devices are not infallible, but they are likely to offer residual function even if say, one mirror survives, or if one telescope in an array gets knocked out, and in fact, could produce useful results from when the first device becomes operational till when the whole array gets retired.

Exploring one option does not preclude others. This one, should it prove workable, may provide a very large mirror more easily than it could be done elsewhere (large in surface area that is - resolution wise, presumably a fleet of spacecraft flying in formation can do a lot better than 100m)

Oh gosh yes! But I am unconvinced that the Lunar giant would be viable at all, though I would not vote against a proof-of-concept toy device of the type; say 1-2 m diameter. After that works properly and satisfactorily, we could explore other toys like 10m jobs. The moon is big enough to support a few I reckon.
Note however that adaptive optic arrays can be made indefinitely large and in space can be pretty light.
In fact, the membrane lens might bake into a good medium for assembling modest-sized components into 100m or even1000m devices. If so, then certain other devices might become practical, such as large-scale 3He collectors, as I hinted earlier in this exchange.
But maybe a membrane lens would prove inferior to a sponge lens. I still am musing about that.
And I still am thinking in terms of modular devices for preference.

For resolution I completely agree. For collecting area, not so sure.

I take your point. In fact I had come to similar thoughts in compiling my previous reply. However, given the ability to assemble anything subtending a sufficient diameter, there is no simple limit to how big an area we could achieve. Even square kilometres, if we let the manufacture and delivery continue long enough; a million1-sq m segments would do it. It would take a long time even with 10-sq m mirrors, but collecting usable data all the time, getting better and better.

 

[mfb]

For that 1 km² project, the secondary mirror should be in the 100 m-range. A few 10 m x 10 m elements won't make a good primary mirror, and you lose even more light at the unfinished secondary mirror.

You probably want to avoid the shadow of Earth due to thermal considerations. Such a thing would experience tidal gravity close to Earth which could make sun-synchronous orbits problematic. In L1 you have the Earth as disturbing bright object, in L2 you have to move the mirror around sometimes to avoid the shadow (Gaia has a limited lifetime due to this). A high Earth orbit could work, or something far away from earth. Certainly a solvable challenge, but scaling things up is not trivial.

 

[Chronos]

Lets not overlook the prospect of erecting a moon based space elevator. That could be a big time game changer.

 

[Jon Richfield]

For that 1 km² project, the secondary mirror should be in the 100 m-range. A few 10 m x 10 m elements won't make a good primary mirror, and you lose even more light at the unfinished secondary mirror.

Thanks, I appreciate these remarks, but would be grateful for a few elaborations (not necessarily detailed, a few hints should be helpful). I accept that a few small elements won't gather as much light as a large mirror, and it might be hellishly hard to control the flock, though it seems to me that they should gather about as much light as the sum of their scattered elements. Also, they should have some advantages over a single mirror with the same area.
However, did you have any other fundamental reason why a few 10 m x 10 m elements won't make a good primary mirror? (I am no expert on astronomic optics!)

You also spoke of "the unfinished secondary mirror"? "secondary mirror" I understand, but "unfinished secondary mirror"? Is that a technical term, or are you referring to its being unfinished in some sense that I missed?

You probably want to avoid the shadow of Earth due to thermal considerations. Such a thing would experience tidal gravity close to Earth which could make sun-synchronous orbits problematic. In L1 you have the Earth as disturbing bright object, in L2 you have to move the mirror around sometimes to avoid the shadow (Gaia has a limited lifetime due to this). A high Earth orbit could work, or something far away from earth. Certainly a solvable challenge, but scaling things up is not trivial.

I was in fact thinking of staying inside the shadow. I realize that working at a few K might be problematic, but I would expect that the constant cold would reduce the thermal cycling problem. For power we then either could use a nearby solar power craft that beams the necessary MW power to the observatory, or we could power the observatory with a few kg of Pu238. Probably the latter would be more practical and less noisy. A worse problem IMO might be the moon barging by every few weeks and causing gravitational disturbances.

I am undismayed by the Earth as a bright object; what you say is true, but the planet would only subtend a degree or two. For most purposes it would be avoidable. The sun though, for an L1 orbit could be a problem, hard to overlook. Of course if we had solar observatories at the L1 points of Earth, Venus, and Mercury (this one would involve some chasing! :) ) they could be immensely valuable and they would present no fuel problems -- ever!

What you say about scaling things up I support heartfeltly!

 

[Jon Richfield]

Lets not overlook the prospect of erecting a moon based space elevator. That could be a big time game changer.

Well, I grant that it would render the problems of establishing a moon-based major observatory negligable, but the only game it would change would be by making any other space-related advances irrelevant forever, because we never could afford them. As destructive social influences go, it would dwarf the Great Pyramid and the tower of babel combined.

 

[mfb]

However, did you have any other fundamental reason why a few 10 m x 10 m elements won't make a good primary mirror? (I am no expert on astronomic optics!)

No fundamental reason, just not much light. If you plan for 10000 segments for the primary mirror and 100 segments for the secondary mirror, 50 segments (~0.5% of the total project) won't give you 0.5 % of the light, but only 25/10000 * 25/100 = 0.0625 % (using 25 for the primary mirror and 25 for the secondary one, let's neglect the others). You get the same amount of light with 7 segments and a smaller design. Aperture synthesis works much better with more segments as well.

Those issues disappear once you have hundreds of segments assembled.

I was in fact thinking of staying inside the shadow.

There is no place of constant shadow* (apart from some valleys on the moon - scnr). All you get at L2 is a partial shadow.

* Sun/Jupiter L2 would be such a point, but that is far away. Not sure about Sun/Mars as the eccentricity of the orbit becomes important there. For circular orbits, the condition is (diameter of sun)/(diameter of planet)*(mass of planet/(3*mass of sun))^(1/3) < 1 which means the planet has to be less dense than 4200 kg/m³ independent of its orbital radius.

 

[wabbit]

Regarding the shadowing, wouldn't it be best to use a separate screen to protect the system from solar radiation and keep it both cool (for cameras etc) and at a constant temperature ? This seems to be the design for the JWST if my understanding is correct.

 

[Jon Richfield]

No fundamental reason, just not much light. If you plan for 10000 segments for the primary mirror and 100 segments for the secondary mirror, 50 segments (~0.5% of the total project) won't give you 0.5 % of the light, but only 25/10000 * 25/100 = 0.0625 % (using 25 for the primary mirror and 25 for the secondary one, let's neglect the others). You get the same amount of light with 7 segments and a smaller design. Aperture synthesis works much better with more segments as well.

Those issues disappear once you have hundreds of segments assembled.

Thanks mfb, that sounds comprehensible and reasonable. Of course, the idea is that the big telescope would be teeeerrriffic when complete and partly useful once the basic structure got past a certain point, even if it would have to advance for a long time before offering better bang for the buck than a few independent baby telescopes. But of course my 1km^2 idea is just an illustrative figure; it is the principle that I was discussing, and your reply did indeed address that principle.
And that principle was in response to the liquid mirror at the lunar pole, which I suspect would cost more than any realistic space rival or group of rivals, and would hardly if ever work at all if it didn't work completely.

There is no place of constant shadow* (apart from some valleys on the moon - scnr). All you get at L2 is a partial shadow.

Mmmm... yes. I hadn't stopped to figure the various penumbral effects. But I reckon that in a lot of cases the penumbra might be desirable, raising the ambient equilibrium temperature from say 3K to say 200K or 250K, far more convenient from the engineering point of view (figures pure thumbsucks of course!) Also of course, I had been far too vague in speaking of L2 points. Generally what I meant was the Earth-moon L2 and the various sun-planet L2 points. The penumbral effects would be drastically different.

But in any case, there are several places where craft could be maintained so as to perform special functions that for the most part would be more expensive or less effective on gravitationally non-negligible bodies. The debacle with the Philae lander was a good example of an unexpected effect that could have turned out a lot worse, but where a non-zero but too-small field caused some serious bother.

 

[jkn]

We need in this order:

1: Cheaper access to orbit. Not much can be be done before this.

2: Permanently manned space station with:
- Radiation protection.
- Energy, air, water and food production.
- Production of basic construction materials: metals, glass, ...

Moon is best place for first station, because of fast escape to Earth and because most work can be done remote controlled from Earth.

After this moon telescopes become reasonable. Heavy parts can be made on the Moon and service is available when something breaks. Again most work can be done by remote control from Earth.

When moon base is well tested, it can be copied to martian moon and to asteroids. Mercury perhaps. Venus no.

 

[tom aaron]

We need in this order:

1: Cheaper access to orbit. Not much can be be done before this.

2: Permanently manned space station with:
- Radiation protection.
- Energy, air, water and food production.
- Production of basic construction materials: metals, glass, ...

Moon is best place for first station, because of fast escape to Earth and because most work can be done remote controlled from Earth.

After this moon telescopes become reasonable. Heavy parts can be made on the Moon and service is available when something breaks. Again most work can be done by remote control from Earth.

When moon base is well tested, it can be copied to martian moon and to asteroids. Mercury perhaps. Venus no.

Made on the Moon! Do you have any idea of the technological infrastructure needed to make some hi tech part for a telescope? Let alone install it. Test it. Remove. Retest, etc.? There is a reason the JWST is a decade behind schedule.