Would an orbiting "sun focus" weapon in space be practical?

Factual Questions
21

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!

22

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.

23

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.

24

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.

25

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).

26

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).

27

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.

28

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.

29

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.

30

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.

31

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.

32

You’d still have heat flowing from lower to higher temperature, which violates the second law of thermodynamics.

33

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.

34

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.

35

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?

36

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).

37

Why, can’t the focal point be adjusted? If you’re talking about inverse square law, not losses through air.

38

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.

39

Ah ok thanks.

40

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 500°F, 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 125°F 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 (55.5°F).

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?