# What's the most efficient way to destroy civilization from orbit?

What's the easiest way for your stereotypical aliens to destroy civilization from orbit? I'm taking "destroy civilization" to mean the destruction of all large-scale forms of governance, but bonus points for extinction.

Criteria: Either cheapest or least energy consumption. The aliens have access to anything that's roughly possible for a Kardashev II civilization. The aliens use a magic-plot-teleporty device to get here, but everything else should be as hard-science as possible.

• If aliens have the technology to travel through the vastness of space and achieve a planetary orbit, causing severe damage to that planetary system should be trivial. Given that inter-stellar travel would require technology way beyond current human understanding, we must infer that equally advanced weapons would also exist. Since you've tagged the question with hard-science we must be bound by these inferences. What is not clear is why the aliens would go to the effort. Some sort of anti-matter weapon targeting stars within the system would destroy all solar dependent life. – Jodrell Aug 13 '15 at 7:36
• "Imagine that our entire Solar System were the size of a quarter. The Sun is now a microscopic speck of dust, as are its nine planets, whose orbits are represented by the flat disc of the coin. How far away is the nearest star to our sun? In our model, Proxima Centauri (and any planets that might be around it) would be another quarter, two soccer fields away. This is the typical separation of stars in our part of the galaxy." nasa.gov/audience/foreducators/5-8/features/… – Jodrell Aug 13 '15 at 8:35
• To clarify your question, please define the context for "efficient" and "easy." Do you mean costs the least in alien resources, takes the least time, has the highest human death per alien resource ratio etc? – ryanyuyu Aug 13 '15 at 13:35
• Nuke it from orbit, it's the only way to be sure. – Zikato Aug 14 '15 at 6:17
• Does the planet need to survive? – Mast Aug 15 '15 at 10:06

As Samuel states, throwing a big rock at the earth will certainly end civilization....and everything else too. Short of interplanetary rock throwing contests, the easiest way to end civilization is to nuke each and every major metropolitan area. They're easy to find, just look for the big shiny lights on the planet surface.

If these areas go away then civilization ends. Banking no longer works, government no longer works, transportation systems no longer work (or heavily degraded), and worst of all the Internet will get really really slow.

You can't get paid because the bank that handled your paycheck isn't working and that's because the people who handle your paycheck can't get to work. How long would you last if suddenly ATMs and credit cards stopped working? What if your phone no longer routes calls or text messages don't go through? Would you panic?

Even if the attacks happened at night when most people are at home, only 2.6% of the work force actually works from home with any regularity which means that 97.4% of have to show up for work. (Admittedly the stats are US-centric but I believe are broadly applicable to any First World economy.) In Central Europe during the Great Depression, unemployment peaked at 20% and output dropped by 40%. Unemployment hit 70% in some areas of the UK. In this situation, unemployment is at best 50%. Nuclear attacks on every major metropolitan area have resulted in an unemployment rate at least double, double(!) the Great Depression. It took a world war to get out of the Great Depression but this time, there is no untouched infrastructure to help bootstrap the process.

Couple this loss of employment with 50% global casualties as 54% of humanity lives in cities and the world as we know it has pretty much stopped. It's not quite a Stone Age civilization reset but definitely back to the Iron Age.

From "Effects of Nuclear War" by the US Office of Technology Assessment:

Most economic damage would occur from lost production...The major damage to the economy, however, would result from deaths and long-lasting injuries (to consumers and producers), and personaI tragedies and other traumas making people less able to work. The magnitude of economic loss could be expected to vary with the number of deaths. The attack would cause considerable economic disruption in the uncontaminated area. Facilities there would need to produce a vastly different mix of goods and cope with the absence of goods that normally come from contaminated areas. Until people acted as if they believed the war was over, it could prove difficult to organize production in the uncontaminated areas. Uncertainties about the legal and financial arrangements that support production (money, contracts, credit, etc.) following a nuclear attack might impede production in the uncontaminated areas. Public disorder could also impede production. The changes and uncertainties would cause some economic disruption; however, the greater effort put forth would probably more than compensate for it.

Emphasis mine.

Basically, questions about who gets paid for what, by who, and by what mechanism would cause a drastic reduction in economic output.

Every dot on the map below is gone. How well is civilization going to work when those are all gone?

Oddly, in this scenario, North Korea might end up doing pretty well but only because they don't have anything to begin with.

• Comments are not for extended discussion; this conversation has been moved to chat. – Tim B Aug 14 '15 at 18:49
• According to this map, New Zealand would be fine ;). – Blake Walsh Aug 15 '15 at 19:59
• @BlakeWalsh It and Antarctica bailed. – Samuel Aug 16 '15 at 1:51
• You could probably do it with around 10 tons of weapons grade plutonium. I imagine you could pick up a suitable space rock en-route. – superluminary Aug 26 '15 at 19:07
• Why is Perth the biggest dot in Australia? – Azor Ahai Oct 10 '16 at 6:30

Hit the planet with an asteroid.

Guide a 100 km wide asteroid at the planet at around 10 km/second and there will be a global extinction event. Anything larger than 3 km can cause extinctions, so the 100 km figure is simply to make sure the job gets done right.

• you beat me to it. I literaly said, out loud "Throw a big rock at it" and lo, your answer says the exact same thing. – Green Aug 13 '15 at 3:34
• For bonus points aim for yellowstone. – Murphy Aug 13 '15 at 9:44
• The moon is in orbit around the earth, and de-orbiting that would easily do the job. – Useless Aug 13 '15 at 11:25
• @Useless Perhaps, but this answer neither mentions the moon nor explains how you would go about deorbiting it. -1. – David says Reinstate Monica Aug 13 '15 at 12:54
• @Samuel en.wikipedia.org/wiki/Ceteris_paribus by which I want to say, OP wanted to change one thing about reality, namely aliens bent on our destruction being in orbit around Earth. Also, a 3km rock, dropped from Earth orbit, would NOT be a planet killer, because it wouldn't have the necessary kinetic energy. You don't only need mass, you also need impact velocity. See what-if.xkcd.com/57 – Hackworth Aug 13 '15 at 15:23

Moving asteroids? Antimatter? Relativistic shells? Surely you jest! That's not energy efficient.

The most efficient way is to beat mankind with their own weapons.

Hack into the President's Red Telephone, or NORAD, or some silo in the Midwest and let the nukes go ballistic. Wait for counterstrike and mushroom clouds to settle. For extra effect, release viruses and spray whatever nerve gas is still in their B and C arsenals. Done.

That advanced civilization would only need to listen in to our military radio communications for a while, use their massively parallel quantum computer to break humankind's most advanced encryption algorithms (their NSA is beyond what you can imagine) and radio down the launch sequence codes. This is wonderfully efficient because the ICBM target coordinates are specifically selected to cause maximum damage or disruption by our finest military intelligence. The aliens wouldn't have to scratch their tentacles just where to point their beams of death. The same argument also applies to B and C weaponry: specifically designed to wreak maximum havoc on human physiology. Why abduct humans and study their weaknesses when they already have incorporated all that knowledge in their weapons of mass destruction^W^W^Wself-defense tactics?

Total energy consumption: a few Joules for carefully modulated electromagnetic waves.

• I particularly like this approach, because none of the survivors would know what had actually happened. Practically everyone would assume that either the US or the Russians pushed the big red button. The aliens can come as savours and be welcomed by the tattered remnants of mankind. – Luke Sleeman Aug 17 '15 at 6:24
• @LukeSleeman and then the benevolent aliens invite all survivors to a great feast... where they are the main dish MUAHAHAHA!!!! – Falco Aug 17 '15 at 9:38
• Fully automated nuclear launch systems probably don't exist: they all probably require manual intervention. Which means this solution requires that the Aliens can intercept hard-wired communication and convince soldiers, often verbally (by faking the people on the other end of the line), that they are supposed to launch the weapons, while preventing other sources of communication from being used that would convince the soldiers that they are under information attack. This would require advanced knowledge of human psychology, and more than just hacking encryption. – Yakk Aug 17 '15 at 13:55
• @Yakk Convince soldiers? It could be as easy as using a Presidential Microphone Test – Jens Aug 17 '15 at 14:06
• @jens Under that assumption, the turing test could be passed by a tape recorder. Soldiers actually interact with other people and push back, especially when a "end the world" order shows up out of nowhere. This, scarily enough, has been tested experimentally. – Yakk Aug 17 '15 at 14:12

An electromagnetic pulse (EMP).

We want to destroy civilization - not destroy the planet.

It's evident the Aliens possess some pretty interesting tech, else how would they have got here? I'm pretty sure that the energy requirements for interstellar FTL are massively high - orders of magnitude greater than anything created or generated on earth.

This means there is some advanced energy generation or storage at play.

The Alien race would be able to potentially rapidly 'discharge' this energy into the ionosphere - instantly knocking out satellites - so there goes communication and GPS.

Up the power and you have a full on Geomagnetic_storm - with all the devastation that this would cause:

• Electrocutions
• Complete power grid failure
• Global communication outage
• Devastating weather effects (Hurricanes, Typhoons)
• increased tidal forces (Tsunami, Earthquakes)

All of which would be impossible to fight against, and all the aliens need to do is observe, and increase the power as required.

• If they had FTL, they could just go to back in time and stop the universe from forming. – PyRulez Aug 13 '15 at 12:25
• @PyRulez FTL does not instantly imply time travel. For instance the universe is expanding FTL and is not simultaneously going back in time. – Ryan Aug 13 '15 at 16:32
• @ryan and space expansion is the basis for the Alcubierre drive, which in theory could allow FTL transportation! – njzk2 Aug 14 '15 at 13:43

Post a query to the human's most popular Q&A site, then wait for the humans to list all of their vulnerabilities and easy ways to kill themselves.

Wait for long enough for the question to go quiet. Then choose the method with the most up-votes and see whether in fact your alien technology can implement it. If so, use that method. If not, continue down the list in up-vote order until you find one that you can implement.

Best part? This will work for any form of life with a large data-centric society.

• This is a way to get various methods, not the most effective method. They would still need to ask this exact same question with the list they obtained using your answer. – Samuel Aug 16 '15 at 1:52
• I think this joke meta-answer is hilarious, and is better as a proper answer, rather than comment. – Peter Cordes Aug 16 '15 at 10:59

The absolutely most efficient way would be to land in contested territory somewhere, and claim that you want to give some superior weapons technology to the leading nation of the world.

Then sit and watch how the nations destroy each other as they try to make sure they are the leading nation. It will help if you have defensive technologies that can shield you from nuclear blasts.

Since it is already for human grade technology a little hard to imagine bringing up all the energy needed to travel and do all the things, lets just add a little more and use

## relativistic projectiles

These could also be a convenient way incorporated into the travel of these species. Adding enough energy to a flipper steel ball to make it travel at significant fractions of $c$ can deliver the amount of energy of a hundreds of megaton nuclear weapon, without the drawbacks of the massive fallout (there will be some, but it is likely much less than with conventional thermonuclear weapons).

It will also be a convenient way to slow down when traveling. It is fair to assume that they travel at significant fractions of $c$ to actually get somewhere in time, so firing enough of these balls gradually at earth will slow their ship down, obliterate the earth (no need for aiming, no one will survive a steady stream of thousands of hundred megaton bombs arbitrarily scattered around the surface), and if timed correctly by the time they get into orbit, everything is ready for whatever they want to do.

• Depends on what you mean by fallout. Kicking up tons of dust into the atmosphere that would cause a "nuclear winter" would still occur. It just wouldn't be all radioactive. – Mason Wheeler Aug 13 '15 at 10:39
• @MasonWheeler: Generally by "fallout" people (and me) mean nuclear fallout, that is, radioactive material. I am not aware of any actual calculations, but it seems to be general consensus that it is orders of magnitude less of what we would produce for a similar powerful thermonuclear weapon (imagine the fallout of fully three staged 100MT tzar bombs), probably low enough to easily live with it. – PlasmaHH Aug 13 '15 at 10:43
• What mass of ball are you talking about? At about a kilogram, even hitting three nines will barely clear 400 MT. Two nines won't even reach 120 MT. – SuperJedi224 Aug 13 '15 at 12:49
• @SuperJedi224: The mass isn't really important, we can always add speed. A few grams, maybe a few hundred. 10MT per impactor is enough to obliterate the surface, given you launch loads of them. And you likely want to, because you have no intel about where to aim. – PlasmaHH Aug 13 '15 at 12:53
• – cobaltduck Aug 14 '15 at 14:00

Does blowing up the planet count? In that case I've always liked Greg Bear's idea in The Forge of God:

Fire two pellets of neutronium (neutron star material) at the Earth, one made from neutrons, the other from antineutrons. Due to their density they won't collide with Earth, instead they'll fall straight through it as if the Earth didn't exist, orbiting around the Earth's centre of mass. The antineutronium pellet will spark a little, but it won't annihilate catastrophically. Until they meet at the centre of the Earth, and then: kablooey.

I don't know how scientifically accurate this is, but it sounds legit. If the alien civilisation's intent is to use the Earth for their own purposes then they might consider this slightly overkill... :)

• These "pellets" would probably have to be suprisingly big. A bullet made of that matter would weight as much as the empire state building. I think I remember somone calculating that the antimatter needed to blow the earth up is at least as the mass of the mount everest (one can check that by googling earths gravitational binding energy, and energy released by antimatter annihiliation). It will certainly fall straight through the core (with a bit of oscillation) but at its way will probably already make the planet uninhabitable. Plus you need a way to contain it, since it will expand rapidly – PlasmaHH Aug 16 '15 at 11:57
• @PlasmaHH Neutron-degenerate matter is extremely dense. Even an amount with the mass of Mount Everest would be very small. According to an estimate of the mass of Mount Everest I found here: answers.com/Q/What_is_the_mass_of_Mount_Everest and the lower bound of a neutron star's overall density given here: en.wikipedia.org/wiki/Neutron_star#Properties an amount of "neutronium" with the mass of Mount Everest would be about 0.017 m³. That's 17 l, or about one and a half bucket full. It would punch a very small tunnel through the Earth which probably wouldn't affect it at all. – Pepijn Schmitz Aug 16 '15 at 12:21
• You seem to be assuming that the majority of the mass will be just pushed away. I don't have anything at hand now to do the calculations, but assuming a diameter of 20cm for the tunnel, and all the way down to the core, this is a huge amount of mass, and I think a pretty significant part will be pushed against the antimatter and will react. I would assume that it is many tons, all to the effect of huge nuclear weapons, and due to the built up pressure on the way down, with the following funny effect of a lava super soaker (i.e. big vulcano). Compare that to the dino asteroid. – PlasmaHH Aug 16 '15 at 19:34
• The hard part is keeping the (anti-)neutronium stable. Neutronium is stable when under intense, intense pressure: removing that intense pressure, and it explodes, as the states the particles are in are states they really, really don't want to be in. They don't want to be in that state so strong, that they are able to push against the amazing pressure they are in and stay stable. Without that pressure, they leave their state. Earth has binding energy of 32.5EJ, which is 15.5Ekg, or 15EL of Earth, or 9km cube of Earth. Or two 18 cm neutromium cubes, or 22 cm diameter spheres. – Yakk Aug 17 '15 at 14:04
• A 11 cm radius tube through the Earth weighs 9.5Ekg, or 26.5EJ after annihilation with anti-neutronium. (Interaction with neutronium will be about half as energetic). That in KE is enough to stop a 1 million km/h anti-neutronium particle to a dead stop. Basically, the idea that it would pass through the earth like a ghost doesn't seem to match: it would sweep up a tunnel through the Earth, react nearly completely, and the energy yield will make any reasonable KE of the neutronium particle look tiny, grinding the neutronium to a near immediate stop (or, having it bounce off the Earth). – Yakk Aug 17 '15 at 14:10

# Enhanced global warming

You would need to put a bunch of giant mirrors and/or lenses in orbit on outerspace and keep them properly aligned to reflect and focus the sunlight to Earth. This is doable with some small robotic mirror-bearing or lense-bearing starships powered by solar power.

With enough mirrors and/or lenses in orbit reflecting the Sun light direct to Earth the result would be that Earth temperatures would rise. With a reasonable area of mirrors you could get global warming, weather disruption and famines. With a much larger area, you could venusform* Earth.

* I coined that verb as something similar to "terraforming", but instead of making something like Earth, it would be like Venus.

• You beat me to this answer. E.E. "Doc" Smith used "sunbeams" in his Lensman series decades ago as offensive weapons. The benefit is that you can target it precisely to control how much life you want to destroy and where. – Paul Chernoch Aug 14 '15 at 20:34

# Hide the sun.

If you find a way to completely block the sunlight to shine on a planet, you can be sure every life form should disappear quickly.

• Any kind of photosynthesis become impossible, so oxygen levels can now only go down, and every plant outside a laboratory shall die quickly. Then herbivorous animals would follow, then carnivorous ones.
• Temperature shall become icy in some days. Except in very heated bunkers, everything would froze to death.

So just prevent the planet from being shined for one week, and you would destroy any vegetable or animal life form on the surface. Even if there would be some survivors that would have hide in some shelters, you can let the sun out to watch them starve to death.

• Your timescale might be a little off but the idea is sound. There are various geoengineering idea to reduce insolation (from orbital mirrors to dust-clouds), a true spacefaring race would have little to no trouble scaling these up. It's not like a few gaps would matter, 1% leakage would have little effect, and even dropping insolation to say 50% of its current value would ruin food production within a season. Bonus: the authorities would probably only be able to keep control for a few days – Chris H Aug 14 '15 at 12:49
• I would add - it must take a lot of energy to power a galactic empire. Why not block out the sun with a half dyson sphere? Absorb all that delicious energy with the teensy side affect of destroying the only civilization that could give you grief. And you could do it all with some intergalactic drones! – C Bauer Aug 14 '15 at 14:41
• Besides the fact that one week is not enough to kill all life, there is no reason why a civilization should be unable to recover from that. We already have Seed banks and food reserves to survive until the plants are recreated. Not everyone would survive but the government which ought to be destroyed is the one which has control over these reserves and the recovery process. On the other hand, hiding or shading the sun for a far longer time is a very efficient way to destroy a civilization as it doesn’t cost much energy. You only have to be patient… – Holger Aug 17 '15 at 10:10

Stolen shamelessly from one of my previous posts.

If you absolutely, positively need to sterilize the surface, you must blast it from orbit. It's the only way to be sure.

The problem you'll quickly run into is that the Earth is big. You can crash 10km sized asteroids into it, and there'll still be some survivors to whine about the injustice of it all. Now, we wouldn't want that, would we? There are two surefire ways.

## 1. Antimatter Bombardment

No, the only way to be sure is to be thorough. You'll need a fair bit of antimatter, um, let's see...

1 Mt is $4.1\times10^{15} J$ so the Tsar Bomba (in the tested config) at $42MT = 1.72 \times10^{17} J$
2 kg of $E=mc^2 = 1.79 \times 10^{17} J$

So each 1 kg antimatter bomb (as it anihillates with 1kg ordinary matter) would have a blast similar to the Tsar Bomba, the most powerful nuclear device ever detonated in the puny humans' history. That gives you a nice $1,200 km^2$ blast incineration area. Now, if we were sloppy, we would just pepper the land area of $148,300,000 km^2$, so about 1,236 MIEVs (Multiple Independently Targetable Entry Vehicles) with 100 1kg (+containment) warheads each would do.

But that would miss all the boats, planes and submarines crowding their primitive buoyancy based transportation lanes. With about 100,000 ships out there, that's a lot of survivors. $361,000,000 km^2$ to cover, you'd need another 3,009 MIEVs. You might still miss a submarine or two, but without space launch capabilities, zero industrial capacity, and an all-male crew, you're set anyway.

Pro: Clean blasts, little of that nasty radioactive slag. Also, once in atmosphere, any attempt to shoot down will only damage the containment system and detonate the antimatter.
Con: You need a lotta bombs.

## 2. A dozen or more relativistic ships.

Thanks to our friends at Wolfram Alpha, I don't even have to do the calculation myself. Take your standard light-crafts, a 3,000 ton craft, push them to a reasonable 0.25 c and crash them on opposite hemispheres (preferably from multiple directions, but you could also do several hours apart). Each will clock in at around $8.8\times10^{21}J$, or about 10,000 times those anti-matter firecrackers from earlier, or about the same as a 10 km asteroid impact. To be really sure, send a few dozen of these.

Pro: Almost impossible to intercept once acceleration is complete.
Con: Your reptilian colonists awaiting in orbit might themselves cook a bit from the impact gamma blasts.

• And queue the NSA watchlist in 3, 2, 1. (On this site, I think that's a compliment!) – corsiKa Aug 13 '15 at 15:00

Meteor

Don't bother nuking major cities. Nuke space rocks onto a collision course. Use a few for a faster results! A small moon would be a great candidate! But that's pretty messy. And after all, as a stereotypical alien invader, you're looking to colonize. Are there any cleaner options?

Virus

You knew you had a reason for all those abductions and experiments. Upload a human population in your brand new quantum simulation program on your ship's super computer. Now in supertime, evolve a virus that is guaranteed to wipe out those pesky sapiens! You could start out with a particularly nasty strain of HIV. When you're ready just drop a pod into a major international city, don't want it to burn up in the atmosphere!

Self-replicating machines

You just realized that you are an advanced race, and biological warfare is a pretty low-blow. And you've been watching some DirectTV while in orbit and an old sci-fi flick with a big Austrian guy inspired a brilliant idea. Drop a single small machine loaded with instructions to mine up a bit of resources and complete, then replicate itself. Finding a location with a suitable amount of resources available your colony will be able to double itself in hours. Within 20 doublings (maybe a week or two) your army will consist of over a million robotic soldiers awaiting your command. And when they are finished with their eradication mission they can start the construction of your new palace. And to think it just cost you one 3d printing of some plans you found on the galaxy-wide-web. Just make sure they say catchy phrases when eliminating the targets, like "Hasta la vista, baby."

Terraform

All this time you've been absentmindedly setting the planet's forests and crops on fire with the ship's laser canon, just for fun, when it hits you! You've been getting ahead of yourself. The planet won't support your new family as is. Now you feel silly for entertaining all these ideas. The eradication will solve itself! You drop one Ready-Made Multi-Purpose Automatic Self-Constructing Terraforming Factory Seed 2000 on the planet. Now you sit back and relax. The planet will be a nice and cool 98 centigrade with plenty of that life giving arsenic in the water in no time.

Note: While the tone of this comment is lighthearted, the science is serious. None of these things require the alien to have huge amounts of energy or resources before hand (like relativistic projectiles or antimatter/neutronium bombs). In addition it accounts for multiple different end scenarios (such as destroying the everything, or just the biology, or just one species). It also leaves room for multiple starting situations like a lack of suitable asteroids/comets, highly exploited or otherwise unavailable resources. It's impossible to say what the most efficient method would be for an unknown alien on an unknown planet wiping out an unknown civilization, but we can make some good guesses I think.

• Note the hard-science tag. You have zero references and have somehow come up with four "most efficient" answers, which doesn't make sense. – Samuel Aug 13 '15 at 6:08
• @Samuel The point is that all 4 would be near-zero costs (for a "stereotypical alien" race) with varying end results. Depending on the end result your looking for, they could have different efficiencies. Not completely sure what would qualify as sources in such a scenario, though I don't see anything that would be outside the realm of plausibility for a suitably advanced alien race. If you can give me a suggestion on what sort of source you'd like to see I can take a look at revising my answer. – Adam Phelps Aug 13 '15 at 6:19
• Check out the description of the hard science tag. You should include peer reviewed articles or equations to back up your statements. – Samuel Aug 13 '15 at 14:56

## Massive Solar-Storm

How About I don't attack earth directly, but rather instigate the sun to fire a massive 24 hour long barrage of solar flares in earths direction. If the flares contain a massive amount of radiation they will kill almost all life by radiation poisoning, burn the face of the earth, and the electromagnetic disturbance will destroy most orbital and local electronics. After 24 hours the whole surface of earth was bathed in the storm and civilization will be wiped out.

And they lived happily ever after.

Make a survey of the planetary surface and collect samples of the most potent types of virus affecting humans. Use your sophisticated biotechnology to engineer them to be more transmissible and more lethal. Release them into major urban areas.

That will require no further action from you and will result in sufficient loss of life that civilisation will break down. There will be humans with sufficiently potent immune systems to survive, but very few of them and they will now be living in a perilous post-apocalyptic world. Your mothership landing will be the least of their worries.

Depends on which definition of "efficient" you're looking at.

Efficient: achieving maximum productivity with minimum wasted effort or expense.

If you're really shooting for minimum wasted effort or expense, couldn't you just do nothing and wait around? On a long enough timeline most all civilizations are ultimately doomed; the sun will super nova or burn out eventually, large asteroid strikes happen every so often, and so on...

Criteria: Either cheapest or least energy consumption.

Patience doesn't cost much and requires very little, if any, energy.

• Technically it would require the energy needed to sustain you while you wait – thanby - reinstate Monica Aug 17 '15 at 15:47
• @thanby Wouldn't you expend that energy even if you weren't bent on destroying a civilization? – apaul Aug 17 '15 at 16:05

Consume all the oxygen. Send down microbes, or chemicals that will react with oxygen, and let them absorb the atmosphere down to under 5% oxygen. There are a lot of consequences due to lack of oxygen well beyond being unable to breathe without aid. For bonus points, getting rid of CO2 would help significantly.

It's a slow process, but it's very, very efficient in terms of cost.

• Unfortunately for this scheme, Oxygen would not tend to be very co-operative, to a certain extent there's so much of it, because it's essentially toxic waste - a byproduct of life's relentless pursuit of useful building materials like carbon. Unlike carbon and silicon, Oxygen can't be used to form long chains, it's no good for building stuff. I believe the stable molecule with the most oxygen by weight is H20, which is 89% oxygen by weight. So this would require about 400 million billion tonnes of hydrogen - a good thing it's the most abundant element in the universe! – Blake Walsh Aug 15 '15 at 20:20

# Gamma-ray burst

The solution would be to bombard Earth with gamma-rays. Normally a supernova in a nearby star would be able to do that, but since the aliens are much more near Earth and can focus their energy much better and in the most suitable part of the eletromagnetic spectrum, they won't need something as powerful as a supernova. Just a bunch of powerful gamma-ray lasers will do the work.

What would happen exactly? Accordingly to wikipedia:

The major Ordovician–Silurian extinction events of 450 million years ago may have been caused by a GRB. The late Ordovician species of trilobite that spent some of its life in the plankton layer near the ocean surface was much harder hit than deep-water dwellers, which tended to stay put within quite restricted areas. Usually it is the more widely spread species that fare better in extinction, and hence this unusual pattern could be explained by a GRB, which would probably devastate creatures living on land and near the ocean surface, but leave deep-sea creatures relatively unharmed.

[...] Gamma rays would not penetrate Earth's atmosphere to impact the surface directly, but they would chemically damage the stratosphere.

For example, if WR 104, at a distance of 8,000 light-years, were to hit Earth with a burst of 10 seconds duration, its gamma rays could deplete about 25 percent of the world's ozone layer. This would result in mass extinction, food chain depletion, and starvation. The side of Earth facing the GRB would receive potentially lethal radiation exposure, which can cause radiation sickness in the short term, and, in the long term, results in serious impacts to life due to ozone layer depletion.

Longer-term, gamma ray energy may cause chemical reactions involving oxygen and nitrogen molecules which may create nitrogen oxide then nitrogen dioxide gas, causing photochemical smog. The GRB may produce enough of the gas to cover the sky and darken it. Gas would prevent sunlight from reaching Earth's surface, producing a "cosmic winter" effect – a similar situation to an impact winter, but not caused by an impact. GRB-produced gas could also even further deplete the ozone layer.

• I don't see how this is going to be the cheapest or lowest-energy solution. Effective, but very energy-intensive to use your own GRASERs. – Peter Cordes Aug 16 '15 at 10:52

I think to an extent, the only way to destroy civilization would be to make humans go extinct; people gravitate towards systems of government, even if only at a local level. At least one system would probably grow over time, so the only way to make sure it's completely gone would be to kill everyone. While you could do that with an asteroid, it would be extremely hard to actually change the trajectory of something that large, unless you're willing to do the calculations, tap it slightly and very precisely while it's far away (100's of years) from earth, and then just wait around for it to hit. A much faster, easier method would probably be to just nuke the place. If you don't have enough to hit everywhere, then probably just hitting the major centers of government, commerce, woods/trees, and agriculture, would be sufficient to end civilization in fairly short order. Just make sure that you destroy enough so that any survivors won't be able to find food, fresh water, and/or shelter, or maybe even heat or oxygen if you can set off enough firestorms and destroy enough forests. Clouds of smoke covering the whole world would deprive it of sunlight and heat, which in turn would greatly facilitate the cold death of the planet. Ain't it depressing that we're capable of doing all that right now.

• Note the hard-science tag. – Samuel Aug 13 '15 at 6:09

Why destroy when you can enslave? Destroy 1 major city, televise it, jam all frequency, and say "Join us or you're next"

If we assume the aliens are travelling at relativistic velocity between the stars (no FTL), then they have several options:

1. Launch a probe at a high fraction of c at the Sun while beginning your braking manoeuvre. The impact on the Sun will trigger massive solar flares and other energy releasing events which will destroy virtually any space based technology that we currently have, and fry much of the electrical and electronic infrastructure of the Earth. The world's economy will crash, and in most of the advanced nations, where farming is highly mechanized, the number of farmers left will not be able to feed the local population (much less the global population), and mass starvation will take care of the bulk of the world's people. If diverting part of the US corn crop to make ethanol cold raise food prices to the point of causing riots in Mexico and other nations around the world due to price increases, imagine the effect of stopping ALL food exports.

2. If they are moving at a very high fraction of c, they might just "buzz" the planet. The Lorentz Transformation of a sufficiently massive spacecraft would be like moving a massive object through the solar system, potentially disrupting planetary orbits. The Earth could suffer disruptions ranging from massive earthquakes to being moved bodily from the habitable zone, depending on how the aliens times their pass and how "massive" their ship appears in our frame of reference. (If the movement brings Earth into "their" idea of the habitable zone, so much the better). In any event, the amount of inertia of the Earth will require a very huge amount of energy to disrupt, and this energy will be felt in the movement of crustal plates, oceans sloshing from their basins and massive magma chambers emptying.

3. Kill the Earth. If the Aliens don't particularly care about the Earth, they could just send a probe at a high fraction of c to strike the planet directly. At .99*c*, even a relatively small object will crack open the crust and expose the mantle (http://what-if.xkcd.com/20/). It might actually be more "efficient" to send a shower of objects timed to strike all areas of the Earth as it turns in its orbit over a 24hr period to sterilize the planet without cracking open the crust, but even the energy release at that point will probably boil the oceans. One could imagine a shower of marbles being dumped from a missile bus moving at .99*c* as the impactor, at that speed, the material hardly matters.

One of the advantages for the aliens is that it will be virtually impossible for Earth to see the incoming projectile (regardless of the target) at high fractions of c, it is coming virtually behind its own light, if you do happen to see it, it will be "in the past", and you won't have much of an idea where it is now. Even with futuristic technologies, there does not seem to be any way to see or stop highly relativistic projectiles coming from space.

We, humanity, already have the technology to destroy ourselves. Why complicate it with antimatter or near-c moving objects? Ever seen The Day the Earth Stood Still?

I see two options that are both cheap. One is relatively quick, the other is kind of slow:

Everything will die in about a month. Cheap, easy, and relatively quick.

How?

Nuclear detonation in the atmosphere in key locations should do the trick. Someone already suggested hacking into the already existing systems on Earth for launching nuclear weapons. That sounds like it would cost only a small team of nerds and a ton of red bull. Alternatively, Uranium and other radioactive elements are pretty easy to find in the cosmos, I'm sure. Mining operations are probably not even marginal in operations management for this alien race.

Problems

The planet's irradiated now. Unless you have some way to reverse it you won't be able to use it for a very long time. All surface life is dead, not just humans.

Pros

The infrastructure remains. Everything you ever wanted to know about humans is now enshrined in the remarkably undamaged ruins of its cities.

## Targeted Biological weapons

This was already suggested, but a virus strain that specifically targets humans and spreads and kills quickly would work very well.

How?

A combination of illnesses, covering multiple contagion pathways, all released in multiple locations around the Earth.

Problems

Not as cheap as irradiation, and not as efficient. You would need time for research, probably years, to determine and develop the most effective pathogens. There would also be survivors, whether immune or unexposed, that you would have to dispatch manually.

Pros

This leaves the planet human-free and relatively unharmed. If you do it right, you will get well over 90% of all humanity and nearly 100% of those in the cities.

All others get pretty excessive (i.e. wasteful).

• Honestly I think a Kardashev II civilization would probably be pretty well-versed in biological engineering (assuming they've seen biology like ours anyways), among other things. That might be the easier solution because it's self-replicating. – thanby - reinstate Monica Aug 17 '15 at 15:51
• @thanby Being versed in a subject doesn't change the amount of time it takes to research effectiveness. They would need samples of humans, samples of diseases, isolated factor testing, and time to observe the results in simulated reality. Coming at a civilization with biological warfare half-cocked would likely lead to the failure of its destruction or unintended results (e.g. cross species effects, etc.). – 458 Aug 17 '15 at 16:56

A pretty efficient way to destroy the earth would be to use a high enough powered laser from space to melt through the Earth's crust at its weakest areas (such as supervolcano caldera) to cause a volcanic winter.

The idea is similar to the other ideas of causing extinction mentioned here, such as nuclear winter and just outright blocking the sun, but i think this has the benefit of really requiring very little extra technology beyond our current capabilities. All it needs is a big enough laser, powerful enough to melt silicon and whatever else is in the crust, with big enough focusing lenses of a material that can withstand the intense energy.

Outcome: These statistics are simulated projections for nuclear winter but i think you could draw a pretty direct comparison. Not the fastest way to do it, but i think probably the cheapest and most energy efficient, as well as being technologically realistic.

Negate or Warp the Earth's Magnetic Field

Either temporarily negating, or warping the magnetic field of the planet such that the sun facing side effective has little to no electromagnetic protection. The solar wind would wreak havoc on anything electrical or electronic and on anything on the surface containing DNA (i.e. people, plants, life in general.)

Negation could be effected via phase cancellation, altering the shape of the field can be effected via the employ of extremely strong and large directable fields. Electromagnetic field manipulation on this scale would be especially feasible by a sufficiently advanced alien race capable of interstellar travel. I do not see anything that would preclude such a possibility as such massive fields would likely be used in shielding, containment, the actual method of interstellar travel via the manipulation of the actual physical "shape" of space surrounding the craft, possibly directed energy inter-vessel weaponry, and so on. They would certainly be capable of harvesting, harnessing and/or producing the requisite energy to do these things. It might even be likely that such a vessel would be powered by some kind of a contained/managed stellar, neutron star, or even a black hole based "power plant" or "engine."

Perhaps even a complete and permanent collapse of the planet's electromagnetic field could be caused by focusing complex, directional and huge fields on the molten core of the planet thereby causing a cessation of its rotation therein!

A few days of this would be sufficient to wipe out any real civilization, kill a large portion of life on the planet, trigger lots of mutations among the remaining life and make earth an all around fun place for the few politicians who hid themselves in bunkers--if they had the time/wherewithal to do so before being affected beyond survivability--as they emerge thinking they'll have something left to run...

• Hello Mce128, welcome on worldbuilding. The question had the "hard-science" tag, could you elaborate how the magnetic field would be eliminated? – o.m. Aug 15 '15 at 5:30
• @o.m. I reworded a bit and added some elaboration on various possibilities for effects on the earth's electromagnetic field. – Mce128 Aug 16 '15 at 7:03

Not a joke: jam their smart phones.

• The population aged under 30 commits suicide for obvious reasons.
• The world economy comes to a grinding halt since top managers are helpless.

Not as sexy as that antimatter device, but sadly much more effective and energy saving. Who said aliens civilizations wouldn't care for green conquer? That's what Douglas Adams would have thought about :-)

I find it unbelievable that no-one has mentioned the single most efficient weapon for destroying civilization, at very low energy and economic cost.

https://en.wikipedia.org/wiki/Biological_warfare

a couple of quotes from the above wikipedia:

Offensive biological warfare, including mass production, stockpiling and use of biological weapons, was outlawed by the 1972 Biological Weapons Convention (BWC). The rationale behind this treaty, which has been ratified or acceded to by 170 countries as of April 2013, is to prevent a biological attack which could conceivably result in large numbers of civilian casualties and cause severe disruption to economic and societal infrastructure. Many countries, including signatories of the BWC, currently pursue research into the defense or protection against BW, which is not prohibited by the BWC.

That is, we are talking about a weapon less legal than nukes. And here's why.

It has been argued that rational people would never use biological weapons offensively. The argument is that biological weapons cannot be controlled: the weapon could backfire and harm the army on the offensive, perhaps having even worse effects than on the target. An agent like smallpox or other airborne viruses would almost certainly spread worldwide and ultimately infect the user's home country. However, this argument does not necessarily apply to bacteria...

Aliens would have an entirely different physiology and would almost certainly be immune to human viruses, which have specifically evolved to reproduce in a human cell. (On the other hand, simple bacteria and parasites might prove deadly to the aliens.)

So if theres a wave of alien abductions, they're probably taking people to use a guinea pigs to breed and incubate the ultimate strain of smallpox. Although the disease has currently been eradicated, we shouldn't get complacent and stop vaccinating, because when aliens attack we're doomed.

https://en.wikipedia.org/wiki/Smallpox

https://en.wikipedia.org/wiki/Smallpox_vaccine

Note that smallpox played a major role in the conquest of the Americas. The Europeans brought smallpox with them, and the natives, having had no contact with the disease, were decimated. (In return, it is believed the Native Americans probably gave the Europeans syphilis.)

Aliens should research our culture and gain knowledge of the planet, before they will destroy us. I believe from perspective of inhuman specie I would've destroy human kind with the most creative way as possible. If we can convince entire human race that after you die, you become into anything you wished. Christians will go to heaven and atheists will become gods of galaxies, muslims will travel to gardens of virgins and etc. Maybe we the aliens will pour some toxic delusional drug into the atmosphere. Make everyone trip into dreamworld of endless possibilities. Countless casualties by suicides. The ones that keep breathing on this planet only because of fear to die. Yeah orbital strike with enhanced LSD is my final answer.

• "If we can convince entire human race that after you die, you become into anything you wished. Christians will go to heaven and atheists will become gods of galaxies, muslims will travel to gardens of virgins and etc." But that's already what people belie-.... oh. – Praxis Ashelin Aug 18 '15 at 12:25
• Yeah, billions already believe they are going to heaven, but they aren't killing themselves over it. – 458 Aug 20 '15 at 15:13
• Cause its a sin to commit suicide, but if they'll see that suicide is still ticket to heaven. They will fly of roof tops quick. – undulationer Aug 20 '15 at 16:45

Destroy the 3rd world countries and wait for civilization to collapse. Modern first-world countries depend on the third world countries for so many resources it would be difficult to function without them. After all, those countries supply large amounts of the resources necessary to build a F-16 or any other modern weapon. The destruction of the 3rd world countries could be carried out via a bioweapon or something else. This has the added benefit of minimizing damage to the planet.

Throw a tether from the Moon.
OK:said like this, it need almost infinite strength. But if you make something that hung until the earth atmosphere, the aerodynamic drag will slowly slow then crash the Moon down.
This still need a science-fictionish tensile strength, but the energetic cost can be very small.

A lot of people have been ignoring the energy-efficient aspect of the request. There's a lot of things that would work but use a huge amount of energy.

The most efficient way to destroy civilization would be from the Kupier belt. Take something say 20 miles across and deorbit it. Gravity does most of the work. It's slow, though.

• Moving a massive object is surely no easy, nor energy efficient task. Just raw power, expended rapidly, and more and more is needed with every spec of acceleration. Yeah, you could give it a nudge at 1000 kph, but then it will take millennia to hit the target. Getting it there quickly would require an acceleration that is not efficient by any means of the word. – 458 Aug 17 '15 at 8:31
• @fredsbend Decades, not millennia. That's why I went with the Kupier belt rather than the Oort Cloud. – Loren Pechtel Aug 17 '15 at 20:07
• My math yields about 500 years. 29 AU from Earth is about 4.35 billion km. At 1000 kph, will take 4.35 million hours, which is 181 thousand days, which is about 500 years. Not millennia, but also a bit more than decades too. If you can manage to hurl it 10x faster, then it would be 50 years, but would take way more than 10x the energy. Either way, starting from the Kupier belt is not a great idea. The Asteroid Belt is much closer, but you still have the energy expenditure to get it moving. – 458 Aug 17 '15 at 23:40
• @fredsbend You're neglecting gravity and I was figuring a small boost to speed the fall. – Loren Pechtel Aug 19 '15 at 1:31

They could use their "magic-plot-teleporty device" to teleport a large chunk of the nearby star's core into the planet. That'll do it.