space mirrors

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

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  1. Jul 14, 2019 #1

    pythonrock

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    Why hasn't any space company - NASA, SpaceX, etc. started building a fleet of space mirrors in solar orbit?? It seems that should be one of the first projects before long term presence on Mars or the moon. and even without that it would be immediately impoortant.
    power a solar sail craft accross the galaxy
    light for telescope observation of Kuiper or Ort etc. like camera flash
    power to solar collectors on earth, Mars, moon etc for increased and consistent elect output
    Earth defence from NEOs by Yarkovski effect (sp)
    and good or bad depending, could also be used as a weapon with reach accross solar system

    BTW also my favorite explanation for Tabbie's star , an incomplete Dyson ring of mirrors
     
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  2. Jul 14, 2019 #2

    jlabrasca

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    >smile<

    Because of math.
     
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  3. Jul 14, 2019 #3

    ThirstyBarbarian

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    Maybe they are just focusing on being able to put astronauts in space again.
     
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  4. Jul 14, 2019 #4

    kuririn

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    I'm all for space mirrors. After all, the astronauts have to look presentable before they step on the moon, don't they?:D
     
  5. Jul 14, 2019 #5

    Marc_G

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    I love space mirrors in fiction. One to increase the light to Mars; another to reduce the light getting to Venus to let it cool down, both to help eventual terraforming.

    Unfortunately right now these ideas are fictional only; the materials science and economics aren't quite there yet. But given time, I anticipate plenty of good applications with them.
     
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  6. Jul 15, 2019 #6

    aerostadt

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    Yes and Robert Zubrin has discussed both in his non-fiction books. Like many things this is easier said than done. For propulsive the mirrors films need to be incredibly thin. Some feeble attempts at propulsion are planned for the near future.
     
  7. Jul 15, 2019 #7

    jlabrasca

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    No earlier than July 21

    http://www.planetary.org/explore/projects/lightsail-solar-sailing/
     
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  8. Jul 15, 2019 #8

    kuririn

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    The force may be weak, but the cumulative effect over many weeks/months can result in an impressive velocity.
    Same with ion propulsion, which I understand has been used in interplanetary missions.
     
  9. Jul 17, 2019 #9

    pythonrock

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    actually the materials and technology are available and mature now .
    the economics -- I'm sure shipping coal and O2 to Mars to make electricity is more expensive than solar with mirrors
     
  10. Jul 17, 2019 #10

    jqavins

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    There's already sunlight reaching Mars; how much more do you propose to send there? A mirror in a near-Mars orbit that doubles the light reaching the planet would have to be about the diameter of the planet. Couldn't one send a heck of a lot of fuel for the price of building that?
     
  11. Jul 17, 2019 #11

    jlabrasca

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    The orbital mechanics of putting a mirror in "near mars orbit" would be a little tricky, I think. You might imagine a constellation of smaller mirrors in transpolar orbits, but, as you say, the effective aperture of these mirrors would have to be very large and every mirror would become a shade as it passed over the sunlit side of the planet.

    Going back to the OP, and looking at a mirror in "solar orbit", the Lagrange points are attractive.

    L1 is on the wrong side of the planet (we will save for another thread, the arguments about putting planet-sized Fresnel lenses into solar orbit)

    L2, would work (although the increased insolation would mean illuminating the night side of mars). Unfortunately, from L2, mars subtends an angle of about 0.34°-- which is just about exactly the angle subtended by the sun at the orbit of mars. That is to say, as seen from L2, the sun is almost entirely eclipsed by mars. So, an L2 mirror would have to be an annulus with an outer radius about 1.4 times larger than the radius of mars. Back-of-the-enevelope; it would take just about 1 billion square kilometers of mylar.

    The trojan points are exactly as far away from Mars as the sun. You could put a parabolic mirror with an aperture about the cross sectional area of Mars (and its focus coincident with the sun) at one or the other of these points and orient it so that the effective insolation of mars was doubled. Mars would have two suns above the horizon for part of each day, and the day would be about 1.2 times longer (about 17% more hours of daylight for a given point on the surface). Again, a Fermi approximation without actually trying to figure out the shape of the mirror that would get you out from under the inverse square penalty, it would require at least 100 trillion square kilometers* of mylar.

    marslagrange.png

    * The 100 trillion sq. km figure is wrong -- but I am resolved NOT to correct it. I have real work to do today.
     
    Last edited: Jul 17, 2019
  12. Jul 17, 2019 #12

    jqavins

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    And the structure stiff enough to do the job, which would weigh a lot despite mylar's nearly insignificant mass. And then, how long would it take to construct a thing - any thing - of that size? What are the overhead and labor rates for construction in space conducted far from any planet?

    And then, of course, it also needs station keeping capability, i.e. rockets (or some other means) to correct orbital perturbations caused by, among other things, the solar sail effect acting on the reflector, and the control and power systems for said rockets (or other things). Communication for ground control wouldn't be much different from any other satellite, but the station keeping for a thing of this size would pose huge challenges, meaning huge expense.

    The answer to the original question seems to become pretty clear: $$$$$$$$$$$$$$$$$$$$$$$$$
     
  13. Jul 17, 2019 #13

    jlabrasca

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    A clever engineer would figure out how to use the light pressure to keep the mirror oriented correctly --

    I got a clean envelope out of the recycling: If you put an ellipsoid mirror at one of the trojans, constructed so that its foci coincide with the sun and mars, you could -- I think -- get the aperture down to about a billion square kilometers (give or take an order of magnitude). Comparable to the annular mirror at L2.

    This is the most effective bit of nerd-sniping I've seen on TRF to date (in a post not-related to the dangers of falling rockets, at any rate)
     
  14. Jul 17, 2019 #14

    heada

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    Which would be a better reflective surface, Mylar or liquid mercury spun into a disc and then allowed to freeze?
     
  15. Jul 17, 2019 #15

    jqavins

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    Mass is critical, and aluminized mylar weighs almost nothing. It also costs very little, so it's total cost in orbit is far, far less than that of mercury.

    Also, things go through a radical change in size/shape on freezing. I just checked, and mercury, like most things, shrinks causing divots, so the frozen mercury would have to be machined to its final shape.

    Aluminum's total reflectivity over the visible spectrum is second (among metals) only to silver, if I remember correctly.

    So, I'd have to assume that aluminized mylar is the better choice.
     
  16. Jul 17, 2019 #16

    pythonrock

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    This is not a telescope mirror!. Each mirror is a flat sheet of mylar 20 to 50 feet accross. it can be unfurled from a 2 or 3 unit cubesat. one Falcon launch could deliver several hundred mirrors to solar orbit. depending on the application they would mostly be in the same orbit as the planet but leading or trailing by 5 to 20 deg.
    For positioning think about Hubble telescope. It can stare at one point in space for hours or days to get clear images. gyros, reaction wheels, and perhaps thrusters as needed. All geo satelites also need to maintain position in orbit so this is not a new problem. and the tech to do it is clearly available.
    Except possibly the weapon version, there is no need to focus, several 50 ft mirrors pointed at a 2000sq ft solar panel array would multiply its output and light it at night ( provided of course the array can tolerate that loading.)
    same for saill craft etc.,
    as a weapon- point a hundred at a large building - it would burn instantly. Or a single miror that does focus to destroy a plane, a truck, a small building or a satelite
     
  17. Jul 17, 2019 #17

    boatgeek

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    I think you're missing the problem of scale that jlabrasca is talking about. Let's say for the sake of argument that you could configure a Starlink satellite to unfurl a mirror 100 meters on a side, 0.01 square kilometers per mirror. I think that's probably a little high for what that size satellite could actually carry, but carry on. Let's further assume that we're only trying to get to 10% additional solar power on Mars, so we *only* need a mirror of about 100 million square kilometers net area (taking jlabrasca's calcs as accurate). That means that you only need to launch 10 billion Starlink size satellites to make this work. With 60 satellites per launch, you only need ... 167 million F9 launches.

    That's the problem--the total cost, effort, and number of launches to get any kind of meaningful effect from mirrors is absolutely enormous.

    And if aiming multiple low-power beams were that easy through an atmosphere, we would have many laser weapons already deployed. The fact that we don't indicates how hard this is.
     
  18. Jul 17, 2019 #18

    jqavins

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    Indeed it is not a telescope mirror, and yes that makes matters much easier. But still far from easy. The tech to maintain orbital position and attitude for telescopes and comm sats is well established; now multiply that by... hmm, let's see.

    If we want to double the incident light on Mars using a whole bunch of mirrors 5 to 20 degrees ahead of or behind the planet then we need a total mirror area about the same as the planet's presented area (by which I mean the area of its apparent disk; I don't know it there's an official term for this.) Mars's diameter is about 4200 miles, so it's presented area is almost 14 billion square miles, or 155 billion separate reflectors if each is is a 50 foot square.

    Station keeping and attitude control for one of these would not be very difficult. The electrical power, physical structure, and fuel requirements for 155 billion of them would undoubtedly be far greater than those of many structures each comprising many of them, let's say (pulling a number out of my backside) 155 million assemblies of 1000 reflectors each. Roll that over in your mind for a moment: each unit has nearly a tenth of a square mile of mylar and there are 155 million of them!

    The design life of a typical GEO com sat is 15 years, and the fuel load is enough for somewhat more than that just in case the equipment hangs in there. Many do last longer; many, not all. But notice, it's not the fuel load that limits life, but rather things like equipment failures in the harsh thermal and radiation environment and degradation of the solar arrays from those same factors and micrometeor damage, among other things. So, let's just suppose our 1000 mirror satellites can do better on life; let's suppose they do lots better and manage 50 years. To replace 155 million satellites in a span of 50 years means manufacturing and positioning 3.1 million of them per year, or about 8500 per day, 5.9 per minute.

    Can we build and launch 5.9 of these units and the launch vehicles to lift them at a rate of 5.9 per minute? Maybe we could do multi-unit launches, so we only need one launch vehicle per minute; that's a whole lot better :rolleyes:. And this isn't the initial deployment, this is the replacement rate for the full-up system, which means it never finishes.

    I'm not even going to try to estimate the total mass of various materials - aluminum, silicon, copper, various polymers, fuel, etc. - being consumed by this and sent off planet every day.

    And all this just to double the incident light on Mars. Which would bring its total incident light almost up to that of Earth.
     
  19. Jul 17, 2019 #19

    jlabrasca

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    Before you quote my -- very hasty -- computations.

    [​IMG]

    -- edit: my age is 100 years (I am older than Randall Munroe), and the cross-sectional area of mars (or Mars -- some of the textbooks I've used capitalize planet names, some do not) is either 10 million or 100 million square km. This means that my estimate of the area of the effective aperture of the annular L2 mirror (taken to be 100% reflective and figured to concentrate 100% or the intercepted sun light onto the surface of m/Mars) is either 10 times or 100 times too large. I rounded UP to 1 billion because I figured it is better to have and not need, than to need and not have.
     
    Last edited: Jul 17, 2019
  20. Jul 17, 2019 #20

    pythonrock

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    You are not paying attention. read my post. there is no problem of scale. We are not trying to double the light on Mars, that would be stupid, only the solar panels, so one 50ft flat square mirror will double the light on a 2500sq ft solar panel instalation. on Mars or on Earth or on the moon.
    and we are not shading Venus at all!!!
    and a hundred 50x50ft mirrors pointed at the earth will burn down a house in minutes
    wasn't there a big issue about a Russian space mirror a couple months ago because it could be weaponized
     
  21. Jul 17, 2019 #21

    pythonrock

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    You are not paying attention. read my post. there is no problem of scale. We are not trying to double the light on Mars, that would be stupid, only the solar panels, so one 50ft flat square mirror will double the light on a 2500sq ft solar panel instalation. on Mars or on Earth or on the moon.
    and we are not shading Venus at all!!!
    and a hundred 50x50ft mirrors pointed at a point on the earth will burn down a house in minutes
    wasn't there a big issue about a Russian space mirror a couple months ago because it could be weaponized
     
  22. Jul 17, 2019 #22

    pythonrock

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    I think Russia has been testing this kind of mirror, suposedly for increasing solar power for about 25 years
     
  23. Jul 17, 2019 #23

    jqavins

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    Then there's an inverse problem of scale. I used doubling the light on Mars because it is a significant but somewhat modest goal in terms of its practical effect. Doubling the light on a single 2500 sq-ft solar array is a spit in the ocean.

    A GEO comm sat, as of when I left that industry some years ago, cost about $200 million on station (satellite, launch, and orbital slot combined). That's a hell of a lot of money to spend spitting in the ocean.

    I have not looked into the Russian mirror project. Assuming what you suggest is true (you don't sound altogether sure yourself) then it says something that after 25 years they still have not claimed any success, either as a power system or as a weapon.
     
  24. Jul 17, 2019 #24

    boatgeek

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    Erm, OK. It still doesn't work. Let's go back to realsies. The Ivanpah Solar Power Facility in CA generates around 1 TWh per year on a concentrated solar power system. Let's pretend that's on Mars, with half of the total solar power input. That means we get 500 GWh per year. That would theoretically supply a city of 50,000 people on Earth. Some energy needs would be greater on Mars (life support), but presumably all electrical systems would be built for maximum efficiency, too. You're also going to lose efficiency to heating up all of the air between your collector and orbit.

    Ivanpah has about two square kilometers of mirror surface, so you'd need absolute minimum 200 or so of my hypothetical Starlink mirror sats, and more likely 300-500. You mentioned space telescope style station keeping and aiming. Hubble cost $4B, and the James Webb telescope will cost around $10B. You'd get some economies of scale building a few hundred, so let's say the total array costs $200 billion. That's awfully expensive for Mars support, without even mentioning that you'd have no power in dust storms.

    To get any real power, you need a vast amount of mirror area, and that's really expensive and difficult.
     
  25. Jul 17, 2019 #25

    jlabrasca

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    Iridium satellite flares

    The antennae on the first generation Iridium satellites are plane mirrors in orbit --- each with an area of about 10 square feet.

    Under especially favorable conditions, the specular reflection of sunlight from one of these antennae, observed from the surface of the earth, might have an apparent magnitude of -9.

    Again, back-of-the-envelope, this means that the flash would measure at about 1 ten-millionth the intensity of sunlight.

    The Iridium satellite orbits keep them within 1000 km of the surface of the earth. Remove that mirror to a geosynchronous orbit (so that you could keep the reflection trained on the solar panels on the earth's surface) about 40 times further away from the earth's surface, and the intensity would drop by a factor of 1600 -- but let us just call it one billionth the intensity of sunlight.

    So, again, you'd need about a billion plane mirrors about the size of an Iridium antenna to double the amount of sunlight on a solar panel on the surface of the earth, or something on the order of a million mirrors of the size you propose.
     
    Last edited: Jul 17, 2019
  26. Jul 18, 2019 #26

    OverTheTop

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    :):):)

    That is remarkable similar to what they were doing with Keplar to keep it generating science. They positioning gyros had degraded so they were using solar wind pressure on the solar arrays to steer the satellite. So current tech.
     
  27. Jul 18, 2019 #27

    pythonrock

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    Yes, let's try to stick to reality.
    James Webb is projected to cost 8.7 billion total, for design (of the biggest most complicccated scope ever) , developement, building, launching AND 5 years of operation including positioning and all the science work. Don't compare that to keeping a flat piece of mylar in a frame in position in solar orbit.
    A flat plane mirror is not a point source of light and therefore does not disperse at distance squared so- irrelevant.
    There is not significant air on the moon or Mars. But it's true the extra light would tend to heat the solar panels which would make most current panels slightly less efficient, hpoefully new panels could be designed to work at higher temps
     
  28. Jul 18, 2019 #28

    pythonrock

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    Thinking into the future - perhaps sci-fi, but hopefuly eventually -- hundreds or thousands mirrors connected into a " Dyson Ring " in solar orbit would essentially stabilize itself so the mirrors could be aimed mechanically and almost no need for "positioning"
     
  29. Jul 18, 2019 #29

    jqavins

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    Yes, an absolutely valid point. Comparing a simple reflector to the Webb is like comparing apples to prize winning pumpkins.
    Welll, not quite. For a perfectly dead flat plane mirror you'd be almost completely right, but nothing is perfectly dead flat, light from the mirror will diverge, so distance does matter. It's not inverse square with orbital altitude, but it does matter.
    Define "significant". On the moon of course there's none. On Mars there is way way less air than on Earth, yet not so little that it could be ignored; there'd be some loss.
    That's an interesting proposition. Semiconductors' performance is effected directly by temperature in a few ways, intrinsically (no pun intended) and unavoidably. I wonder to what extent the conversion efficiency vs. temperature characteristic can be altered. This isn't the time for me to dig into it. Of course, arrays could be build for better cooling.
     
  30. Jul 18, 2019 #30

    jlabrasca

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    Well, that is an extraordinary assertion.

    Maybe, try thinking about it this way: The source of light from your plane mirror is, in fact, an image of the sun produced by that mirror. The rays will diverge from that image in exactly the same way that they diverge from the sun -- just in a different direction.

    sunargument.png

    Or, you could consider that if you were to approach the sun it would not appear to be a point source. That its point-like character is entirely due to our distance from its luminous surface.

    Upthread I suggested a parabolic mirror, or an elliptical mirror, that would either collimate, or focus, the light from the sun onto your target. It would still require a bananas-large area of mylar, but it would require less than what you are proposing.
     

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