This writeup is part of the background scenario for a lot of the stuff I've been linking from the Unfinished Stories node.

Alien Invasion
Aliens in movies and books mostly suck. They're too human. But that's probably a rant for a separate node. Here, I just want to talk about what an alien invasion would be like if it was done right. First of all, one needs to ask what would be the motivations behind an invasion? Here are some I could think of:
  1. Motive:Our sun is conveniently located, and has enough matter orbiting it, to be the perfect place for a Dyson Sphere.
    Method:The process itself would destroy life on Earth if humans did not resist. If they did, the most cost effective response would probably be to plow several huge asteroids into the Earth.
    Prospects for Survival: If there is enough human manufacturing infrastructure already in outer space to build new orbital and asteroid settlements, Humans might persist indefinitely, clinging to life in zero gravity.

  2. Motive:Oxygen/water worlds in just the right temperature zone are rare. The goal of such an invasion would be to cleanse the Earth of humans and maybe higher native lifeforms altogether and recolonize it with alien ones. Depending on how advanced/inexpensive their terraforming is, they may or may not take precautions to protect the plants and lower animals.
    Method: Taylor self-reproducing nanites to devour all mammals (or all mammals that have a particular set of antigens on their cells, or whatever... this can be made pretty specific). When characters talk about Plague in my stories, it's usually a reference to this type of invasion, or the alien civilization that launched it.
    Prospects for Survival: It sounds like an infallible plan for genocide, but it isn't. At least, not for the purposes of a sci-fi story. First of all, nanites are subject to distribution by wind and water currents... it's not as if the entire surface of the Earth will instantly be coated in grey goo. Some areas will be hit first, and others will have time to prepare. People who for whatever reason are living in hermetically sealed environments might survive. People living in isolated areas, such as deserts, mountain tops, the Antarctic, and remote islands may also survive. If humanity knows enough about nanotech at the time it happens, and antidote might even be developed. Furthermore, after the initial infestation, and the death of most of the human race, the nanites would either starve off or more likely go into some kind of dormant state. If the survivors could figure out a way to keep them dormant, they could move about with impunity and eventually build up a counter offensive.

  3. Motive:Irrational reasons. The aliens might be a warlike culture that keeps on its toes by picking on primitive races that are just sufficiently advanced to put up a spirited fight (eg, Predator). Other irrational reasons might include religious/ideological assimilation (eg, the French in Africa), or protecting nature from human defilement. This type of alien invasion has been greatly overplayed in science fiction. Just because our motives for invading places are often irrational...
    Methods: Anything ranging from the nanotech blitz in the previous scenario down to advanced conventional warfare tactics.
    Prospects for Survival: If you have to survive an alien invasion, this is the best! The underlying theme to this type of invasion is that there is something the aliens want from humans themselves, so a speedy extermination is not an option. The aliens may even be forced to undergo the time and expense of learning to communicate with humans. This gives buys humans time to hide, wage guerilla war, and reverse engineer captured alien technology. Probably the one thing that could bootstrap us to the next level the fastest would be a botched invasion attempt by aliens just a little bit more advanced than we are.

  4. Motive:The aliens are actually in the solar system doing something that doesn't directly encroach upon human interests, but jingoistic governments decided to 'teach those green slimebags a lesson' and the aliens have reluctantly accepted the challenge.
    Methods:Any of the above, depending on how ruthless the aliens are, how much we pissed them off, and what they need from this solar system.
    Prospects for Survival:As above, depending on the methods used. In addition, if communication can be established, a responsible representative could try make ammends and plead for mercy.

  5. Motives that simply don't stand up to scrutiny and should be banished from science fiction:
    • Enslavement (Come on! Even we can make machines that work better than enslaved humans.)
    • Cannibalism aka "How to Serve Man" (Incompatible biochemistry, no nutritional value at best, poisonous at worst. Sorry, V.)
    • Copulation gimme a break. Do *you* want to invade a chimp colony and abduct them to be your love slaves?
    • Humans as Incubators (sorry, Alien) if this process is dependent on biochemistry, then it's not going to happen for the sam reasons as cannibalism, above. If reproduction merely requires a warm, wet body to deposit eggs into, it's got to be possible to find or breed one nearby instead of having to travel 20 light years and conquer a planet.

Stories that at some point involve an alien invasion (mostly in the distant past and mostly type 2): Green Triskelion, InBetween City
See also: Terra Irridenta

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