2013-11-17

BSinSF: Space Weapons

In my past Bad Science in Science Fiction posts I’ve covered the obvious problems faced by scifi writers. Unless someone comes up with a suggestion, this will be the last in this tiny series.

What is the best possible weapon for an advanced culture to use? It depends. Believe it or not, if you’re interested in colonizing a planet that happens to already have an indigenous species living on it, your best tactic is to throw a few rocks at them—big ones. Change the course of a few asteroids and have them impact the target world and you have yourself a nonradioactive equivalent of an ecosphere destroying nuclear attack. Wait a decade or so and then just mop up the few survivors and move in. This is an example of a kinetic energy weapon.

What about fighting in space? Kinetic energy weapons would be useless for long-ranges because the target can maneuver out of the way. High acceleration missiles can also be used for close-in combat. But for long-range battles with the combatants at a distance of hundreds of thousands of kilometers then the only choice is a directed energy weapon—beam weapons.

Near light-speed particle beam weapons can deliver a lot of energy but require an enormous amount of power to operate. Is it worth the power needed to generate the particle beam? Perhaps. You must have a way of accelerating the particles and that usually requires a very long, difficult to aim, exceptionally power-hungry machine. Lasers are a much better choice.

In space, the choice of laser frequency would depend on what type of target you’re aiming for and how easily you can focus the beam. If your target is a very reflective surface you probably want to stay away from visible-spectrum lasers. If you’re trying to destroy something on the surface of a planet from space you need to consider how fast your beam will be absorbed in the atmosphere. The type of atmosphere will make a difference as well. If you want to keep the science real, look up the absorption spectrum of the major gas of the atmosphere. This means your beam weapon must be tunable.

I am a very big fan of science fiction movies. I love the high-tech weaponry and awesome special effects. Even though I am well-aware that every scifi film I’ve ever seen violates the known laws of physics when it comes to weapons I still agree with Hollywood when it comes to showing visible beams in space. Star Wars, Star Trek, and Battlestar Galactica would be less interesting if the weapons in use were invisible. Making them visible definitely adds to the visual effects.

Why are lasers invisible in space? Think about it for a minute. How do your eyes work? In order to see something a few photons of light from the source must enter your eye. A laser is a coherent beam of energy with all the photons moving in synchronization from the source to the target. If you can see the beam, then some of those photons are not moving in the right direction which represents a loss of power. The only way for those photons to leave the beam and enter your eye is if they are forced to change direction by hitting something. There’s not much of anything in space to hit.

The other factor involved is the frequency of the laser. If there was something causing the beam to leak photons then those photons would have to be in the visible spectrum to be seen. That’s a pretty narrow band of frequencies. If the beam is not composed of energy but is, instead, a beam of particles, then there is no way anyone could see the beam. The bottom line here is if you want your novel to follow pure science, then all beam weapons should be invisible.

I work at a nuclear power plant and I’m quite familiar with the units of measurement for power and energy—the two are NOT the same but they are closely related. It took me a long time to completely understand the difference. Even today, I sometimes get confused. I don’t know how many times I’ve read that a beam was delivering xx megawatts of power to the target. A megawatt is a measurement of the amount of energy consumed over a period of time. It is not what the experts use to denote the amount of energy delivered to a target. For this, they use Joules. Depositing energy is what causes damage. The more energy you can deliver to the target the more damage you can do. For a very good discussion of this topic take a look here.

While I’m on the subject of energy, the science-minded writer must consider how much energy it takes to actually damage a target. Unless your weapon is backed by a titanic energy source you won’t see the type of damage depicted in the movies. (Huge power sources have their own issues as I pointed out in a past post: Thermodynamics, the bane of science fiction.) A hit from a laser will not cause a ship to instantly explode unless it happens to cause a secondary explosion. I recommend reading up on how lasers work and how they affect their targets. You should also be familiar with how much energy it takes to melt a given thickness of material.

A final note—in order for a beam weapon to penetrate the skin of a ship, it must maintain its focus on a very narrow spot for a period of time long enough to deposit enough energy to burn its way through. Doing that at a distance of a few hundred or thousand kilometers when both target and weapon platform are moving is quite a task. If you want to damage your target, then you had better give your weapons mind-boggling power levels (or is it energy levels?).


By the way—I just finished season one of Falling Skies. I love it!