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There are a few uses for ground-to-space lasers in science fiction settings. One is the laser broom, used to shoot space debris until it deorbits and burns up. Another is beam propulsion, or sending energy to a launching spacecraft via laser. These technologies pose a pretty serious question; how do you prevent millions of people from going blind when they inadvertently look up at the sky?

As I understand it, lasers can cause eye damage even if you aren't in the path of the beam due to scattered radiation and diffuse reflections. An obvious solution would be to provide laser safety glasses; in a small-scale, single-room laboratory, this works fine. However, I can't wrap my head around how you could possibly distribute enough glasses to reach every single person on this half of the Earth, and communicate to them when and where the laser will fire no matter their language or literacy level.

This kind of logistics would require knowing the location of every single human being within a horizon's length, including isolated rural families, remote indigenous tribes, homeless folks, etc etc, and being able to send them protective equipment, time and date without timezone errors, direction, and an order to lock themselves indoors without windows. Plus, there are animals, who you can't just put safety glasses on. I've considered the unethical approach of having the government/corporation/military just ignore the people and animals they aren't able to reach, but I think this would be financial and political suicide in the age of social media.

My question is this: how can you safely fire a laser at an orbital (or beyond) target without blinding millions of people?

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    $\begingroup$ If this is really a problem for humans, it is going to be much, much worse for wildlife, to the extent that using this thing at all is going to be an environmental catastrophe. You don't want to cause a complete collapse of the ecosystem, right? Fortunately, it seems this should not be an issue... $\endgroup$
    – Matthew
    Oct 5, 2020 at 13:36
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    $\begingroup$ Also, I am confused (and I seem to be not-alone); are you asking about a laser firing from orbit, as in your final sentence, or from the ground, as implied by the entire rest of your question? $\endgroup$
    – Matthew
    Oct 5, 2020 at 13:51
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    $\begingroup$ The whole scenario of mass blindness was explored in detail in The Day of the Triffids - Dated now but still an excellent read. Has been filmed several times. $\endgroup$ Oct 5, 2020 at 15:09
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    $\begingroup$ @Matthew I must have misunderstood the meaning of "orbital laser", I thought it meant "a laser that shoots into orbit." Apologies for the error $\endgroup$ Oct 5, 2020 at 16:50
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    $\begingroup$ Incidentally, a 30W green laser (way stronger than your usual laser pointer) was fired into the sky from an institute building of the university in Bonn (Germany) that is located more or less in the city centre. No safety measures were taken for the general population without ill effects. $\endgroup$
    – arne
    Oct 6, 2020 at 15:07

5 Answers 5

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Although laser light is collimated, the bloom it creates in atmosphere will not be. Any reflected light is reflected in all directions more or less equally. (Unless you have something in the air capable of specular reflection, and it's all aligned in the same direction - in which case, stop shooting your laser at the giant floating mirror!) Most of the light that doesn't reach the target is absorbed rather than reflected, and this is re-radiated in all directions as black-body radiation.

The effect of non-collimated light falls off with the square of the distance from the origin. Even if the laser light is dangerous at a distance of a few meters, it won't be dangerous over kilometers. If it were, you wouldn't be worrying about blinding people, you'd be worrying about destroying your laser by superheating the air in front of it into plasma.

It's probably prudent to avoid getting too close to the beam, but you shouldn't let people get too close to your space infrastructure for any number of other reasons. Rocket launch sites and testing ranges are usually built in fairly out-of-the-way locations for this very reason, and laser installations would follow suit.

Inevitably, some animals (mainly birds) will wander too close to the beam and be harmed by the light and/or heat, but it should not be terribly out of line with other infrastructure. Environmental impacts are rarely "none", but in all likelihood it wouldn't be worth raising a fuss about.

tl;dr: as long as you practice safety measures appropriate to an industrial site, you'll be fine.

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    $\begingroup$ "The effect of non-collimated light falls off with the square of the distance from the origin": the effect of any light source falls off with the square of the distance; better collimation just changes the constant of proportionality, but the overall relationship still holds. $\endgroup$
    – AlexP
    Oct 5, 2020 at 11:37
  • $\begingroup$ Thank you for the explanation, I'd assumed it would be dangerous by default $\endgroup$ Oct 5, 2020 at 17:38
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    $\begingroup$ Minor nitpick, the light that is reflected along the path of the beam through the atmosphere would be a line source. So as long as you are much closer to it as its length, its intensity decreases linearly, not quadratically. (As you say, the intensity from each point of it decreases by the square, but at the same time the amount of it you see when looking in a certain direction increases linearly.) Still, the intense part will be the very first few kilometers, where the air is dense and 100% of the original beam is still present, so the square law will take over soon enough anyway. $\endgroup$
    – mlk
    Oct 5, 2020 at 19:22
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    $\begingroup$ @mlk When you're close enough to the beam for the intensity to decrease linearly, the beam will also subtend a significant angle in your field of view, which would spread the image over a larger area of the retina with correspondingly lower intensity per unit area. Whether this actually helps is beyond my knowledge; and I don't think it would not help at all against corneal damage. $\endgroup$
    – David K
    Oct 6, 2020 at 12:50
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    $\begingroup$ Reading deeper into this answer, doesn't that mean that the atmosphere will dissipate enough energy from such a laser it would be nearly useless for propulsion? $\endgroup$ Oct 6, 2020 at 15:59
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When you read that laser beams are very well collimated, it doesn't mean they do not diverge at all. It just take them long distances to show some appreciable divergence, but they DO diverge.

For example, in the Lunar Laser Ranging experiment, the laser beam which is shot from Earth is no longer a point when it reaches the Moon surface

At the Moon's surface, the beam is about 6.5 kilometers (4.0 mi) wide.

Just to give you a measure, every $10^{17}$ photons fired, only 1 is collected back on Earth (after the roundtrip).

As you see, distance is your friend, spreading the laser beam over a large area.

If you pick a frequency where the atmosphere is opaque, you have further attenuation.

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  • $\begingroup$ The distance to the moon is about 1000 times that of a low Earth orbit, at LEO range divergence won't make it as harmless. And I doubt anyone would pick a frequency where the atmosphere is opaque for a laser that should penetrate the atmosphere (beam propulsion requires a land-based laser, after all, and laser broom is supposed to be land-based as well, also it could be placed in orbit in theory.). $\endgroup$
    – Alice
    Oct 5, 2020 at 10:33
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    $\begingroup$ @Alice, OP is asking about firing an orbital laser, not a ground based laser. $\endgroup$
    – L.Dutch
    Oct 5, 2020 at 10:39
  • $\begingroup$ If you pick a frequency where the atmosphere is opaque, you have even more reflection coming back down to the ground. $\endgroup$
    – user253751
    Oct 5, 2020 at 15:11
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    $\begingroup$ @user535733 The question body says ground-to-space lasers. $\endgroup$
    – user253751
    Oct 5, 2020 at 16:50
  • $\begingroup$ @L.Dutch-ReinstateMonica OP is asking about a ground-based laser, they just didn't realize what orbital means. (They stated as much in a comment.) $\endgroup$
    – Kat
    Oct 5, 2020 at 17:14
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No need for goggles.

The inverse square law is out friend here.

One of the things that makes laser's a danger to the eye, while other sources of light (lightbulbs) are not is that laser light is aligned into a beam. This means that even a modestly powerful laser can generate a much brighter point of light than a powerful light bulb where the energy is far more spread out.

Now, when laser light scatters from these orbital debris and asteroids the light scattered will come out at all kinds of angles, and so it will spread out in all (or at least many) directions.

Near Earth orbit is 20,000 km above the ground. Lets say a laser is hitting some debris or the back-plate of a spacecraft using laser-assisted launch at that height. Lets further assume (for simplicity) that the laser light is scattered evenly in all angular directions. So at the 20,000km mark the energy is evenly spread over a sphere with surface area of $5\times 10^{15}$ meters (a 5 followed by 15 zeros).

On Earth sunlight has an intensity of about 1 kilowatt per square meter. To be as bright as the sun (as seen from the Earth's surface) the power you need to be comming off this piece of space debris is about $5\times 10^{18}$ Watts. This is 5 exajoules per second.

Lets put that in perspective, according to wikipedia (https://en.wikipedia.org/wiki/World_energy_consumption#/media/File:Japan_energy_and_GDP.svg) the energy usage of Japan (I think only on the power grid) was about 20 exajoules in the entirety of 2010.

This hypothetical laser is dissipating a Japan's worth of energy into wasted scatter light every 4 seconds.

Now their are lots of assumptions here, factors of 10 or even 100 or possibly 1,000 could be altered by adjusting them. The biggest weakness is the assumption of even spread, the asteroid you are melting might have one big flat metallic facet that acts like a mirror. But the point remains, no plausible laser system operating in space has any chance of getting anywhere near outshining the sun due to laser scattering. I assume that causing widespread blindness would require reaching intensities in this kind of ballpark.

Their are still dangers. If some computer error (or terrorist attack, cyber-attack, other problem) caused a laser to turn it beam to directly face the Earth then the danger is potentially much higher.

A final point. If one found that the dangers were much greater than indicated by my estimates, and blindness was in fact a danger. Then a possible solution is to choose lasers that operate at a wavelength that is absorbed strongly by the atmosphere. This would have drawbacks (all the laser stations would have to be in space), but it would ensure that Earth was safe.

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  • $\begingroup$ If there is a mirror surface, it won't cause widespread blindness, but it can cause blindness to a few dozen (or maybe even hundred) unlucky people that would be in the area where a significant part of the beam would be reflected. It's the same as with ordinary lasers: while a small-power laser wouldn't pose a threat were it evenly dispersed, there is always a risk of it hitting a tiny reflective speck and hitting your eye. $\endgroup$
    – Alice
    Oct 5, 2020 at 10:28
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I agree with the observation about the benefits of the inverse square law to mitigating this problem. I add also that the wavelength of the laser's operating mode is also an important factor

enter image description here

Wavelengths greater than 1.5 $\mu$m are absorbed by water -- this makes them eye-safe since the water in the eyeball absorbs the radiation before it can damaged the rods and cones of the retina.

If your planet based lasers operated in the pass bands at ~5.5 $\mu$m and ~6.5$\mu$m, were the attenuation is minimized, the energy would pass through the lower atmosphere with minimal losses and the inverse square law would refracted light would be too attenuated, both by atmosphere and viscous goo in the eye, to injure a passers-by vision.

That said the laser systems would want to operate either atop mountains -- to lower atmosphere density and absorption -- or in arid places with low atmospheric water content. These two conditions will maximize the power transfer to the target.

There would also be some concern with dust since it could absorb energy and flare in different wavelengths which could be in UV range. Think thermalized electrons or fluorescing mechanisms -- there are many processes available to cause matter to generate photons in response to energy.

This master's thesis discusses transmission and absorption of IR in the atmosphere, if you want more info.

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  • $\begingroup$ It's rather far fetched to call "water absorption" a means of making lasers eye-safe. See CO2-lasers for the sort of damage done to eyes by IR-lasers. $\endgroup$
    – Ghanima
    Oct 6, 2020 at 19:59
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how do you prevent millions of people from going blind when they inadvertently look up at the sky?

This is going to be really obvious, but...

If the laser is on the ground and pointing at the sky, that means the light is going into the sky. In order to get blinded by a laser, the laser light has to go into your eyeball. That means you have to be in the sky, not just looking at the sky.

Unless the laser accidentally reflects off a plane.

So don't point it at planes, or the ISS.

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    $\begingroup$ As the OP notes, just seeing the beam of really high-powered lasers is reportedly dangerous. (You might want to ask this guy about that...) That said, others have stated that this is only the case at short distances and/or due to accidental reflections. $\endgroup$
    – Matthew
    Oct 6, 2020 at 2:54
  • $\begingroup$ As far as I know, the main reason you shouldn't be anywhere near the beam of a high-powered laser without safety equipment is because of accidental reflections. $\endgroup$
    – user253751
    Oct 6, 2020 at 9:29

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