space mirrors

Discussion in 'The Watering Hole' started by pythonrock, Jul 14, 2019.

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  1. Jul 18, 2019 #31

    jlabrasca

    jlabrasca

    jlabrasca

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    I totally missed this one.

    It is called a Niven Ring (or a Polyakov Ring for a non-rigid arrangement of tethered structures https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19780010146.pdf).

    Such structures are, famously, unstable.

    https://www.popularmechanics.com/space/deep-space/a11183/could-we-build-a-ringworld-17166651/

    also

    https://en.wikipedia.org/wiki/The_Ringworld_Engineers#Plot_summary

    Your choices for locating a mirror in solar orbit so that it is always in the same position with respect to the other thing in solar orbit that you want to illuminate are -- really -- only the trojan points L4 and L5 (the antipodal point L3 is stable, but the sun would always be in the way).

    For a mirror in orbit around the earth, if you want to keep the mirror in the same place in the sky, you've only got the geostationary orbit.
     
    Last edited: Jul 19, 2019
  2. Jul 18, 2019 #32

    jqavins

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    That "image of the sun" explanation is good, but not good enough.

    Your first assertion was that the apparent intensity of the source would fall off with the square of the reflector's distance from the observer. That's clearly wrong, as your explanation above shows. If the satellite were a point source then the change would be ((Lower orbit radius minus Earth radius)/(Higher orbit radius minus Earth radius))². Which is wrong.

    By your explanation above, the reduction is far, far less. Consider the figure below, which is not to scale.
    upload_2019-7-18_9-19-32.png
    The change is ((Direct Path)/(Reflected Path))². The scale of the solar system being what it is, that's utterly insignificant. But that's not the whole story.

    The mirror isn't perfect. Rays will not reflect from the mirror with geometric accuracy, and the effect of this is that light will diverge more after reflection that it does before. One should be able to approximate this by an inverse square law with a different baseline distance: ((Lower orbit radius plus reference distance)/(Higher orbit radius plus reference distance))². The reference distance depends on the quality of the mirror (including its mounting). I don't know what is the best it could reasonably be expected to be, but it certainly is anywhere near 1 A.U. All the same goes for a parabolic reflector; it can't ever be perfect so light will diverge from the designed path. And since this divergence is a result of limited random scattering you can't get around it by changing the focal length.

    The orbital altitude matters, neither as much as you initially stated nor as little as you stated next. I strongly suspect that the difference between a reflector at 1000 km and one at GEO would be quite important, though not nearly as drastic as the 1/1600 you first stated.
     
  3. Jul 18, 2019 #33

    boatgeek

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    OK, fair point on [edit] the JWST [/edit] cost. However, I'm not sure it will be more than two orders of magnitude lower cost. You still need telescope-level aiming precision, and the mirrors still need to be launched, positioned, tweaked, etc. Aiming will take significant resources and near-constant mirror moves to keep the reflected sunlight on your solar array (or solar thermal collector). If you have to go through any atmosphere at all, there's going to be shifting distortions. You can accept a less tightly focused beam (either from non-flatness of your mirrors or less accuracy in aiming), but that means you need a larger solar array on the ground or a larger mirror array in space. It still doesn't deal with the Mars dust storm problem, either.
     
    Last edited: Jul 18, 2019
  4. Jul 18, 2019 #34

    Reinhard

    Reinhard

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    The sun is not a point light source, its angular diameter when viewed from the earth is a bit more than 0.5 degrees. The light reflected from the mirror won't be parallel, regardless of its geometry. A light ray originating from the north pole of the sun, hitting a point X on the mirror will be reflected in a different direction than a light ray originating from the suns south pole hitting the same point X.
    Everybody can see the effect at home. Use a mirror to reflect light on a nearby surface and you will see a sharp contour. Now increase the distance and you will observe the edges getting blurred. If you increase it even further, it will further transform in a blurred spot of increasing size and decreasing brightness.
    For space applications, the effect is very significant. Light reflected from a mirror in low orbit will be spread over miles just from this alone and it gets proportionally worse in higher orbits or even Lagrangian points. Accordingly, the light from a small mirror will be very dim. The only way to counteract this, is by using enough mirrors. In the space weapon scenario this boils down to, yes, you can theoretically use enough space mirrors to burn down a house, but only if you are also willing to burn down the whole city it is in.

    There are other limitations how well the light can be focused (e.g. diffraction), that will limit the use of space mirrors for "small", that is, non science fiction sized, applications.

    Reinhard
     
  5. Jul 19, 2019 #35

    jlabrasca

    jlabrasca

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    Mirrors in the sky: Status, sustainability, and some supporting materials experiments

    https://www.seas.upenn.edu/~lior/do...lityandsomesupportingmaterialsexperiments.pdf

    In Section 2: A Few Key Equations for Space Mirrors the author presents a formula for figuring the spot size ("image size" in the paper == the area of the ellipse illuminated by reflected sunlight on the earth's surface) in terms of the area of reflector for a circular plane mirror at some distance above the earth.

    I thought I remembered something about the PRC lofting an "artificial moon"

    http://www.astronomy.com/news/2018/10/why-chinas-artificial-moon-probably-wont-work

    at the end of that article is mention of the Russian Znamya experiment:

    "...a circle of aluminum-coated plastic 65 feet in diameter that was launched from the Mir space station and unfolded in space like a fan.

    The artificial moon worked — briefly. As The New York Times reports, astronauts aboard the station could detect a faint beam of light arcing toward the ground, though observers on Earth saw only a momentary bright glimmer in the sky.
    "

    A little further down the google rabbit hole you bump into Lewis Fraas

    https://aip.scitation.org/doi/pdf/10.1063/1.4822239

    and Kraft Ehricke's Lunetta and Soletta (Power Solettta, Biosoletta, etc.) proposals for constellations of geostationary or sun-synchronous mirrors.

    I can't find anything from Ehricke that isn't behind a pay-wall, but this one

    https://www.researchgate.net/public...conomically_feasible_supply_with_solar_energy

    showed up as a citation in this paper

    Space-Enhanced Solar Power for Equatorial Regions

    http://eprints.gla.ac.uk/137570/1/137570.pdf

    -- which goes into crazy levels of detail on a proposal for two parabolic mirrors of about the size that pythonrockets' proposes (~4600 sq. meters each) "...in a flower constellation rotating with the Earth to deliver a repeat ground track."

    A good read if you are a fan of Lagrangian dynamics and integro-differential equations (okay, they are just integral equations, but they would be integro-differential if the authors were just a little more bloody-minded)

    From the (bloody-minded) Bonetti & McInnes paper

    "Another interesting option is the use of electrochromic layers to effectively control the attitude of the mirror. The reflectivity of the surface would be locally different generating a torque that can be exploited to passively control the attitude of a large mirror. The electrochromic layers should be on the edge of the mirror in order to not affect the collection of solar power. Electrochromic coatings have been successfully employed on the IKAROS solar sail for attitude control in 2010"

    I am embarrassed to admit that I either forgot about, or was never really aware of, the Ikaros mission. The idea of putting LCD windows at the edge of the sail to steer it is just brilliant.
     
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  6. Jul 20, 2019 #36

    pythonrock

    pythonrock

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    If you redraw this with lines from accross the face of the sun and to both edges of the miror, BUT make the sizes and distances correctly proportioned, all of your lines will appear to be almost perfectly parrallel
     
  7. Jul 20, 2019 #37

    pythonrock

    pythonrock

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    Because the mirror will be at a significant angle to the sun , the apparent size will be narrowed in that direction proportional to the angle.
     
  8. Jul 20, 2019 #38

    pythonrock

    pythonrock

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    The apparent size of the mirror does become narrowed as the angle increases ( 90 deg - on edge - no mirror )
     
  9. Jul 20, 2019 #39

    jlabrasca

    jlabrasca

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    I am not sure if you are responding to my post but Reinhard did a much better job of explaining it than I did.

    You do not notice the divergence of sunlight over small distances -- but it does diverge.

    From here

    Mirrors in the sky: Status, sustainability, and some supporting materials experiments

    https://www.seas.upenn.edu/~lior/do...lityandsomesupportingmaterialsexperiments.pdf

    solarmirror gemometry.png
    Put your 50 ft x 50 ft (230 sq. meters or about 0.0001 sq. miles) plane mirror on the ISS, at an orbital height of about 400 km. The light from that mirror would illuminate an area of about 10 square km (about 4 square miles) on the surface of the earth.

    The illuminated area will increase, and the overall intensity of the reflected sunlight will decrease, as the square of the distance between the mirror and the collector (because the sun is an extended light source there will be a center-to-edge variation in intensity, the center of the illuminated region will be brighter than the edges, but this too will tend to flatten out as h gets large compared to the the diameter of the mirror and collector).

    Again, if you consider Iridium flares* The glint of sunlight that came off the antennae was visible across a ground track much wider than the dimensions of the antenna.

    *I really wish that there were some other well-positioned plane mirrors in orbit to offer as an example, now that the Iridium satellites have been decommissioned. One of my favorite things to do for my astronomy classes; during observing sessions I'd have the students look in the right direction just before a flare. Actual "oohs" and "ahhs" would follow. I am going to miss that.

    edit: I am going to be in trouble with the center-to-edge thing. Consider a collector of fixed size -- say a circle 1 km in diameter and near the center of the ground track for the mirror. As the space mirror moves higher the center-to-edge variation in intensity measured
    across that collector will become less pronounced.
     
    Last edited: Jul 20, 2019
  10. Aug 10, 2019 #40

    pythonrock

    pythonrock

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    Even better. checkout non-Keplerian orbits. Statite and quasites,. put the mirrors in stationary positions from the sun
     
  11. Aug 10, 2019 #41

    pythonrock

    pythonrock

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    or in solar orbit between Venus and Mercury with orbit of 365 days to light Earth solar farm
     

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