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‘Shielding Swarms’ & SETI Observables

If you’re on the Moon and learn that there has been a major solar eruption, your best course of action is to get inside an appropriate shelter somewhere below ground, where you can be shielded from its effects. By analogy, wouldn’t a future civilization on Earth be able to shield itself from the effects of a supernova or gamma ray burst by burrowing into the planet?

In their paper on stellar explosions and risk mitigation, Milan Ćirković and Branislav Vukotić argue against the idea, which runs into problems on multiple levels. For one thing, while the duration of gamma ray emissions is generally short — on the order of a hundred seconds or less — the pulse of accelerated cosmic rays from a supernova or GRB blast is likely to last much longer, perhaps a matter of months or even years.

Digging to avoid the worst of the effects would take you deep into the ground indeed. The authors cite work showing that you would need to burrow up to 3 kilometers below the surface before the incoming flux would drop to 1% of its initial value. And finally, Earth’s atmosphere could not itself be shielded this way, opening the door to ecological catastrophe.

A system-wide infrastructure involving asteroid mining and perhaps planetary colonies is likewise at risk. For all these reasons, Ćirković and Vukotić think a future or extraterrestrial civilization would choose space-based shielding in its own planetary system as a response to the threat of any nearby stellar explosion. Here the most natural building material is ice, found in great abundance in the outer Solar System. What emerges is the concept of a ‘shielding swarm’ far from the Sun whose bulk density can be adjusted as necessary.

Here the notion of ‘smart dust’ inevitably occurs, which gives us a way of describing the swarm:

…we envision a swarm of particles confined by electromagnetic forces interspersed by smart dust particles controlling the swarm and enabling more precise manipulation, in addition to controlling ionization necessary for the ice particles to be moved around. They could provide essential telemetric information and the data on conditions within the swarm necessary for self-regulation actions. Since various forms of carbon, including fullerenes, is currently thought to be the best material for building smart dust, as well as other nanotechnological applications [89], and the Kuiper Belt objects are carbon-rich, it seems natural to assume that fragmentation of the very same icy body or bodies creating the bulk of the shielding swarm might provide material for construction of smart dust particles as well.


Image: Artist’s impression of a supernova blast. Credit: NASA.

So let’s drop back to the core idea. Assuming a threatening source can be identified and its likely explosion predicted, an advanced civilization could choose the appropriate icy objects and change their orbit to reach the staging area, where construction of the swarm can begin. Such a swarm could itself reduce cosmic ray flux by several factors, while additional active shielding could be provided by the same system of electromagnetic confinement that would be used to manipulate the particles in the swarm during its formation.

Searching for Extraterrestrial Technology

When contemplating what an extraterrestrial civilization might do (or indeed, what we might ourselves do when we reach a sufficient technological level), it’s useful to consider the thought processes involved. Freeman Dyson is relevant here — we are saying that anything we can hypothesize about our own future course should apply to at least some extraterrestrial species. This is how, without understanding the intricacies of a future technology, we can make broad predictions about possible astroengineering and how we might detect it.

Searching for Dyson swarms or spheres is an example of this, the search being motivated by our belief that such a swarm would be an intelligent way of maximizing energy resources for a society around a particular star. The fact that we can imagine it — although we are a long way from being able to do it — means that more advanced cultures have probably run across the same idea, given that it contradicts no physical laws. Thus it is at least worth the attempt to figure out what a Dyson swarm would look like if we stumbled across one in our data.

In a similar way, Ćirković and Vukotić believe, it would be rational for any society to attempt to reduce large-scale risk, in this case in the event of the explosion of a close supernova. Shielding swarms of the kind the duo discuss in their paper are an extrapolation of what we, as a Kardashev 0.7 culture, might do if we had the energy and resources. Nudging forward into Kardashev 1 and beyond, we might look for signs of the presence of such swarms as a potential SETI signature around other stars. The paper suggests these possible observables:

  • Planetary size structures with unusually small mass and non-Keplerian motion. In other words, find something the size of a terrestrial planet but with a mass as little as 10-8 Earth mass and suspicion should mount that it is an artifact.
  • A swarming shield made predominately of ice may show unusual optical properties like polarization and non-equilibrium temperatures, as well as strong absorption in the far infrared. And this is interesting — a shield that is transiting its star would produce a small optical transit depth but a much larger infrared transit depth.
  • Continuing fragmentation of small bodies in a planetary system without accompanying physical causes (collisions), along with anomalous loss of kinetic energy and momentum.
  • Unusual observables in a planetary system occurring simultaneously with the last phases of evolution of a nearby supernova progenitor.

Image: The above two photographs are of the same part of the sky. The photo on the left was taken in 1987 during the supernova explosion of SN 1987A, while the right hand photo was taken beforehand. Supernovae are one of the most energetic explosions in nature, equivalent to the power in a 1028 megaton bomb (i.e., a few octillion nuclear warheads). Credit: NASA.

A shielding swarm of ice particles with infused ‘smart dust’ is attractive on a number of levels, not the least being that we can think of no physical laws such a swarm would violate, in sharp contrast to issues like faster than light travel or even the problems attendant on building a solid Dyson sphere (as opposed to a shell). The materials involved in a shielding swarm are those that would be available to any growing technology, and the construction and maintenance of such a swarm would be inexpensive to an advanced spacefaring culture.

…we conclude that successful mitigation of cosmic explosions risk is viable for sufficiently advanced technological societies, both future terrestrial and extraterrestrial. We suggest that building and maintaining shielding swarms of small particles/components is (relatively!) cheap and efficient way of achieving that goal and creating a durable planetary and interplanetary civilization. The technology required partially overlaps with that required for mitigation of asteroid/cometary impact risk, which could provide some clues for future technological desiderata and even convergence. Finally, this new type of macro- or astroengineering could not only enrich the spectrum of astroengineering possibilities, but also provide another opportunity for bold and innovative SETI programs to detect advanced technological civilizations elsewhere in the Galaxy.

Thus we have a possible new astroengineering signature to look for, and on that score, I think back to something Freeman Dyson said in a 1966 essay called “The Search for Extraterrestrial Technology.” Here he explains his view that if there are millions of places in the universe where light might develop, then we should not be thinking about average technological societies but those that are the most conspicuous. It is these we have the best chance to detect, and they will be the ones doing the biggest possible artificial activities.

Can we really rely on our extrapolations from our own technology to study this question? Here’s how Dyson stated the answer:

I assume that all engineering projects are carried out with technology which the human species of the year 1965 A.D. can understand. This assumption is totally unrealistic. I make it because I cannot sensibly discuss any technology which the human species does not yet understand. Obviously a technology which has existed for a million years will be likely to operate in ways which are quite different from our present ideas. However, I think this rule of allowing only technology which we already understand does not really weaken my argument. I am presenting an evidence proof for certain technological possibilities. I describe crude and clumsy methods which would be adequate for doing various things. If there are other more elegant methods for doing the same things, my conclusions will still be generally valid.

The paper is Ćirković and Vukotić, “Long-term prospects: Mitigation of supernova and gamma-ray burst threat to intelligent beings,” accepted at Acta Astronautica. No preprint yet available but I’ll insert it when it appears. The Dyson paper I quote above is F.J. Dyson, “The search for extraterrestrial technology,” in: R.E. Marshak (Ed.) Perspectives in Modern Physics, Interscience Publishers, New York, 1966, pp. 641–655.


Comments on this entry are closed.

  • Nelson Bridwell November 8, 2016, 15:21

    I suspect that it depends upon the wavelength of the gamma rays that you are talking about”
    “For example, gamma rays that require 1 cm (0.4″) of lead to reduce their intensity by 50% will also have their intensity reduced in half by 4.1 cm of granite rock, 6 cm (2½″) of concrete, or 9 cm (3½″) of packed soil.”

    “Earth’s atmosphere is very effective at absorbing high energy electromagnetic radiation such as x-rays and gamma rays, so these types of radiation would not reach any dangerous levels at the surface during the burst event itself. The immediate effect on life on Earth from a GRB within a few parsecs would only be a short increase in ultraviolet radiation at ground level, lasting from less than a second to tens of seconds. This ultraviolet radiation could potentially reach dangerous levels depending on the exact nature and distance of the burst, but it seems unlikely to be able to cause a global catastrophe for life on Earth.”

  • James Franklin November 8, 2016, 15:32

    Interesting item Paul, thank you. I wonder though, if a society is so technically advanced, do they need to shield themselves from impending disaster?

    Supernova are not predictable to us, but it may be possible, at some point of technology, to mske a reasonably informed guess as to when a given star may explode, within say 500 years.

    So you are smart, you have technology we can only dream of, what are your options..

    1. Do nothing, accept your fate and die.
    2. Look to shield yourself within the planets of your home system.
    3. Build a Dyson swarm out of material in the outer part of your planetary system.
    4. None of the above.

    Well point 1 would not be a sign of intelligence so I think we can dismiss that for all societies with super advanced technologies.

    Point 2. Well it may not be a solution, but perhaps you have advanced terraforming technology too so you don’t care about the atmosphere and surface, you can replenish once the event has passed by.

    Point 3. This may be doable, but you civilisation could be so advanced it has already cannabalised the outer reaches of your planetary system and this may not be an option for you. Alternatively you may have insufficient time to construct the shield between when you prediction is mature and tge event will occur.

    4. None of the above. Space is allegedly filled with tiny black holes left over from the big bang. Perhaps you have the technology to detect one and move it into an orbit close enough to the star to start feeding on it, but in a degrading orbit so the star consumes the black hole, but the black hole grows by consuming the star enough to prevent catastrophic events…at least gor the time being, by which time you develope new technology to mitigate the altered threat.

    The last point…would a society that reaches Kardashev level 1.5 or above stay on a world likely to get decimated by cosmic events, or would they turn a free floating world into a habitat and use this as a lifeboat to a new star system?

    • ljk November 9, 2016, 11:42

      James, your list reminded me of how folks in the Nineteenth Century viewed the intelligent, civilized beings that they thought lived on Mars and were the ones responsible for those networks of canals supposedly covering the planet.

      Percival Lowell, the Boston aristocrat, viewed the Martians as not only more advanced technologically but also intellectually. Faced with a dying planet (the view at the time was that Mars being farther from the Sun, formed sooner than Earth at the beginning of the Sol system and therefore was farther along in its evolution, which included its eventual demise), the Martians built huge canals to get the water at the planet’s poles to the thirsty cities near the equator.

      I have not been able to find any reference if Lowell ever considered that “his” Martians would try to escape their dying planet by means of rocketships or any other method. Apparently, being more enlightened and therefore noble beings, the Martians would slowly die out with dignity, with only the canals and their cities remaining as tributes to their former greatness.

      Science fiction author and working class Englishman H. G. Wells took a different tack with those same canal-building Martians. His beings did not take the fate of their world lying down. Instead, “minds
      that are to our minds as ours are to those of the beasts that perish,
      intellects vast and cool and unsympathetic, regarded this earth with
      envious eyes, and slowly and surely drew their plans against us.”

      Wells’ Martians used their higher intellects and technological superiority to build not only the means to reach Earth for permanent colonization but formidable war machines to ensure that they would be the new dominant life forms of the third rock from Sol.

      As a side note, since the various film versions of WotW never cover this, when the Martians ultimately failed in their conquest due to their lack of immunity against terrestrial microbes, they launched a new colonization effort at the planet Venus. Since that planet was even closer to Sol and therefore even less evolved, which would include any intelligent species being little to no threat, the Martians probably had better luck in ruling the second world from Sol.

      This is how the various classes of humanity have viewed ETI ever since in general. If you are well educated and wealthy, you tend to see them as enlightened beings who pursue mainly noble pursuits and would never dream of impinging on the homes and property of others.

      However, if your state in human society is decidedly less glamorous, you might see those with more power and resources as either rivals or conquerors. What the real state of affairs is beyond our pale blue dot is why we need to ramp up our SETI and METI efforts.

  • Nelson Bridwell November 8, 2016, 15:43

    An additional observation: If the premise of these authors is correct, and kilometers, rather meters of shielding are required in order to provide protection against the most energetic, nearby supernova/GRB, perhaps that might argue for the inhabitation of very small asteroids or cometary nuclei, where it might be possible to easily engineer large open volumes in the center, particularly for an aggregate pile of rubble, rather than a solid (or molten liquid!) body. Their premise sounds to me like a justification for a cool thought experiment, rather than a plausible engineering reality.

  • Alex Tolley November 8, 2016, 16:27

    Here he explains his view that if there are millions of places in the universe where light might develop,


  • Alex Tolley November 8, 2016, 16:37

    It is an interesting idea. It is predicated on being able to predict supernova explosions. If not, then the gamma ray burst would have to be tolerated and the shield deployed for the subsequent particle showers.

    Ideally, superluminal communication would allow detectors situated light years away could provide the needed detection and communication with the solar system so that deployment could be made to protect biospheres.

    Without detectors, the prediction will need to be good so that deployment can be done within a window of the detonation period. Will telescopes in sol system be sufficient or will satellites in orbit about potential supernovae monitoring the star’s state be needed?

  • Robert G November 8, 2016, 17:19

    Another way of dealing with an impending nova would be to move it far, far away. Bat it out of the stadium.

    Homo sapiens has yet to figure out a way of moving a star. But we do have plenty of telescopes for measuring their motions. A star travelling at a speed and in a direction that looks dynamically unusual, particularly if that star looks like it is on the way to going nova, would be a rather big giveaway.

    I would imagine we could do a data dig for this already, and improve on it as we improve our understanding of the motions of stars within our galaxy.

    • Jim Franklin November 9, 2016, 6:20

      I don’t think this is a realistic solution at this time with our current understanding of physics. The sheer mass involved would require so much energy to move the star it would likely destroy it. The only other way would be gravitational interaction, but again that would require such a large mass to have an impact on the star we are back to square 1.

      We must bear in mind the angular momentum a star has in in motion around the galaxy is incredibly large..off the scale, especially as we are talking about stars that are 10 solar masses or more in size.

  • Robert G November 8, 2016, 17:29

    … We don’t know how to move a star, but if we found one nearby that was about to go bang, I think it would concentrate our minds wonderfully, and the same would be true for ET

  • Andrew Palfreyman November 8, 2016, 17:30

    A kilometres-long, metre-wide rod, accurately pointed, would form an effective shield.

    • Michael Spencer November 9, 2016, 7:17

      That’s thinking out of the box for sure – similarly Clarke/ Baxter in “Sunstorm”, though because they were dealing with a solar event; in the case of a remote GRB/SN, it’s actually a point source.

  • Harold November 8, 2016, 19:34

    This is an interesting idea. It is economical. I am picturing swarms forming meta-materials with unique properties that increased the protective effect of the magnetic control field.
    For any given star, wouldn’t there be a known number of potentially hazardous neighbors? A civilization would only need maintain a swarm at hazardous stars and once the hazard passed they would need to find another use for the material or abandon the swarm. Would be interesting to model long term decay of a swarm.
    Would blowing up a comet in just the right place and way, roughly duplicate a swarm? Not including the protective effect of the magnetic field.

  • craig watkins November 9, 2016, 1:14

    So, would Boyajian’s Star be a candidate for this sort of activity? Comets are mostly ice, right?

  • Dmitry Novoseltsev November 9, 2016, 5:19

    “Planetary size structures with unusually small mass and non-Keplerian motion. In other words, find something the size of a terrestrial planet but with a mass as little as 10-8 Earth mass and suspicion should mount that it is an artifact.”

    See http://lnfm1.sai.msu.ru/SETI/koi/articles/Arhipov_1SWASP.pdf

    1SWASP-J161732.90+242119.0 (TYC 2042-7-1) ?

  • Dmitry Novoseltsev November 9, 2016, 5:21

    In this case in the foreseeable near future, “Shielding Swarms” to protect against the ion flow may consist of a multilayer stack of thin electric solar sails like this (https://jour.space/issues/issue-8-2014/). This will take a little mass and they can be quickly deployed to protect the Earth from the distant flash of a supernova, or, more important, from powerful solar emissions. And to the place of deployment, they all can be delivered quickly to traditional rocket engines with high thrust.

  • Brenda Kalt November 9, 2016, 9:48

    A Dyson swarm cuts you off from observing anything outside of it.

  • ljk November 9, 2016, 12:01

    As usual, it probably won’t be the ETI that the residents of Earth need to worry about:


    The other big issue will be communication. Some folks assume that aliens will automatically learn English or some other human language to make talking between us and them easier (for us). Others, especially scientists, assume that mathematics is the literally universal language.

    As usual, though, the Universe rarely ever bothers to conform to human wants and needs, and why should it?


  • ole burde November 9, 2016, 13:14

    Perhabs we shouldnt dismiss the idea of burrying underground , just because its not as cool as some of the alternatives ….actually a planet like Mars might be just cool enough for the purpose of creating a true underground -civilisation , capable of sustaining itself in a very wide range of disaster-conditions . Such a civilisation would need LOTS of energy , and it might cover the complete surface with energy harvesting systems of many different kinds , from photovoltaic to organic enhanced photosyntesis …the question is if and how this would influence the radiatiion spectrum of the planet

    • ljk November 10, 2016, 9:37

      The same situation may apply to Venus:


      If life can exist and thrive around boiling hot hydrothermal vents deep in the ocean and in highly acidic and equally boiling hot geyser pools, then the relatively milder subsurface conditions are just as appealing if not more so. This of course expands the possibilities for extraterrestrial life by quite a bit.

  • ljk December 28, 2016, 17:03

    The way we might protect ourselves from an exploding star could help us find alien civilizations

    Kevin Loria

    December 19, 2016, 4:28 PM

    Full article here:


    To quote:

    “For the present purposes, we note that no part of any spiral galaxy [like our Milky Way] can be considered safe from cosmic explosions in the long run,” the authors write in the paper. “And as the timeframe considered by an intelligent species grows longer, more relevant becomes the issue of mitigation.”

    The longer we hope to live and more powerful we become as a species, the more we’ll have to consider these possibilities.

  • Mike Serfas January 2, 2017, 18:38

    I think we should be wary of supposing that aliens will try any approach we think of. Something that requires planetary level effort should first face planetary level skepticism and research of alternatives. So I suppose that if a better way exists, they will very likely find it.

    For example, it occurs to me that what we’re looking for is basically a silencer for a supernova: something that will quell the motion of protons or other particles in our direction. Like our primitive ancestors, we shouldn’t immediately suppose that “a rock” is the best possible technology we can invent. Looking up “gamma shielding metamaterial” I quickly find news like https://news.ncsu.edu/2015/07/rabiei-foam-rays-2015/ and people recommending papers like http://arxiv.org/ftp/arxiv/papers/0801/0801.2223.pdf . I won’t pretend to be an expert on the quantum physics involved in cloaking matter waves, but I’m guessing there’s a fair chance that people like this will eventually think of something that could be floated as a membrane in the atmosphere even by our own civilization, if we really made it a priority. If folks come up with more suitable attire for tourists at Chernobyl and Fukushima in the process, so much the better.

    P.S. I found the preprint at https://arxiv.org/ftp/arxiv/papers/1611/1611.06096.pdf . Also, it’s worth noting that asteroids like Vesta are predicted to have comfortably warm interiors several km below the surface, which might provide some other survival or diaspora possibilities, particularly if warfare has already made planetary surfaces untenable by stage 1.0. Of course in that situation it would be best not to place any interstellar phone calls!