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So, it's several hundred years in the future and the majority of humanity now live on artificial habitats, either clustered together into nation-sized groups or scattered individually across the solar system. Technology has advanced pretty far, and while we haven't figured out stuff like Alcubierre drives or artificial gravity; we do have reliable, powerful, fusion for ship drives capable of efficiently giving a ship several hundred km/s of delta-v.

Out of the depths of space comes an automated alien ship. It has spent centuries traveling from another star a hundred light years away; sent by a race of aliens that had just been taken over by their own version of the Third Reich when their astronomers reported that a little planet a hundred light years away had the perfect conditions for life. Being completely reasonable beings, they decided the most logical thing to do would be to send a ship to make sure that whatever is living on that planet never has the opportunity to develop into something that can threaten them.

So, it's a single automated alien warship vs. the collective military might of somewhere around 20-30 billion humans, most of whom live in space. The human ships are armed with kinetic kill missiles capable of hitting with the energy of a tactical nuke, cannon capable of firing heavy projectiles at several dozen km/s, Casaba howitzers, and lasers. For defense, they have defense grids of railguns, lasers and particle cannons for vaporizing incoming projectiles, as well as powerful magnetic shields for protection against radiation, particle beam weapons, and the vaporized debris from incoming projectiles. They have engines that use some undefined method of fusion to provide several hundred kilometers of delta-v at a fairly sedate acceleration, or throw themselves around at several g-s in an emergency. Pretty agile for something that masses over a hundred thousand tons.

This level of human technology is pretty impressive... in fact, it comes pretty close to the most advanced level of technology that we humans are capable of predicting within the bounds of hard science, and that's the problem. What technologies can I give the alien probe that would enable it to utterly outclass the human fleets, while remaining within the bounds of hard science fiction?

EDIT: To be clear, in order to be a threat, this alien ship has to be capable of dispatching hundreds, if not thousands of warships like the ones I've described, and then still have the capability destroy every habitat scattered across the solar system, and THEN still be able to wipe Earth itself clean of life. About the only limitations I'm putting on it are: No future fantasy (hard light shields, warp drives), and an upper limit on size of somewhere around a billion metric tons.

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    $\begingroup$ We already fail at making forecasts on the future of technology based on what we know, see all the "how the world will be in 2000" publications back from the past century. And you are even asking to make forecast to go past those! $\endgroup$ – L.Dutch Sep 22 at 3:48
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    $\begingroup$ I don't know exactly what the question is but huge upvote from me anyway as half way through the question imma already dying to know how the story pans out and half ready to die for the human race meself $\endgroup$ – Sentinel Sep 22 at 23:46
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    $\begingroup$ @Sentinel I'm not sure where I want to go with it at this stage, but my original idea was for a short story where the probe shows up with terrifyingly advanced technology, contemptuously sweeps aside our defenses... then gets blasted when a SECOND alien ship shows up. The second ship sends a transmission to humanity explaining that in the five-hundred years since the probe was launched, their culture changed to become less genocidal, and they developed FTL in order to catch up with the probe. They apologize for the inconvenience and leave. Humorous, but not really satisfying. $\endgroup$ – FlyingLemmingSoup Sep 23 at 2:23
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    $\begingroup$ "It has spent centuries traveling from another star a hundred light years away". This means they can travel at a significant fraction of c. With that kind of energy, they can do whatever they want. $\endgroup$ – RonJohn Sep 23 at 3:36
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    $\begingroup$ Not with anyone in charge. Fusion reactors aren't what enthusiasts claim, and we're at the bottom of a really deep gravity well. Just handwave the tech and write your story... :) $\endgroup$ – RonJohn Sep 23 at 5:30

19 Answers 19

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This might not be the answer you are looking for, as it doesn't result in lots of exciting space battles between the alien warship and the humans. However, if your aliens are interested in winning rather than putting on a show, then this is how a grown-up civilisation would wage an interstellar war. The warship's advanced alien technology is...

Not slowing down

In order to cross a 100 light year distance within hundreds of years, the alien space ship has an average velocity of somewhere between 0.1c and 0.9c. If we take the lower bound of 0.1c, then as a rough order of magnitude estimate, every kilogram of the alien spaceship has as much kinetic energy as is released by a 100kt nuclear warhead.

The attack plan of the automated ship is simple. While it is still speeding through interstellar space, the warship would disgorge millions of small, independently targeted kill vehicles, ranging in mass from a few grams up to a tonne. These would lock onto the heat signature of everything emitting more than a gigawatt (which would include all the fusion drives, habitats and planetary colonies in the solar system). Using the same advanced drive technology that allowed them to accelerate the warship in the first place, these kill vehicles would use the occasional burst of engine power to maintain a collision course for their targets.

If they were looking in the right direction, Human astronomers may puzzle about why they are seeing millions of bursts of radiation, all coming from an expanding constellation in one corner of the sky. However before anyone has put together any serious plan of action, the swarm of vehicles has entered the solar system. Every habitat and ship that happened to not be occluded by a planetary body has been hit by a cloud of near-relativistic shrapnel. Every planet has been hit by hundreds of 100 megaton kinetic strikes, vaporising populations on one side of each planet, and levelling the other with the ensuing earthquakes and dust clouds. With careful timing and delaying the arrival of the munitions over a couple of days, there would be very little in the way of a hiding place. The human presence in the solar system would be decimated before space-navies even had a chance to react.

No strike is perfect, and it is likely that some lucky circumstances may allow a handful of ships and habitats to survive. However, human civilisation would be no more. Colonies would be wrecked, and those that managed to survive would face massive infrastructure failures. The survivors would quickly find themselves running low on food, fuel, propellant and medical supplies. Although it may be possible to rebuild, the solar system isn't going to be a threat to anyone for a very long time.

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    $\begingroup$ Very cool! I think it doesn't necessarily kill the story. Astronomers could very well identify the threat decades in advance. There is still not much you can do to protect the planets. (Try to shoot at them to change their course, maybe?) But we could hide a lot of stuff behind the Sun and other safe spots. Humans could also try to figure out the origin of the attack. And do something about it! $\endgroup$ – Daniel Darabos Sep 23 at 16:06
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    $\begingroup$ This makes quite a lot of sense, though it makes my job a little bit harder. :D I could come up with reasons why it might not work entirely, mostly down to the fact that if the alien ship doesn't slow down, it only has one shot at hitting every target, and physics may not allow it to hit targets too far off its flight path. If the human forces CAN see it coming, then its own velocity might work against it, since a 10 ton missile going the other way would carry enough energy to vaporize, well, just about anything. $\endgroup$ – FlyingLemmingSoup Sep 23 at 16:52
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    $\begingroup$ Don't the aliens want the planet for themselves? Why would they simply destroy it? $\endgroup$ – CGCampbell Sep 23 at 17:59
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    $\begingroup$ @FlyingLemmingSoup This method could be the whole reason humans are in so many colonies in the first place. Astronomers saw the threat, then the planet stepped space colonization into high gear. All your nations and city clusters may be slow relative to the incoming shrapnel cloud, but that cloud is not the size of a planet, let alone a system, so big sky small object principle is in play. Human weapons and defense would primarily be to push projectiles off course, shield from fallout, otherwise track the targets and move 10' quickly to dodge...or stay on the move. $\endgroup$ – wolfsshield Sep 24 at 20:49
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    $\begingroup$ Projectiles have to slow down to turn. It's space version of the case of the mighty Knight with Greatsword charging in with his mightiest swing, to simply have the plucky youth dodge and hit him from behind before the Knight can bring the sword to bear. So once you dodge the projectiles, use the lasers blast it from behind before it can try to sling shot around the sun and hit you again in fifty years. Though if it accelerates fast enough, it could aim at taking out the sun. $\endgroup$ – wolfsshield Sep 24 at 20:52
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von Neumann Machines

aka Gray Goo, one of the more horrific potential apocalypses facing the humans race. Raw numbers are probably the strongest of force multipliers you could ask for, which means that unless you give this alien warship weapons and technology which just outclass the humans, which, given that humans have kinetic projectiles at sizeable fractions of c means you need the stuff you don't want, i.e. anti kinetic shields or dimensional drives.

So the solution is to give the alien ship a means to counter the numbers, and just have their automated ship made up of self-replicating robots (not even necessarily nanobots) and spends a decent chunk of time in the asteroid belt snagging asteroids and comets to use to create a fleet.

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    $\begingroup$ Depending where on the Hardness scale OP falls, Gray Goo may not qualify. Our planet is covered in self-replicating machines ranging in scale from the lower limits of what can be built out of atoms up to 10's of meters. None of them have succeed in eating the planet, in spite of trying very hard. If a Grey Goo is possible, it will not be perfect or magic. This is not to say self replicating machines aren't a good idea, just that they're probably not sufficient. $\endgroup$ – ShapeOfMatter Sep 22 at 13:50
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    $\begingroup$ @ShapeOfMatter: Our planet is covered in self replicating machines thrown together by the cosmic equivalent of a blind bingo caller trying to give instructions to a deaf watchmaker using the medium of dice rolls. Frankly it’s amazing they function at all. $\endgroup$ – Joe Bloggs Sep 22 at 21:00
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    $\begingroup$ @JoeBloggs The shop of Intelligent Engineering is based on search in tool space and accumulation of reusable, composable mini-tools. The shop of Evolution is based on search in tool space and accumulation of usable, composable mini-tools. Evolution IS Intelligent Engineering, highly parallel, over vast cubes of space and time and unconstrained by ethics, possibly boosted by quantum computing at nontrivial points. You WILL find best design in the shop of Evolution. If humans are cool enough, both shops will join forces at some point. Not yet. $\endgroup$ – David Tonhofer Sep 23 at 20:40
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    $\begingroup$ Not gray goo: the entire Solar System will be converted into paperclips. $\endgroup$ – Monty Harder Sep 23 at 21:33
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    $\begingroup$ @DavidTonhofer: you can’t breathe and eat at the same time, some of your nerves take torturously circuitous routes to get where they’re going, you spend a third of your life unconscious and your body has an organ whose sole purpose is to harbour bacteria that might go rogue and kill you. ‘Best’ is a human term. Evolution is just flailing around searching for things that kill you slightly less that your peers. $\endgroup$ – Joe Bloggs Sep 24 at 6:54
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"The Three Body Problem" by Liu Cixin has several ideas for crazy advanced technology and I recommend reading it for the full details (it's also really good).

The two most applicable to you:

multi-dimensional entities

The ship is just the 3D projection of something that is actually an eleven-dimensional object. Among other things, that means its internal "volume" (or whatever the 11D term for that is) isn't x^3 but x^11 - if it is a cube of 10m each side, it isn't 1,000 m^3 inside, but 100,000,000,000 m^11

Plus, of course, it basically won't care much about things that hit it in 3D space. The same scaling factor would apply to damage, only in reverse.

strong-interaction material

The 2nd book in the series (I think) actually has a similar scene. An alien scout ship enters the solar system and the entire human armada goes to meet it - a show of force. The scout is small and unmanned. But its surface is strange. Then it accelerates and simply flies through every ship in the fleet, barely slowing down. Turns out it is made of a meta-material condensed towards the strong-interaction force being dominant. In essense, it is orders of magnitude more dense than metal, and any solid ordinary material is little more than a puff of air to it. It doesn't need weapons, it just flies straight through you, turning you to dust in the process.

It is similar to neutron matter, if you want.


In essence - look to the crazy outlandish ideas in modern physics. Many of these things have been theorized about - quantum materials, neutron matter, miniature black holes, etc. etc.

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  • $\begingroup$ Maybe to support the strong interaction material concept they could use a sort of propellant-less drive that uses tachyons in a similar manner to a photon drive creating a ship capable of accelerating near infinitely without using fuel as the tachyons have imaginary mass so it wouldn't take energy to produce them. $\endgroup$ – Efialtes Sep 23 at 11:27
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    $\begingroup$ Also, the Sol system was completely flattened in book 3. $\endgroup$ – Draco18s Sep 23 at 22:34
  • $\begingroup$ I like the multi-dimensional idea as it gives plenty of room for interesting narrative. $\endgroup$ – Darren Rogers Sep 25 at 6:47
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    $\begingroup$ @Efialtes tachyons are definitely magic. They make for good math for unworkable theories tho! $\endgroup$ – Varad Mahashabde Sep 25 at 10:03
  • $\begingroup$ @Tom Ships of ultra-stable strange matter maybe? $\endgroup$ – Varad Mahashabde Sep 25 at 10:04
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Trojan horse

The automatic warship limps into the system, obviously disabled, flying blind. The long dead corpses of its crew are still aboard. Earth recognizes its alien nature and realizes that it is a ghost ship. The military wants that alien tech! And the ship does have excellent and complex tech.

It is brought back to earth for study. As its creators expected, because that is what they would have done. Once on the surface the ship releases a prion-like polymeric bioweapon that acts on reduced carbon. This rapidly spreads across the surface, purging the earth of life.

The military in their spacecraft wonders why things have gotten so quiet. If they return to the surface to find out, they will find out.

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    $\begingroup$ Why would you need a trojan for that? Wouldnt you be able to hollow out several tiny asteroids (football sized or so), place a sphere in there with heat shielding and the bioweapon, then send a few hundred of those crashing into earth? Also how would this affect the hundreds if not thousands of space stations? $\endgroup$ – Demigan Sep 22 at 20:27
  • $\begingroup$ If they see this alien drone ship come into the system and release a bombardment it will pretty much give away what is up. The idea is to not be perceived as a threat. Re stations: if the polymeric bioweapon is slow to act (again like a prion) many of the space habitats might get infected by commerce with earth. $\endgroup$ – Willk Sep 22 at 20:48
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    $\begingroup$ Or you would deliver a superior crucial tech component blueprint. Today that would be for example a highly miniaturized computer chip. Wait till they are everywhere, because they are better and cheaper and then make them go haywire. $\endgroup$ – openend Sep 22 at 21:45
  • $\begingroup$ @openend It's a cookbook! $\endgroup$ – RonJohn Sep 23 at 3:34
  • $\begingroup$ LOl i had to look that up :) Nice reference and clip on youtube. $\endgroup$ – openend Sep 23 at 8:57
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With all due credit given to Roddenberry's masterpiece...

Cloaking Field

If the alien vessel can absorb all of the energies (including visible light) that our active sensors use, and if it can also store/conceal all of this collected energy plus its' own emitted energies from our passive sensors, then all of our weapons and maneuverability won't help.

Such an invisible vessel could stalk around our solar system, quietly cataloging every human habitation and major ship, only attacking each at their moment of greatest vulnerability.

The ship's computer has been patient enough to travel for hundreds of years just to reach us. There is no reason it has to hurry with our extermination now that it is here.

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    $\begingroup$ A cloaking field certainly falls into the category of future fantasy, especially when you consider it would also need to cloak the exhaust from any drive it uses. $\endgroup$ – Turksarama Sep 22 at 22:39
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    $\begingroup$ @Turksarama, does it? What if the alien ship propels itself by dragging itself across the underlying fabric of the universe rather than farting around in the vacuum like today's space craft do? All they would need is some way of grabbing onto that fabric which gets bent by mass to produce gravity. If they can create and destroy fixed points on that fabric, they can pull them selves along from point to point without leaving any tell tale exhaust in their wake. $\endgroup$ – Henry Taylor Sep 23 at 0:49
  • $\begingroup$ What do you mean by "destroying fixed points on the fabric of the universe"? $\endgroup$ – The Bosco Sep 23 at 8:57
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    $\begingroup$ Energy has a gravity well. van.physics.illinois.edu/qa/… $\endgroup$ – Jay Lemmon Sep 23 at 15:09
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    $\begingroup$ @Henry Taylor "Does it? What if the alien ship propels itself by dragging itself across the underlying fabric of the universe" Replacing one form of magic with an even more unlikely form of magic lol. $\endgroup$ – Kevin Kostlan Sep 23 at 15:56
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The other idea's here are all good, but there isn't going to be a silver bullet.

A human civilization like you're describing is a large, diverse ecosystem of adaptable self-replicating intelligent agents. (And it may have access to it's own super-intelligent AI, or it may not, IDK.)
Even if you can get 99% fatalities in your first volley, the invader is going to need to hang around for decades hunting out that last 1% and responding to their every batsh*t scheme.
Remember: 1% of a trillion is ten billion.

The invader will want to be able to build more of itself. If that's infeasible, then the invader will want to remain hidden as much as possible and let disposable probes that it can make more of do most of the dirty work.

The invader will want to remain hidden. How it does this is open-ended, and multiple levels of obfuscation are good. Information is power, so don't let your prey learn anything about you if you can help it.

The invader will be clever and deceptive. At some point the humans will understand they're under attack, and/or that there is an alien visitor. Information is power, so the more they misunderstand the situation the better.

Specific technology:

Some mechanism for venting heat quietly. I don't know what kind of surveillance technology the humans have. A general purpose cloaking device is a good idea, and can help with different kinds of active detection. What's still needed is a way to not show up on conventional infrared telescopes. Maybe the invader can vent heat as a neutrino beam pointed out of the solar system?

Ironically, defense is easy. The weapons you list the humans having are all either easy to dodge (by always accelerating randomly) or can be defended against using defenses that you also list the humans having. The invader can dodge or shrug off human attacks using human-level technology as long as it doesn't let itself get cornered by an overwhelming force simultaneously.

Offense is hard. The best defense is a good offense, so if you're going to get in a fight with a human ship then you need a weapon it can't dodge or armor against. You're trying to avoid speculative science, so I can't think of any magic thing the humans don't already have, but if the invader has more energy at its fingertips, and can vent heat faster, then it will win a laser fight eventually.

But how to start?

The opening volley should take out Earth completely, and ideally every other target-able population center as well. I suggest item #3 here.

In the best case, at the same time you're seeding earth with micro black holes (because of course you won't send just one), you're also seeding the Sun. I have no idea if stars are actually vulnerable to micro black holes, but let's assume they are. It's going to take a while for the sun to get eaten, during which time it'll be cooking the entire solar system with extra radiation. This is actually a problem for you, because it's an opportunity for humans to escape to other stars. (If they haven't already done so, the extra radiation from the sun will let them set up solar sails that much easier.)

Suggested timeline:

  • First decade: Hide at the edges of the solar system doing reconnaissance.
  • Six months to two years: Bio-weapons. They won't wipe out the humans, but they'll sure be distracting.
  • D-day: Micro black holes arrive simultaneously at the Sun, the Earth, and as many other inhabited bodies as you have black holes to spare.
  • Simultaneously: More traditional surprise attacks on as many other structures as you can target. Depending on the timeline for planets and stars to succumb to micro black holes, you should probably target Earth cities too.
  • The next 100 years: Make more of yourself and hunt down all the survivors.
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  • $\begingroup$ I recently asked a question on how to feed a small black hole. Even with the hawking radiation turned off, which creates a huge outward pressure for small black holes, the size of the black hole is smaller than the very atoms. How do you feed a black hole that is smaller than the very matter and energy it tries to absorb that simultaneously has such a high outward pressure close to its surface that you need to ram the matter at the smaller-than-atom event horizon? $\endgroup$ – Demigan Sep 22 at 21:02
  • $\begingroup$ Well, looking at the top answer on your question: they're discussing masses in the range of 10^11kg. qntm, linked above, estimates that a "Mount Everest" massed black hole would work. That's about 10^15kg. Whether or not that's big enough to mitigate the problems in question would depend on a firm understand of what happens near an event horizon. Also hand-waved are all the questions about sourcing and manipulating micro black holes in the first place. $\endgroup$ – ShapeOfMatter Sep 22 at 22:04
  • $\begingroup$ I was considering having the ship itself be powered by a miniature black hole, which would be a point of terror for the human protagonists on its own because creating such a black hole would require technology and scale of energy production far, far beyond what humanity can accomplish at this point in time. I don't think there's any hard sci-fi way this thing could create more, though; not without dyson sphere levels of energy. It might use its power core as a weapon against Earth, but a black hole of that size won't eat a planet, it'll just expel heat for a few thousand years then blow up. $\endgroup$ – FlyingLemmingSoup Sep 22 at 23:15
  • $\begingroup$ @ShapeOfMatter there are several ways to calculate the size and weight from the mount Everest, I saw numbers ranging from 10^11 tons to 10^13 (10^15kg should be the 10^11 tons unless I get the numbers wrong). According to this site: space.geometrian.com/calcs/black-hole-params.php you would still have event horizon sizes of 10^-13, and atoms according to the first google result are 10-10, 3 magnitudes larger. So the problem would be the same. Even if the pressure of the earth was enough to push atoms into the black hole we dont know if it can absorb enough to keep growing. $\endgroup$ – Demigan Sep 23 at 5:16
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I would strongly recommend relaxing your rules for the aliens A LOT.

I will assume that you want to have some kind of narrative. To really make the alien alien, the other, I would give them future-fantasy devices. Not necessarily the most used in future fantasy, but offense and defense impossible by our current understanding of physics.

Examples: Short-Range Teleportation of the ship, Energy Apsorption, Terror/Rage inducing Fields and the like.

This way their technology reinforces their alienness and you can have a focused narrative with seemingly invulnerable and deadly invaders.

ADDITION AFTER THINKING ABOUT IT: If you do not want to take this path, maybe you could use a complete alien paradigm. Thats why I like @Willk's answer. It's not that the alien just have a bigger hammer to kill humans with small hammers. They have rats to kill the humans before they can use their hammer :)

Examples: Bio/Techno-Plague (As answered by Willk), Psychoactive Field Emitters, everything not related to maximizing energy impact (the dominant human paradigm in warfare)

Bottomline: Who wants powerful but ultimatly 'linear-stronger' (on the same tech tree as humans) alien invaders? Make them quasi-magic to incite awe and terror.

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There is no need for the alien ship to do much at all.

This sort of existing armament ready to deploy means we're still in mutually antagonistic groups. Without outside threat there would not be this sort of armament.

The addition of an unknown ship shooting anything without warning would trigger a war and the probable destruction of humanity.

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  • $\begingroup$ Humans are pretty good at teaming up against outside threats, though. $\endgroup$ – nick012000 Sep 24 at 12:21
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    $\begingroup$ So the alien ship arrives and posts fake news to Facebook until everyone starts fighting? $\endgroup$ – Robin Bennett Sep 24 at 12:22
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    $\begingroup$ @nick012000 it's a robot ship, hasn't got a flag on it, could be anyones, they'd attack the ship because t's not theirs, and attack everyone else because it's probably those dodgy backstabbers. They have it all planned out already, just got to hit a button. $\endgroup$ – Kilisi Sep 24 at 13:39
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I'm going to focus on two kinds of weapon. Information and Energy weapons.

It arrives in a wave of relativistic shrapnel. The humans see it coming -- a bunch of blue-shifted drive plumes and Cherenkov radiation caused by relativistic matter moving through the interstellar medium.

Some humans hide, dodge, try to intercept. Hiding behind planets works reasonably well. Dodging doesn't. Interception runs into the problem that (a) enemy drive tech is way better, and (b) if you intercept too close the debris are as dangerous as the projectile.

Still, there is some success. 1% of humanity survives the initial attack, the economic collapse caused by the rest of humanity being destroyed, and the flotsam and jetsam flying around the solar system after it.

Humanity at least knows that the alien ship is hostile. The largest body of the enemy ship decelerates as more relativistic projectiles continue to bombard the system. For the rest of the war more relativistic "thunderbolts" arrive, directed by the enemy ship, at irregular intervals. Singularity powered craft with near limitless energy budgets constructed out of exotic chemistry held together with active magnetic fields.

That was the energy attack. Next comes the information attack.

The ship splits into two. One part starts self replicating and building up an industrial base (at dozens of seed-sites). The other starts hunting the remaining humans.

Lifeforms that are adapted to space and eat your habitats. Not grey goo, but custom-built pseudo-life, as dangerous as O2 emitting organisms where to early Earth life (as in, very).

Drones that spy on all movement and heat and communcations channels in the solar system. That break human communications security as if it wasn't there. That pattern match on human communications, replicate, and do a cross between a Chinese Room and a Deepfake to insert chaos into human communication systems.

Basilisk attacks on the human visual cortex that paralyze, kill, or induce specific behavior on (some) humans on sight.

Reasonably accurate predictor machines that model human future actions with high fidelity from tiny fragments of information.

This "information war" is intended to keep any remnants of humanity (a) under watch, and (b) ineffective and distracted. It knows it cannot manage a 100% cleansing of humans using these techniques, so it doesn't try. I mean, the death rates are horrifying, with 10%-50% of the remaining population of humans dying every year, but that is just to provide incentive. Instead, it uses humans themselves to find other humans and find information about humans, and gets them to band together.

Humans are distracted away from the von Neumann factories producing an industrial base for the aliens while they are vulnerable. They are convinced to focus on the singularity hunter-killer ships, who blatantly destroy human habitats and ships at seeming random and are extremely good at dodging.

30 billion humans drop to 300 million on first strike.

150 million are lost in the first year of the war.

Then the losses drop. 20 million, 10 million, 5 million, 15 million, 5 million, 2 million, 8 million, 30 million, 10 million, 5 million.

More and more humanity hides in its remaining refuges. Communications channels talking of rallying arrive, they send their best and brightest, and they never return.

Strange energy readings are picked up from the moons of Jupiter, together with communications stating that the humans there have managed to outfox the alien invasion and are rebuilding to strike back. Actually, area is quite sterile, and new ships arriving are escorted into meat grinders.

Then a few sterilization sweeps clean up 99.97% of the remaining humans. The remaining thousands live in a stealth habitat buried under Mars. They survive for a few centuries before the aliens disassembly of the planet digs up their habitat. They are then placed in museums as an example of yet another threat removed from the glorious Empire's worlds.

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  • $\begingroup$ This is a truly terrifying scenario... I like it! :D $\endgroup$ – FlyingLemmingSoup Sep 24 at 15:25
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Not meant to be a top answer but as something to consider: what if it isnt superior technology but superior design?

The human ships are all efficient, they dont waste space and are build as compact and capable as possible. That makes turning easier and gives you a smaller profile to hit. Great atteibutes right?

But the aliens come with an absolutely enormous ship. It's armor can't stop the projectiles of the humans who can easily shoot holes and they can verify their shots coming out the other side. But no matter how much they shoot it the ship just doesnt stop functioning, and every time it escapes the battle it comes back repaired and ready in a relatively short time and on top of that it seems to adapt it's array of weapons and engine's each time. So how do they do it?

They use a hollow ship. Most of the hull is just that, a dead piece of bulkhead that looks functional but actually is nothing more than some armor designed to leave as small a hole when penetrated as possible. The size of the hull is used to radiate the heat generated and for obfuscation of what goes on inside the hull. A secondary and tertiary thinner hull are on the inside that can be rotated to make it look like the ship has more internals after it has been penetrated.

Along the inside of the ship several components can move around. The command module(s), weapons and engines can move around the inside, anchor themselves at several places and open up the modular hull to do any sensing, propulsing or firing they need to do, then move again in case of retalliatory fire to prevent damage. Then after the battle the ship moves to a few factories hidden in the kuiper belt that produce new hull modules for any damaged area's and weapons. The humans first need to find out what is happening and hope they can win the battle of attrition and chance, their hope being that they can kill enough command modules containing aliens to stop their ability to progress with the war.

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Local singularity.

Deploy AI into the local system that is more advanced than whatever is present.

This frightening prospect is real even for people today.

Secondly, hallucinogenic weapons. For some reason never deployed even in the worst horrors of human war. LSD bombs.

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Asteroid Launcher

The ship sneaks into the asteroid belt and starts manufacturing engines on the asteroids. When enough is made, it launches them at the Earth and/or other targets.

Sure the Earth forces can try and blow them up but that's not really going to help as it just changes a single shot round into a shotgun round.

Multiply that by thousands of asteroids all at the same time and Earth forces will be overwhelmed.

A single automated ship with manufacturing capabilities and enough time could launch a meteor swarm capable of taking out the Earth

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The Straight forward solution

I can't help but notice that your humans have incredibly slow railguns. You could potentially give your aliens railguns several magnitudes faster. This allows them to outclass individual engagements against humanities flagships.

Additionally, your humans don't have AI. that means that their thought can only be done at the speed of human thought. Maybe make your alien ship controlled by some living network that is able to intake, parse, interpret, and make counter-strategies to whatever humanity can throw at it. If you want to go really out there, you can make the ship either living itself, living material wrapped around a mechanical ship, or controlled by some biocomputer composed of quadrillions of neurons.

Adding on to that last point, perhaps you could have the ship constantly changing it's configuration.

If you want to borrow a page from another book, maybe have your ship nest itself in our asteroid belt (or a planetary ring, whatever) and build up its own small armada, turning it into a von Neumann probe. If it plays it's cards right, it could take out the weaker human settlements without anybody figuring out where the main probe is. Depending on how advanced you want to go, your ship could also make it's aliens inside of it, allowing them to settle in old human settlements.

Maybe your alien ship has a more advanced version of humanity's magnetic fields, such that all radiation misses it except for a few blips to let the ship know where it is. This shield is so large that it's small armada can hide inside of it, and the energy and matter from the exhaust is dispersed enough to be considered anomalous.

Perhaps your aliens have some manner to harvest antimatter. Your ship now, seeing that there are 11 planetoids in this system, prepares 11 antimatter bombs, and launches them in an intercept trajectory with each planetoid.

If the inside of your ship is pressurized (or if it isn't), then you now have diseases from the original planet. All the ship has to do is go into the atmosphere of one of humanity's planets and release these. The same goes for intentionally cultivated diseases.

Perhaps your alien ship could have the capacity to launch small planetoids (Pluto, Chiron, etc) through some kind of propulsion device (antimatter charge, turn itself into a tunnel-shaped railgun, etc). This object is has too much momentum for any of humanities technologies to do anything to it.

The Really creative Solution

Your alien makes a persona with which to proselytize humanity into some new religion. Maybe it offers immortality in it's computer system. Maybe it offers the chance to go past the solar system super fast. Either way, for whatever promise that is made, the end effect has to 1) divide humanity into believers and non-believers. The believers would win in a fight because alien tech is cool, however the alien keeps it's best stuff completely hidden, leaving the believers faction incredibly decimated.

Your alien then uses the psychology of humanity, that it learned over the course of the war, to push them to exhaust all of their resources towards whatever plan it has for them. Some split will inevitably be caused within the believers, leading to whatever efforts have been made to come crumbling down. Eventually, humanity will be too small to effectively put up resistance, or your ship can say "It seems you weren't worthy after all," and all of humanity can be killed with minimal expenditure.

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The automated alien ship contains a huge radioactive mass that sterilizes everything getting too close.

It emits dangerous levels of radiation even intact, but if humans perforate its capsule, it would only increase the amount being emitted and make things even more dangerous.

If humans attempt to use powerful explosives against the ship, the radioactive mass could either:

a) Scatter across the solar system, effectively poisoning it and making it uninhabitable for billions of years.

-or-

b) Fuse and explode with such a force that it obliterates the Sun and many of the planets and human settlements.

Such a ship meets all stated criteria:

  • It's capable of dispatching thousands of advanced warships, as its radiation penetrates even the thickest feasible shields in current use at that time.

  • It has the capability to destroy every habitat scattered across the solar system by simply getting close enough to lethally irradiate them.

  • Can similarly wipe Earth itself clean of life.

  • It needs no advanced technology beyond being able to contain and lug around a huge mass of radioactive material.

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  • $\begingroup$ Unless the radiation is magic, it is unlikely to be effective. Remember that at the centre of the solar system is a colossal nuclear reactor continuously throwing heat, light, short wavelength radiation and fast particles out in vast and deadly quantities at the same time as the whoel system is bathed in even more powerful cosmic radiation. The OP's setting is already well equpiped with radiation shielding, space is big and the inverse-square law is cruel. Mere radiation ain't enough, unless you brought a supernova with you. $\endgroup$ – Starfish Prime Sep 23 at 13:09
  • $\begingroup$ @StarfishPrime There are various degrees of "vast and deadly". Between "manageable with radiation shielding" and "OMG, two neutron stars collided nearby and a gamma-ray burst is about to cook us all instantly", I reckon there are levels that pose a threat when they move close, emitted by sources that future technology could contain within starships. $\endgroup$ – luvieere Sep 23 at 14:09
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    $\begingroup$ Neutron-star level radiation in under billion tonnes of ship? Mimicking gamma ray bursts caused by the death of supergiant stars without the aid of a supergiant star? Firmly in the "future fantasy" category $\endgroup$ – Starfish Prime Sep 23 at 14:11
  • $\begingroup$ @StarfishPrime Regarding the feasibility of such a source existing in a mobile form-factor, coming from an alien civilization, there is a degree of speculation, of course. Since theoretical physicists of today are already speculating on how to create micro black-holes and other such seemingly-fantastic constructs that could wreak havoc on the entire planet, I figured that another 300-400 years in the future could take us in quite interesting directions in this regard. $\endgroup$ – luvieere Sep 23 at 14:13
  • $\begingroup$ soooo... future fantasy, then? $\endgroup$ – Starfish Prime Sep 23 at 14:27
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Directed dark energy weapon.

Similar in principle to a laser but made of dark energy. A rather trivial amount of dark energy would annihilate positive energy and or matter. Human civilization may not have sensors to detect it. And even if detectable, the attack is travelling at the speed of light. Combine this with @MadScientist answer and you'd annihilate vast portions of the solar system without a trace.

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  • $\begingroup$ Dark energy isn't negative energy. Dark energy doesn't annihilate anything. It isn't obvious that it is even something that could have a "speed", either. $\endgroup$ – Starfish Prime Sep 23 at 15:49
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Since it would be sufficiently difficult for the invaders to predict what human society would look like by the time that they arrived they need to create contingency plans in the unlikely event humans discovered the perfect counter to whatever attack plan they initially had.

Some suggestions come to mind:

A giant universal constructor, the alien ship is basically a giant 3D printer capable of producing anything as long as it can procure raw materials. Luckily there are plenty of asteroids and enemies that can be harvested for carbon that can then be molded into whatever automaton or weapons that is needed at the time.

Alternatively your aliens could attempt to blow up the sun, surely killing off most life on earth and significantly reducing the ability of any space stations that escape the initial blast to survive. Even if they do they will be hard pressed to ever be capable of posing any kind of significant threat in the future.

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Genetically-engineered bio-weapons capable of exterminating all life on Earth.

They're super-advanced aliens, so the bio-weapon doesn't have to be based on anything possible as a result of nature, either, so you could have something like a bacteria-analog which forms biofilms, aggressively attacks and breaks down all other biological material it comes into contact with, has a cell wall composed of high molecular-density polymers to prevent it from being broken down by other microscopic life (and also probably results in the bioweapon releasing toxic waste products as a result of its metabolism producing these polymers), which can eat right through any form of plastic or rubber to allow it to break through biohazard seals, which is immune to all known antibiotics, and whose spores are capable of surviving anything short of a full-blown centrifuge.

The end result would be a slowly-but-inexorably expanding zone of black sludge that kills anything that comes into contact with it by dissolving their bodies into more of the black sludge.

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Using advanced computers the ship can perturb trajectories of some asteroids to impact Earth. This would not need much energy because of the stochastic character of their movement: the better calculations you do on how and when to perturb them, the less energy you need.

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This problem isn't actually that hard. Some ideas that come to mind:

  • Teleportation
  • Gravity manipulation
  • Cloaking, for visual, heat, and radar, etc, and both for the large vessel and any smaller bodies or persons that leave the vessel.
  • Shapeshifting/Imitation. Imagine small fighting ships that can exactly mimic human vessels, fooling all our sensors, even the the human eye. Imagine a similar device for individual soldiers, letting them mimic people, down to medical checks and disection. Maybe these devices are operated remotely.
  • Super-powers. Give individual solders skin-tight armor where each soldier has Iron-Man, Superman, or Goku-like power, or whatever else you want.
  • Phase shifting. Allow the ship to slip into and out of an alternate dimension, so missiles, kinetic projectiles, and energy attacks sent against it just harmlessly pass through.
  • Time manipulation. Not absolute time travel... just the ability to make small, limited adjustments to the flow of time. Think "Omega 13" from Galaxy Quest, or bullet time from the Matrix.
  • Immortality. I'm thinking how Guardians work in the Destiny video games, where there's a separate entity that looks much like technology that can reconstruct the soldiers every time you kill one. The large ship could be your "Traveler".
  • Perfect Battle AI. No matter what the humans do, the AI has the perfect counter-strategy already in operation.

One additional concept that has worked well for many others is taking things that right now must be large and making them small, take things that right now must be expensive (and rare), and making them cheap (and common), take things that right now need too much energy and finding ways to supply that energy or have them need less.

That was just a couple-minutes worth of thought. To me, the hard part is finding appropriate limits for the aliens. Otherwise, the aliens are too strong, and there's no struggle left from which to build an interesting story.

Now, you might say some of these ideas violate this requirement:

"While remaining within the bounds of hard science fiction"

But I'll counter with:

Any sufficiently advanced technology is indistinguishable from magic.
-- Arthur C Clarke

Arthur C Clarke is about as "hard science fiction" as you can get. Remember, you don't have to explain the technology. It just is. If the humans in your story could explain how it worked, they could also perhaps copy, reproduce, or counter it.

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protected by L.Dutch Sep 24 at 7:57

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