# Would an orbiting "sun focus" weapon in space be practical? **URL:** \ **Category:** Factual Questions\ **Created:** [July 2, 2012, 7:15am UTC](https://boards.straightdope.com/t/would-an-orbiting-sun-focus-weapon-in-space-be-practical/626981 "2012-07-02T07:15:16Z")\ **Posts on this page:** 15\ **Page:** 3
**Author:** ![md2000](https://avatars.discourse-cdn.com/v4/letter/m/73ab20/32.png) [@md2000](https://boards.straightdope.com/u/md2000)\ **Post date:** [July 3, 2012, 4:43pm UTC](https://boards.straightdope.com/t/would-an-orbiting-sun-focus-weapon-in-space-be-practical/626981/41 "2012-07-03T16:43:33Z")
> [@cmyk](#): > > 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? 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). ---
**Author:** ![billfish678](https://avatars.discourse-cdn.com/v4/letter/b/7bcc69/32.png) [@billfish678](https://boards.straightdope.com/u/billfish678)\ **Post date:** [July 3, 2012, 5:01pm UTC](https://boards.straightdope.com/t/would-an-orbiting-sun-focus-weapon-in-space-be-practical/626981/42 "2012-07-03T17:01:29Z")
> [@cmyk](#): > > ] > > 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. ---
**Author:** ![M.Constant](https://avatars.discourse-cdn.com/v4/letter/m/3ab097/32.png) [@M.Constant](https://boards.straightdope.com/u/M.Constant)\ **Post date:** [July 3, 2012, 5:39pm UTC](https://boards.straightdope.com/t/would-an-orbiting-sun-focus-weapon-in-space-be-practical/626981/43 "2012-07-03T17:39:23Z")
> [@scr4](#): > > 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. ---
**Author:** ![cmyk](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/cmyk/32/3353_2.png) [@cmyk](https://boards.straightdope.com/u/cmyk)\ **Post date:** [July 3, 2012, 6:08pm UTC](https://boards.straightdope.com/t/would-an-orbiting-sun-focus-weapon-in-space-be-practical/626981/44 "2012-07-03T18:08:15Z")
Of course I forget that focal lengths are _fixed_ for lenses/parabolic mirrors. :smack: Thanks for the correction in my understanding! ---
**Author:** ![ZenBeam](https://avatars.discourse-cdn.com/v4/letter/z/3ab097/32.png) [@ZenBeam](https://boards.straightdope.com/u/ZenBeam)\ **Post date:** [July 4, 2012, 3:05am UTC](https://boards.straightdope.com/t/would-an-orbiting-sun-focus-weapon-in-space-be-practical/626981/45 "2012-07-04T03:05:42Z")
> [@M.Constant](#): > > 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. ---
**Author:** ![billfish678](https://avatars.discourse-cdn.com/v4/letter/b/7bcc69/32.png) [@billfish678](https://boards.straightdope.com/u/billfish678)\ **Post date:** [July 4, 2012, 5:31pm UTC](https://boards.straightdope.com/t/would-an-orbiting-sun-focus-weapon-in-space-be-practical/626981/46 "2012-07-04T17:31:04Z")
> [@cmyk](#): > > Of course I forget that focal lengths are _fixed_ for lenses/parabolic mirrors. :smack: > > 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. ---
**Author:** ![md2000](https://avatars.discourse-cdn.com/v4/letter/m/73ab20/32.png) [@md2000](https://boards.straightdope.com/u/md2000)\ **Post date:** [July 4, 2012, 6:33pm UTC](https://boards.straightdope.com/t/would-an-orbiting-sun-focus-weapon-in-space-be-practical/626981/47 "2012-07-04T18:33:21Z")
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. ---
**Author:** ![supery00n](https://avatars.discourse-cdn.com/v4/letter/s/5f9b8f/32.png) [@supery00n](https://boards.straightdope.com/u/supery00n)\ **Post date:** [July 4, 2012, 6:51pm UTC](https://boards.straightdope.com/t/would-an-orbiting-sun-focus-weapon-in-space-be-practical/626981/48 "2012-07-04T18:51:18Z")
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… ---
**Author:** ![billfish678](https://avatars.discourse-cdn.com/v4/letter/b/7bcc69/32.png) [@billfish678](https://boards.straightdope.com/u/billfish678)\ **Post date:** [July 4, 2012, 6:55pm UTC](https://boards.straightdope.com/t/would-an-orbiting-sun-focus-weapon-in-space-be-practical/626981/49 "2012-07-04T18:55:50Z")
> [@md2000](#): > > 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. ---
**Author:** ![md2000](https://avatars.discourse-cdn.com/v4/letter/m/73ab20/32.png) [@md2000](https://boards.straightdope.com/u/md2000)\ **Post date:** [July 4, 2012, 9:58pm UTC](https://boards.straightdope.com/t/would-an-orbiting-sun-focus-weapon-in-space-be-practical/626981/50 "2012-07-04T21:58:52Z")
> [@billfish678](#): > > 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. ---
**Author:** ![billfish678](https://avatars.discourse-cdn.com/v4/letter/b/7bcc69/32.png) [@billfish678](https://boards.straightdope.com/u/billfish678)\ **Post date:** [July 4, 2012, 10:16pm UTC](https://boards.straightdope.com/t/would-an-orbiting-sun-focus-weapon-in-space-be-practical/626981/51 "2012-07-04T22:16:47Z")
Because thats a really low brow way to go. Personally I like my shock and awe to have a little class and high tech pizazz. BTW, you kind have missed the point. Nobody here is claiming this was something practical. Just how (un)doable is it? ---
**Author:** ![cmyk](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/cmyk/32/3353_2.png) [@cmyk](https://boards.straightdope.com/u/cmyk)\ **Post date:** [July 4, 2012, 11:28pm UTC](https://boards.straightdope.com/t/would-an-orbiting-sun-focus-weapon-in-space-be-practical/626981/52 "2012-07-04T23:28:30Z")
What about something that _looks_ like a small moon, but is really a giant space station? ---
**Author:** ![Leo\_Bloom](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/leo_bloom/32/10377_2.png) [@Leo\_Bloom](https://boards.straightdope.com/u/Leo_Bloom)\ **Post date:** [July 5, 2012, 12:04am UTC](https://boards.straightdope.com/t/would-an-orbiting-sun-focus-weapon-in-space-be-practical/626981/53 "2012-07-05T00:04:36Z")
> [@M.Constant](#): > > … > > Surely you’re familiar with little boys cooking ants with a magnifying glass. And little girls never did this? ---
**Author:** ![cmyk](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/cmyk/32/3353_2.png) [@cmyk](https://boards.straightdope.com/u/cmyk)\ **Post date:** [July 5, 2012, 4:48am UTC](https://boards.straightdope.com/t/would-an-orbiting-sun-focus-weapon-in-space-be-practical/626981/54 "2012-07-05T04:48:04Z")
> [@Leo\_Bloom](#): > > And little girls never did this? Never did they once! Cuz it’s _oogy_ and _gross!_ Hmph! _skips away_ ---
**Author:** ![bldysabba](https://avatars.discourse-cdn.com/v4/letter/b/ecccb3/32.png) [@bldysabba](https://boards.straightdope.com/u/bldysabba)\ **Post date:** [July 5, 2012, 6:20am UTC](https://boards.straightdope.com/t/would-an-orbiting-sun-focus-weapon-in-space-be-practical/626981/55 "2012-07-05T06:20:04Z")
> [@](#): > > Would an orbiting “sun focus” weapon in space be practical? It might be. If built in the 1920’s. Flees [Previous page](https://boards.straightdope.com/t/would-an-orbiting-sun-focus-weapon-in-space-be-practical/626981.md?page=2)