There was a fad in science fiction for a while for relativistic weapons, where material projectiles moving at a high fraction of light speed were an unstoppable superweapon. The concept was introduced to science fiction by Charles Pellegrino and George Zebrowski in the novel The Killing Star. It was used elsewhere as well. I seem to recall an old TV movie wrapped around the concept.
The aliens in the novel were absolutely genocidal, considering the existence of any race capable of relativistic travel is a threat, to be destroyed preemptively. Well, I can stipulate that for argument’s sake, though I don’t have much respect for it. In the first place, the logic chain of exists, therefore threat, therefore annihilate has some pretty large holes in it from where I sit. I’ve pointed out elsewhere in this blog that Genocide Doesn’t Pay the Bills. Still, there have been some human regimes with similar thought processes.
I have gone into some detail here, and those who wish to skip to the takeaway can do so just by scrolling down.
Moving on, here, we come to the choice of weapons. An object moving at a high fraction of light speed does indeed have a huge amount of energy in it. That energy will be released as soon as it hits something solid.
The virtues of this weapon are supposed to be that it is impossible to dodge, or see coming once fired, plus it is incredibly destructive. Basic physics on makes that correct on all counts. If, however, we go a little further down into the weeds, we do find some problems our Evil Genocidal Alien (EGA) engineers are going to have to overcome, and detection is a bit more of an issue.
Dumb Projectiles
The energy stored in a projectile moving at high speeds is indeed high. A table of representative values will give an idea. Energy is given in Megatons of TNT equivalent.
| Projectile Mass | Projectile Velocity | Kinetic Energy (MT Equivalent) |
|---|---|---|
| 1 kg | .95 c | 47.3 |
| 10 kg | .95 c | 473 |
| 100 kg | .95 c | 4731 |
| 1 kg | .98 c | 86.5 |
| 10 kg | .98 c | 864.6 |
| 100 kg | .98 c | 8646.4 |
Well, that’s certainly quite a large amount of energy. Enough to ruin days on a planetary scale, no doubt of it. It also raises some significant problems for our EGA Engineering Corps.
The first problem is generating all that energy. It then has to be stored and directed into one burst to accelerate the projectile to relativistic velocity. Being generous, we can allow them fifty percent efficiency. Most weapons systems don’t do nearly that well. All that waste heat has to go somewhere. In space, no one can sweat. It has to be radiated. That means really really huge radiators, which are going to be a beacon visible for light-years.
The platform on which this weapon is mounted will have to be large enough, and stoutly built enough, to deal with the very considerable recoil.
If that projectile is to go out the weapon barrel intact, it has to be kept below its melting point.
Running the numbers for a projectile made of tungsten, the ne plus ultra of dense high melting point materials, we find that the energy to vaporise the projectile is a tiny fraction, as in a few billionths, of all that waste heat. Any more than that, and the slug is plated all over the inside of the weapon. Any other material, such a nickel-iron, will be even more demanding.
The EGA Engineers are going to have to burn some midnight oil on that one, but assume they can find a solution.
Pressing on, the solid shot emerges from the muzzle of the cannon into the vacuum of space. It promptly runs into something. Empty space isn’t completely empty. There’s about one hydrogen atom per cubic centimetre. It may not sound like much, but it adds up. Every hydrogen atom that projectile hits creates heat. How far it will go before it is vaporised is determined by how much heat it takes to vaporise it.
This is not going to be a stealthy weapon. The muzzle flash, the launch signature in military parlance, is going to be as much energy as the weapon delivers to the projectile. The flight path will be an incandescent streak across the stars until it’s gone.
How long is that range limitation? When the round hits enough hydrogen to create that much heat. It will be losing mass all the way, of course.
A one hundred kilogram sphere of tungsten has a radius of 10 centimetres, and a diameter of 20 centimetres. It will drill a circular tunnel of that diameter through the interstellar medium, hitting enough hydrogen atoms in its flight path to vaporise it in 11 AU. In order to command the inner Solar System, it will have to approach the sun to, at minimum, the distance of the orbit of Jupiter. Closer is better. No hanging out in the Oort Cloud.
The range stated above assumes nice clear space. When you get down into the inner Solar System, that assumption starts to fray. There is gas and dust. The amount of mass required to detonate the projectile in a collision is incredibly small, about 350 nanograms. If the projectile encounters a grain of dust that size, it will premature. That could happen right outside the weapon’s muzzle. Bad things will happen to the platform in such an event. The EGA Overlord will need a lot of expendable minions.
This also suggests a very cheap and simple defence against such a weapon. Surround the target with a cloud of gas and milligram dust. The round will premature at a good distance from the target. While the physics of such collisions are beyond me, the energy would most likely be released as hard radiation, gamma and X-rays. Shielding against the radiation would still be needed, but it’s a lesser problem.
Give the problems outlined above, adding a guidance system would be useless waste of time. The same reasons it has almost no warning time are the ones that mean any plausible guidance system would be unable to change its course in time to have any useful effect.
The Takeaway
Much of the above may be TL;DR for some people. Here are the bullet points.
- The platform is huge and incredibly energy intensive, with massive and vulnerable heat radiators.
- The weapon has a maximum range of 11 AU with a 100 kg projectile. Effective range is less.
- There is a high risk of premature detonation. A tiny dust grain will do it.
- A cloud of gas and dust is a cheap and simple passive defence against such a weapon.
- When you run the numbers, this concept is fatally flawed.
For the genocidally inclined, there are better options. See my post on Blowing Up Stars With Impactors for just one.
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