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Old 07-02-2012, 03:15 AM
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Would an orbiting "sun focus" weapon in space be practical?

I'm talking about the kind of weapon that was in "Die Another Day", it was a satellite orbiting the Earth that focuses sunlight into a tight beam that is highly destructive, easily ripping through buildings and creating a crater where it hits.

Could this kind of weapon be practical in real life? Especially in terms of whether it can be made to be destructive enough to be worth the trouble.
Old 07-02-2012, 03:54 AM
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One big problem (as per high energy optical lasers) is thermal bloom - the focused sunlight heats the air it passes through, causing thermal distortion (heat shimmer) that damages the focus, and thus the effectiveness.

Also, clouds. It will take such massive amounts of energy to drill through a heavy cloud layer, and your opponent will figure out what you are aiming at before it gets hit. You would really have to wait for a sunny day. The "Die Another Day" weapon was only really useful against the static minefields and other fixed emplacements of the Korean DMZ.

Finally, size. You need really big space-based mirrors with great orbital and directional coordination. Easy to make (foldable mylar) but not really a surprise weapon.

Si
Old 07-02-2012, 04:41 AM
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Interesting. I was wondering the same thing. Except using it as a weapon we could use it to drill trough the top layer of ice on Jupiter's Europa or Calisto (Which ever one has an ocean underneath).

After that, we could send a submersible probe down there and see if we can find any interesting lifeforms.
Old 07-02-2012, 04:55 AM
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Originally Posted by Shakes View Post
Interesting. I was wondering the same thing. Except using it as a weapon we could use it to drill trough the top layer of ice on Jupiter's Europa or Calisto (Which ever one has an ocean underneath).

After that, we could send a submersible probe down there and see if we can find any interesting lifeforms.
Sounds like a good way to destroy whatever ecosystem there is there.
Old 07-02-2012, 05:05 AM
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It would, however, be a possible tool to create/steer hurricanes.
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Old 07-02-2012, 05:21 AM
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Originally Posted by Shakes View Post
Interesting. I was wondering the same thing. Except using it as a weapon we could use it to drill trough the top layer of ice on Jupiter's Europa or Calisto (Which ever one has an ocean underneath).
It would have to be a big mirror - the solar energy density out at Jupiter is much lower than at earths orbit (~1/27th from the inverse square law). Also, it would be much harder to keep the mirrors in a suitable orbit, as the space is gravitationally complex. Finally, there are more bits of ice and rock moving around Jupiter. You're gonna break a few mirrors, and that means lots of bad luck.

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Originally Posted by Bosda Di'Chi of Tricor View Post
It would, however, be a possible tool to create/steer hurricanes.
I prefer the Quantum Weather Butterfly for that. Of course, you either don't know where the hurricane is going to go, or how fast it is going to get there.

Si

Last edited by si_blakely; 07-02-2012 at 05:23 AM. Reason: added correct energy density value
Old 07-02-2012, 07:13 AM
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Sounds like a good way to destroy whatever ecosystem there is there.

I'm sure the Monolith would intervene before that happened. Us being the dumb monkeys we are.
Old 07-02-2012, 08:06 AM
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The mirror would need to be incredibly massive in order to have any practical effect from orbit. Imagine a truly enormous satellite big enough that it takes up about the same % of the sky that the sun does. So even the best a mirror of that size could do is to make that spot on the ground seem like it's getting double the sun's normal output. So it'll certainly get warmer, but that's about it.
Old 07-02-2012, 09:36 AM
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What if there was a massive field of solar reflectors in the Sahara that could send a beam through a network of reflecting satellites? To redirect it back to Earth. That sounds like a more practical way of doing something like this. The only major issue is that those satellites would have to be positioned exactly right.
Old 07-02-2012, 10:38 AM
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Originally Posted by s0meguy View Post
What if there was a massive field of solar reflectors in the Sahara that could send a beam through a network of reflecting satellites? To redirect it back to Earth. That sounds like a more practical way of doing something like this. The only major issue is that those satellites would have to be positioned exactly right.

Seems to me that would be less efficient. You have loss of focus and power anytime your light beam passes through an atmosphere, and if you make the beam pass through the atmosphere twice, losing power and focus on the way up from the Sahara and on the way down, then you'll find it much more difficult to create a useful weapon.

Not that it's a bad idea, I'm actually all in favor of useless weapon systems that supervillains create, thinking they'll be able to hold the world hostage, but that can't actually hurt anyone.
Old 07-02-2012, 11:42 AM
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If you are just using reflective optics, the smallest spot you'll be able to make on the earths surface is the distance from mirror to ground divided by 114. Lets say your mirror/mirror system is in low earth orbit. 250 mile divided by 114. Lets call that 2 miles wide.

For focusing lenses and mirrors there is a specification called the F number. That is the focal distance divided the diameter of the mirror/lens.

So lets pick a fixed focal length. The sun's image at the focus/target area is X wide. The mirror is Y wide, reflecting Z watts of sunlight into that X wide image. Now lets make the mirror 2Y wide. It will be reflecting 4 times as much light (the surface area goes as the square of the diameter), but it is still "pumping it" into the same sized target. So the intensity in that target zone is 4 times higher.

A F2 system has 4 times the intensity of an F4 system. An F1 system has an intensity 16 times that of an F4 system.

From playing around with various lens and mirrors over the year, you can use an F4 or so system and focus it on your hand. About all it does is get warm (of course sending that directly into your eye would cause nearly instantaneous and permanent eye damage). To actually get the intensity high enough to actually cause stuff to catch fire in short order you need something in the F2 to F1 range IIRC. And thats stuff thats rather dry and easy to catch on fire like paper and cardboard.

So, if you want things bursting in flames or melting you are going to need a mirror system in orbit thats about 120 miles across at a minimum. And all that is going to do is set some things on fire. Its not going to be like one of the Star Trek ship phasors blasting the crap out of the target area from orbit. You could probably scale it down by a factor of 2 to 4 if you just wanted to melt roof tops, ruin car's paint jobs, and kill of some types of vegetation in the area.

Last edited by billfish678; 07-02-2012 at 11:44 AM.
Old 07-02-2012, 11:50 AM
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Oh, I could be of by perhaps a fator of 2 to 4 (WAG) due so scaling effects (or way the hell off due to doing the math in my head wrong). But it's pretty obvious the mirror's goinga have to be pretty damn big.
Old 07-02-2012, 12:11 PM
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Originally Posted by wevets View Post
Seems to me that would be less efficient. You have loss of focus and power anytime your light beam passes through an atmosphere, and if you make the beam pass through the atmosphere twice, losing power and focus on the way up from the Sahara and on the way down, then you'll find it much more difficult to create a useful weapon.

Not that it's a bad idea, I'm actually all in favor of useless weapon systems that supervillains create, thinking they'll be able to hold the world hostage, but that can't actually hurt anyone.
The reason I'm saying to put it in the Sahara is because there is 1000's of square km of empty hot (no clouds, less moist air?) space there, perhaps that would offset the loss of energy because of passing through the atmosphere.

Last edited by s0meguy; 07-02-2012 at 12:12 PM.
Old 07-02-2012, 12:35 PM
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Just using mirrors, you'll never get a mirror with the same apparent size as the Sun to look brighter than the Sun. Adding more mirrors will just increase the size of your spot on Earth, not make the space mirror look brighter at some location on Earth. So using mirrors in the Sahara will never work better than using the Sun directly.

ETA: And no, there's no way to somehow use the Sahara mirrors and the Sun to overcome this. You could get two spots on Earth, but not one brighter one.

Last edited by ZenBeam; 07-02-2012 at 12:37 PM.
Old 07-02-2012, 01:04 PM
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The Giant Space Mirror could easily be defeated by filling the air with particles similar to volcanic ash over any target. This would also stop incoming aircraft.

Last edited by TriPolar; 07-02-2012 at 01:04 PM.
Old 07-02-2012, 01:55 PM
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I recall some discussion that the USA considered such a mirror system during the Vietnam war. Not for random incineration, but to illuminate the jungle at night. It would be the brightness of the full moon or so, making it easier to spot infiltrators. One of the disadvantages, apparently, was that it would also contribute to heavier jungle growth. You win some, you lose some...
Old 07-02-2012, 02:08 PM
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On thought occured to me. Even such a system that could not catch things on fire quickly probably would kill anybody not lucky enough to be in or near a building. Just a few minutes of breathing hot assed air is gonna kill you. And if it's hot enough to be near or past the boiling point of water even faster than that.

Then again, the commuters in Atlanta will probably wonder where this cold front came from.
Old 07-02-2012, 04:08 PM
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Originally Posted by s0meguy View Post
The reason I'm saying to put it in the Sahara is because there is 1000's of square km of empty hot (no clouds, less moist air?) space there, perhaps that would offset the loss of energy because of passing through the atmosphere.

It's not relevant how much room is in the Sahara since there's no limitation size on the lens in space in the OP.


For any given size mirror in the Sahara, your effort would be better spent on a larger lens/mirror in space, since Saharan mirrors would just lose most of their efficiency in the atmosphere on their way up to the satellites anyway.
Old 07-02-2012, 04:43 PM
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I don't see why you can't create really high temperatures; much smaller solar furnaces can generate temperatures as hot as the surface of the Sun. See example here, where they melt rock and metal in seconds and wood instantly bursts into flames, which certainly sounds like a "solar death ray" as described by the OP.
Old 07-02-2012, 04:50 PM
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The difference is that the space mirror would be many miles away from its target, and would be separated from that target by a thick atmosphere. The intensity of the mirror's radiation would decrease as the square of the distance, so it would have to start out extremely powerful in order to deliver any appreciable amount of heat onto its target--even ignoring the effects of atmospheric diffusion.

A solar furnace is not subject to these limitations.
Old 07-02-2012, 04:55 PM
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Originally Posted by Michael63129 View Post
I don't see why you can't create really high temperatures; much smaller solar furnaces can generate temperatures as hot as the surface of the Sun. See example here, where they melt rock and metal in seconds and wood instantly bursts into flames, which certainly sounds like a "solar death ray" as described by the OP.
That is about an F 0.25 system (the focal length being 0.25 times that of the mirror diameter judging from the video). Yeah, those are scary strong. But they are 16 times more intense than an F 1.0 system. And 64 times as strong as an F 2.0 system. And keep in mind an F2 system is what I WAGed as being about what you needed to start getting workable. The thing would take up most of the visible sky as it passed overhead.

So, for an F 0.25 system orbiting at a low Earth orbit of about 250 miles the mirror in orbit would have to be a thousand miles across! If it was in geosynchronyos orbit it would have to be roughly a hundred times bigger than that!

Last edited by billfish678; 07-02-2012 at 04:58 PM.
Old 07-02-2012, 05:00 PM
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Originally Posted by YamatoTwinkie View Post
The mirror would need to be incredibly massive in order to have any practical effect from orbit. Imagine a truly enormous satellite big enough that it takes up about the same % of the sky that the sun does. So even the best a mirror of that size could do is to make that spot on the ground seem like it's getting double the sun's normal output. So it'll certainly get warmer, but that's about it.
This makes no sense. Are you unaware of how curved mirrors work? Its all about energy. The solar energy is a function of area of the mirror, and that energy can be focused to an extremely small spot depending on the manufacturing accuracy.
Old 07-02-2012, 05:09 PM
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This makes no sense. Are you unaware of how curved mirrors work? Its all about energy. The solar energy is a function of area of the mirror, and that energy can be focused to an extremely small spot depending on the manufacturing accuracy.
Ummmm...are YOU aware of how curved mirrors work? To reach a focal point of many miles away, the curve of the mirror would have to be EXTREMELY shallow--and for any appreciable amount of energy to be concentrated, the mirror would have to be enormous.

ANY concave mirror will have a single focal point. The more concave the mirror, the closer its focal point is to the reflecting surface, so a severely concave mirror will be more powerful in that the focused energy will be less diffused at the focal point, having traveled less distance to get there. However, since the space mirror would have to have a focal point at extreme distance (the earth's surface), a highly concave space mirror would have to be enormous.

There is also the problem that the amount of sunlight that said mirror could capture would be proportional to the cross-sectional area of the reflecting surface--which would be smaller with a highly concave mirror.
Old 07-02-2012, 05:12 PM
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This makes no sense. Are you unaware of how curved mirrors work? Its all about energy. The solar energy is a function of area of the mirror, and that energy can be focused to an extremely small spot depending on the manufacturing accuracy.
Thats really not true either. I could make a 1000 mile wide mirror with a focal length of 250 miles out of a bunch about a mile on a side flat mirrors. The smallest spot they would make would be about 2 miles across. Or I could have one giant mirror that was the perfect curve to within a fraction of a wavelength of light. The smallest spot would still be about 2 miles across. The first is crude as hell and the other impressively accurate but the results will be about the same.

The fact that the sun is NOT a point source, but has an apparent size of half a degree across basically means you can only focus the spot so small.

Now, super high accuracy and diffraction effects are a consideration when you talking about an optics resolution. But thats a totally different thing from the minimum size of the sun's image you can produce.

Last edited by billfish678; 07-02-2012 at 05:15 PM.
Old 07-02-2012, 05:15 PM
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Originally Posted by billfish678 View Post
That is about an F 0.25 system (the focal length being 0.25 times that of the mirror diameter judging from the video). Yeah, those are scary strong. But they are 16 times more intense than an F 1.0 system. And 64 times as strong as an F 2.0 system. And keep in mind an F2 system is what I WAGed as being about what you needed to start getting workable. The thing would take up most of the visible sky as it passed overhead.

So, for an F 0.25 system orbiting at a low Earth orbit of about 250 miles the mirror in orbit would have to be a thousand miles across! If it was in geosynchronyos orbit it would have to be roughly a hundred times bigger than that!
I think you are giving undue importance to the focal length of the mirror, when really, the effectiveness of a space mirror weapon is dependent on the solar energy caught by the mirror (a function of its area).
Old 07-02-2012, 05:19 PM
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This makes no sense. Are you unaware of how curved mirrors work? Its all about energy. The solar energy is a function of area of the mirror, and that energy can be focused to an extremely small spot depending on the manufacturing accuracy.
That's what I was thinking as well; for example, suppose you want a beam that is 10 meters across and 1000 times more intense than regular sunlight; you'd use 1000 (plus extra to account for for losses) 10 meter mirrors (NOT one big mirror, something like this) which are all focused on the same spot. With mirrors arranged in a square, they would cover an area of 100x100 meters, hardly the entire sky, especially when it is hundreds of miles up. Note that the beams aren't focused smaller either but kept the same width from mirror to ground (like a laser beam).
Old 07-02-2012, 05:21 PM
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I think you are giving undue importance to the focal length of the mirror, when really, the effectiveness of a space mirror weapon is dependent on the solar energy caught by the mirror (a function of its area).
And the SIZE of the spot is a function of the focal length (no easy way of getting around that (if any for tha matter)). And the collection area is a function of the size mirror.

The two cancel out and it's the F ratio that determines the intensity of the final spot.
Old 07-02-2012, 05:27 PM
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Originally Posted by Michael63129 View Post
. Note that the beams aren't focused smaller either but kept the same width from mirror to ground (like a laser beam).
That won't work.

I could take a flat mirror 1 inch across. Reflect the sun from that a distance of 250 miles. Guess how big that reflection will be? About 2 miles across. Make it a perfectly shaped curve. Guess how big the sun will be on the ground? About 2 miles across.

And this ignores diffraction effects. When you throw those in you flat mirror method can only have mirrors so small before too small becomes a problem. And once your mirrors start becoming as big as spot you want they start getting too big. I ain't working out the exact numbers but the principle is sound.

Its a small but important fact. The sun is NOT a point source. The light comes from slightly different directions in the sky. You can't take light like that and make it magically parallel with simple reflective optics.

Last edited by billfish678; 07-02-2012 at 05:29 PM.
Old 07-02-2012, 07:24 PM
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And the SIZE of the spot is a function of the focal length (no easy way of getting around that (if any for that matter)).
Yeah, you can't focus Sunlight to make a spot brighter than the Sun without violating the laws of thermodynamics. If you could, then you could use the Sun to heat something up to a hotter temperature than the Sun. That's going to translate into a restriction on the spot size.
Old 07-02-2012, 07:40 PM
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Yeah, you can't focus Sunlight to make a spot brighter than the Sun without violating the laws of thermodynamics. If you could, then you could use the Sun to heat something up to a hotter temperature than the Sun. That's going to translate into a restriction on the spot size.
Yes, but that IS going to be hot enough at some point. See the video given by another poster above. Whether that size is practical is another matter, but you can get stuff plenty hot enough.

Last edited by billfish678; 07-02-2012 at 07:44 PM.
Old 07-02-2012, 08:21 PM
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Gawd, I'll probably regret even saying this (and I think it should be ignored for this conversation) but in the interest of techinical correctness I'll say it anyway.

I've said a system from 250 miles away can only focus the sun down to a spot 2 miles across or so. But that is based upon simple geometric optics. And as Zenbeam has noted that limits how hot the spot can get (though that can still be pretty damn hot as your system gets absurdly large).

However, using fancy diffractive optics it MAY be possible to make a spot hotter still. The downside to such a system would be you would probably "throw away" most of your energy. Something like you have a spot hotter than sun a fraction of a mile across but the rest of it gets diffracted all over the place and is wasted.

You would still be conserving energy. And I am not saying it can be done. I am saying it might be possible. Though it would be so MUCH more complicated than a giant assed regular mirror or a gigantic fleet of smaller mirrors that its practicality level is at a whole nother level.
Old 07-02-2012, 09:37 PM
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Originally Posted by billfish678 View Post
Something like you have a spot hotter than sun a fraction of a mile across but the rest of it gets diffracted all over the place and is wasted....You would still be conserving energy.
You'd still have heat flowing from lower to higher temperature, which violates the second law of thermodynamics.
Old 07-02-2012, 10:21 PM
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You'd still have heat flowing from lower to higher temperature, which violates the second law of thermodynamics.
It's (imo probably) not the same thing. To me thats like the optical equivalent of saying you can't have voltage amplifiers.

And until someone here that does diffractive optics for a living comes along and says they have looked at exactly this sorta of problem and says it can or can't done I suspect all subsequent discussion about it will be mostly a bunch of hand waving.

And as I said before, even if it IS doable, its at such a higher level of complexity compared to regular mirrors in space (which is impractical enought as is) that such a discussion isn't relevant to the OP's question.

Last edited by billfish678; 07-02-2012 at 10:23 PM.
Old 07-02-2012, 10:45 PM
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That won't work.

I could take a flat mirror 1 inch across. Reflect the sun from that a distance of 250 miles. Guess how big that reflection will be? About 2 miles across. Make it a perfectly shaped curve. Guess how big the sun will be on the ground? About 2 miles across.

And this ignores diffraction effects. When you throw those in you flat mirror method can only have mirrors so small before too small becomes a problem. And once your mirrors start becoming as big as spot you want they start getting too big. I ain't working out the exact numbers but the principle is sound.

Its a small but important fact. The sun is NOT a point source. The light comes from slightly different directions in the sky. You can't take light like that and make it magically parallel with simple reflective optics.
This. Very simple, here's another way to look at it. Create a pinhole into a closed room, a camera obscura. How big will the image on the far wall be? Bigger than the pinhole. Because the light from the sun hits the pinhole from sources up to half a degree apart (the suns disc) the pinhole creates an image bigger than the pinhole - it spreads at the rate of half a degree. If the far wall was 250 miles away, the image would be that 2 miles across. A mirror is just a pinhole that can be aimed.

So you are spreading the mirrors' reflection over 2 miles diameter. In order to equal the intensity of existing sunlight, you reflecting mirrors would each have to be aimed at the same spot, and have a total area of 2 miles diameter. (ie bunch of small aimable mirrors, totaling 2 mi. Diameter. ) 4 mi gives 4 times the intensity; 8 miles, 16 times the intensity. 16 miles diameter active mirror segments, 64 times regular sunlight intensity. That should be enough to fry anyone's bacon.
Old 07-03-2012, 12:24 AM
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Hmm I've heard of a ~1m2 fresnel lens melting asphalt. Would the reason this is harder in space be that the lens is much farther away?
Old 07-03-2012, 12:33 AM
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Well, the one in space 250 miles away, rather than a foot or so away from the asphalt, would have to be about a million times bigger (literally).

Last edited by billfish678; 07-03-2012 at 12:36 AM.
Old 07-03-2012, 12:37 AM
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Why, can't the focal point be adjusted? If you're talking about inverse square law, not losses through air.
Old 07-03-2012, 12:42 AM
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Its late and I am about to call it a night.

Short story. You can only make the point so small. That smallness is directely related to the focal distance (the distance from lens to the target in this case). 10 times further away? Spot is 10 times bigger. Want the same intensity at the focal point? Gotta make the mirror/lens 10 times bigger too, hence the million or so factor for 250 miles vs a foot or so.
Old 07-03-2012, 01:30 AM
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Ah ok thanks.
Old 07-03-2012, 04:43 AM
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Basically, the focal point increases at the square of the distance.

So, let's say you have a magnifying glass placed a foot from the ground that perfectly focuses the sun's image, and therefore its heat, to an area of 5mm in diameter.*

Now, move that lens away to 2 feet away from the ground, and that focal point, which is really just a clearly focused image of the sun, is now the square of 5mm in size (25mm), and the inverse square in brightness and intensity. So, if at a foot off the ground, the spot produced heat of 500F, then at two feet away, the temperature of the 25mm spot would be the inverse square, so 4x less intense (2x2=4), producing a maximum of 125F within the 25mm image of the sun. It'd also be a quarter as bright, too.

So, keep going, at three feet from the ground (3x3=9), the spot can't get any smaller than 45mm, 9x less hot and bright than at 1 foot (55.5F).

So you can see, that 250 miles up, this weapon would have to be massive and accurate. Especially to overcome the insulated effects of the atmosphere.

And like billfish678 says, it's because you're really just projecting an image of the sun, and the sun isn't a point source, but has and angular diameter in the sky, so the distance of the lens or mirror, can only project a non-diffuse (or non-blurry) image of the sun on the ground of a minimum size and intensity based on this angular size.

At least I think! Please correct my understanding if I'm wrong!

*these numbers are arbitrary just to keep it simple.

ETA, looking at billfish's last post, it seems the focal spot increases linearly, not squared?

Last edited by cmyk; 07-03-2012 at 04:47 AM.
Old 07-03-2012, 12:43 PM
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Basically, the focal point increases at the square of the distance.

So, let's say you have a magnifying glass placed a foot from the ground that perfectly focuses the sun's image, and therefore its heat, to an area of 5mm in diameter.*

Now, move that lens away to 2 feet away from the ground, and that focal point, which is really just a clearly focused image of the sun, is now the square of 5mm in size (25mm), and the inverse square in brightness and intensity. So, if at a foot off the ground, the spot produced heat of 500F, then at two feet away, the temperature of the 25mm spot would be the inverse square, so 4x less intense (2x2=4), producing a maximum of 125F within the 25mm image of the sun. It'd also be a quarter as bright, too.

So, keep going, at three feet from the ground (3x3=9), the spot can't get any smaller than 45mm, 9x less hot and bright than at 1 foot (55.5F).

So you can see, that 250 miles up, this weapon would have to be massive and accurate. Especially to overcome the insulated effects of the atmosphere.

And like billfish678 says, it's because you're really just projecting an image of the sun, and the sun isn't a point source, but has and angular diameter in the sky, so the distance of the lens or mirror, can only project a non-diffuse (or non-blurry) image of the sun on the ground of a minimum size and intensity based on this angular size.

At least I think! Please correct my understanding if I'm wrong!

*these numbers are arbitrary just to keep it simple.

ETA, looking at billfish's last post, it seems the focal spot increases linearly, not squared?
Seems to me your math is confused (but correct in part).
The lens makes a 5mm DIAMETER perfect focus at a foot.
A different lense with a 2-foot focal length, can only make a perfectly focused 10mm image at 2 feet, not 25mm. The intensity of the light, anywhere in the image, goes down by 1/4 (area=diameter squared); or your lense needs 4 times the area to produce the same heat. And so on...
20-foot focal length 100mm sun image.

OTOH, the 1-foot focal as you move it out (or any other lense) the sun image becomes unfocussed and evern larger; the perfect focus is the smallest you can make the image of the sun.

So at 250 miles, the sun's image will be 1/2 degree wide; or, about (250/114) if you know how to work in Radians, 1 radian is about 57 degrees, so half a degree is about 1/114 of a radian, and using sine law, sin(height/distance) is approx equal to (height/distance) for small angles.

(Basically, the image of a disc at effective infinite distance, but 30 minutes or 1/2 degree in diameter - with a spread of half a degree from the lens, lens focal distance 250 miles).
Old 07-03-2012, 01:01 PM
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Originally Posted by cmyk View Post
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ETA, looking at billfish's last post, it seems the focal spot increases linearly, not squared?
Twice as far away? The sun's image is twice its previous diameter (assuming you are focusing so the diameter is at a minimum). If you keep the same F ratio then your lens/mirror also is twice the previous diameter.

So, the lens/mirror has collected 4 times a much light (area goes as diameter squared). But the spot/image of the sun also has 4 times the previous area (twice the size, four times the area again).

So, if you keep the F ratio constant, the intensity of the light (or power per amount of area) stays constant.

An I think the North Koreans have lauched their solar weapon and are now using it where I live. Its pushing 100 degrees and the heat index is hovering around 130right now. And I'm in the deep south and it sure as hell aint one of those nice dry heats either.
Old 07-03-2012, 01:39 PM
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Originally Posted by scr4 View Post
You'd still have heat flowing from lower to higher temperature, which violates the second law of thermodynamics.
It's still the same amount of energy (accounting for losses due to inefficiencies), it's just concentrated in smaller area, hence higher temps.

Surely you're familiar with little boys cooking ants with a magnifying glass.
Old 07-03-2012, 02:08 PM
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Of course I forget that focal lengths are fixed for lenses/parabolic mirrors.

Thanks for the correction in my understanding!
Old 07-03-2012, 11:05 PM
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Originally Posted by M.Constant View Post
It's still the same amount of energy (accounting for losses due to inefficiencies), it's just concentrated in smaller area, hence higher temps.

Surely you're familiar with little boys cooking ants with a magnifying glass.
From the ant's point of view, the Sun looks big. It occupies a larger angular area than without the magnifying glass. That is why the ant burns.

But the ant will never heat up more than the Sun. It will never get to a higher temp than the Sun, from radiation from the Sun, no matter what magnifying glass or mirrors you use.

Last edited by ZenBeam; 07-03-2012 at 11:06 PM.
Old 07-04-2012, 01:31 PM
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Originally Posted by cmyk View Post
Of course I forget that focal lengths are fixed for lenses/parabolic mirrors.

Thanks for the correction in my understanding!
They typically ARE but they don't have to be. The important part to remember for this discussion is that whatever distance you optical system is from the target will be by definition the focal distance. That distance will determine the minimum size you can focus the sun down to. And once you have that, then that also determines how big your solar death ray machine will have to be achieve whatever "power level" you think you need.

This recent heat wave got me thinking. The OP was probably thinking along the lines of tanks exploding, buildings burst into flames, and people getting charr broiled in short order. And, I think we have shown that to do that your orbiting thingamabob is going to have to be absurdly big.

Now, if you are patient and don't need all that drama you could probably get by with something much smaller. And used the savings to bedazzle your new lair.

Lets wait until you have a heat wave in some big city. Now, lets use our orbiting mirrors to say double the amount of sunlight hitting the area. If you could keep that up for a few hours I think you could heatstroke a buttload of people. And you ain't evacuating a big city fast enough probably.

Or, even if your enemy comes up with a plan to evacuate the cities or build shelters or whatever you still have another option.

Hit agricultural areas. I suspect during a hot day there are plenty of crops/plants that if they were exposed to a modest increase in the amount of sunlight for not a terribly long period of time you'd kill em. Or for that matter an abnormal amount of heat/light during some other time of year might mess up their growth cycle.

The impact of being able to wipe out a few square miles of crops/plants every day would be an economic pain the ass at least for the target country even if nobody died or no infrastructure was damaged.
Old 07-04-2012, 02:33 PM
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The big problem is, it would be pretty obvious what was happening and it would be an act of war. If you weren't the USA, Russia or China, you'd be a friend of one of those. whoever is on your side would gladly demonstrate (with warheads) why the mirror in space violates far too many rules to be left alone, the primary one being no weapons in space, please.

So many square miles of active mirror would be a sitting duck. Just take out the command/control units and the mylar sheets would drift away aimlessly; I suspect with that surface area it would re-enter pretty soon, all the while being a tangled mess on unaimed garbage.

Last edited by md2000; 07-04-2012 at 02:33 PM.
Old 07-04-2012, 02:51 PM
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If there was a large enough sun focus in space, and it were big enough, then it would be terrible. Imagine if our earth was an ant, and the increase in intensity over the whole earth was by a factor of 100. This would be as if the Sun had become a red giant in the area in which the magnifying glass was focused. If that area was as large as the Earth, (and if it is actually possible to focus sunlight by a factor of 100 through an area as large as the Earth itself), then the oceans would boil, we would all die, and what is supposed to occur naturally through the evolution of the Sun through its red giant stage will be caused a billion years early.

Of course, the other planets will not be affected.

Don't try this at home...

Last edited by supery00n; 07-04-2012 at 02:53 PM.
Old 07-04-2012, 02:55 PM
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Originally Posted by md2000 View Post
So many square miles of active mirror would be a sitting duck. Just take out the command/control units and the mylar sheets would drift away aimlessly; I suspect with that surface area it would re-enter pretty soon, all the while being a tangled mess on unaimed garbage.
Oh, quite true.

Such a weapon would only be workable, if not practical, if the country that used it used against a non-space capable country and the user country could also make it quite clear to any other space capable country that they best not be fucked with when doing their business elsewhere.
Old 07-04-2012, 05:58 PM
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Originally Posted by billfish678 View Post
Oh, quite true.

Such a weapon would only be workable, if not practical, if the country that used it used against a non-space capable country and the user country could also make it quite clear to any other space capable country that they best not be fucked with when doing their business elsewhere.
If you can get away with that, why ot just send a wave of bombers and cruise missiles in to blow the country into submission with shock and awe, followed by ground forces who expect to be greeted as liberators... that should get the mission accomplished and would be a lot cheaper than a billion dollars worth of mylar - wouldn't it?

Actually, anyone capable of this tech has already agreed not to weaponize space, because that's a really bad and expensive arms race toget into.
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