The Stackexhangerians live on a habitable, earth-like planet near Alpha Centauri. Problem is that their world is dying. Let's say it was attacked by yet another race of hostile aliens, and most of their civilization annihilated. But some of the population managed to escape in generational ships that don't have FTL travel. They went into cryosleep and it took them 500 years to reach the outer limits of the Solar System.

To make things more "logical": the aliens have more advanced tech than humans, yes, but nothing that would allow to them to take on Earth head-on as they are VASTLY outnumbered. They are also tired and worn out from their travels, and they don't have a lot of military personnel to begin with. Also, this is a near-future Earth wracked by climate change, destroyed resources, overpopulation etc. There is no space for these aliens to land on Earth and grow their own population. They don't have the means to land on Mars or the Jovian/Saturnian moons and "start from scratch": they need a ready-made world. And since they can expect resistance, they decide to soften up the Earth first to make colonization/conquest easier.

How do the Stackers tackle this issue? I was considering two options:

  • They send spies to Earth and over the years, the spies not only learn about human tech and human biolology, they also indoctrinate disenchanted humans to their side, and together, they develop a biological super-weapon. the bio-weapon decimates most life, but some people prove immune. Now all the Stackers need to do is put boots on the ground and pick off the remaining survivors

  • They use their last remaining weapons to either bust up Earth, or push an asteroid into it, or something similarly destructive. They will then try their luck at, once again, picking off the survivors using their advanced technology

Do any of these make sense or is there another way they can approach this?

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    $\begingroup$ I'd avoid blending Cryo-Sleep with Generation Ships, as they're distinctly separate strategies for long distance travel and will confuse and infuriate the more scrupulous reader $\endgroup$ – Stephan Oct 8 '19 at 18:38
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    $\begingroup$ Plan before to beat mankind selected before 500 years of cryosleep? Who would worry about civilisation which top military tech are musketeers and galleons? :D $\endgroup$ – Shadow1024 Oct 8 '19 at 19:10
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    $\begingroup$ They should consider tripods, heat rays and "Black Smoke". It came pretty close to working before.... $\endgroup$ – Thucydides Oct 8 '19 at 19:53
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    $\begingroup$ Seems the easy way is to pay us to kill each other. $\endgroup$ – SRM Oct 9 '19 at 0:09
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    $\begingroup$ "earth-like" is so bad description of a planet. Even the Earth wasn't "earth-like" for most of it's history, and wouldn't be. $\endgroup$ – user28434 Oct 9 '19 at 12:27

18 Answers 18


Can't beat em? Join em!

Your refugees contact a selected Earth government. They ask for help. They say that they are fleeing their dead world and need a place to live. They have advanced tech they are willing to share. Plus they are xenocurious humanoid hotties who only wear skin tight silver lamé space jump suits.

There is no government on Earth that would pass up the chance to get that tech with benefits. It would give them an edge against their rivals. It might help fix some of the problems Earth has come to. If they blow the aliens out of the sky, there goes the tech. The government would make room and the aliens would move in.

It sounds like for your story you want violence! That is fine. There is no better way to learn how to war on humans than to team up with some humans. Once the aliens are received by their Earth state patron, the aliens might mention their interest in having some lands to call their own - still under the protection / supervision of their host, of course. A neighboring hostile country would do perfectly. With the alien tech, a war of conquest is easy for the host country. The humans of this country are sent packing or exterminated, or remain as a subjugated population according to the tone of your story, and the aliens move in.

  • $\begingroup$ This is actually another direction I wanted to go in! The aliens make contact and offer to trade tech for shelter. But things go wrong, and certain Earth leaders get the idea of wiping out the aliens with their own tech. Because now they have the tech and the numbers - why bother sharing already dwindling resources and territory? But the aliens came prepared with a counter-measure - as soon as these backstabbers try to harm them, they use their secret weapon. Which turns out to be the ability to call down an asteroid on Earth. So now most of humanity is dust and the aliens REALLY have a reason $\endgroup$ – Faz Oct 8 '19 at 19:43
  • $\begingroup$ @Faz - instead of an asteroid, have them signal the hostile aliens that chased them off their own world. Then when those things show up and start kicking butt, the alien hotties can ask "How you like that, <insert acquired earth slang>?" $\endgroup$ – Willk Oct 8 '19 at 19:53
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    $\begingroup$ @Faz If you are already thinking of the aliens counter-backstabbing humans (setting up an already somewhat morally gray story), another suggestion would be that perhaps the aliens expected to find an unoccupied planet. If you use the hypersleep (rather than generational ship) setup, you could have the aliens awaken from hypersleep together, putting them on a pretty hard timer (the ship is designed for shipping sleeping aliens, not feeding a whole alien society). Why wake the whole ship at once? Perhaps the protocol was to wake one guy, but his chamber got damaged in transit and he mummified. $\endgroup$ – Zwuwdz Oct 8 '19 at 20:06
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    $\begingroup$ +1 for tech with benefits $\endgroup$ – infinitezero Oct 9 '19 at 10:11
  • $\begingroup$ Yes, offering tech to one nation only - what could possibly go wrong? I'm sure all of the others will be happily welcome the new world dominators. $\endgroup$ – ths Oct 9 '19 at 11:25

Pushing asteroids isn't a terrible idea.

They've already travelled hundreds of years, so coordinating a cluster of small asteroids to take out all the major cities of the planet in one bombardment wouldn't take much resources, there wouldn't be much we could do about it, and what's another 10 years to wait.

The collapse of human society that would surely follow such a major disaster would likely prevent any coordinated response from humanity at all.

At that point, the aliens just need to wait to find a region of earth that is suitable for them with a low population density, set up shop, and use their advanced tech to create a defense zone that keeps humanity away.

At that point humanity is not likely to catch up to their advancement faster than the aliens can advance their own tech, or develop counters to the human tech.

  • $\begingroup$ Yeah I was leaning towards this. The idea is that these guys are exhausted and don't have much left in the tank, both figuratively and literally. So they use whatever juice they have left to drive an asteroid into the earth in the hopes that it'll take care of the hardest part - the overwhelming number of humans. $\endgroup$ – Faz Oct 8 '19 at 19:12
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    $\begingroup$ a single asteroid isn't a good idea. anything large enough to seriously reduce mankind will leave earth in a bad state for recolonization for a long time. $\endgroup$ – ths Oct 8 '19 at 19:37
  • $\begingroup$ agreed, it'd need to be small asteroids with the right size to level cities. nothing crazy, just a small boom. it's the chaos and the fact that all the major cities are hit that would destabilize humanity enough to prevent a coordinated response $\endgroup$ – Stephan Oct 8 '19 at 20:11
  • $\begingroup$ @Faz You don't exactly need a lot of material to do this. Any alien race with enough technological prowess to make an interstellar journey most likely also has the tech to calculate a trajectory for their vessel that can pull a couple hundred asteroids from the asteroid belt through gravity assists. $\endgroup$ – Nzall Oct 9 '19 at 12:15
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    $\begingroup$ The asteroid would still need to be massive enough to hit the surface with enough force to damage things. Gravity is very weak. Unless the ship is so massive its effectively a moon, it would take a mellenium for them to arrive and do damage, as the course change would be miniscule $\endgroup$ – Stephan Oct 9 '19 at 12:34

They don't need to.

Simply put, a civilization capable of traveling vast, interstellar distances to colonize another world already has the technology sufficient to sustain themselves without needing to colonize another world, much less commit xenocide in order to do so.

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    $\begingroup$ This is totally correct. $\endgroup$ – Fattie Oct 9 '19 at 17:13
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    $\begingroup$ +1 because they probably have the ability, but it should not be assumed that the two go hand in hand. They have a cryosleep technology that allows them to put their life on pause for centuries, maybe longer. If we had such cryosleep tech we could have launched a crewed mission to Alpha Centauri decades ago, but they could not have sustained themselves there alive long-term without a resource cache similar to Earth. $\endgroup$ – Loduwijk Oct 9 '19 at 18:27
  • $\begingroup$ I do not find this very convincing. The ability to maintain a minimum viable space habitat for ~150 years is not the same thing as having an entire planet. $\endgroup$ – ikrase Oct 31 '19 at 5:32
  • $\begingroup$ If you can "maintain a minimum viable space habitat for ~150 years", while transiting between star systems, you can do so indefinitely. ESPECIALLY within a solar system where you can (relatively) easily harvest energy and material. All without ever setting foot in a gravity well of any significance. $\endgroup$ – Theo Brinkman Nov 1 '19 at 17:12

Part 0: observe and infiltrate, if possible. Learn the language, the people, the conflicting factions, the lie of the land. Get you some high quality, high resolution surface images of everywhere. Note everything that looks like a power generation facility (lots of cooling towers and or lots of smoke), an industrial facility or a major population area.

Part 1: sweep earth's orbital space clean. This can be done without much fanfare... maybe send in little automated spacecraft to simply push everything out of orbit rather than going full-on Kessler syndrome, but depending on how you want to get to the planetary surface afterwards and your ability to clean up or avoid the debris you could just blow everything up.

Once the various GPS networks are down, there will be chaos below.

Part 2: drop a large number of relatively small kinetic impactors onto the targets you identified in part 0. You don't need to annihilate every single human, or everything they've ever made. Just smash up a load of stuff. Hitting the population centres might not even be necessary, given that you've just utterly wrecked the foundation of technological society... the ability of the system to support a huge population has been wrecked.

Don't use silly giant asteroids. They'll just wreck the environment and cause any number of awkward long term ecological and meteorological problems. Large numbers of small impacts are much more efficient, neat and clean.

Part 3: wait a bit. Doesn't need to be long... maybe a few months. Watch the panic and rioting and disease and civil collapse below. National armies may mobilise, but they can't fight you anymore, even if they knew where or what you were. Wait for the worst of things to calm down (eg. mass die-offs of people unable to feed themselves, or deal with injury or illness). Wait a little bit longer. You should be able to see groups reforming to try and rebuild civilisation from the ashes... larger groups and organisations should be visible from space with suitable sensors, and from closer to via autonomous drones.

Crush them, too. Rocks or rods from god, your choice. Again, you don't need to do a total 100% cleansing strike... let them build up hope, and take it away from them. The survivors of the second smiting will likely be broken and demoralised, and those who aren't will simply be unable to offer a realistic defense of their world, minus any support or industrial base.

Part 4: land somewhere that seems quiet and start rebuilding your world. If necessary, establish a quiet zone with the aid of autonomous reconnaissance and hunter-killers. Any humans still around will probably just surrender. You don't even have to be a cruel victor and enslave or execute them all... sterilise them, where practical, and let them live out the rest of their lives in relative peace. Oppression builds resentment.

Keep forces in space, though. Don't take any anti-air or anti-satellite weaponry to the surface, and don't bring any humans up to orbit. There's always the possibility that you overlooked something, or a particularly clever group might seize control of a spacecraft. Space security is paramount. If it doubt, smite.

  • $\begingroup$ Part 3) alternative: Wait for nuclear missiles you didn't find in part 0) (as they don't have neither cooling tower nor other indicators) of several armies combined blow you out of orbit. $\endgroup$ – Oleg V. Volkov Oct 10 '19 at 1:30
  • $\begingroup$ @OlegV.Volkov they have no communications grid, long range radar capability or GPS target assistance. They can only be fired at predefined ground targets, which will be other humans cities and bases, and their accuracy will be terrible. None of them will be suitable for shooting at space targets, and even if they were they don't have the supporting infrastructure required to find a target. Those missiles are utterly, utterly useless. $\endgroup$ – Starfish Prime Oct 10 '19 at 7:12
  • $\begingroup$ @OlegV.Volkov I'll also point out that this attack can be orchestrated entirely remotely, or run at a relatively safe remove like the lunar L2 point, or even just from a very high orbit where incoming missiles, especially ones from a non-spacefaring civilisation, will be clearly visible for hours and travel in predictable trajectories, because they weren't designed for combat in space, because of course they weren't. $\endgroup$ – Starfish Prime Oct 10 '19 at 7:15
  1. Niven-Pournelle's "Footfall" handles this scenario so well that you need to read it to avoid copying them without having read it :-). They even start out from the same general location. Worse/better they have tech that they inherited and do not fully understand.

They use space domination, kinetic weapons (from orbit ANYTHING is a weapon). They have advanced LASER technology and use launch-LASERs for ground to orbit. They have nukes but are not keen to mess up the playground with them. They start off destroying all major dams and bridges, and go from there. They aim at domination, not extinction.


  1. Create an organism with

    • infectiousness of the common cold,

    • payload when triggered like Ebola (mechanism may differ but Ebola is "pretty effective"

    • Silent transmission like HIV.

It doesn't trigger until we all have it and then it presents an Ebola like worldwide effect with nobody to help thos who may otherwise survive with proper care. Charming.

Y'All die, y'hear.

There is a lot of help available.
A few decades back Australian scientists came close to making a excessively effective mouse destroyer by genetically engineering Mousepox, but lnfectiousness was low - just as well for mice and humans.

One can be sure that the major powers took it from there and by now have a human targeted wipeout weapon too terrible to use.
The aliens just need to track it down.


Australian mousepox development:

Populist outline{Guardian} - Lab creates killer virus by accident

7 page open access paper here The mousepox experience

Legion - web search.


I would go for the horror factor. The aliens infiltrate our solar system and take up residence in the planetary neighborhood, without tipping off Earth to their presence. They then use evolution to their advantage, altering and introducing key terrestrial crop plants to Earth capable of out-competing our plants. These invasive species were given an alternate genetic alphabet (other than C,A,T,G).

Once the crops take hold, humans the world over find that no matter how much they stuff themselves, they and their loved ones are dying of starvation in ever increasing numbers. Humans starve on a world of plenty because the invasive crops are not digestible!

By the time those pathetic humans realize that they have a problem, the crops have thoroughly polluted the food chain and the scale of human die-offs has snowballed beyond the humans ability to control. Society collapses, human capabilities degrade, and the humans still have no idea as to the true source of their woes. The doom of the human infestation is assured! Muwa-Ha-Ha-Ha!!

...Or is it??? Every plan goes awry, and the story of how humans discover the nature and true source of their woes, identify the aliens only weakness, then ultimately overcome them is the stuff of a great novel. But PLEASE! Do not hinge human salvation on the the aliens' unfortunate choice of Windows-98 for their operating systems. (I'm looking at YOU, War of the Worlds 2005.)

  • $\begingroup$ Interesting fact! The Stackexhangerians are from the planet Toe, which is in orbit around the second sun of the binary system Mon. (In the alien tongue, their home world is known as "Mon-San-Toe.") $\endgroup$ – user10637953 Oct 9 '19 at 19:43
  • $\begingroup$ Minor biological correction: I believe that we could digest non-CATG-based material just fine, if that were the only difference; however, if the organisms had incompatible sugars, fats and/or amino acids, or were essentially poisonous (e.g. had weakly-bound cyanide-groups throughout), that'd be a different story... $\endgroup$ – poncho Oct 9 '19 at 22:45
  • $\begingroup$ Thanks to Poncho, I think we have a question to throw to the crowd! A new topic is brewing... Watch for it. $\endgroup$ – user10637953 Oct 10 '19 at 0:11
  • $\begingroup$ I found poncho's comment concerning non-CATG-based material interesting enough to post on it. But in composing the question I discovered related questions which lead me to believe that the answer is, 'almost certainly not.' The reason being that humans are incapable of digesting most compounds already found on Earth, before alternate alphabets are even considered. The question did not seem unique enough to warrant a new thread. I invite additional comments to this thread though, if anyone has specific information. Thanks! $\endgroup$ – user10637953 Oct 10 '19 at 0:24
  • $\begingroup$ @user10637953 As a biologist and geneticist, I believe than humans could digest non-ACTG bases just fine. Why? Pure biochemistry. Once it's degraded into simple molecules, your small intestine really shouldn't have any problem absorbing anything that looks like our nucleotides. Unless, as poncho said, they are severely different from ours, but in that case they would most likely be toxic, not just indigestable. If you want to find out more, I'd suggest looking at the chemical structure of our ACTG alphabet. There's already a fairly large difference between AG and TC, so as long as [[CONT]] $\endgroup$ – Whitehot Oct 10 '19 at 9:02
  1. Land on Ceres or another suitable body.
  2. Build a mass driver capable of launching 1000 ton payloads at Earth.
  3. Start making payloads. You can just use a metal skin and fill it with whatever is on the surface of the planetoid you wind up at. When you have a few thousand of them, start firing them at Earth.
  4. Wait for the pulverization to be finished, then make a tasty human smoothie out of the remains and start moving in.

This has the advantage of leaving the planet habitable and ready for occupancy, once you hose down the impact areas. A single large asteroid big enough to wipe out humanity could also leave the planet uninhabitable.


How does an alien race from a dying world annihilate most of humanity to colonize the planet for themselves?

Is this not what the rich and greedy already do?

Jungle munching machines, mass forest fires, and incomes high enough to numb oneself from the guilt of killing without question. Taking, taking, taking. Eyes always on the next conquest, never stooping low to see the trail of death behind you.

I know this is not the answer you wanted, but I say, if you're looking for an out of this world story we'll believe in, look at the wake we've left behind on our own planet. We treat Earth like we don't depend on it, like its an entity entirely separate, that if we have to do without, we'll find a way. Wilfully arrogant, destructive aliens on our own planet. We are literally alienating our species from its home base - a dying world - and annihilating most of humanity to colonize the planet for those of us greedy enough to steal it from all who we share it with.

  • $\begingroup$ Welcome to the Worldbuilding Stack Exchange! You you please explain your answer in more detail? Currently it appears to be but a statement and not a reasoned argument. Examples, sources and quotes would all increase the quality of your answer. $\endgroup$ – A Lambent Eye Oct 9 '19 at 7:16
  • $\begingroup$ You could make that argument, but it's also a particularly inefficient way to go about it. I get the feeling these aliens want to do the job in a few weeks if not days, not a couple of centuries. $\endgroup$ – Separatrix Oct 9 '19 at 9:12

Land in a region of civil war.

Conquer a small region large enough to stay for some month. Attack the fighters, and allow civilians to flee. When the acquired region is occupied, defend it, and make it clear to the fighter of the civil war. The fighters will just be fully occupied with fighting themselves, and understand that they do not even need to fear you currently. It would not be rational for them to divert resources fighting you. If necessary, take care to keep the war active, not won by either side.

Stay in the location as long as you like, and start interacting with humans when you are ready. Prepare for fighting, but possibly cooperate. Acquire intelligence as part of the preparation.

Now, continue to fight globally or cooperate aggressively. You had much time to decide, and to prepare, so it is much easier than any other kind of attack could be, because of intelligence and preparation as much as you want.

Optionally, use multiple locations.

Optionally, use Afghanistan to Land, and make the local tribes cooperate by providing superior weapons and fighters. At first, attack by air, fighters are vulnerable by air attack, but nothing else. The geology is hard to attack without local knowledge, and the local humans have fighting experience since multiple generations. (Russians fought there until they gave up after long time. Then, the USA did the same). The local people are very few because of the geology.


We come in peace, but the Facebookites don't!

(Yes this is basically the plot of that Doctor Who episode from 2005.)

Land somewhere not too populous but connected, maybe Australia or the U.S. Midwest. "Crash land" a small pod in a desert next to a decent sized city, leaving your ship in orbit around Mars or something. Sightings in every newspaper, people going mad. Is it true? Have we finally seen a UFO? After a few days, show up to the highest government official you can find. We come in peace!

Tell them that you are survivors of a far off world, wrecked by war with the Facebookites, who wish to conquer the galaxy! Now that your pod has "crashed" on earth, they will surely come looking for the last survivors, and find your semi-civilised world. Their tracker vessel is already in this solar system, scanning the surface of Mars! They will surely crush you on sight with their warp bombs! Unless... you give us access to your planet's defense systems, so that we can upgrade them and help you combat them!

When you finally have access to all of Earth's nukes and other assorted weaponry, it'll be too late for them to do anything. Set each country against its neighbours, maybe don't nuke everything or you'll probably ruin the environment.

And now you have a nice little planet all for yourselves!

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    $\begingroup$ So basically "I need your gun really bad, and it's for your own good too. I promise I won't shoot you." shoots you $\endgroup$ – Loduwijk Oct 9 '19 at 18:32

How about this? Slightly tongue in cheek, but would allow you to have the Stackexhangerians have arrived a few years ago, and do it all without those silly Earthlings knowing anything about it.

Spend a couple of years learning all the languages and infiltrating all electronic systems you can.

Then, encourage mass communication on a global scale (push that internet!).

Then, encourage more and more systems to use it (cloud solutions). Encourage all encryption etc - the Stackexhangerians are advanced enough to think quantum encryption is quaint, let alone standard prime multiplication etc!

Then, start to "modify" what's going on a little. You want governments that are at the more extreme ends of the spectrum. Shade a little of the automatic translation that hopefully by now you have going on, to sow distrust between allies. Chose a couple of the larger powers that are relatively isolated politically and shade blame on them (but not enough evidence to actually call them out on it). For the more technologically savvy countries, try and make the politicians and the technical elites have different enough agendas that they actively start working against each other.

it now depends on how you want to push them over the edge! Let's assume we want a nice stable planet at the end of the process, with as little cleanup as possible.

So, in order of easiest to fix for a very high tech group.

  1. Push that environmental disaster (and maybe plant enough false data so any warnings are ignored until too late). 40% of the world population lives near the coast (https://sedac.ciesin.columbia.edu/es/papers/Coastal_Zone_Pop_Method.pdf), so rising the sea levels quickly (eg 4-5 degree global warning over a short 20-50 year timescale) would nicely destabilize things and start several low level wars (with luck!), that should start dropping the population.

  2. As they're nicely reliant on the WWW, crash it! (you don't want to do this straight away, as this might encourage people to throw nukes around, and that takes a while to clean up). This should stop any interference in what the Stackexhangerians are doing, and reduce the likelihood of discovery. Hopefully a few more deaths from here. And for kicks, corrupt the codes for the nukes so they can't be used against the Stackexhangerians even if someone spots it!

  3. Maybe at this point trigger a few carefully crafted viruses, both against the humans (careful here, you don't want them to jump species!) and also against their primary foodstuffs (may something against grasses). Again, a few weaker ones are less likely to jump to different types, especially of you concentrate on the more heavily modified food strains.

  4. By this point you should have mass die off. Now it's just a choice of do you want to have a slave race, or eliminate the last survivors.


Nuke them from orbit; its the only way to be sure.

But with a little more nuance than that. Our intrepid Stackers have come to Earth in a nuclear-powered spaceship. Knowing that they don't have enough power left to go anywhere else anyway, they craft a series of small, nuclear devices meant to be detonated 30-50 km from the ground and drop them over key technological centers of the world. This results in an EMP that knocks out the backbone of human communication, the internet, and a good deal of all other forms of communication (see the linked article). (They also roll over the various emergency communication satellites that various governments have installed). No body knows they did it, or if someone does, they have no way of getting this information out. There is no preparation for the coming invasion only a lot scared people on the surface.

And what is this surface like? It's the near and entirely paperless future. Everything is online. Physical stores don't exist anymore (or are a novelty). Physical money is a rarity. People rely on the internet to get their food and to get their paycheck. Governments rely on the internet to move their resources and organize their efforts. Armies rely on the internet to command their forces. Every soldier is wired to and depends on the net. Or did, anyway. The result is pure chaos without a known cause and no outside source for relief.

I don't think humanity would completely destroy themselves in the resulting collapse of society; this isn't as ultimate a scenario as the other comments are giving, but it would do the trick. The Stackers could carve out a big of territory for their own and it may be decades before the rest of the Earth even knows they've landed. Their preserved future-technology would give them a huge advantage over mankind. Importantly, I think this scenario allows for a wide range of plausible human resistance. In this story, humans could be reduced to sparse and feudal tribes for generations, or the ever-intrepid humankind could dust off their old tech, retool their factories and give battle within years.


A depressing answer is that they don't even intend to do that. They come here in peace, hoping to share everything they know with humanity. Unfortunately, their micro-organisms "have other ideas".

There are four possibilities when separately evolved biospheres meet:

  • 1: Mutual incompatibility. They cannot eat our biochemicals to any dangerous extent, and vice versa. What they hoped for. They brought food synthesizers to make food out of simple feedstocks like simple hydrocarbons and ammonia.

  • 2 and 3. Superiority of one or the other. In HG Wells' "War of the Worlds" it was Earth's microbes that triumphed over the Martians. In this tragedy it is the other way around.

  • 4: Microbial war. Their microbes consume all Earthly higher-level life, and vice versa, leaving a planet full of slime which will evolve into new higher organisms if the Sun lasts long enough. The Fermi paradox is answered if very few civilisations work this out in time.

  • $\begingroup$ Why they then even need Earth if they're incompatible? Won't some other planet do? $\endgroup$ – Oleg V. Volkov Oct 10 '19 at 1:58
  • $\begingroup$ @Oleg_V._Volkov They don't know about compatibility in advance. There's a limit to what they can know in advance from tens of light-years distant, and it's a one-way trip at sub-light speed. $\endgroup$ – nigel222 Oct 10 '19 at 9:27

Grey-ish Goo

If you're talking entirely separate evolutionary process, these aliens will have absolutely no compatibility with our ecosystem. They will have to transplant their own crops, environment, etc to be able to live and sustain themselves. Therefore, they need to eradicate our ecosystem first and foremost.

Therefore, they design an organism, likely small, like a bacteria, fast replicating, that consumes key terran proteins (or something similarly universal). It will quickly spread and reduce all earth life to nutrient sludge, and the aliens can terraform to their liking in its wake. Our scientists will be too concerned with trying to devise counter measures to this pathogen to have much immediate concern about these strange plants growing in the dead zones.

  1. With the threat of orbital bombardment
  2. By exerting influence at upper echelons of global government, through mind control / infiltration via shapeshifting tech / religion / cabal
  3. By engineering global catastrophe through nuclear war or accident / metrological / biological / toxiclogical / mind control / killer robots etc, and then sleeping in orbit while humanity withers / dies / tears itself apart
  4. just asking for help!

Use their mastery of advanced technology to hijack mass and social media in order to subtly influence the politics of the most advanced nation and cause them to select the worst possible leadership. Political, economic, ecological, and martial disaster ensue.

  • $\begingroup$ Sounds like you're describing the world exactly the way it is right now. $\endgroup$ – overlord Oct 9 '19 at 16:51
  • $\begingroup$ @overlord that's only if you bellieve sources who want that leadership position to themselves. $\endgroup$ – Oleg V. Volkov Oct 10 '19 at 1:59
  • $\begingroup$ So shocking that this comment comes from a Russian. No offense intended to the great Russian people. $\endgroup$ – Jeremy Brown Nov 22 '19 at 21:27

The biological weapon:

The ability to hibernate in cryo-sleep and then revive implies an incredible control of their own biology. They can repair cellular damage from ice crystals and also repair the DNA (or equivalent means of inheritance) damaged by radiation.

And no, you can't just ignore the radiation by saying the cryo pods are shielded. Over a time period of centuries, even the radiation coming from their own bodies would cause enough damage to have to be repaired.

So in short, these beings are clearly capable of manipulating biological systems with great precision. Crafting a biological weapon after studying us is very plausible.

Rock Dropping

If they can make it here from another star, then have absurdly good long-term power generation. Shoving a few rocks around once they get here should be no problem.

Other weapons

Again, if they made it here from another star, their ship can produce a lot of power. They might be able to just irradiate us from orbit.


Easy. Stay in a geostationary orbit behind Mars. Meanwhile, research biological vulnerabilities of humans to build a deadly virus that could be dispersed with very small one-way entry vehicles.

In 2 or 6 months, the Earth population could be 10% or 15% so the planet can be invaded easily.

  • $\begingroup$ "geosynchronous orbit behind Mars" That is not how orbits work. A geosynchronous orbit is closer than our moon, let alone Mars. If you mean a geosynchronous orbit around Mars (probably wouldn't be called geosynchronous - not sure what the term would be, but I'll gloss over that for now), that would not keep them "behind Mars" either. The closest thing to what you described would probably be a Mars lagrange point, but even then you would have to time it right because it would only be out of view for a certain amount of time... maybe more than the 2-months you mention, I'm not sure. $\endgroup$ – Loduwijk Oct 9 '19 at 18:23
  • $\begingroup$ @Loduwijk how about on the dark side of the moon? Would they be noticed, or would our satellites pick up on them playing Pink Floyd on repeat? $\endgroup$ – Whitehot Oct 10 '19 at 9:06
  • $\begingroup$ If it's an orbit around the moon, eventually you'd be back around to the not-dark side. If it's a lunar lagrange point, then that might keep you out of sight indefinitely, though I'm not sure how stable that would be, or if you'd drift out of that eventually. Lagrange points are basically special orbits where certain gravitational pulls from different bodies allow for special kinds of orbits you couldn't normally have. If you landed on the dark side of the moon though, that would be the best. Just smile for the Chinese moon lander's camera if it wanders into your area; it's on the dark side. $\endgroup$ – Loduwijk Oct 10 '19 at 19:08
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    $\begingroup$ @Loduwijk My bad when I said "Geosynch orbit", I was thinking in "Geostationary Orbit". en.wikipedia.org/wiki/Geostationary_orbit Why did I mention Mars? Because a spaceship fleet positioned behind the Moon using a Lagrange point would be detected sooner or later by any satellite or the ISS when any of them reflect light from the Sun, otherwise they should keep very close to the Moon canceling the Lagrande point strategy. A solution, if you plan to keep using the Moon would be landing on it. $\endgroup$ – Luis Arriojas Oct 31 '19 at 3:56
  • $\begingroup$ @LuisArriojas A geostationary orbit around Mars still would not keep it out of line of sight from Earth except for a few hours per day. Still, because nobody is expecting to find something there, it would probably go unnoticed for longer than anything close to Earth, that is correct. +1 to your comment $\endgroup$ – Loduwijk Nov 1 '19 at 17:11

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