attack of real aliens

[CraigD]

Originally Posted by Moontanman

We have nerve agents powerful enough to pretty much kill everyone on the planet with a very small amount of agent. Dropping it on us from orbit should be fairly easy. so we would go down like insects and they would be Terminex

That it’s possible to kill large, dispersed populations with small amount of chemical weapons such as nerve agents, is, I think, a misconception.

Even the most effective nerve gas weapons must reach concentrations in air high enough to deliver enough toxin to kill people before they can escape to clean air (or the gas blows away). Traditionally, this is done by releasing large amounts of it near the intended victims, via bombs and shells. The lethal dose (LD50) of the most toxic known nerve agents, VX gas, is about .01 g. A human respires and otherwise contacts about 10000 L of air per day. So, to kill half of a population in one day, the VX gas concentration must be about 0.000001 g/L, or about 1 ppm. Blanketing the bottom 1000 m of Earth’s atmosphere (about ) with this concentration – a pretty reliable way to make sure not too many would be able escape to clean air - would take about , about the same mass as all the humans on earth. (sources: wikipedia articles “VX (nerve agent)”, “Earth’s atmosphere”, “Earth”, “respiratory rate”, and “Orders of magnitude (mass)”)

While such quantities may not be beyond the ability of genocidal invaders from space, it certainly isn’t what you’d call a “small quantity”.

I’ll stick with my pick of “drop space rocks ‘til nearly everyone’s dead, then mop up on foot with guns, etc” conquest of Earth plan. However, I’ve got to offer a nod to L. Ron Hubbard’s mega-novel “Battlefield Earth”, in which ruthless space invaders don’t exterminate humans with a deadly gas, but cause them to suddenly be too stupid to understand or use technology with a “stupid gas” – pretty good space opera.

[Moontanman]

Originally Posted by CraigD

That it’s possible to kill large, dispersed populations with small amount of chemical weapons such as nerve agents, is, I think, a misconception.

Even the most effective nerve gas weapons must reach concentrations in air high enough to deliver enough toxin to kill people before they can escape to clean air (or the gas blows away). Traditionally, this is done by releasing large amounts of it near the intended victims, via bombs and shells. The lethal dose (LD50) of the most toxic known nerve agents, VX gas, is about .01 g. A human respires and otherwise contacts about 10000 L of air per day. So, to kill half of a population in one day, the VX gas concentration must be about 0.000001 g/L, or about 1 ppm. Blanketing the bottom 1000 m of Earth’s atmosphere (about ) with this concentration – a pretty reliable way to make sure not too many would be able escape to clean air - would take about , about the same mass as all the humans on earth. (sources: wikipedia articles “VX (nerve agent)”, “Earth’s atmosphere”, “Earth”, “respiratory rate”, and “Orders of magnitude (mass)”)

While such quantities may not be beyond the ability of genocidal invaders from space, it certainly isn’t what you’d call a “small quantity”.

I’ll stick with my pick of “drop space rocks ‘til nearly everyone’s dead, then mop up on foot with guns, etc” conquest of Earth plan. However, I’ve got to offer a nod to L. Ron Hubbard’s mega-novel “Battlefield Earth”, in which ruthless space invaders don’t exterminate humans with a deadly gas, but cause them to suddenly be too stupid to understand or use technology with a “stupid gas” – pretty good space opera.

Why drop space rocks, just use nerve gas type chemicals to wipe out all large population centers and them mop up with guns, no need to destroy the environment with space rocks but then again a engineered virus that targets only humans and has a 85 or 90% kill rate would do the trick with minimal effort.

[CraigD]

Originally Posted by Moontanman

Why drop space rocks, just use nerve gas type chemicals to wipe out all large population centers and them mop up with guns ...

Because, as I tried to show in my last post, it takes a lot of nerve gas to wipe out a large population center, especially one on a hill with a breeze. Another drawback with nerve gas and other chemical weapons is that the people you most need to kill – soldiers who can effectively shoot at you when you try to mop up – can fairly easily protect themselves with antiCW suits and drugs.

... no need to destroy the environment with space rocks but then again a engineered virus that targets only humans and has a 85 or 90% kill rate would do the trick with minimal effort.

An engineered superdisease could be devastating, but it’s uncertain if such a thing is possible. Humans and other plants and animals have an amazing ability to combat even apparently new diseases, and modern pharmacology is increasingly able to combat a broad spectrum of pathogens, even viral ones.

A really, really smart, nanomechanical pathogen, while science fiction by human standards, might be within the technological capability of a more advanced civilization. Such a pathogen combining biological, mechanical, and information technology, might indeed be irresistible.

My affection for killer space rock weapons, while in part a bias due to my personal success with such weapons in science fiction war games, a dubious simulation of possible reality, is mostly due to how difficult it is to defend against such “dumb” weapons, and the modest technology necessary to make them (especially, one might assume, for a civilization adept enough at spaceflight and engineering to travel to and attack another planet). As MTman notes, though, being “extinction-level event weapons”, they'd almost surely make a severe mess of the environment.