Death lasers in space and diffraction

[mr friendly guy]

This just occurred to me, but I thought I might put it in SLAM rather than sci fi because supposedly militaries do consider building such a hypothetical device.

Wouldn't there be a problem with diffraction of the laser as it passes from the upper atmosphere to hit the target closer to the ground? The upper atmosphere is colder than the atmosphere closer to the ground, so its diffraction index is different, the same principle why some images on a really hot day look "wavy". How can a theoretical space based laser overcome this as I imagine if the laser was to target something "small" like a building, it might throw out the aim.

[Darth Yoshi]

Move directly over the target? It doesn't matter what the diffraction index is, n sin 0 is always 0.

If that's not an option, if you knew the absolute position of a target then a robust enough targeting computer could probably calculate the indeces from the discrepancy between the absolute and observed positions, and from there determine the proper angle to hit the target.

[Purple]

I don't think hitting it is what's the problem. IIRC. The issue is that the laser would waste huge amounts of energy heating and boiling the air around its path. And that would make it hideously inefficient as a weapon.

[someone_else]

Wouldn't there be a problem with diffraction of the laser as it passes from the upper atmosphere to hit the target closer to the ground?

The main issues with diffraction are usually the ones caused by the laser itself, that heating up the air it moves thorugh creates such random "lenses" of hot air that eventually defocus the beam. You end up with a very warm instant-ignition spot, but not with a serious weapon. Unless the output is far into the unreasonable like with most sf shows anyway.

Diffraction caused by such phenomenon would mainly be annoying just as wind is to modern artillery. You look at how you aimed and where the shot landed and compensate with the next shots. Given that lasers can do it without actually shooting the full-power shot, its easier.

The flying laser (that aircraft with a laser turret in the nose and a ludicrously huge gas laser assembly in the rest of the fuselage) does this trick to some extent using a couple additional (non-weapon) lasers to measure where the air diffracts the beam before shooting teh main lazor.

[LaCroix]

Alternatively, if you can fire a continuous beam - just slide in the right direction after the impact point is known.

[someone_else]

yeah, if you don't have the cooling issues that plague modern lasers.

[LaCroix]

yeah, if you don't have the cooling issues that plague modern lasers.

Hence, "if". I'd say cooling problems are your least problem if you managed to get one of those puppies into space...

[Simon_Jester]

Alternatively, if you can fire a continuous beam - just slide in the right direction after the impact point is known.

That doesn't help you with the "random lenses of hot air" problem, though.

[LaCroix]

Alternatively, if you can fire a continuous beam - just slide in the right direction after the impact point is known.

That doesn't help you with the "random lenses of hot air" problem, though.

To my understanding, that can't be helped by any method. The impact point will oscillate within a certain area all the time.

So you can either shower the target in pulses and hope for similar deviation that results in a hit, or use a continuous beam that dances over the ground and swipe it over the target area. I believe your chances are better with the last option.

[starslayer]

There are two different concepts being thrown around here - diffraction and refraction. Diffraction is the bending of light around obstacles, just like what water waves do around jetties and such. Refraction is the bending of light as it passes from one medium to another. There is no such thing as an index of diffraction - however, there is an index of refraction, which measures how much light is bent if we presume it encounters the medium coming from pure vacuum. The index of refraction for sea-level air is already very close to one in visual wavelengths (1.0003), and as you move into the upper atmosphere, it gets even closer to one, so the bulk refractive properties of air are of no real consequence unless you want super accurate targeting, and even then, it's more on the level of "aim slightly to the left" than "oh crap, huge problem."

As has been said, however, since the atmosphere is not a nice isothermal slab of gas with an exponential density profile, there is turbulence at all times. This can cause the beam to get quite a bit more diffuse, even though in terms of actual targeting, it doesn't have much effect (the worst seeing usually gets is a few arcseconds; at a distance of 500 km, the upper edge of LEO, this translates into a targeting error of about 2.5 m/arcsec). To counteract this, you can use adaptive optics, which will ensure that, at least at first, your beam remains fairly concentrated all the way down. What will throw this out of whack is the fact that your beam, if it's powerful enough to do much of anything on the ground, is going to heat and ionize the air in at least a portion of the beam column fairly rapidly, greatly worsening the atmospheric turbulence and seeing.

However, there is another problem that does actually have to do with diffraction; namely, the fact that the smallest spot size you can generate is limited by the size of your laser opening. This is because you cannot perfectly collimate the beam inside the laser, and due to the fact the beam will partially diffract around the circular aperture of the laser system. This effect will get smaller and smaller as you increase the aperture size, and gets worse as you move to longer wavelengths. For a laser with a 6" aperture, for example, the smallest possible spot size is about an arcsecond for a green laser using the Rayleigh criterion, which may not be fully appropriate, but should be close enough. This means that your spot will be at least 2-3 m wide shooting from LEO even ignoring atmospheric effects, and there's nothing you can do about it.

[Psawhn]

Also; if you're worried about missing due to the refraction of light due to the atmosphere, ignoring natural and laser-generated turbulence and the other concerns that have been brought up, that can be solved by targeting using a camera. One of the basic ideas in optics (and if we treat the whole laser/atmosphere system as an optical system) is that the path traced by a light ray is bidirectional. This means that you just point the laser where you see the target, no matter if that's not where it "actually is," that's where your laser will hit. This differs from the classic spear-fishing problem in that with a spear you are trying to hit the fish where it actually is, not where it looks like it is. If you went fishing with your laser, you only have to aim at the fish.

Of course, this assumes that your camera is up to the job and requires very precise and accurate alignment between the camera and your laser emitter. If your target is obscured by clouds or dust, that will also pose a big problem, even if your laser is not affected by that.

On the other hand, if you use a targeting solution that relies on your position and the target position being known in a common reference frame (like GPS), then you have the spear-fishing problem in reverse. In this case you must model the atmospheric effects to adjust your aim.

[Sky Captain]

Isn't there a possibility of firing several short powerful laser pulses to superheat the air and force it out of the way so the main killing laser shot remains relatively unaffected by atmospheric effects? It might be desirable for laser system to work in pulsed mode anyway because it would also cause not only thermal but also explosive damage to target.