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For the Universe I'm creating Humanity has devolved into groups of scattered tribes. This is due to the fact we ran afoul of a really nasty alien race, who decided to leave us homeless. A coalition of races had banded together to fight this menace, but mankind ended up with the sort end of the stick. It is the Great Diaspora, and we are only hanging on by our fingernails.

The problem I'm having is how to so thoroughly destroy, or at the very least render uninhabitable, the places Man once called home. If it were only Earth it would be a simple enough matter, but by the time the attack takes place he also has colonies on Luna, Mars, Ceres, Callisto, and Titan. How can I muck them up badly enough that we have to now seek a Promised Land elsewhere?

Ceres I figure is small enough it could just be reduced to rubble, but what about Luna? If you destroy the colony there, what's to prevent them from setting up new ones a few hundred miles away?

At the moment all I can think of is a "Gray Goo" weapon, short of blasting multiple planets and moons to smithereens. I'm hoping there's another option out there I haven't considered.

EDIT: I'd like to hold onto the Sun. You know, sentimental reasons. :)

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    $\begingroup$ This seems way too obvious, but... destroy the Sun? $\endgroup$ – F1Krazy Jun 11 at 20:11
  • $\begingroup$ @F1Krazy would make sense, I was thinking along the lines of giving the Sun an inert core exceeding the Chandrasekhar limit. Adding 1.4 solar masses of Iron would do the trick and create a core collapse supernova if I am not mistaken. Sure this is a lot but it's not technologically impossible hell current tech could do it even as it's ultimately simply a problem of applying sufficient brute force to accelerate the desired mass of material to the desired target. $\endgroup$ – MttJocy Jun 11 at 20:15
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    $\begingroup$ To be pedantic: Aside from Earth, the Solar System is utterly uninhabitable without major changes. Thus without knowing further details on what changes have happened to your version of it, the simplest answer would be to revert it to it's current state - but that would be cheating and not providing you any help, so I write it as a comment.. $\endgroup$ – dot_Sp0T Jun 11 at 20:30
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    $\begingroup$ Frame challenge: you are vastly underestimating the cost of creating any kind of colony outside Earth. On Earth, if your city is destroyed you flee and start anew elsewhere. If a lunar habitat is destroyed, everyone dies and if there's any survivor they won't be able to build another habitat with their own bare hands - or even with technology and machines: they will run out of either energy, food, water or air way before. The only challenge is to end Earth's inhabilability, the rest of the Solar System is already lethal enough, you just have to burst those little breathable envelopes. $\endgroup$ – Rekesoft Jun 12 at 10:09
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    $\begingroup$ @Rekesoft Yeah. As Professor Lord Rees (former Astronomer Royal) likes to say "there is nowhere else in the solar system that is as hospitable to human life as the top of Everest or the depths of the Antarctic in winter". $\endgroup$ – Martin Bonner Jun 12 at 10:25
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Rocks

Simply put you throw rocks at stuff you don't like.

Fortunately our solar system is full of rocks just waiting around doing nothing, so we just grab a few (OK, more than a few) and direct them gently at the source of the problems (any inhabitable place) and wait for nature (and gravity) to takes it course.

In the due course of time you can not only wipe out all life, but ensure it won't be producing or supporting any life for millions of years.

Everyone wins.

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  • $\begingroup$ The problem there is that humanity is already assumed to be a spacefaring race, so anything that has been "gently" nudged into an offending orbit will be spotted promptly and despatched in short order. You'll need to push quite a lot of rocks at quite high speed to overcome any defenses; probably you'll need to fit them with engines and stuff. $\endgroup$ – Starfish Prime Jun 12 at 6:12
  • $\begingroup$ In the question it's clear humanity has managed to go no further than the solar system. By the standards of a coalition of alien races (who implicitly must have FTL of some form to get together at all), they're not going to have any problems with the technology of a bunch of glorified monkeys who can't even get out f their own solar system. They'll easily be able to send the rocks at speeds the monkeys can't do anything about (maybe FTL them !). I'm confident these irritating monkeys are about to go extinct. :-) $\endgroup$ – StephenG Jun 12 at 7:33
  • $\begingroup$ Sure, but "use FTL to unstoppably kinetically bombard everything" is not quite the same as "gently direct rocks and wait for gravity to do the rest", hence my comment. $\endgroup$ – Starfish Prime Jun 12 at 17:51
  • $\begingroup$ @StarfishPrime You don't actualy need high speed to overcome defences with a decently sized asteroid - it's sheer weight will be hard enough to stop. And how do you throw an asteroid? Well, attach a couple of big thrusters, deorbit it and speed it up so that it's new orbit accidentally intersects with your target. Perhaps newer generation of strong engines (ion?) have been developed to provide constant thrust and avoidance from launched contermeasures? (comment might be slightly based on Netflix's "Salvation") $\endgroup$ – Rachey Jun 13 at 13:58
  • $\begingroup$ @Rachey a high speed interception will do a slow, heavy rock no good at all. It will be defected or disrupted. If you can support a colony on titan, you can be reasonably expected to have sufficiently capable rockets that interception of even quite large bodies is within the realms of practicality. You will also not be doing much evasion of any kind if you're a big asteroid, and ion engines will not help you in the slightest. $\endgroup$ – Starfish Prime Jun 13 at 19:17
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How to reduce things to rubble easily

Relativistic kill missiles are your best bet for thoroughly destroying inhabitable places in the solar system (which is technically every one of the nine planets and most of the dwarf planets, given enough technology) is to use relativistic kill missiles; the aliens could just send kilometer long projectiles moving at a decent percentage of the speed of light, which, on impact, would convert matter into energy at an efficiency possibly better than even matter-antimatter annihilation (which loses matter-energy efficiency by radiating out neutrinos).

A relativistic kill missile need not have any explosives, because its mass is all it needs, along with some thrusters, which can use fusion reactors, or pushing lasers, to accelerate to well over 50% the speed of light. Just a few teratons of iron or some other dense material, the procurement of which would be child's play to any K2 or above civilization (I'm assuming these aliens aren't from our solar system), would be needed to build the missile; and, when it hits its target at a decent fraction of the speed of light, can bake even a planet and render it uninhabitable. The K-pg impactor caused a mass extinction, but was moving nowhere near the speed of light. A larger asteroid, or one moving a lot faster, could have rendered the planet uninhabitable. And that's what a relativistic kill missile could do.

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A small stream of strangelets in a series of flyby trajectories around the sun will do the trick. As long as the sun is not touched, it is safe. Everything else becomes lethal to the touch.

Since the ΔV from even Mercury to the Sun is immorally huge, the sun should be safe. Add to that, solar wind pushes stuff away from it... Over millenia other stars around the sun will be disassembled, but the Sun will be fine.

As for the Earth, Moon, asteroids, comets... Each and every one of them will disintegrate. Wikipedia, for example, says this could happen if even one strangeled touched the Earth:

This "ice-nine"-like disaster scenario is as follows: one strangelet hits a nucleus, catalyzing its immediate conversion to strange matter. This liberates energy, producing a larger, more stable strangelet, which in turn hits another nucleus, catalyzing its conversion to strange matter. In the end, all the nuclei of all the atoms of Earth are converted, and Earth is reduced to a hot, large lump of strange matter, the size of an asteroid.

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    $\begingroup$ Strangelets are a bit inconveniently overpowered. If you can use them on a whim, it doesn't bode well for the existence of the other alien races forming the coalition mentioned in the OP. $\endgroup$ – Starfish Prime Jun 12 at 6:19
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Self Replicating Drones

The enemy race drops a 3d printer/mobile factory on an asteroid. It starts building more of itself, and creates swarms of drones. It keeps doing this. Eventually there are so many drones you can't have a civilization anymore, either because enough of the mass of the solar system is converted into drones, or you give up and leave and the factories go into hibernation mode.

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    $\begingroup$ Grey goo and self replicating death machines are problematic because they aren't things you can realistically run away from. They can self-seed between stars, so everywhere you run away to already has a pre-fab army, well equipped and entrenched, waiting to mop up stragglers. The other nations preparing to fight the big bad? They've probably already been eaten. $\endgroup$ – Starfish Prime Jun 12 at 6:26
  • $\begingroup$ @StarfishPrime Presumably the aliens who built the self-replicating death machines didn't program them with blueprints for insterstellar starships. $\endgroup$ – nick012000 Jun 13 at 6:01
  • $\begingroup$ @nick012000 they might only program them to use harsh language instead of projectile and energy weaponry too. $\endgroup$ – Starfish Prime Jun 13 at 8:11
  • $\begingroup$ @StarfishPrime There might be practical reasons to avoid giving your self replicating killer robots the ability to travel interstellar distances- for example, to avoid friendly fire against their own holdings or those of allies. $\endgroup$ – nick012000 Jun 13 at 8:21
  • $\begingroup$ @nick012000 There are two things there. Firstly, there's nothing to stop you seeding your replicating weapon systems to select target systems yourself. If you were actually trying to wipe out the humans and their rag-tag band of allies, you'd certainly do that... anything otherwise starts looking like an idiot plot. Secondly, you're going to have problems preventing your goo from spreading anyway, piggybacking on fleeing spacecraft or simply flying through space like a malign panspermia (though by the time the latter becomes an issue, you'll be long gone or changed). $\endgroup$ – Starfish Prime Jun 13 at 10:20
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The answers all suggest multiple plausible ways to render the Solar System uninhabitable, but suffer from some flaws, such as requiring a long time frame to work, or require a great deal of resources.

I will refine the process and suggest the aliens simply have to fire one RKKV (Relativistic Kinetic Kill Vehicle) into the Sun. Striking the sun with that amount of energy and using the extreme speed of the RKKV to deliver the energy deep into the Sun, many processes will be disrupted and the Sun will likely emit massive flares and other radiation events which will scour life from all exposed surfaces. Since the Sun will be unstable for years, there is really no hunkering down and waiting to fix the blasted surface installations, especially since we are talking about the sorts of flare events which would strip large portions of the atmosphere from the inner planets. Larry Niven's short story "Inconstant Moon" describes what such a scenario would look like from Earth.

enter image description here

Use SPF 10,000,000

http://news.larryniven.net/biblio/display.asp?key=52

This story raises the question: How would you spend your last night on Earth? When the Moon suddenly starts shining brighter, Stan and Leslie realize the sun must have gone nova and they only have a few hours until the Earth rotates into the deadly sunshine. This story won a Hugo Award and was made into an episode of The Outer Limits.

As a possible bonus, after the flares settle down, the Sun may also go into a Maunder minimum, producing less energy and making any sort of biological or industrial recovery even more difficult.

Even K2 civilizations have limits, so being able to deal with the opposition using only one RKKV is much more efficient than trying to strike every planet, moon and free flying object in the Solar System.

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  • $\begingroup$ An RKKV big enough to disrupt the sun, huh? Isn't it going to have to be at least as big as the moon? And if you can throw rocks around that big at relativistic speeds on a whim, you're not an enemy that can be practically fough, as the OP requires. That kind of firepower ensures that the plucky underdogs were all cooked to a crisp long ago. $\endgroup$ – Starfish Prime Jun 12 at 6:24
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    $\begingroup$ That’s unlikely to make Titan uninhabitable, since it’s a long long way away from the sun and the flares. $\endgroup$ – Mike Scott Jun 12 at 7:15
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Your aliens could flood the moons, planets orbits with high intensity radiological materials. Fast Neutron radiation is very bad for humans and things like the magnetic field surrounding the Earth and Jovian moons won’t provide protection since neutrons have no electric charge and aren’t effected by magnetic flux lines.

So dusting the regions our habitats orbit through with isotopes of Californium would rain down deadly radiation.

And, depending on how much energy they have to work with, they could generate intensive E-M fields. Our natural protections like magnetic field will attenuate the E-M radiation, but at enough high intensity it will cook us like a hotdog in a microwave. Populations living in totally enclosed metal cages (faraday cages) would survive if the cages didn’t melt. And the biospheres would be wrecked.

And, biological attacks would be effective.

If humans had sealed colonies on hostile moons like Io, they might be protected against these attacks — except E-M since they’d qualify as faraday cages, or faraday cages in faraday cages.

And, then there is good old fashion sabotage. Infiltrate and overload the reactor or poison `water supply.

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I'm going to take a simplistic shot at this, forgive me if I don't 'color inside the lines'. In the OP "uninhabitable" and "diaspora" made me think that a civilization capable of such destruction would also likely have no issues with Biological weapons.
In the basic premise of the pathogen seen in films like "Alien: Covenant", I suggest that a universally lethal-to-all virus/prion or other pathogen would fit the job.
As a self-replicating, incurable weapon...deploy once, then step back and wait.

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    $\begingroup$ The issue is that, as dot_Sp0t comments, in most of the solar system you can only live inside artificial, isolated habitats. So you could sprinkle all of the surface of all of the planets and satellites with the most deadly prion/virus and it would not matter: men would build their own sterile habitat isolated from it (if it were not isolated it would not be habitable, even without the virus/prion). $\endgroup$ – SJuan76 Jun 11 at 21:37
  • $\begingroup$ @SJuan76 Course it also depends when this event were to happen, were it to happen around now while we at least in theory have the tech the engineering designs we have for anything remotely large enough to be self sufficient are early concept drawings at best. It would likely take years just to finalise the design let alone begin construction, aliens could also probably delay this a lot maybe use a bunch of devices lets call them Kessler Bombs, basically orbital fragmentation weapons designed to induce Kessler Syndrome. Not an implausible idea area denial weapons are a thing after all. $\endgroup$ – MttJocy Jun 11 at 23:50
  • $\begingroup$ @MttJocy the OP states that humans also have colonies elsewhere in the solar system which woudl also need to be wiped out. Even if you managed to clutter the orbits of every single one, there's no guarantee there are no big space habitats, and given the abiltity of enough interplanetary travel capability to set up those colonies, the technologies to deorbit all that debris safely and remotely will doubtless also exist. $\endgroup$ – Starfish Prime Jun 12 at 6:14
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Depending on what kind of technology your alien race has, you could use something with a very high mass to gravity slingshot the planets into the sun (or at least near it). I believe that vaporising a planet makes it uninhabitable.

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