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Planets at near stars?

Posted by: JonHogan - Sat Nov 02, 2013 12:23 am
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Planets at near stars? 
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Post Re: Planets at near stars?   Posted on: Thu Nov 07, 2013 5:40 am
Challenging, definitely, but the price seems right compared to other proposals. Each mirror unit would be its own little mass-produced autonomous lunar lander. The more units we drop, the higher the resolution. The idea is to make the process pay-as-you go so low-resolution images become available quickly and then just keep getting better and better.

The hardest part is delivering an L2 component (I really need to find a good name for that thing) that can hold steady and consolidate the reflections from all those ultra-precise mirrors roughly 36,000 miles (~58,000 km) away into a single coherent image. I think of the “receiver” as a wide-angle Hubble-type telescope in its own right pointed directly at the moon and using the mirrors on the surface to look at a distant planet behind it, like Annie Oakley doing a trick shot by using a mirror to shoot a target behind her with a rifle over her shoulder. The difference is that the lunar optical array would also be a singularly powerful imaging amplifier.

Most space-based telescopes, such as the Hubble, use guide stars to fix their position. Registration targets dropped onto the lunar surface would serve the same purpose and might even be more reliable. Natural geological features probably would not work well as registration marks because most astronomical imaging would occur during the lunar night. Surface features would be very difficult to resolve with sufficient accuracy within the inky blackness of the lunar umbra, even taking into consideration the ambient starlight. Artificial targets are much simpler.

I enjoyed your “artist’s impression,” James. The sky’s a little off. Maybe slightly less vegetation? You’re right about the units being much farther apart. In fact, I suspect if you could stand next to one of the mirrors on the lunar surface, the landers might actually be so far apart you wouldn’t even be able to see the next one in the array. That would make a much less interesting postcard, though.


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Post Re: Planets at near stars?   Posted on: Thu Nov 07, 2013 2:39 pm
Blame Bing's image search engine. It was the best I could find in the first couple of pages.

Ideally you would want the lunar primary mirrors made insitu of lunar materials and then be self-deploying. Just give them a small rocket and a location on the lunar surface and they can do a little ballistic flight. Maybe even self-replicating? Each reflector is like a tree, it roots into the regolith, finds materials to assemble "seeds", that contain the instructions, the rooting and self-assembly machinery which the parent then throws out to "grow" more reflectors. After a while you'd have a forest of telescopes...

Not only will the "receiver" need to know where it is in relation to the primary reflectors, but the reflectors will need to know precisely where the receiver is too. All of them. As instantly as physically possible. I imagine each one would need a laser with which to do ranging and tracking (remember you want to be able to steer the foci of the array too). Also it could serve as the communication link for the whole network, if you can encode & duplex the signal wide enough (tens of thousands of individual reflectors).

Sufficient to say, we won't be building this anytime soon. LOL. But it would still be easier & cheaper than mini-starships...


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Post Re: Planets at near stars?   Posted on: Thu Nov 07, 2013 6:18 pm
My goal has been to imagine a system using existing astronomical and spaceflight technology so Jon can get his “nice photo” of a distant world sometime soon and at reasonable cost. (NASA and I disagree about what “soon” and “reasonable” mean) “Growing” a forest of self-replicating astronomical mirrors from lunar material is ideal, but unfortunately beyond our present capabilities.

I mentioned earlier that most space-based telescopes use guide stars in their field of view to aim accurately. A lunar telescope would have the benefit of a second very stable and predictable frame of reference, the moon itself. Registration targets dropped onto the far side of the moon would be even more useful than guide stars because the lunar surface is spherical. The distant stars that are used to aim the Hubble and other similar telescopes appear only as a two-dimensional pattern. The L2 “receiver” of the lunar system would use laser ranging reflections from widespread prismatic retroreflectors (similar to those used by surveyors) to provide the software with extremely precise three-dimensional global positioning coordinates for every component in the system.

After being individually calibrated, the mirrors would have no need to find the “receiver.” For any given astronomical observation, the primary reflectors would all simply be guided by the software to direct the incident light toward a single three-dimensional coordinate in the sky. The L2 “receiver” would have the responsibility of finding and staying in that spot. There is no atmospheric distortion so the “adaptivity” required of the optical array is minimal. Tidal, seismic and thermal effects would make periodic recalibration necessary. Robotic refueling of the station-keeping reaction control system is also within present capabilities.

Optical links would allow much faster data transmission as well as minimize radio interference in the event a large radio telescope array also were to be established on the far side of the moon. The relay orbiter would be a fairly ordinary satellite, nothing special. Both telescopes could use the same relays at the same time.

Guide stars would continue to be needed for finding the planet to be observed, but the laser beams between L2 and all of the registration targets scattered over the lunar hemisphere can be thought of as the struts of an immense trusswork made of light which allows the far-flung components of the telescope to function almost as a single rigid structure, and still completely within present engineering capabilities.


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Post Re: Planets at near stars?   Posted on: Thu Nov 07, 2013 7:32 pm
Fabricating, lifting, and emplacing, even by remote means from Earth will be... impractical financially and technically in the near term IMO. Esp. if you want one that has any significant advantage over terrestrial or LEO telescopes. It HAS to be done there. Insitu and even self-assembly/replication isn't as far off as you might think.

The mirrors themselves would be the registration targets.

Registration (actually "collimation") will probably need to be a pretty much constant process because:

Differential thermal effects as the Sun creeps across the array will introduce weird effects.
Gravity influences tugging on the receiver. Its easier and quicker to adjust the mirrors than push a satellite around.
And most of all, you are aiming at a "moving" target as the Moon rotates and orbits across whatever you are trying to image. So as your receiver moves, the mirrors have to follow it. Easier to have them dynamically do it themselves than hope they aim to where the receiver is "supposed to be".


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Post Re: Planets at near stars?   Posted on: Wed Nov 13, 2013 5:23 pm
Once SpaceX gets done lowering the cost to space, we can talk about all kinds of projects. But just imagine - pics of extrasolar planets - that would be awesome!

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Post Re: Planets at near stars?   Posted on: Fri Nov 15, 2013 2:09 am
It will be interesting to see how that works out. If its actually possible to take a portrait shot of a planet, or if the exposure time etc. required will mean they are blurred streaks that are mostly just good for spectral analysis (which would be more important that a pretty picture anyway), and any images will have to be so digitally manipulated that they might as well be "artist's impression of".


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Post Re: Planets at near stars?   Posted on: Fri Nov 15, 2013 6:39 am
JamesG wrote:
It will be interesting to see how that works out. If its actually possible to take a portrait shot of a planet, or if the exposure time etc. required will mean they are blurred streaks that are mostly just good for spectral analysis (which would be more important that a pretty picture anyway), and any images will have to be so digitally manipulated that they might as well be "artist's impression of".


It's more important to get the pictures people can see well - public interest is more important than science, at least at this stage. Without public interest, there is no science, while vice versa isn't the case. Unless we find little green men, or something like that. :D

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Post Re: Planets at near stars?   Posted on: Sun Nov 17, 2013 8:17 pm
Sorry for the delayed response. More mundane matters have insisted on having my full attention. I have attempted to address all your points one by one.

The installation of an optical array on the far side of the moon capable of high-resolution imaging of planets around other stars is definitely financially unlikely (to use the most generous term), but technically well within present capabilities.

I wish I shared your optimism regarding the prospect of ultra-precise astronomical tracking mirrors extracting, refining and assembling all the necessary elements from the regolith to replicate and transport themselves over hundreds of miles. I disagree fabrication has to be done on the moon.

SuperShuki makes a couple of valid points. The privatization of space transportation (by competition among several companies) will only continue to bring down the cost of delivering such components by more conventional methods and the more we send, the lower the per unit cost will be. A lunar optical array would be ideally suited to benefit from such economies of scale. Not having to deal with life support really brings down the cost of space transportation as well. Deployment of an array of identical mass-produced astronomical mirrors by rockets from Earth will be feasible long before deployment by self-replication of optical, mechanical, guidance, communications, power, transportation and semiconductor microelectronic data processing systems using only lunar soil and a factory on the far side of the moon.

Also, public satisfaction with the results is essential for public support. A lunar optical array would not disappoint, delivering the biggest bang for the buck, for both hard science and public satisfaction. Terrestrial telescopes will never achieve the necessary resolution with the atmosphere in their way, even with the best adaptive optics. A single Low Earth Orbit telescope will never be able to collect enough light from exoplanets to overcome the blinding brightness of their suns. We could put a few dozen to work observing one planet at a time simultaneously, but a lunar optical telescope would cost less than a constellation of Hubbles, be much easier to aim at hundreds or potentially thousands of Earth-like worlds spanning the celestial sphere and produce images many times finer in the bargain.

The whole system is little more than a single “orbiter” of Hubble-type sophistication boosted to L2, an “array” of identical small autonomous all-aluminum programmable tracking mirrors scattered fairly randomly on the lunar surface using landers that are essentially mass-produced updated versions of Space Race technology from the 1960s and the twenty-year-old software needed to run it all.

Picture a single launch of a lunar transfer “bus” tumbling out dozens of mirror landers over the lunar surface and just letting them fall nearly randomly (with autonomous navigation to a safe landing, of course). Perhaps even adapting the airbag landing systems some of the Mars rovers have used to function in the vacuum and lower gravity of the moon (with tethered rockets as the “parachutes”) would reduce costs still further.

The exposure time needed to acquire a useful image is a function of the aperture (light gathering capacity) of the system. The degree of resolution (clarity) depends largely on the collimation (“parallelness”) of the incident light captured to produce the image. A lunar optical array with even just a few hundred widespread tracking mirrors would have an aperture large enough to allow exposure times to be sufficiently brief to produce full disk images rather than streaks. The great distance from the array on the lunar surface to the receiver at L2 (~ 36,000 miles, or ~ 58,000 km) combined with a comparatively “narrow” array would result in exceptionally fine collimation of the incident light. Offering much more than digitally manipulated ‘artist’s impressions,’ an array of thousands of mirrors confined to an area only a few hundred miles wide would make possible true high-resolution ‘portraits’ of extrasolar planets that would leave us speechless.

Consider the effect on the public imagination after capturing images of a distant blue white globe with a resolution fine enough to discern continents, oceans, polar ice caps, cloud formations, volcanic plumes, glaciated mountain ranges, large islands and lakes and perhaps a moon or two. Money well spent, if you ask me.

Then imagine the day when astronomers announce unambiguously confirmed precisely periodic observations of extensive webs of artificial lighting on the night side of a faraway world.

In previous posts, the term “registration” refers to fixing the position of the L2 component with respect to global positioning coordinates in the lunar frame of reference, not collimation with respect to the primary reflectors or positioning within the celestial frame of reference. The mirrors would definitely have retroreflectors as well to mark their individual positions within the system, but they would not make the best registration targets for the receiver at L2. Very small, inexpensive and easily deployed retroreflectors spread much more widely than the mirror array would make better use of the three-dimensional curvature of the lunar hemisphere for calculating extremely precise (millimeter-scale) global positioning coordinates for every component in the system.

L2, the focal “point” in the sky for the lunar array, is effectively stationary with respect to the lunar surface (except with respect to lunar libration) even as the moon revolves around Earth and around the sun. The periodic motions of the moon (including libration) have been very thoroughly studied and are precisely predictable. Conventional astronomical software can easily guide programmable tracking mirrors to compensate for any aspect of lunar motion and focus the field of view of any target identified on the celestial sphere onto precise lunar global coordinates along the axis passing through L2 without any communication between the receiver and the array during the observation.

I used the analogy of a solar power plant earlier. The mirrors play the role of the heliostats, the receiver takes the place of the stationary boiler at the top of a (very) tall tower and the distant exoplanet moves across the sky more or less like the sun. The heliostats of a solar power planet don’t need to communicate with the boiler; they just need to know where they are, where the sun is supposed to be and where the boiler is supposed to be.

The lunar optical array would work in basically the same way. All of the relative motions of the system can be written into the software to guide the tracking mirrors with extraordinary steadiness. The L2 receiver looks into the mirrors, sees the field of view “over its shoulder” and uses guide stars to stay precisely aimed at the distant planet throughout the period of observation. Except for the reorientation and amplification of the field of view by the mirror array this is exactly how the Hubble has been aimed for more than twenty years.

The movement of the receiver to any predetermined position is very slight, not nearly as much as you might imagine and certainly no more complex than what the Hubble does now. The mirrors are told to focus the incident light onto a particular point along the axis of the system and the receiver is instructed to be there at the same time. Laser registration of all the components with respect to the lunar frame of reference allows these movements to be exceptionally precise. The once unprecedented steadiness of the Hubble rapidly became the standard for all space telescopes and has since been surpassed.

The influence of lunar gravitational anomalies at L2 (36,000 miles up) is trivial compared to the wobbles of a spacecraft in a low-altitude short-period orbit. Tidal movement shifting the position of the mirrors is a much greater concern, but even that is periodic and therefore predictable. In fact, the regular rise and fall of the lunar optical array would function as a gigantic geological instrument as well, helping lunar geologists (selenologists?) to learn more about tidal effects on the lunar surface then we presently know.

Seismic activity is the least predictable variable and therefore the most certain to play havoc with the schedule for routine recalibration.

Imaging is not likely to be attempted during the lunar day primarily because of the blinding effect of the reflected sunlight, but the wave of thermal deformation passing over the array of mirrors and mounts would certainly interfere with good “seeing” as well. The lunar night, on the other hand, is a uniform cryogenic blackness, a lovely place for planet spotting.

All in all, every aspect of the system, including deployment, is well within proven practical technical experience.

Now, about the money...


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Post Re: Planets at near stars?   Posted on: Mon Nov 18, 2013 1:16 am
Look into how complicated the James Webb telescope is. And it is a simple dime store spy glass compared to a lunar optical array.


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Post Re: Planets at near stars?   Posted on: Tue Nov 19, 2013 10:02 pm
The hardware of the James Webb Space Telescope is much more complicated. The complexity of operating a lunar optical array lies primarily in the software necessary to orchestrate the performance of so many discrete components. The components themselves are comparatively simple and software available today is sufficiently sophisticated to run the whole system. Delivering all those components to their places is basically a very expensive “shipping” problem that has little to do with actually operating the telescope.

The myriad identical landers scattered over the lunar surface would carry small, mass-produced, surface-stabilized tracking mirrors. The benefit of having surface-based mirrors has been mentioned before. The mass of the moon itself eliminates the complex hardware necessary to deal with the reaction forces arising from trying to aim a huge multi-element mirror in the weightlessness of orbit, allowing the technology required to aim the primary mirrors of a lunar telescope to be much simpler and more durable. Software does the hard work.

The one truly complex element, the receiver at L2, would be only as challenging as the Hubble Space Telescope. Compared to the Hubble, the Webb has many more complexities designed into its hardware, not the least of which is that the entire telescope (including a primary mirror several times the diameter of the individual mirrors of a lunar array) must be folded like a blossoming origami flower, packed into a single rocket and boosted all the way to the L2 point in the Earth-Sun system, well over a million miles farther than the Earth-Luna L2. We already know all about building, deploying and operating Hubble-type observatories.

Three-dimensional laser registration of all the elements of the system within the lunar frame of reference is no more difficult than the calculations needed to make our own Global Positioning System work. A laser scan of a hemispherical array of retroreflecting targets spread across the far side of the moon would illuminate a unique three-dimensional “constellation” of benchmarks with only one solution for the coordinates of any point within the frame of reference. Once again, comparatively simple hardware can be coordinated by well crafted software to yield extraordinarily precise results.


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Post Re: Planets at near stars?   Posted on: Tue Nov 19, 2013 10:33 pm
It must be such a burden to know everything.


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Post Re: Planets at near stars?   Posted on: Wed Nov 20, 2013 1:43 am
James, when JonHogan started this thread by proposing a miniature exploratory probe launched by a space-based linear accelerator as his “solution” to the challenge of acquiring detailed images of an extrasolar planet, you suggested that a cheaper faster alternative would be to build a “really REALLY big telescope” instead. The concept of a practical lunar optical array is offered only as an exercise of the imagination in an attempt to respond to Jon’s idea and your suggestion. Do you have your own preferred concept for a “really REALLY big telescope” with the power to resolve details on a planet orbiting a distant star? You never did say whether you had a specific design in mind.


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Post Re: Planets at near stars?   Posted on: Wed Nov 20, 2013 3:35 am
Image

Your idea is fine. But you are incorrect on the technical details.


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Post Re: Planets at near stars?   Posted on: Fri Nov 22, 2013 1:59 am
Thank you for your persuasively detailed opinion. Had I known you were in possession of alien technology I never would have presumed to question your authority. With Marvin the Martian as your technical consultant your insistent attachment to the much more practical notion that an exquisitely precise astronomical array would have to be “grown” from lunar soil seems perfectly rational now, not Looney Tunes at all.


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Post Re: Planets at near stars?   Posted on: Fri Nov 22, 2013 2:29 am
Don't make me whip out my discombobulator.


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