I came across one space weapon, known as the Sun Gun, which uses a concave mirror on a satellite to concentrate sunlight onto a small area at the Earth's surface, making a hot beam of death. Apparently, according to a German scientist, 9 km^2 of reflectors are enough to burn oceans and cities, but that is not the point.

Assuming that the contraption even works the way it should, we should also find a way to turn off the weapon when not in use. After all, we only want to cause mayhem when we need to, and we don't want a weapon we cannot control. So, just what is the best way to deactivate the Sun Gun, to stop it from forming a heat beam when we don't need one?

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    $\begingroup$ This is similar to the Archimedes death ray, which to my understanding was likely a series of polished shields properly shaped and used together. web.mit.edu/2.009/www/experiments/deathray/… $\endgroup$
    – DWKraus
    Dec 28, 2020 at 14:36
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    $\begingroup$ Notice that it's hardish to make biggy mirrors, and even more biggy concave ones that redirect at a very precise point. You'd need to split the mirror in hundreds or even thousands of tiny mirrors and make lots of tweaking on site ^^. $\endgroup$
    – Tortliena
    Dec 28, 2020 at 16:01
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    $\begingroup$ @Tortliena given that comment, I'm sure you're aware that this is how modern giant telescopes are created. $\endgroup$
    – RonJohn
    Dec 28, 2020 at 18:51
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    $\begingroup$ Could your world use that heat productively? Redirect the sun gun to something useful, like electrical energy production or aluminium smelting? That way it doesn't turn off, it just re-targets to a point designed to take the heat, or put a Night Shade on it? Comment cos not answering the question as asked. $\endgroup$
    – Criggie
    Dec 28, 2020 at 23:35
  • $\begingroup$ You moon it!!!! $\endgroup$
    – Hot Licks
    Dec 29, 2020 at 14:38

5 Answers 5


Just deform it and lose focus.

The problem with your death ray is that if you don't have a quick way to turn it on and off you can't just take out a city. You have to take out an entire swath leading up to the city.

The solution is to build your concave mirror with variable geometry. Once the mirrors are not forming a perfectly concave mirror that focuses your death ray on New York City the rest of the state will just receive a little more light, no burning.

In fact it doesn't have to be concave. Just line up as if it was. It's called a Fresnel Reflector. Looks like this:

enter image description here

Compact Linear Fresnel Reflector technology by CNIM - Concentrating solar power plant

Just that easy. Please don't use your death ray for evil.

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    $\begingroup$ Surely you want to burn a whole swathe of stuff leading up to the city, no evil overlord wants to just win, they need people to know. $\endgroup$
    – Separatrix
    Dec 28, 2020 at 11:24
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    $\begingroup$ @Separatrix don’t need swaths for that when I can carve my name into the moon. Can the kids still read cursive? $\endgroup$ Dec 28, 2020 at 12:11
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    $\begingroup$ @candied_orange No, but they'd probably recognize an emoji 💩 $\endgroup$ Dec 28, 2020 at 21:37
  • $\begingroup$ @candied_orange, but that lacks the glorious sense of anticipation as the beam approaches their homes and they accept their inevitable deminse. $\endgroup$
    – Separatrix
    Dec 29, 2020 at 14:41
  • $\begingroup$ This is essentially the path taken in John Ringo's Troy Rising series (and there, it's many individual arrays, spread throughout the solar system). $\endgroup$ Dec 29, 2020 at 18:37

Nope, sorry. Your Sun Gun is permanently disabled

Because the Sun is not a point source of light, the mirror will not be able to perfectly focus its light on one spot.

Indeed, no mirror reflecting a real-world light source can achieve a greater apparent brightness than matching the temperature of its source. See conservation of etendue for the technical details of this.

The mirror as proposed by Oberth's "Sun Gun" proposal would be no more than a nice flashlight in the sky. At 9km^2 surface area , the mirror will have a diameter of just 3.4km

3.4km at 8500km altitude is a mere 0.023 degrees. The "Sun Gun" will be able to illuminate at most 1/475 as bright as noonday sunlight.

Even if we change Oberth's design, and put the reflector in VERY low orbit (120km), it will only span 1.6 degrees, and provide at most 10.2 times solar illumination. This might, might be enough to start fires. Remember that the light focus will rapidly diminish from this hottest center focal point, most of its light will be a diffuse spot about 5km wide, and that the mirror would be zooming past at more than 7.8km/s thus requiring very rapid adjustment of the focal point to keep one spot targeted.

But, assuming you set up a sufficiently large reflector with suitable controls: All that is needed to "safe" it is to minutely de-focus the beam. If instead of focusing on the ground it focuses 1/10th of the distance (still an almost perfectly flat mirror), then the light intensity on the ground is less than 1/1000th the peak achievable level, and your sun gun is only a tiny bright spot in the sky, from the minute fraction of its surface that manages to reflect the sun towards you.

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    $\begingroup$ @PcMan some of your arguments look too refutable to me: (1) It's not conservation of etendue that limits achievable temperature but radiation equilibrium. (2) Solar radiation lifts surface temperatur from 3K to -17°C (ca. 250K) so a tenfold increase would achieve 2500K/2000°C (very order-of-magnitude), this would be more than enough for military use. (3) Oberth wouldn't have risked his position with a plan that's so easily refuted in the planning phase: he would have been unable to shift blame and his rivals would have instantly used this weakness against him (Nazi Germany kindled rivalry). $\endgroup$
    – toolforger
    Dec 28, 2020 at 19:53
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    $\begingroup$ Bear in mind that CSP power plants do indeed concentrate Sun's energy enough that they are a concern to wildlife, as birds flying to close to the focal point get boiled/burned. If a set of mirrors pointing at a solar tower can burn a bird it is completely conceivable that a much larger mirror can be weaponised. $\endgroup$
    – JANXOL
    Dec 28, 2020 at 19:59
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    $\begingroup$ @JANXOL The surface of the mirrors can appear to be as bright as the surface of the sun itself; that is the limit of using mirrors to focus light. At a few dozen meters distance, this can fry things. But at orbital distance, you need a ridiculously large set of mirrors for that to be hot enough. $\endgroup$
    – Yakk
    Dec 28, 2020 at 22:03
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    $\begingroup$ Obligatory xckd ... OK not an xkcd cartoon but one of the What-If essays: Fire From Moonlight This What-If is about moonlight, but as part of the answer, the question "Why can't you just concentrate more and more of the Sun's energy down onto the same point?" crops up, and is of course answered. $\endgroup$
    – straycat
    Dec 29, 2020 at 0:37
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    $\begingroup$ If New York's sky is entirely filled with orbital mirrors, then you could raise New York's temperature to at most half the temperature of the sun regardless of the height of the mirrors. Practically it would probably be quite a bit less. But that's fine, even a fraction of the temperature of the sun would kill everybody. $\endgroup$
    – Robyn
    Dec 29, 2020 at 3:37

Direct the Mirror Away from the Sun

Spacecraft routinely have systems to keep their solar panels aimed at the sun for maximum power generation. You would have to have stationkeeping and actuators to keep the mirrors pointed at the sun at the appropriate angle to focus light on earth.

When you want it switched off, redirect the mirrors so they aren't reflecting the sun's light. Done.

  • $\begingroup$ But doesn't rotating the mirror away means gradually dragging the beam on the ground until it is no longer pointing at Earth? - As @candied_orange puts this in their answer above - "The problem with your death ray is that if you don't have a quick way to turn it on and off you can't just take out a city. You have to take out an entire swath leading up to the city." $\endgroup$
    – G0BLiN
    Dec 28, 2020 at 16:20
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    $\begingroup$ @G0BLiN - nope. Having one obscenely large mirror is not particularly flexible and would require a perfectly circular orbit to keep its focal distance at ground level. There would be thousands of mirror panels making up the concave mirror, all of which could be rotated independently. So there could instantly be no more concave mirror.\ $\endgroup$
    – jdunlop
    Dec 28, 2020 at 18:47
  • $\begingroup$ jdunlop - that's a valid point - consider adding it to the answer? Seems that OP assumes a single large mirror rather than a cluster of them. $\endgroup$
    – G0BLiN
    Dec 28, 2020 at 18:52

Make the mirrors transparent. Perhaps by rotating them so they do not catch the sun, perhaps by using "smart glass" and running an electric current through them, which causes glass to become transparent.

The light has to transmit. Turning them non-reflective would burn up your sun gun.

  • $\begingroup$ Smart glass is a good idea, but making the mirrors non-reflective wouldn't burn up the sun gun, it would only heat it up a bit. The whole point is that it's focusing natural sunlight, so a non-reflective surface would just experience normal sunlight. $\endgroup$
    – jdunlop
    Dec 28, 2020 at 18:48

Polarized Glass

You ever take 2 pairs of polarized glasses (like you get in a 3D movie screening) and overlay them on top of each other? If they're lined up you can see through them just fine. Rotate one of them 90 degrees and it becomes completely opaque. So what you need is 2 layers of polarized glass over your mirrors (one of them can be the glass of the mirror itself). One layer is fixed and the other is on a rotating actuator. Simply rotate the glass filter 90 degrees and your mirror becomes non-reflective. (You can also rotate them partially to attenuate the reflectivity - give the city a taste of the heat you can produce before you turn it on full-blast.)

Now ideally, you're also following some of the other advice and making a large array of small mirrors rather than one big mirror, so rotating a bunch of small polarized filters can be done fairly quickly. To avoid any side effects of rotating elements affecting the attitude of your satellite, alternate the rotations of each filter between clockwise/counter-clockwise, and the total angular momentum will cancel out. (90 degrees in either direction works equally well for blacking out the mirrors).

  • $\begingroup$ fun fact: if you put a third polarizer at 45° between the other two, you can see through again, due to quantum weirdness. $\endgroup$
    – ths
    Dec 29, 2020 at 18:43

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