I'm writing a story in which a cloud of grey goo is entering the solar system and humanity must prevent any particle of it from reaching Earth (and potentially other worlds too). Alternately, I need a way to deal with grey goo after it has landed and begun spreading, but keep in mind that it would be intelligent and actively working against humanity. The diameter of each nanite is on the order of 100 nanometers during travel, making them hard to detect.

The cloud more or less spans the solar system but is primarily centered on the planets. Going in humanity's favor is that this cloud is extremely thin—only a hundred nanites or so will be on a collision course with Earth.

Also in humanity's favor is that they can be a lot more technologically advanced than 2016 Earth. (But not in the distant future! Within 50 years is best.) Molecular manufacturing, fusion power, and jovian space stations might be a reality. Is it possible to manufacture an event horizon shield? Maybe that's a viable option, though I'm imagining railguns and lasers.

Going against humanity, however, is that this cloud is moving at relativistic speeds. They have little warning—maybe two months since detecting its effects in deep space. Yes, imagining a nanite like that 'landing' on Earth without exploding might be improbable, but I have some ideas for that.

In summary, humanity must, in two months or less, make preparations to destroy ~100+ nanites of 100nm each heading towards Earth at speeds of .9c or more. Potentially without knowing the nature of what's coming. I.e. That they are nanites. Secondary is preventing them from landing anywhere else; Mars, Jupiter, the asteroid belt, Sol.

  • $\begingroup$ Do we have any idea about what the grey goo is made of? Like, is it electronic/robotic (vulnerable to emp), biological (vulnerable to hard radiation), something else? It might be easier to come up with a defense if we can guess at weaknesses. Maybe create a black hole and throw it toward the cloud, so that it will pull in the nanites and make them less dispersed. Otherwise I don't know how to protect every rock and pebble in the system from them... $\endgroup$
    – AndyD273
    Commented Nov 18, 2016 at 18:02
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    $\begingroup$ I fear that if anything was coming towards the Earth at 0.9C there wouldn't be enough time to do anything about it. We'd have to see it, figure out exactly what it is, realize it's a threat, determine how to stop it, and implement it all in a fairly small window. And if they are that small it would be even less time if not impossible to notice them before it's already reached Earth. $\endgroup$
    – Virusbomb
    Commented Nov 18, 2016 at 18:09
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    $\begingroup$ If a nanite can survive planetfall at .9C, and could "survive" on the sun, there's nothing humanity could possibly do to hurt it. Even if we put aside the difficulty of tracking and hitting these things, they're just too tough for us to hurt. $\endgroup$
    – Deolater
    Commented Nov 18, 2016 at 18:22
  • $\begingroup$ @AndyD273 Well, they can survive anything in interstellar space, but they aren't built to handle supernovae. (Actually, that's not strictly true. There are, in all, tons of these things and the reason for that is precisely because it is cheaper to mass produce them than to make them survive everything.) And they don't really need to protect every rock and pebble. If grey goo takes over, say, Venus, then conceivably we could drop a tiny black hole on top of them. Too bad for Venus and future (or present) colonies though. $\endgroup$ Commented Nov 18, 2016 at 18:45
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    $\begingroup$ @Virusbomb They've detected the swarm in advance because of its effects in deep space, so they know something is coming. They might place sensors in the outer solar system to acquire a map of the swarm as they pass by. Perhaps by detecting the warping of space-time. $\endgroup$ Commented Nov 18, 2016 at 18:50

6 Answers 6


In order to set up defenses, you need to know what you're up against. The first observations will be those of a dust cloud moving at 0.9c, and as interest increases ("how did it get moving so fast?"), it would turn out to be more metallic than expected.

At this point a large portion of space-based telescopes will be aimed at the cloud, and they might start detecting communications between the nanites (assuming they communicate using radio (or at all)). People might start to realize that it's a nanite cloud, and once it's been confirmed, humanity would be scrambling to analyse the communications.

If the data is encrypted, hacking the network would be hard, if not impossible. Although the encryption would be at the cost of latency. Otherwise, if it is possible to hack the nanite network, there are 4 options.

  • Increase transmission power to drain energy.
  • Increase transmission power for better targeting.
  • Disable the nanites. (maybe there was a kill switch installed by the creators in case the swarm attacked their own system?s)
  • Redirect them. (might not be fast enough given that they are moving at 0.9c)

Not much energy would be needed to disable a nanite, and a heavy-atom particle beam would do the trick (diffraction is very low, so high accuracy/long range). If humanity had managed to increase the transmission power of the nanites, the particle beam would not need to be as powerful as it would have to be if the nanites were transmitting at normal power (or not at all), as it could be more focused.

  • $\begingroup$ Somewhat surprisingly, in retrospect, I think this is the answer that most closely resembles what I am going with in the end. So, acceptance goes to you. It is probably the most reasonable possibility, anyway. $\endgroup$ Commented Dec 2, 2016 at 21:26

Um, guys, there isn't a problem here with nano-machine infection. There may be a radiation problem, but probably not. These things are coming at 0.9c, or faster, yes? So their kinetic energy is equal to about 80% of their mass energy. The situation we have here is a lot of very small versions of xkcd What-If #1.

When they hit anything, such as an atmosphere, or even a very thin gas cloud, they're going to be torn apart into very small blobs of plasma. The energy released in a collision is orders of magnitude above their nuclear binding energy, never mind the chemical binding energy that holds their various atoms together. They were probably spotted as a collection of small and brief gamma-ray bursts from the same direction. I have no idea how we're supposed to know that they're nano-machines at this range, and it doesn't matter anyway.

They can't slow down enough to make a difference. They'd have to convert 80% of their mass directly into kinetic energy to do it, and that's an impossible level of efficiency, and leaves them without enough mass to be a convincing nano-machine seed anyway.

That was always implausible, because at 100nm across, they only contain about a million atoms, which makes storing as much as a million bits of data (128Kbytes) in them highly implausible on quantum-mechanics grounds. 128KB doesn't seem remotely enough for a seed for grey goo.

Now, if there were a lot of these, we'd be at risk of their killing us by the radiation released from their collisions. But if there are only about a hundred that will collide with Earth, there's no problem. It will be about like a hundred instances of the Oh-my-God Particle. Spectacular for astronomy, but harmless.

  • $\begingroup$ Yes, that premise may well be impossible in practice, but I'm willing to rely on suspension of disbelief on that point. Nonetheless, there may be heat shields that persist long enough to allow them to penetrate an atmosphere, or they are not made out of atoms at all. Femto-tech maybe? Built out of quarks? The point about information density sounds like an issue though. Is that a fundamental limit? $\endgroup$ Commented Nov 19, 2016 at 0:50
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    $\begingroup$ Storing a bit per atom is already very dubious. Going below that just isn't on: nothing stays the same for long enough, except statistically, and that isn't good enough for reliable information storage. If you're going to postulate stable quark matter or the like to enable the nanos to survive collisions, then we're dealing with a technology so far beyond what you postulate for defence that there's no point trying. There isn't anything we can do that is a hundredth as damaging as a relativistic collision. $\endgroup$ Commented Nov 19, 2016 at 1:04
  • $\begingroup$ Hm. Ok, here's an idea, data transmissions are received from their progenitor that instruct them on what to do. Prior to entering our solar system a transmission arrives commanding groups of ten to approach one another and merge, resulting in a tenth the number, but over ten times the storage space. This might also improve their durability for atmospheric entry. The next transmission just tells them how to be grey goo at the target, and also to build a radio receiver (or gamma or neutrino or whatever they'd use) within, say, a week so it can receive the full transmission. $\endgroup$ Commented Nov 19, 2016 at 1:35
  • $\begingroup$ As for the technological discrepancy if given sub-atomic architecture; the nano-machines are arriving with virtually zero resources. If they do much other than build colonies, they fail. Though if they do, humanity's doomed unless they manufacture a black hole. (Is that feasible?) Though, if using the staged transmission scheme I outlined above, we have a viable defensive technique. That aside, they're designed specifically for handling relativistic collision, not concentrated firepower. These are construction tools, not war machines. There are surely weaknesses they didn't think mattered. $\endgroup$ Commented Nov 19, 2016 at 1:48
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    $\begingroup$ Having the nanos join up is not feasible: they don't have the space for sensors to find each other, nor for engines to let them move towards each other. No, there is no feasible way of manufacturing black holes, and if there were, they'd likely end up eating the Earth. Or the Sun. And the idea of materials that survive relativistic collision, but not other attacks, is ridiculous. The centre of a nuclear explosion is cold and calm compared to relativistic collision. It would be a great deal easier to send the nanos in bulk on a ship, which can slow down, and disperse them where they're useful. $\endgroup$ Commented Nov 19, 2016 at 2:05

The problem is that individual nanites are far too small in far too large of an area to pick out indivdually. Anything like a laser defense system is simply unfeasible.

We need to think wider, with forces that can affect a wide area without needing to know the nanites' exact location. The two that come to mind are radiation and electromagnetism.

For instance, perhaps you can setup a series of gamma bursts radiating in the direction you think the cloud is coming from. While the nanites likely have some degree of protection to radiation, hopefully they can't handle the radiation from nearby nukes going off. Coincidentally, "a bunch of nukes" would be my solution for dealing with nanites landing, too.

The other possibility is electromagnetism. If you can setup a strong enough field, you might be able to incapacitate the nanites, or simply redirect them away from the earth (and being nano-sized, it wouldn't take much force to veer them completely off course).

  • $\begingroup$ I'm thinking that each position could be determined through either signals or throwing together some type of early warning system in Neptune's orbit. (Repurposing a telescope array? Gravitational wave detector?) But an area-effect defense would be great. The nanites transmute their structure, so I can see gamma ray bursts being particularly effective if they hit during this period. (Sorry, forgot to mention that originally. I designed these awhile ago and it totally slipped my mind that they even do this.) Redirecting them would be nice, but they almost certainly maintain a net neutral charge. $\endgroup$ Commented Nov 19, 2016 at 0:42
  • $\begingroup$ I'm not sure a nano-sized object can even produce radio waves big enough to be detected, at least at the kind of ranges we're talking about. And again, they're so small that they'd be impossible to distinguish from random dust and other particles you find in space. You've mentioned tiny black holes before. If those are possible, then you could possibly redirect them using gravity instead of EM. $\endgroup$ Commented Nov 19, 2016 at 1:39
  • $\begingroup$ That is absolutely right, thank you for reminding me about basic radio physics, haha. And I just realized that their effect on spacetime is also negligible—relativistic or not. As for a tiny black hole, that would have no notable amount of gravity. In order to redirect them gravitationally we would effectively have to move something with the mass of Jupiter. Which is totally infeasible given the timeframe available. My thoughts on black holes are either shielding or razing tactics. $\endgroup$ Commented Nov 19, 2016 at 2:02

It's all about detection.

If we can see them we can try to hit them with big bombs, burn them with powerful lasers or try to push them around with magnetic fields. Even one landing on Earth wouldn't be a problem if we could detect it before its colony dispersed very far, we have a lot of nuclear weapons laying about and we can always build more cities.

If they are forming a cloud of sorts they probably talk to each other. If they do over space relevant distances we can probably detect it once they get near, and with how many antennas we have triangulation should be possible.

Unfortunately you say they are smart, so once some of them die some others will try to be stealthy, and we haven't got a chance of detecting a nanite pretending to be dust.

As for the other planets the situation is even worse. We can't even get to mars with only a few months warning so even if we have stuff over there it will be on its own, and I doubt space weapons are a priority for any colonies.

And if they take a planet they probably would relaunch in uncountable numbers from there in aimed attacks at the rest. We probably don't have the resources to fight a Kardashev Type I civilization and certainly don't have the firepower to destroy a planet they take.

  • $\begingroup$ Oh, actually, it is mainly only their colonies that are smart. They are specifically designed to have as little mass as possible (makes them cheaper to bring up to speed) so they have very little computational hardware on-board. They would certainly relaunch, yes, but it could take a little time. These are designed on the assumptions that there are no defenders so their colonies primary goals are to survive, absorb energy, and grow. If one winds up inside a gas giant, it might not bother to look 'outside' at all for quite a while. $\endgroup$ Commented Nov 19, 2016 at 0:48

Question contains 3 incompatible premises

  1. it would be intelligent and actively working against humanity
  2. cloud is extremely thin
  3. this cloud is moving at relativistic speeds.

they are incompatible in following combination: 1+2+3, 2+3
they are valid in following combinations: 1,2,3, 1+3, 1+2

  • with the premise that they do work on principles which are already known to us, if not then resistance is futile, and the only author may save the planet and the system, by any plot he likes.

Defense 2+3

The first thing to do is to think about where it comes from because it can't be from a far away, and to think about why this pointless thing just happens. Except that, do nothing - they will burn on collision as John Dallman pointed

They are launched not from far away, or they are part of a way much much much more a bigger problem, galaxy sizes, something just enormous even for me, who argues about the viability of disassembling of planets in this century, and for which dismantling Jupiter in 100 years is not fast enough. It looks more like an invasion from another galaxy by enormous force, or someone is testing this civilization at close proximity, some thoughts here starting at "Where is a big thing, though."

Smaller the system is, and more complex tasks it has to accomplish - more fragile is the system. Probability of collision of 100nm particle with hydrogen atom of interstellar medium(at lowest 0.2 particles per cm3) is about 2e-09 chances per 1 m traveled. 10% chances not to collide and 90% chances to collide with one hydrogen particle is the 1'151'292'514m distance traveled or it is slightly more than 1/150 a.u.

Ok, let say one collision isn't fatal for it but traveling 1 a.u. mean high chances to get about 150 of such collisions, but let say it has a probability of 0.5 to switch off that unit after 150 collisions, damage it completely. And I'm very generous here because one act of fission is enough to evaporate that thing into a small plasma cloud. Which will form some kind of protective layer for the rest of them, a bit later about this situation.

So in the worst case scenario half-life of the front layer of that cloud will be 1 a.u. or about 500 seconds(as it moves with speed of light).

Calculate half-life of the cloud is a more challenging task, as plasma from front particles will shield the rest of the cloud, but still, this cloud will wear out because of some particles will sneak inside the cloud, and because that plasma shield will ablate too.

But at some extent we may have examples of such cloud, there are clouds after supernovae blasts, as an example https://en.wikipedia.org/wiki/Crab_Nebula and such

It has a diameter of 3.4 parsecs (11 ly), corresponding to an apparent diameter of some 7 arcminutes, and is expanding at a rate of about 1,500 kilometers per second (930 mi/s), or 0.5% of the speed of light.


Such extreme catastrophes may also expel much, if not all, of its stellar material away from the star,[5] at velocities up to 30,000 km/s or 10% of the speed of light.

Gravity pull is the smallest percentage for breaking force, most of the pulling force comes from the interstellar medium.

By other words thin cloud at relativistic speed without forming a more complex structure to protect itself is incompatible with intelligence in this galaxy, or you will notice it by how all stars in the galaxy are dimming, as they(GGC masters) prepare to launch that cloud. It is like shooting pump gun underwater.

Defense 1+3

This will be a problem.

And this problem will be not the: ~100+ nanites of 100nm each heading towards Earth at speeds of .9c

But more likely some more complex structure, which is divided in chunks proportional for masses of all significant bodies, including the star (but really it depends on the size of that GGC system, it can't be too small though, or it will have problems to slowdown in our system)

Traveling as a bigger and more compact(dense) system allows to reduce of the deterioration rate of the system and allows it to repair itself etc.

1% of the sun mass at 0.9c speed is enough to evaporate our system completely, and take all heavy elements (everything except H), use H as a propellant and travel further. 2 waves expected - one evaporates the system, another collects separates extract elements from that cloud. The effect will be similar to supernova. The time between waves, 1-100 years.

The first wave may be anything, not necessary GG, just hydrogen cloud. And this thing will be seen from far far away, in the first place because it consumes the energy of previously dismantled systems.

defense - take all bio-samples from the earth, evacuate libraries and people, and head in the perpendicular direction from that thing, and pray for your gods.

Another way is to try to take control over that thing, but the most likely the star system will be lost, because it may be impossible to stop the attack even after gaining the control over the system.

Defense, more like OP it wishes, earth.

The main part here is that you have to have similar systems. It is pretty hard to do something with a stone against a tank, not impossible, but it is not likely that something good will happen there if there are no proper/matching tools that can be used.

Individual units of GGC are not a problem. They may be a bit tougher than usual microbiological live but hard gamma will efficiently disinfect them, the same way as it used for medical instruments disinfection.

They still need an energy source, and energy source will limit them how fast they can grow etc. Attack them, cut their energy sources.

This way if individual particle lands on earth somewhere you have to detect it, and do it fast enough(days, month, years). And it highly depends on which kind of disguise it may use. But small unit probably can't use any kind of disguise, because it is not native infection which spreads trough living things (they are most abundant sources of energy which is easy to extract from, another alternative is sunlight)

Sunlight will limit their grow speed by 100W per square meter on average. Consuming biological matter may be a faster way for them to grow. But as a small unit is most likely dumb as usual microbiological life, and it can be noticed by observation from satellites. Both cases can be observed from satellites. Also, entry trajectories will be seen because of gamma rays generated during the descent and it will help to determine the place of its landing.

Tool from this answer, starting from "Note about Venus scrap, snake elephant" can help to detect those particles fast(in different ways, let say just cover overall planet), most important it may help to fight those GG systems and it is less advanced (as I think) in creation as a molecular assembly.

The strength of Gray Goo is not in the capabilities of a single nanite, but in a system which it may create from themselves(or other matter as well) en masse. The difference is like to compare a human consisting of a big set of cells and some microorganism consisting of one cell.

And if it is intelligent, it will be intelligent not as single nanite, but as significant size system of those nanites. But rules of physics still apply to that system, essentially it needs energy and its capabilities will be limited by available energy. Most energy comes from sunlight, biological materials on the surface, gas/oil in the earth's crust.

GG is not some kind of an unstoppable force, a doom whatever. The problem begins when someone is too late to apply countermeasures when it is grown bigger than someone's capabilities to counteract it.

Coordination, observation, energy sources control, fast enough reaction - are keys to eliminating the problem at its start, at its source.

  • Here is discussion and it drifted away from materials for space habitats to about discussing nanites threat, maybe will be interesting for someone.

Defense, more like OP it wishes, space/orbit

Single nanites can't maneuver, again they are useful as some arrangement of them, as a system of them. And as a big system, they may easily be an invading force even at 0.9999c, but as a cloud of single independent units, it is helpless. Just place a foil 100nm thick on its way and it will just annihilate. A gas cloud on its way will lead to the same effect. Basically take an asteroid and evaporate it with a thermonuclear blast or in a less spectacular way - grind it, spread the dust. Effectively, on average, it has to be like 100nm thin layer of foil(as the average density of that dust cloud on the way of nanites) to be as effective as that foil barrier.

to protect the earth in the way, it needs 1'920'000'000 tonnes of water as an example, or 2-3 times more (by mass) tonnes of rocks. Thus an asteroid of 10 cubic kilometers of rocks will be enough if it is dispersed correctly.

This method will work with single units up to pretty low speed, less than a few km per second (speed difference between a nanite unit and the shield, I'm just eyeballing that speed, maybe 10km/s.)

Defense against the carefully planned attack

If they acting like a dumb swarm, it way less a threat compared to a situation of a planned attack against humans driven by an evil intelligence.

If humans control the system(including Oort cloud), and all energy in the system, all matter sources, and attacking forces are not big enough and have less energy then available for humans - then humans can counteract. They may fail or may succeed.

But if humans do not control the system and if initial invade force is small, the attack can be already happening right now, thousands of years, somewhere in Oort cloud, or on Gas Giants, as an example, Great Red Spot may be an observable mark of such attack, we do not know why it exists, and why it is red. If the attacking system will gain more strength than we have, then there will be no chances to stop the intelligent invasion, or a self-learning system, without to create a more efficient and faster-growing and intelligent system to counteract the threat.

The only chance in this situation is to create a better system that is faster, more efficient, more intelligent, operate it in a better way than other intelligence.

Everything boils down to Defense 2+3 case, it has to be understood what is the source of those attacking particles, the reasons for the situation. Are they just symptoms of an illness in the solar system or what, and how big is the problem really.

  • $\begingroup$ Your accent is familiar. By the way, do note that the numbers that I've given are subject to change. I consider 100 nanites headed toward Earth to be a minimum; if it seems feasible then I'll be increasing that number by a great deal. The half-life you cite seems too small to be effective, so I'll have to rely more on them rebuilding their numbers after passing through each star system, with millions+ nanites minimum. I'll be considering everything that everyone's said so far and will make adjustments to the story as necessary. I will come back to pick an answer soon. $\endgroup$ Commented Nov 21, 2016 at 22:20
  • $\begingroup$ @Silhalnor Numbers are not so important for the answer, maybe except the size of a unit. When/If they form a system - they may successfully invade any planet or asteroid in solar system and brake from 0.9c speed. Or they can do other interesting things,and it depends on their mass as a system. But as a thin cloud of independent units it is almost useless. But as beam of independent particles it it will live longer, deacceleration still will be a problem trough, but maybe doable. Regenerating, yes it is a way to solve the problem of damage, no way trough for one particle to repair itself. $\endgroup$
    – MolbOrg
    Commented Nov 22, 2016 at 7:50
  • $\begingroup$ I like how this answer covers a lot of possibilities and scenarios—both the more realistic ones and the ones that would better suit the story. But you really need to work on your grammar if you are going to be writing paragraph after paragraph of stuff. $\endgroup$ Commented Dec 2, 2016 at 21:27
  • $\begingroup$ @Silhalnor tnx. Grammar, yes I know about that, kingledion was kind enough to edit few of my long posts, and I see the difference. I my self made an edit to one of my long answers, I hope it improves it in the grammar sense. It looks like I improve myself, slowly but nevertheless. I appreciate edits made to my answers, when they improve the grammar. $\endgroup$
    – MolbOrg
    Commented Dec 3, 2016 at 9:36
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    $\begingroup$ If you're interested, I recently found an app that checks your grammar as you type it and suggests edits. You leave out a lot of articles ('the', 'a') and it points those out. It seems to be very accurate and when it comes to correcting basic grammar, it's free. Sorry to be advertising an app on here, but I don't see a PM system and if you're like me, you might appreciate it. app.grammarly.com $\endgroup$ Commented Dec 3, 2016 at 16:02

I cut this up in different parts:

  1. The Travel

0.9c is not possible without an external power source, nor possible to slow down from without an external power source. Thus, I presume the most logical source of energy: radiation. The nanites could be flat, riding radion to speed up and slow down. Like solar sailing, though, they could ride any energy field.

*Using this means much slower deceleration, spread out over months, easily doable.

  1. Surviving the Trip

At 0.9c they die before arriving. Particles cannot be wiped out of the way fast enough. Even if they shield behind a wave of a supernova. Decreasing speed makes much more sense in this regard.

The point of artificial life is that it is in no hurry; it makes no sense to waste energy traveling that fast if you have eternity.

As such I presume much slower speeds: 0.1c tops. Add a simple rotate ability (think a tube of 100 atoms long, 1 thick, 1 high) and most collisions can be evaded).

  1. Intelligence

I circumvent the storage issue by using links more like how a brain works. The order of atoms makes the data; the number of unique sequences within a 100 number is insanely high.

That could be the passive coding, activating as communication as the nanites grew in number. Again nanites have time though, their brain processes need not go as fast as ours. Mortal lumps of meat need thousands of impulses per second; nanites can work fine with one one every thousand years.

  1. arrival

So now you have dust particles nearly undetectable since at best you pick up one communication impulse, and they appear dormant and have slowed down before you noticed them. Nothing you can do.

At the much lower speeds the descent is much more doable. They might use the same solar sail tactic to parachute down, or just drop to the ground as given their small size the gravitational pull on them is marginal too. Likely the bodies with atmosphere and of smaller size are prioritized to land on.

  1. Growth

They have a sun nearby and energy they will start replicating but again they are in no hurry, they can use billions of years if needed to consume the solar system. As such the goo is relatively harmless. We won't detect it for millennia, but once we do it is neither an immediate threat nor a problem to contain.

But presume they can speed up their normal slow reproduction if enough energy is available. It would make little sense to do as a standard tactic. The energy of the sun over billions of years outweighs anything else, so such a short rapid burst only makes sense to counter an immediate situation.


  • If peaceful it might defect life, and rapidly multiply to get to the numbers to communicate with us. (No threat)

  • If defensive it might counter danger by multiplying faster and/or relaunch part of their population prematurely. (Again no threat)

  • If offensive it might eliminate any threat to itself and if we are seen as such we will be killed since we cannot remove them all from all planets.

As such, best defense would be: act peaceful, let it be.

  • $\begingroup$ Welcome to the site, Dutchstudent. I've made a few edits to your post to improve readability. If you disagree with my changes, feel free to roll them back. Also, feel free to take the tour and check out our site culture when you get the chance. $\endgroup$
    – Frostfyre
    Commented Nov 15, 2018 at 16:52

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