Space laser weapons!

[Everten P.]

I am wondering how you could do damage to an enemy ship with laser beams and bursts. I would like to know A) which frequency you would need to do the most damage how much range would be it's optimal C) what could it punch through? I have done absolutely no research on the subject so I am relying on you completely

[Scotius]

A) You would want to have most energetic beam possible - so X-ray or even gamma ray lasers would be ideal. Energy source would have to be incredible though

Range is tricky. Lasers hit at the speed of light, so you might think even 300 000 kilometers (a.k.a. one light second) is not too much. But even coherent beams disperse - the farther out, the "dot" is bigger - delivering less energy per square centimeter.

C)...No idea. It depends heavily on parameters of your hypothetical laser cannon, and parameters of the target it has to chew through.

[K^2]

A) As Scotius said, hard X-Ray or soft gamma is your best bet. There are a lot of reasons for that. Besides these Scotius mentions, it's very hard to reflect hard X-Ray or higher frequency. And it will punch through the armor, depositing energy, potentially, meters into the ship. These are all nice qualities for weapon.

Of course, these are also the reasons why it's very difficult to build an X-Ray or gamma laser. For a good X-Ray beam, you need a particle accelerator. For gamma, at least a nuke and a very large superstructure for focusing the beam.

Take diameter of your emitter squared, and divide it by the wavelength. That will give you maximum range at which you can deliver almost 100% of the power. Past that, the beam will suffer roughly the inverse square law. So, for example, if you take an emitter 10cm across and 100keV X-Ray beam, the effective range will be 10¹⁰ meters, or about 30 times distance from Earth to the Moon. Which is huge.

Of course, that's a theoretical limit. If you don't manage to get the beam perfectly collimated, you'll suffer range penalties. But in either case, your main problem will be with aiming, not making the beam have sufficient range.

C) Given time and sufficient power input, anything. The rate at which it will burn through armor will depend on material composition, what means of cooling are available to the target, and energy density of your beam. It's hard to estimate all of these without knowing specifics.

[Idobox]

X-ray or Gamma are good for anti-personnel, but for sheer destructive power, you want longer wavelengths and pulsed laser.

As said, with X-ray, you penetrate deeper in the material, which means you heat things more evenly, and a ship will have significant heat radiation, and will take a massive amount of energy to destroy that way.

With longer wavelengths, ie UV, all the energy is deposited in a shallow layer of material, and with pulsed operation, the heat doesn't even have the time to be conducted. The result is ablation of the surface, and a lot less losses to blackbody radiation.

The main problem with any laser weapon is that it will generate copious amounts of debris that will get in the way. Once again, pulsed UV is the best solution for that, as the debris will hotter, and therefore will move away from the target faster.

[Tommygun]

Wouldn't UV lasers be easier to defend against though?

If you use ablative armor and constantly rotate the ship so the bean is hitting a larger surface area as the ship moves.

Edited July 9, 2014 by Tommygun

[KerikBalm]

You want the highest frequency/ lowest wavelength that you can focus (X rays with grazing incidence mirrors basically), this reduces beam dispersal.

Approaching hard X ray wavelengths.... if you had say... a 20 meter diameter focusing array (or a phased array of about 20 meters), and a laser output of a few hundred terawatts (we've made peta watt lasers already), then you could still explosively vaporize targets dozens of AU away with a pulse of only about 2 gigajoules.

I did all these calculations before using an online calculator. Ranges of nearly 1 light hour are feasible.

You don't want a continuous beam, you want a pulse that will vaporize the target, ie the target's surface will literally explode. Abalative material and spinning like mentioned above would work well against a continous low intensity beam... but a few gigajoules of laser energy delivered onto a small area in a thousandth of a second... the surface detonates, explodes, sends a shockwave through the vessel that will not react kindly with its occupants.

In practice, it is easier to make lasers with longer wavelengths more efficient. So you'll have less energy wasted going into heat when firing them (at close ranges, at long ranges, there is much wasted due to the beam diverging), and heat buildup would be a major concern for a spacecraft firing gigajoule level laser pulses...

[Everten P.]

So another question I have is if they can be blocked in any way

[sal_vager]

So another question I have is if they can be blocked in any way

Yeah, ablative materials

[shynung]

Yeah, ablative materials

Works well against low-frequency lasers (i.e. UV/visible/IR), but doesn't do anything against X-ray or Gamma lasers. Not sure against microwaves, though.

[K^2]

Are we talking about real weapons, or something I can defend against with kitchen foil? Anything bellow hard X-Ray is just not serious for space combat.

[Idobox]

Are we talking about real weapons, or something I can defend against with kitchen foil? Anything bellow hard X-Ray is just not serious for space combat.

Infrared is used very commonly to cut through steel. A 5kW pulsed IR laser will cut through tin foil like paper, a 5kW x-ray laser would do pretty much nothing to, because it would deposit significantly less energy.

If you have a laser that can emit 1GJ per shot (roughly 250kg of TNT), and you're shooting at a large target, you want that energy concentrated. Since you're shooting from far away, you can't hope to have it focused, but with long wavelengths, the energy will be deposited in the first few µm, while hard X-ray and gamma would deposit less energy (because they get through), and deposit it in the mass of the ship.

If your ship weighs 100t (empty space shuttle) and is mostly made of aluminium, a 1GJ X-ray shot would elevate the temperature by 11K, which is a lot, but the same energy with UV would behave like 250kg of TNT spread over the surface.

If your ship is a drone running with super rad-hard electronics (ie vacuum tubes), it would barely notice the first shot, but would be reduced into a could of debris by the second.