MAD with Aliens? Interstellar deterrence and its implications


Previously, I made the case why space aliens, or ETIs (short for ExtraTerrestrial Intelligences) would probably not exterminate humanity out of fear of future competition from us, because a) interstellar warfare either is an uncertain business at best, or, b) if we postulate a civilization so advanced that it could destroy us with impunity, it would not seem to benefit from doing so.

Now the argument has been sharpened considerably, thanks to a publishing process which culminated in my article being accepted to Acta Astronautica (Korhonen 2013). I encourage you to get your hands to the paper, either directly from Acta or, if you’re so inclined, via the manuscript version at the arXiv repository.

For those ADHD’s who don’t wish to read the whole paper (I can’t imagine why you wouldn’t, but I’m an accommodating person), here’s the gist of the argument. I make six assumptions about the nature of interstellar warfare, namely,

  1. All civilizations have a concept of risks and benefits, i.e. they do not simply act randomly.
  2. A civilization that does not need to fear retaliation has little need to destroy other civilizations. We humans could destroy all the anthills we find without having much to fear, but are we doing so just because they may be our competitors one day?
  3. There are practical limits to technological development. At some point, civilizations cannot greatly reduce their vulnerability to attack through technological improvements; even if our laughably primitive weapons won’t get through, some weapons will.
  4. No defense can be guaranteed to be 100% successful 100% of the time. See above.
  5. No attack can be guaranteed to be 100% successful. Even the most sophisticated attack will have a non-zero probability of failing to achieve its complete objectives. It also seems that the marginal cost incurred by the attacker as it tries to increase destructiveness of the attack increases rapidly: sure, killing off 50% of humans may be trivial, but killing all humans may not be!
  6. Verification of peaceful intentions over interstellar distances may be difficult. Regardless of assurances, there is little that can be done – short of a physical visit – to verify the truth of any statement a civilization’s representatives may make.

According to these assumptions, the only surefire way of preventing another civilization from attacking your civilization, ever, seems to be the complete destruction of the said civilization, if not the said species. Unfortunately (from the attacker’s viewpoint, that is), creating absolutely effective first strike weapons and gathering timely intelligence required for their use will always be massively more complicated than creating effective deterrent weapons. After all, the deterrent needs only to threaten unacceptable damage to the attacker; if the marginal cost of the attack rises along with the desired amount of damage, an attack causing severe damage may be orders of magnitude simpler and cheaper than an attack that eliminates the species.

But what is an unacceptable level of damage? This is the tricky part, and I acknowledge I don’t have perfect answers. No one does. However, our history strongly suggests that as civilizations become more technologically advanced, their tolerance for death and destruction diminishes greatly. This trend has continued for centuries, and there seem to be no good reasons to believe it would be reversed as technology advances. Several reputable studies – some written by former high-level nuclear weapons specialists and military officers – make a compelling case that today, deterrence is achieved by being able to destroy about ten random cities. Even when rounding up the numbers, this suggests that the expected value of credible deterrent is – very rough estimate here – in the order of 0.1 x total loss. Even smaller numbers have been put forward, and it is difficult to believe that human decision-makers would willingly lose even a single city for any conceivable political gain.

Now, then, how do we achieve this level of deterrent? Consider that any interstellar attack is likely to cause severe damage to the target civilization. As an example, a rather primitive 1970s reference design for interstellar probe – the Daedalus – could, with just a programming change, deliver some 146 gigatons of kinetic energy to any largish target (=planets, moonbases, battle stations the size of a small moon) within 25 lightyears. When one compares this to the approximate explosive energy of the entire world’s nuclear arsenals (ca. 6.5 gigatons), one may conclude that a hit from even a single interstellar probe would be Very Bad News Indeed.

Even an advanced civilization might have problems defending against such an attack. The sluggish Daedalus, coming in at measly 12% of the speed of light (0.12 c), would be theoretically detectable with Hubble Space Telescope-level sensors at about 10 to 20 Astronomical Units (AU). If the sensor is pointing to the right direction, and if the lack of apparent motion – which is what most detection systems rely on – doesn’t bother. Detection at such ranges would give only some 24 hours of warning before impact. Is this long enough for an interception?  And consider that considerably faster, smaller (more difficult to detect) probes may be possible.

What, then, is the likelihood that we (or some other civilization) can launch such a retaliation? I simplified the argument to four probabilities, namely

  1. P(identified), which is the probability that an attacking civilization’s military intelligence can identify the victim’s essential centers of gravity correctly,
  2. P(hit), which is the probability that the identified targets are hit with sufficient force,
  3. P(destroyed), that once hit, the ability of the victim to retaliate is destroyed permanently, and
  4. P(no witnesses), that there are no other civilizations which might detect such unprovoked attacks and see them as causes of alarm, precipitating their pre-emptive strike against the attacker.

Even if the attacker is 95% certain of each and every single variable, the joint probability of a successful attack is only 0.815; in other words, the complement probability of a retaliation is uncomfortably high 0.185. I leave it to the individual readers to judge, given the arguments in the paper (e.g. that it’s impossible to be sure whether “primitive” radio transmissions are really the sign of a “primitive” technological civilization, an equivalent to renaissance fair, or a deliberate lure intended to attract hostile civilizations) whether certainties of 95% are achievable across interstellar distances. Considering that even one hit may engender 90%+ losses, it would therefore seem that attacking someone simply because they may be threats in the future would be a losing strategy.

The good news, therefore, are that attempts to contact extraterrestrials are likely to be low-risk affairs. The bad news is that waiting for an answer may be a long wait, as the ETIs might be fearful to take any actions that might be misinterpreted as hostile acts. For example, sending a high-velocity interstellar probe and launching a high-velocity interstellar kill vehicle look the same to the recipient…

This should also be remembered when discussing our own interstellar missions. If one has the capability to send interstellar probes to someone else’s home system, they also have the capacity to hit them with interstellar kill vehicles. We must make sure that our exploratory efforts, if and when they happen, are not mistaken for attacks!

Thanks to Markku Karhunen and to two anonymous referees of Acta – their comments and suggestions improved the story considerably!


Korhonen, J. M. (2013). MAD with aliens? Interstellar deterrence and its implications. Acta Astronautica, 86, 201–210. (manuscript available at arXiv:

The article manuscript is published in arXiv repository according to Elsevier’s publishing policy regarding permitted scholarly posting.

About J. M. Korhonen

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6 Responses to MAD with Aliens? Interstellar deterrence and its implications

  1. Tuomas Huovinen says:

    This is good stuff. I am happy that someone posted your Fukushima tritium comparison, it was also a good read.

    However I have two questions and I hope you can deliver some answers.

    1. Are kinetic interstellar kill vehicles even possible? I am not suggesting that the acceleration to 0.12c, or any low fraction of c is impossible. What I think is that space itself is too “dirty” for a c-speed thing to survive the trip between source and destination.

    Here is a very basic calculation (I have no education and I suck at maths, yet I find it very interesting): light year length is 9.46*10^15 meters: if we assume (and this estimate is out of magicians hat) that there is a grain of sand in every cubic kilometer of space then a vehicle with 1m x 1m faceplate will travel completely through volume of 9.46 billion cubic kilometers.

    I think that the kinetic vehicle and the grain of sand will meet at some place.

    Collision energy is going to be enough to either destroy or seriously damage the vehicle. Ok – this is based on pre-school math and a assumption that space would be that dirty. Still if you make the space ten or hundred times, or perhaps ten thousand or million times cleaner the probability based on this calculation says that the collision with something is pretty much unavoidable.

    I tried to find info how to calculate collision probability in 3D space but couldn’t find anything that would be understandable. So, if you know maths better than I do, and I believe so, please write about the collision probability of interstellar “kill vehicles”.

    2. I had a really good second question but I fucking forgot it. Sorry.


    Tuomas Huovinen

    • Hi Tuomas, and thanks for comments and a good question :).

      I think your estimate of the density of interstellar medium is too high. Let’s assume a very small speck of sand of 0.1 mm diameter; it might weight something like 0.000002 grams. From Avogadro’s number (6.023 x 10^23 atomic units per gram) we can nicely compute that that speck of sand would have some 1 204 600 000 000 000 000 atoms in it – give or take a few. Thus, a cubic kilometer (10^15 cubic centimeters) containing one speck of sand would have an average density of some 1204.6 atoms per cubic centimeter.

      Space, however, is far emptier; the average density of interstellar medium in the Milky Way is around 1 atom per cubic centimeter, although this varies region by region. Molecular clouds can have a lot more, up to 1 000 000 atoms per cubic centimeter; obviously that would be bad for interstellar spaceship.

      However, the consensus seems to be that while interstellar medium poses some problems for high-velocity starships, they are not insurmountable – especially compared to problems of generating enough energy to reach high fractions of light speed. Several reference designs have been made over the years, the first complete assessment being the Project Daedalus ( from 1978. The Wikipedia page on interstellar probes has a list of other proposals (

      Especially if we consider automated probes, shielding will almost definitely be feasible solution. For starships crewed with fleshy meat-sacks, the problem is that collisions will produce highly energetic radiation that is difficult to protect against (besides the additional problem of cosmic radiation); that is one reason why I don’t believe humans in our current form will ever venture very far from Earth.

      In order to estimate how far away from target the RKV could be detected, I actually calculated how much the probe would heat – and hence radiate IR radiation – from impacts with interstellar and interplanetary medium (the latter is much more dense). For the 0.12 c probe, the Daedalus reference design, having a frontal area of 3220 m^2, when accounting for relativistic effects and assuming that all impact energy is absorbed by the probe (which is not true), the maximum energy flux to the probe would be around 230 kW in interstellar medium and some 640 kW in interplanetary medium (the latter would be less than 200 watts per square meter, or about 1/5th of solar irradiation on Earth’s surface on a sunny day). To shield against impacts from particles larger than individual atoms and molecules, Daedalus design proposes a beryllium ablative shield whose exact dimensions I can’t recall any longer.

      Even larger particles, like those specks of dirt, would have to be detected and either destroyed by e.g. on-board lasers, diverted by charged fields, or avoided. All these are feasible at least to an extent; during the final approach of the target system, the Daedalus spacecraft would be projecting a “smoke screen” capable of vaporizing rocks (or interceptor missiles…) weighing up to 500 kg! Of course, there remains a risk of something slipping through, but the risk has usually been believed to be minor.

      If you recall your second question, please go ahead and shoot :).

      • Tuomas Huovinen says:

        Wow, thank you for the answer – yeah it seems that RKVs may be flying out there somewhere. Now I actually remember the second question:

        How do we know that we have not been RKV’d already?

        It is a bit of a joke question but the reasoning is this:

        It was only this year when the Internet told via TED talks that all this radio telescope SETI stuff is not the only way to detect ET life in space. The other way and also probably the best way to do that is to measure planets spectrum and determine oxygen content of its atmosphere. If less than 1% then most likely no life, if more than 1% or ungodly 20% then there has to be life there. High oxygen content is unnatural thing in atmospheres.

        Now, when you compare earth radio-visibility (max 100ly or so) and oxygen-visibility (more than one billion ly) it is obvious that every civilization in the Milky Way have had at least one billion years to detect life here. That means that some civilization might have hit us already (bye bye dinosaurs) – is there a chance that RKV leaves a special isotope fingerprint which would be detectable even today?

        Also, what comes to being an asshole to other civilizations the game plan is this:

        1. Radio silence
        2. Hide your ecosystem and revert atmosphere to pre-life conditions
        3. Send autonomous RKV systems going around and around the Milky Way looking for ET oxygen atmospheres, don’t forget the radio detection if something more exotic springs to life 🙂
        4. Profit… wait! I meant to say wrath of god arriving from time to time out of nowhere.

        If some civilization would be a real asshole like that they probably would have made this a science, RKV probes everywhere and AI algorithms calculating when to hit again (and with what).

        This also leads to the possibility that we will not detect ETs because they take the autonomous RKV threat seriously and play dead. Kinda reverse MAD I’d say, We All Dead, WAD.

      • Yeah, measuring atmospheric spectra for reactive elements like oxygen seems to me the best bet for finding life in distant star systems.

        I’ve thought for many years that listening for radio waves is doomed to fail – as a civilization, we are rapidly “going dark:” most of our most powerful omnidirectional transmitters (analog TV and radio) are shutting down because information is increasingly transferred through cables and/or with far less power. Our “window of visibility” in radio spectrum seems to be less than 100 years, give or take, and there seems to be no a priori reason to believe other civilizations would be much slower. Compared to radio emissions, indirect signs of civilization in planet’s atmosphere (like increase in carbon dioxide) are much more difficult to hide.

        Another option that’s technologically available even to us right now is direct imaging of exo-planets, using very large scale space-based telescope arrays of multiple independent telescopes. In space, the baseline (which largely determines the resolution) can be arbitrarily big, and one could easily imagine a cloud of space telescopes that can cooperate to take high-res pictures of planets several light years away. These might be good enough to show signs of civilization.

        But in any case, it’s completely possible that ETIs have detected us and it’s even possible that RKVs are on their way already. But for the reasons I try to lay out in the full paper, I don’t think that’s likely. There is no reason to believe we’re living in any special point of time, and given that other civilizations might have had billion years to exterminate life on Earth, why would they do it just now? If a civilization has a power to sterilize planets, why take the risk of having some pesky intelligent monkeys escaping just before the extermination fleet arrives? We might, after all, have that capability in 100 or 200 years, which is shorter than a blink of an eye in galactic timescales.

        It’s also possible that we’ve been RKV’d already, but whether that’s likely is another question. As far as I understand, the dinosaur killer was probably a 10-km asteroid or perhaps a cometary nucleus, given the single crater and iridium deposits found all over the world. In other words, evidence is consistent with the simpler explanation – a natural disaster. It’s also worth noting that the dinosaur die-off was not instant but happened over a period of million years or more, which would indicate a rather poor attack design. (Of course, it’s just plausible that there had been a civilization of intelligent dinosaurs, maybe with their centers right where the crater now is, and we’re simply not seeing any record of it – that’s possible given the paucity of fossil record, but if there were a civilization, it probably wasn’t very advanced, given that fossil fuel and mineral deposits seem to have been untouched before humans showed up.)

        The strategy you describe is known in the SETI discussion as “Berserker hypothesis,” after a series of sci-fi books where automated warships known as Bersekers, originally built to fight some long-forgotten war, have taken as their mission to exterminate all life. It’s also precisely the plot of Greg Bear’s “The Forge of God” and “Anvil of Stars:” a paranoid civilization has sent out self-replicating killer probes that wipe out other species. However, the Anvil of Stars explores what happens afterwards: if civilizations are common, it’s in the interests of others to gang up against killer civilizations, seek them out no matter where they hide, and exterminate them.

        As laid out in the books, sending killer probes is a high-risk strategy that may backfire very badly, unless one is very sure that one’s technology is invincible. That’s a risky assumption to make; after all, there well may be older and far more advanced civilizations that are better at hiding and simply listening – maybe even actively creating decoys (or perhaps even using lesser developed civilizations as a bait) to draw out genocidal civilizations and terminate them before they can even pose a real threat.

        So, it’s quite possible that other civilizations are hiding and at the very least not answering our calls precisely because they fear what we might do. However, it would seem to me that any would-be hostile civilization would have to assume that any signals they detect – maybe even planets apparently inhabited by our kinds of civilizations – could be decoys designed to lure them out. Therefore, either way, it would seem to be unlikely that they’d send out killer probes just because they can.

  2. Tuomas Huovinen says:

    Yep, I agree that anyone fancying after auto-RKVs to show who is the king of the universe must take in consideration that older “lurker” civilization(s) may have droned the entire Galaxy while enjoying endless Big Brother broadcast from the “real world”. Their Discovery Channel must be something else. 🙂 So RKV builders, who knows, maybe even us building some quick probe, may gain automated attention with a broadcast message “Please be careful with that”. 🙂

    I don’t share that pessimistic view of civilizations radio-visibility – as long as there is air and ship traffic we have various radars online, and some of them are really good beacons. Specially those which track space junk and missiles. So, unless we implode to some post-human virtual reality I am quite certain that some kind of radio communications are used. It is cheap and it works.

    I would also claim that all civilizations, whether flesh or bits, must keep either optical or radar surveillance on, even if the RKV threat is near zero the random asteroid coming out of nowhere is too dangerous to be missed. Furthermore I think other civilizations would most likely have HAM nerds using age old technologies just because it tickles their creative mind. Just take a look what our countrymen did: – if the trend continues like this they are building their own space radio arrays in 100 years or so. 🙂

    Anyway, it is fun talking about this, in IRC things get superheated too quickly and forums are full of trolls and aspies who focus on irrelevant things. I keep following your unpublished notebooks.



    • Thank you too, it has been a nice discussion. It’s of course true that some radio transmissions will continue, but I think it’s not clear how detectable they would be. From what I’ve been led to understand, the trend towards more energy-efficient radars on one hand, and towards less jammable and less detectable radars on the other, has also made them less useful as beacons. In any case, a civilization that has the capability to detect radio transmissions from exoplanets will relatively quickly also have the capability to even directly image them.

      There may be a follow-up article for this paper, if I get around to doing it. Hope you’ll hang around!

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