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Interstellar Travel and Stellar Evolution

The stars move ever on. What seems like a fixed distance due to the limitations of our own longevity morphs over time into an evolving maze of galactic orbits as stars draw closer to and then farther away from each other. If we were truly long-lived, we might ask why anyone would be in such a hurry to mount an expedition to Alpha Centauri. Right now we’d have to travel 4.2 light years to get to Proxima Centauri and its interesting habitable zone planet. But 28,000 years from now, Alpha Centauri — all three stars — will have drawn to within 3.2 light years of us.

But we can do a lot better than that. Gliese 710 is an M-dwarf about 64 light years away in the constellation Serpens Cauda. For the patient among us, it will move in about 1.3 million years to within 14,000 AU, placing it well within the Oort Cloud and making it an obvious candidate for worst cometary orbit disruptor of all time. But read on. Stars have come much closer than this. [Addendum: A reader points out that some sources list this star as a K-dwarf, rather than class M. Point taken: My NASA source describes it as “orange-red or red dwarf star of spectral and luminosity K5-M1 V.” So Gliese 710 is a close call in more ways than one].

In any case, imagine another star being 14,000 AU away, 20 times closer than Proxima Centauri is right now. Suddenly interstellar flight looks a bit more plausible, just as it would if we could, by some miracle, find ourselves in a globular cluster like M80, where stellar distances, at the densest point, can be something on the order of the size of the Solar System.

Image: This stellar swarm is M80 (NGC 6093), one of the densest of the 147 known globular star clusters in the Milky Way galaxy. Located about 28,000 light-years from Earth, M80 contains hundreds of thousands of stars, all held together by their mutual gravitational attraction. Globular clusters are particularly useful for studying stellar evolution, since all of the stars in the cluster have the same age (about 12 billion years), but cover a range of stellar masses. Every star visible in this image is either more highly evolved than, or in a few rare cases more massive than, our own Sun. Especially obvious are the bright red giants, which are stars similar to the Sun in mass that are nearing the ends of their lives. Credit: NASA, The Hubble Heritage Team, STScI, AURA.

These thoughts are triggered by a paper from Bradley Hansen and Ben Zuckerman, both at UCLA, with the interesting title “Minimal Conditions for Survival of Technological Civilizations in the Face of Stellar Evolution.” The authors note the long-haul perspective: The physical barriers we associate with interstellar travel are eased dramatically if species attempt such journeys only in times of close stellar passage. Put another star within 1500 AU, dramatically closer than even Gliese 710 will one day be, and the travel time is reduced perhaps two orders of magnitude compared with the times needed to travel under average stellar separations near the Sun today.

I find this an interesting thought experiment, because it helps me visualize the galaxy in motion and our place within it in the time of our civilization (whether or not our civilization will last is Frank Drake’s L factor in his famous equation, and for today I posit no answer). All depends upon the density of stars in our corner of the Orion Arm and their kinematics, so location in the galaxy is the key. Just how far apart are stars in Sol’s neighborhood right now?

Drawing on research from Gaia data as well as the stellar census of the local 10-parsec volume compiled by the REsearch Consortium On Nearby Stars (RECONS), we find that 81 percent of the main-sequence stars in this volume have masses below half that of the Sun, meaning most of the close passages we would experience will be with M-dwarfs. The average distance between stars in our neck of the woods is 3.85 light years, pretty close to what separates us from Alpha Centauri. RECONS counts 232 single-star systems and 85 multiple in this space.

Hansen and Zuckerman are intrigued. They ask what a truly patient civilization might do to make interstellar travel happen only at times when a star is close by. We can’t know whether a given civilization would necessarily expand to other stars, but the authors think there is one reason that would compel even the most recalcitrant into attempting the journey. That would be the swelling of the parent star to red giant status. Here’s the question:

As mentioned above, this stellar number density yields an average nearest neighbor distance between stars of 3.85 light years. However, such estimates rely on the standard snapshot picture of interstellar migration − that a civilization decides to embark instantaneously (at least, in cosmological terms) and must simply accept the local interstellar geography as is. If one were prepared to wait for the opportune moment, then how much could one reduce the travel distance, and thus the travel time?

Maybe advanced civilizations don’t tend to make interstellar journeys until they have to, meaning when problems arise with their central star. If this is the case, we might expect stars in close proximity at any given era — ruling out close binaries but talking only about stars that are passing and not gravitationally bound — to be those between which we could see signs of activity, perhaps as artifacts in our data implying migration away from a star whose gradual expansion toward future red giant phase is rendering life on its planets more and more unlivable.

Here we might keep in mind that in our part of the galaxy, about 8.5 kiloparsecs out from galactic center, the density of stars is what the authors describe as only ‘modest.’ Higher encounter rates occur depending on how close we want to approach galactic center.

Reading this paper reminds me why I wish I had the talent to be a science fiction writer. Stepping back to take the ‘deep time’ view of galactic evolution fires the imagination as little else can. But I leave fiction to others. What Hansen and Zuckerman point out is that we can look at our own Solar System in these same terms. Their research shows that if we take the encounter rate they derive for our Sun and multiply it by the 4.6 billion year age of our system, we can assume that at some point within that time a star passed within a breathtaking 780 AU.

Image: A passing star could dislodge comets from otherwise stable orbits so that they enter the inner system, with huge implications for habitable worlds. Is this a driver for travel between stars? Credit: NASA/JPL-Caltech).

Now let’s look forward. A gradually brightening Sun eventually pushes us — our descendants, perhaps, or whatever species might be on Earth then — to consider leaving the Solar System. Recent work sees this occurring when the Sun reaches an age of about 5.7 billion years. Thus the estimate for remaining habitability on Earth is about a billion years. The paper’s calculations show that within this timeframe, the median distance of closest stellar approach to the Sun is 1500 AU, with an 81 percent chance that a star will close to within 5000 AU. From the paper:

Thus, an attempt to migrate enough of a terrestrial civilization to ensure longevity can be met within the minimum requirement of travel between 1500 and 5000 AU. This is two orders of magnitude smaller than the current distance to Proxima Cen. The duration of an encounter, with the closest approach at 1500 AU, assuming stellar relative velocities of 50km/s, is 143 years. In the spirit of minimum requirements, we note that our current interstellar travel capabilities are represented by the Voyager missions (Stone et al. 2005); these, which rely on gravity assists off the giant planets, have achieved effective terminal velocities of ∼ 20 km/s. The escape velocity from the surface of Jupiter is ∼ 61 km/s, so it is likely one can increase these speeds by a factor of 2 and achieve rendezvous on timescales of order a century.

My takeaway on this parallels what the authors say: We can conceive of an interstellar journey in this distant era that relies on technologies not terribly advanced beyond where we are today, with travel times on the order of a century. The odds on such a journey being feasible for other civilizations rise as we move closer to galactic center. At 2.2 kiloparsecs from the center, where peak density seems to occur, the characteristic encounter distance is 250 AU over the course of 10 billion years, or an average 800 AU during a single one billion year period.

You might ask, as the authors do, how binary star systems would affect these outcomes, and it’s an interesting point. Perhaps 80 percent of all G-class star binaries will have separations of 1000 AU or less, which the authors consider disruptive to planet formation. Where technological civilizations do arise in binary systems, having a companion star is an obvious driver for interstellar travel. But single stars like ours would demand migration to another system.

We can plug Hansen and Zuckerman’s work into the ongoing discussion of interstellar migration. From the paper:

Our hypothesis bears resemblance to the slow limit in models of interstellar expansion (Wright et al. 2014; Carroll-Nellenback et al. 2019). In a model in which civilizations diffuse away from their original locations with a range of possible speeds, the behavior at low speeds is no longer a diffusion wave but rather a random seeding dominated by the interstellar dispersion. Even in this limit, the large age of the Galaxy allows for widespread colonization unless the migration speeds are sufficiently small. In this sense our treatment converges with prior work, but our focus is very different. We are primarily interested in how a long-lived technological civilization may respond to stellar evolution and not how such civilizations may pursue expansion as a goal in and of itself. Thus our discussion demonstrates the requirements for technological civilizations to survive the evolution of their host star, even in the event that widespread colonization is physically infeasible.

It’s interesting that the close passage of a second star is a way to reduce the search space for SETI purposes if we go looking for the technological signature of a civilization in motion. Separating out stars undergoing close passage from truly bound binaries is another matter, and one that would, the authors suggest, demand a solid program for eliminating false positives.

Ingenious. An imaginative exercise like this, or Greg Laughlin and Fred Adams’ recent work on ‘black cloud’ computing, offers us perspectives on the galactic scale, a good way to stretch mental muscles that can sometimes atrophy when limited to the near-term. Which is one reason I read science fiction and pursue papers from people working the far edge of astrophysics.

The paper is Hansen and Zuckerman, “Minimal conditions for survival of technological civilizations in the face of stellar evolution,” in process at the Astronomical Journal (preprint). Thanks to Antonio Tavani for the pointer on a paper I hadn’t yet discovered.


Comments on this entry are closed.

  • Rona l d February 16, 2021, 7:53

    Hmm, so stellar evolution time is seen as a bottleneck? I would rather think that survival time of a technological civilization is a much greater bottleneck.
    Having said that, how regrettable that we are not part of a (wide) binary system with a living planet orbiting both stars. And, additionally, that we don’t have another planet or two within our own HZ. We now know that our HZ could easily fit in 2 or 3 planets in stable orbits.

  • Michael Fidler February 16, 2021, 9:41

    In the not too distant future we could land on a comet and convert it into a livable sphere. Just pick the right one and it would be pulled into an orbit around the passing star. Large deep space tracking telescopes could find the right comet centuries before and robotics AI could transform the comet. Of course just put a magnetic reconnection plasmoid propulsion unit on it and be their in a human lifetime…
    Plenty of fuel for the rocket engines.

    • Robin Datta February 16, 2021, 16:19

      And of course call and put options from speculators!

      • Michael Fidler February 17, 2021, 9:42

        I just put $99.00 down on StarLink here in the Philippines with plan for 3rd quarter Beta. The 25 degree low angle may just work for Starlink Coverage here:

        • Alex Tolley February 17, 2021, 21:20

          Isn’t there a $500 upfront charge for the kit too?

          My understanding was that it might be quite bandwidth-limited where there is heavy demand for use, i.e. in cities. If you are an early adopter, the service might be fine (albeit expensive even in the US), but what happens if residents in Manila take to it in droves in due course? Or is this a case that internet service in teh Phillippines is so bad that almost any other option is desirable?

          • Michael Fidler February 19, 2021, 14:15

            The internet service in the Philippines is so bad that almost any other option is desirable! Yes it is that bad! The up front money you need to put down is only 99.00 for the beta testing, the 500.00 comes due when they send the kit. I paid 300.00 for a cable modem some 20 years ago but 2 years later they were giving them away free, always high for early adopters. Cites do not really matter it is the number of satellites overhead, right now high number in northern US and Canada and just saw 420Mbs download speed from user in southern Canada, the orbit coverage is much higher at the higher latitudes. Take a look at the StarLink coverage chart, but that improves over time closer to the equator because of more satellites launched, plus laser transmissions between satellites coming soon. They say 10Gbs for users when system is fully operational! Yes, 10Gbs!!!

            • Alex Tolley February 19, 2021, 15:04

              If I could reliably get 10Gbs for $100/m that would be far better value than what I pay Comcast for currently. Amortize the $500 kit over 24 months and that adds just adds $21 for those 2 years. I look forward to reading about real experiences after the swarm is complete and many users have signed up in the US.

              • Michael Fidler February 22, 2021, 20:29

                INVESTING IN SPACE
                Elon Musk says SpaceX ‘will double’ Starlink satellite internet speeds later this year.

                SpaceX CEO Elon Musk says the company’s Starlink satellite internet service “will double” speeds to customers “later this year.”
                Starlink is a capital-intensive project to build an interconnected internet network with thousands of satellites, designed to deliver high-speed internet to consumers anywhere on the planet.
                “Speed will double to ~300Mb/s & latency will drop to ~20ms later this year,” Musk said in a tweet on Monday.
                Musk added that Starlink will reach customers around “most” of the Earth by the end of 2021, and is expecting to have complete global coverage “by next year.”


      • Project Studio February 18, 2021, 7:46

        Can Starlink help our friends behind the Great Firewall and other totalitarian regimes attempting to censor the Internet?

        • Ron S. February 18, 2021, 15:59

          No, not really. It’s quite easy to find the ground stations because they are (unlicensed in these cases) transmitters on known frequencies. It is also quite risky to import or smuggle the equipment, dish included. Older generation satellite data and phone services have been used in the past to escape restrictions, and it’s a game of cat and mouse, even when the equipment is hand carried or in a vehicle. It doesn’t end well when the mouse is caught.

  • Alex Tolley February 16, 2021, 11:36

    The obvious SciFi reference is “When Worlds Collide”, with the close passing of Bellus and the flight of a small colony to the habitable world, Zyra.

    However, to me, the equivalent deep-time analogy is continental drift and the dispersion of animals when continents collide. For example, when the north and south American continents joined, the mammalian fauna of N. America invaded and largely displaced the marsupials of S. America. When a continent split, it allowed the separate evolutionary trajectories of the now separated populations, something that would happen to humans after some of the terrestrial population migrated to another close passing star.

    However, these time scales for close passing stars may be far to0 long for a technological civilization. Our starships may be more akin to animals making sea journeys on natural rafts to populate nearby islands rather than waiting for possible land mergers.

    If only there was the stellar equivalent of an ice age so that reduced sea levels would allow land bridges to form. This allowed European people to colonize Britain via Doggerland, and Asian peoples to reach N. America via Beringia.

    I tend to side with technology-aided migration. Heyerdahl’s attempt to reach the Polynesian islands from S. America on the Kon-Tiki raft, or the migration to S. America from Africa with the Ra expedition, seems the more likely approach for humans to use technology to reach another star within a millennium. We can imagine such starship technology today, although these imaginings will look very primitive and naive when the technology to do so actually matures.

    Lastly, let us not forget that these stellar encounters can be increased by moving a star, such as with the Shkadov thruster. This could reduce the time for a stellar encounter allowing these more primitive starships to successfully migrate populations to the target star. Can we do this in the blink of the cosmic eye?

    • Robin Datta February 16, 2021, 17:23

      Of course the stars in consideration would be those with suitable planets in orbit or otherwise available for “planet moving” to more clement locations.

      Humans replaced by another organism of earthly origin is a rather tall order. Brachiation gave us three-axis range of shoulder movement, binocular stereoscopic vision and prehensile hands (and feet in primates, lost through bipdal gait in humans) that were needed to make and use tools, with an adequate thumb length for strong pinch to hold those tools prior to vises, with the shoulder movement needed to wield and throw weapons.

      Control of fire gave us cooking, with shrinkage of teeth which together with alterations of the airway from upright stance enabled modulation of sound to speech.

      A fairly continuous overhead arboreal canopy is needed for brachiation. That is found in tropical rainforests. Vertebrates that brachiate are primates.

      If another animal will replace humans as a civilization on Earth, it will likely be another primate.

      • David Jernigan February 18, 2021, 4:11

        Unless octopi make whatever leap is keeping them from making tools (probably air breathing to power enough neurons). After all, they also have the limbs for it, and exquisite control of colors and patterns for communication.

        Unlikely, I’ll grant, but don’t assume that our path from jungle to savannah that ignited civilization is the only one possible.

        • Robin Datta February 20, 2021, 17:01

          Octupuses, even the largest of them, have a short life span. And moreover all of them have zero overlap with the next generation: the adults die after reproduction before the hatching of the next generation. Behaviors can be passed on by instinct, but not by cultural means. No science, no art, no literature, no language.

          They also lack blood corpuscles to keep down viscosity while carrying oxygen-bearing compouds in large quantities.

          And smelting of ores for metal and fabrication of electronics in an aqueous environment could be problematic.

          • Alex Tolley February 21, 2021, 2:15

            Behaviors can be passed on by instinct, but not by cultural means. No science, no art, no literature, no language.

            I don’t see that is implied. Just because the parent-child relationship is broken, octopi can operate just as our orphanages do. The children are educated by adults that are not their parents. There is no reason why permanent cultural artifacts like books cannot be created and used either. If octopi or other cephalopods have intelligence unrestricted to instincts, then there should be the possibility of them creating a civilization. As the phylum mollusca examples like slugs and snails can live on land, there is no inherent reason why the cephalopod class could perhaps adapt themselves too. I suspect size is the issue that has prevented that evolutionary path, but just maybe they could create surface platforms on the ocean to achieve technologies that need air to work, while making it possible to only expose their tentacles to do the surface work while keeping their bodies in the ocean below the platform. Unlikely, I think, but not beyond the bounds of possibility. If not, then their civilization will have to be constrained by the limits imposed by their aquatic environment – e.g. no fire to work metals.

  • Brett February 16, 2021, 13:04

    On time-scales that long, you could potentially move the Earth itself to another star that’s passing that close to the Sun, although you’d have to replace solar lighting/heating with either beamed light or massive fusion reactors until the passing star captured it. It would take a long time, but we’ve got plenty of time – longer than 1.1 billion years, since if we’re still around we can build a solar shade to dampen the amount of light hitting the planet.

  • Thomas R Mazanec February 16, 2021, 13:50

    (32.6/3.85)^3 = 607.
    I imagine that the (232+85) = 317 total reflects brown dwarfs, white dwarfs and dimmer red dwarfs in the outer range of the sphere not yet known? So there are probably several hundred such stars still to be discovered?

  • Mike Serfas February 16, 2021, 14:04

    If you could get a colony started on Sedna, then with that sort of time frame to fiddle with the orbit they could hand off their whole planet to a passing star. Not that they would feel threatened by anything happening on Sol…

  • Astronist February 16, 2021, 15:29

    A slightly more recent SF reference is Fred and Geoffrey Hoyle’s “Fifth Planet” – one of my favourites!

    Responding to Alex Tolley: my own finding is that Shkadov thrusters would not be practical, since they would be slower, more dangerous, less versatile and altogether more expensive than building a fleet of conventional worldships. (http://www.astronist.co.uk/astro_ev/2020/ae156.shtml)

    • Alex Tolley February 16, 2021, 18:43

      Thanks for the reminder about Fifth Planet. I haven’t read that book in ages.

      While teh energy costs of moving a solar system are going to be huge compared to building worldships, there is the advantage that you know that the Earth will continue to support its biosphere with breakdown. Woldships may be neither reliable over the journey times, nor able to support the Earth’s various ecosystems. Moving the Earth in its entirety may be easier than the solar system, although now it will be subject to the limitations rocket equation.

      The greater problems may be what happens when the Earth population arrives. If it is a sterile planet, then the terrestrial biosphere will need to be built up slowly during teh terraforming process. That means that the terrestrial flora and fauna will be mostly kept on board the transport system until they can be transplanted. If teh world is living, it may either need to be sterilized as much as possible, or terrestrial life introduced in the hope of being able to coexist or outcompete the local life.

      If we can build worldships, and they are reliable, then maybe the best option is just to remain on them. If so, is there any reason at all to wait for close stellar encounters?

      If, OTOH, post-humanity is non-biological, then the longest time to travel to any star in a switched-off or unbuilt state is likely to be shorter than waiting for a close encounter, so why wait at all, especially as the possible target stars are far greater in number than the chance encounter with single stars over the next few million years. earth might remain a precious exemplar of evolved biological life in a galaxy mainly populated by artificial, machine life and civilizations.

      • Paul Gilster February 17, 2021, 7:13

        I’m with Alex on this. I had totally forgotten about Fifth Planet, though I’m a fan of Hoyle’s fiction. Thanks for the reminder, Astronist.

  • Armando Gascón February 16, 2021, 16:13

    Perhaps if SETI studies two stars very close to each other but not linked, they could find evidence of interstellar travel between them.
    It ought to be a priority, there can not be an enormous number of them near Sol.

    • ljk February 17, 2021, 12:58

      We should be searching globular star clusters as well as regions of space which are detectable in infrared but not in the optical range:


      SETI really needs to move beyond radio waves coming from star systems similar to our own. I am not saying we should stop searching for radio signals, just that we are going to have the best chances for now to find the Big Boys and Girls of the galaxy based on our limitations, which are substantial.

      Just as the first exoplanets we found were the monsters circling close to their stars, which most astronomers did not even suspect before 1995 because they didn’t match our Sol system. That should be a major hint right there.

  • Robert Flores February 16, 2021, 16:32

    I don’t think this mechanism for spurring interstellar travel applies in
    most system with potential ETI emergence.

    The conditions when is does apply
    1) Planets whose stars have a Short time as main sequence stars
    2) That is so short, that any planets in HZ are not affected by the
    cooling and solidifying of their cores.
    3) so only Cooler type A(A7-A9, and type F stars.

    In my View stellar lifetime is not the controlling
    factor of the time span of habitability in the vast majority stars and their HZ planets , core shutdown is.. Core shutdown results in no magnetic field, no recycling of materials and atmospheric gases, means very quick death to any proto-sentient species.

    • Ronald February 17, 2021, 7:04

      You may be right, though probably not for our sun and analogues, see other comments here: we may only have between 200 and 500 my left for higher life, before our sun becomes too bright.
      I have mentioned, under other posts here on CD, that the optimal star, i.e. with maximum stable lifespan and at the same time bright enough to prevent tidal locking in the HZ, is probably somewhere around late G, early K (G8 – K1). Such a star would allow a maximum of around 20 gy (16-25) of stable and continuous HZ time.
      In that case, as you state, planetary geological death would probably occur sooner for an earth-sized planet.
      What characteristics would a terrestrial planet need to have, to extend its geological lifespan to match its max. stellar HZ time and hence be an optimal planet? A slightly bigger planet? More thorium in the mantle?

      • Bruce D Mayfield February 18, 2021, 9:59

        Yes more thorium, as well as uranium in planetary interiors should be beneficial to long term habitability. The good news is that such very long lived radioactive elements keep being produced until a galaxy stops forming massive, short lived stars. The metal content of interstellar dust is still slowly increasing in galaxies like ours, so younger planets on average might have longer durability as far as internal circulation (magnetic field, tectonics etc.) is concerned.

        Supernovae are good, since we are looking here at very long time frames. (:

  • Geoffrey Hillend February 16, 2021, 17:33

    The Earth’s maximum habitability is only two hundred million years at the most for by three hundred million years Earth will look like Venus with a runaway greenhouse effect since the Sun’s brightness increase about ten percent every billion years.

    Also this paper does not consider scientific progress and innovation which are driven by the creative mind or the sudden intuitive flashes of insight of the scientific intuition. I like doing thought experiments with today’s technology, but I have to think that in one million years interstellar travel will be easy with a FTL warp drive, so we probably should be able to go anywhere in our galaxy at around ten million years. We will have a lot of time to make improve and refine FTL with more energy efficiency and higher energy.

    • Ronald February 17, 2021, 6:48

      I agree, as I said in my other comment, stellar habitable lifespan can hardly be a bottleneck, technological civilization lifespan is.
      Having said that, I am mildly shocked by your estimate of 200 to 300 my left (for higher life I presume), I thought it was closer to 500 my, still only the time we have had since the Cambrian diversity explosion, sobering thought.
      Do you have a source for your estimate?

  • Robin Datta February 16, 2021, 17:58

    If we want to wait for the next stellar “bus”, the timeframes will be enough to have humans evolve beyond races to separate species. We’ve been around for just over 300,000 years. When we first arrived on the scene, around a half dozen species of our lineage or close to it were still around, including australopithecines, Homo erectus, Homo neanderthalis and Homo denisovanis. They’re all gone now.

    And taking into consideration that we are evolving faster than other sepcies (not surprising, since our civilized milieu is so alien to “nature”), there is an excellent chance that our descendants will be several lineages (species?) very substantially different from us both physically and mentally.

  • Eniac February 16, 2021, 19:32

    Great article, Paul! I also love the long time epic considerations, although I tend to think that we humans are far too impatient to wait for a close pass. We’ll find a way to make the passage much sooner than that!

    It is worth considering, though, that the constant motion of stars in the galaxy drastically shortens the time it takes for humanity to spread over the entire galaxy, similar to how a drop of milk can be distributed throughout your morning coffee much faster by stirring than by waiting for diffusion to do its work. This strengthens the implications of the Fermi paradox, considerably, I think, as it limits the time to full occupation to a few galactic revolutions, at most.

    • Harold Shaw February 17, 2021, 21:48

      Wouldn’t depending on or integrating close encounters slow down humanity’s or other people’s galactic expansion? Waiting for close encounters has to take longer than traveling at will. Fermi’s paradox can’t be ignored because it would only take a few million years of willful travel to saturate the galaxy. The Earth takes about 230 million years to circle the Milky Way.

      I don’t see how close encounters significantly strengthen Fermi’s paradox.

      • Eniac February 19, 2021, 0:09

        Spreading through willful travel is accelerated not by close encounters, but the opposite, the dispersion of stars over time. I think you underestimate the time it takes to spread in the static model, because I think you have to allow a few hundred years for colonies to grow to the point where they can and will do another hope to the next system. In the dynamic model, colonies will spread out from each other even while they don’t travel.

        • Alex Tolley February 19, 2021, 15:07

          I suspect it is not as clear-cut as you suggest. There is a trade-off of even getting to a star, how fast the ships can travel, as well as teh residency time of a colony before continuing travel. Seems like a good project to model.

          • Eniac February 19, 2021, 19:16

            That residency time is the key parameter, really, and I think it will generally far exceed the travel time, making the latter less relevant. It is really the time it takes to build an industrial base large enough to provide all the parts needed to build an interstellar seed ship. But the expansion will not be in a spherical front, rather it will be an increasingly extended swirl of some sort, due to the relative motion of the stars that sustain the settlements.

            • Alex Tolley February 20, 2021, 12:18

              Let’s run some numbers of stellar velocities:
              The alpha Centauri example => 10 km/s
              Gliese 710 => 15 km/s

              Assume starships can travel at 0.1c => 3E4 km/s.

              For the residency of a colony to benefit from stellar movement, the non-starfaring time of the colony needs to be less than 2000-3000 years. This seems to be an awfully long time, possibly even exceeding the duration of a technological civilization capable of interstellar travel. This also assumes that handy close stellar encounters occur on these time frames, which the 2 examples indicate they do not.

              I would draw these conclusions:

              1. If viable star travel is possible at fractional c, then unless the travel exhausts the colonists for thousands of years before the next expedition, galactic colonization is best performed directly, rather than waiting for stellar encounters. A complete colonization of the galaxy could be accomplished within the time frame of just the near approach of Gliese 710.

              If however, star travel cannot be accomplished without a close encounter (i.e. a “Worlds in Collision” type encounter) more like the Gliese 710 encounter, then there should be a few scattered species well separated in space after just a few successful close encounters with star hopping. These species may be well separated in space but no longer advanced technological species. They may even have evolved such that only their base biology shows that they are from common ancestry. Depending on the number of galactic civilizations that emerged and were lucky to be able to star hop at least once during their technological peak, we should expect to find a binomial distribution of species or archaeological signs of such species on multiple worlds separated in space.

              LJK’s suggestion that globular clusters are favorable for interstellar travel indicates that only there will multi-system species show up due to the proximity of the stars.

  • Harold Shaw February 17, 2021, 0:09

    These close passing star systems pose a threat, possibly demanding a mitigation strategy. We, or any other people, would have to harden our space faring capabilities in anticipation. This could include preemptively sweeping up comets and asteroids, sheltering habitats or making them more mobile. We would need to maintain a rapid response system for centuries.

    The cost of mitigating the threat could exhaust the resources available for space fairing. However, I think it is significant the temptation of a close encounter is likely paired with unavoidable responsibility. A people may not spread but will likely take steps into space.

    This is a rich seed for fiction. A system could bring a flood and an ark or bring two people to war. Wormholes could keep people connected as their home systems drifted beyond the range of ships.

    Close encounters set the lowest cost and technological sophistication for migration. We could calculate a galactic saturation rate proportional to people survival rate and demand for room to spread. If we depend only on close encounters perhaps we need high assumptions for both to reach Fermi significant saturation.

    Close encounters would benefit planet natural people much more than hypothetical space habitat people or machine embodied people. Planet natural people would be the least fit for space travel. Beyond a sophistication threshold, both space faring people can function at the average distance between systems, close encounters aren’t essential.

    Close encounters would have to increase the odds of panspermia.

  • Alex Tolley February 17, 2021, 0:11

    The movie version of Aniara has the Mars-bound spaceliner get lost in space with all its passengers. At the end of the film, set over 5 million years later, the now long-dead ship enters a solar system in the constellation Lyra.

  • Adam Crowl February 17, 2021, 7:31

    Contra Geoffrey Hillend’s estimate, the end of Life-As-We-Know-it was pushed back to 0.9-1.5 gigayears from now by Caldeira & Kasting in 1992. This estimate has barely changed since – compare this 2020 estimate.

    And possibly even further ahead in time if the atmospheric pressure declines, rotation slows or some other change in the usual assumptions. The biggest unknown is Earth’s geodynamics – we don’t yet know what killed Venus as current modeling suggests it should be habitable today, if not for the planetary resurfacing Event. If Venus could be killed so dramatically, so could Earth…

    • Ronald February 17, 2021, 12:32

      Adam, your 2nd link is the same as the 1st one, to the ’92 paper.
      And scientific views on this matter differ:
      O’Malley-James et al. (2012, 2014), Swansong Biospheres, part 1 and 2, do place it at 500 to 600 my for most life on earth.
      Heath, Doyle (2009), Circumstellar Habitable Zones to Ecodynamic Domains, place it at about 800 my at the latest for nearly all photosynthetic life.
      Some life may persist until 1.2 or 1.3 gy, but not most (higher) life, if I understood the authors well.
      And in addition, because of ocean water either evaporating or escaping into the mantle, plate tectonics will be impaired.

    • xcalibur February 17, 2021, 20:19

      But we do know how Venus became uninhabitable. Its slow rotation rate means that its core dynamo never activated, so it never developed a significant magnetosphere; this in turn meant that it lacked protection from solar wind, which broke down h20 and stripped hydrogen away, leaving the remaining oxygen to combine with carbon, forming co2. That process eventually caused a runaway greenhouse effect, resulting in the planetary oven that Venus now is. This all goes back to the point that a magnetosphere is essential for habitability.

      • Adam Crowl February 17, 2021, 22:52

        I’m not convinced it’s that effective. Venus’s present day ionospheric protects it against almost all the Solar wind. Same as Mars. The role of the magnetosphere is oversold IMO. The empirical data says the Solar Wind (now) isn’t very effective at removing gas.

        • xcalibur February 19, 2021, 20:50

          But it is important. Of course, now that Venus has a massive atmosphere primarily composed of a stable compound (CO2) it’s resistant to solar wind. In the distant past, it was otherwise — Venus was a tropical planet with water on the surface, until the process I described reached a tipping point, unleashing a runaway greenhouse effect and creating a hellscape.
          Mars, too, was once more habitable, with liquid water on its surface, until it fell victim to solar wind, albeit in a different way. The Martian dynamo shut down, not because of slow rotation, but probably due to small size and lower gravity. Its atmosphere was then gradually stripped away, due to both its lack of an intrinsic magnetosphere and lower gravity, a process that continues to the present. Given that Mars’ current atmosphere is very thin, one would expect ‘diminishing returns’ from solar wind ablation. Thus, the effect of solar wind may not seem dramatic now, but over deep time, it ‘anti-terraformed’ Venus and Mars, transforming them from hospitable planets to an oven and cold desert, respectively.
          So yes, the presence of a core dynamo and magnetosphere are crucial to habitability. This also means that the lack thereof makes terraforming very difficult, with uncertain returns, unless the effect can be replicated somehow (and the engineering of which would be beyond our current capabilities). This is another reason why I think that in the future, life off-earth will be dominated by space stations with centrifugal force at 1g, with most activity on other planets in our system confined to research/mining (exoplanets may be another story!).

  • Henry Cordova February 18, 2021, 0:01

    How easily we allow ourselves to fall into the trap of seeing our own technological and organizational/cultural accomplishments become as fundamental and long-lived as biological, geological, or even astronomical processes.

    For most of our history on this planet, our knowledge of our physical universe and our resultant technology has advanced at truly glacial rates, and only by fits and starts. Only in the last few centuries has the growth become asymptotic, or in the contemporary parlance, “exponential”. We blithely talk about galactic travel, moving planets, and even entire solar systems, around the galaxy as if these capabilities are just around the corner. May there not be physical, engineering, or human obstacles to this manifest destiny?

    No, no, no. On the average, when considered over truly human time scales, technological innovation is a slow process. True, our technological explosion since the Enlightenment has been truly remarkable; but can we really think this pace will continue indefinitely? And that rate of increase will continue to increase without limit as well? Forever? Matrioshka Brains and Dyson Spheres may NOT be right around the corner. We have limits, both physical ones imposed by the laws of nature, and mental ones determined by our experience in the trees and later in the savanna. We simply haven’t been around long enough to be able to predict our future history with any certainty. Especially when our past history has revealed us to be short-sighted and stupid on truly cosmological scales.

    Are we familiar with any biological or social process that grows “exponentially” forever? Plot any human activity as a function of time and you get either a monotonous stability, an alarming periodicity, or an explosive growth followed by total collapse: bacteria in a Petrie dish.

    Oh I don’t imply that, properly managed, human technological development couldn’t be harnessed for truly long-term projects. But our psychological and cultural history suggests our management skills are not up to the challenge of controlling our technological development for truly significant (biological, geological, astronomical) periods of time. Our oldest and most stable civilization (like the Chinese and Egyptian) lasted at best several thousands of years, but their cultural stability may have been purchased by a loss of innovation and technical creativity. We aren’t ready to make a jump 0f several orders of magnitude. And if we do, its going to require a form of social tyranny and psychological regimentation that’s going to make the Emperors and Pharaohs of old carefree hippies by comparison.

    A quickly evolving technical civilization will quickly overrun its raw materials, its Lebensraum, its energy sources and its social stability. And no, we won’t just go mine the asteroids to make up the difference, not overnight, anyway.

    Maybe some hive entity around a distant sun can pull it off, but I doubt genus homo, or his silicon surrogates will be able to get away with it. And even if we can, do we REALLY want to live like that?

  • torque_xtr February 18, 2021, 1:26

    The Rare Earth hypothesis sonds less and less convincing. At least it’s parts considering going interstellar, as we can imagine many settings which are actually much more suitable to development of spacefaring civilization than our own. Wide binaries and dense stellar environments which promote interstellar flight. Tightly packed habitable worlds where there would be the exact opposite of both Mars and Venus Disappointments. Late K and early M stars, where strong gravitational lensing begins less than a hundred local AUs from the star and you could look at exoplanets from your system’s backyard. All these things combined, surely the case somewhere in the galaxy, given vast number of stars. There are many worlds compared to which our Solar System looks like desolated outback in the middle of nowhere. And the life development, too. On a 3 Me-super-Earth with a dense atmosphere and no climate-destabilizing glaciers, with their precarious ice-albedo feedback, all these talks about the precious magnetic fields and axis-stabilizing moons convince no one. Only, you need nuclear rockets to launch satellites. But does that matter when the crescents of nearby worlds beckon from nightskies, with exactly the same biosignatures in their spectra as your own world should show from space?

  • Al Jackson February 19, 2021, 12:03

    All this reminds me of Niven’s Puppeteers moving their planetary system from close to the Galactic center because of impending deadly astrophysics there. This was chronicled in “Fleet of Worlds” series (clever title).
    In recent times there has been the Benford and Niven Bowl of Heaven Series , Bowl of Heaven, Shipstar and Glorious with advanced civilization constructs bigger than Ringworld. Shkadov propulsion is featured not sure that has been done in SF before?

  • Alex Tolley February 19, 2021, 15:37

    For the patient among us, it will move in about 1.3 million years to within 14,000 AU, placing it well within the Oort Cloud and making it an obvious candidate for worst cometary orbit disruptor of all time.

    I would think that if Oort cloud disruption was a problem for Earth, we would have seen evidence for it in the fossil record. I am not aware of any, although there is speculation that a couple of craters could be comet rather than asteroid impacts. It should also show up on the Moon. I would have thought lunar craters caused by comet impacts would leave little foreign material evidence, like iron or carbon, as the bulk volatile material of the comet would have dispersed. A survey of craters by orbiters and surface examination would tell us of any like periods of extensive comet impacts (but this is guesswork on my part).

    Avi Loeb has recently suggested that the KT event was caused by a comet fragment, which might suggest that other fragments hit the moon at around the same time. Could that be established? While I am skeptical of this theory given the evidence we do have of the KT impactor, it would strengthen the idea that close encounters would cause terrestrial disruption sufficient to cause extinction events.

    • Michael Fidler February 19, 2021, 22:50

      This is one very good reason to return to the moon, most impactors evidence has been destroyed on earth; 70% ocean impacts plus weather and plate tectonics destroying it on the land. I have a hunch that if the Apollo Moon program was not curtailed by the Nixon administration and the full extent of President Kennedy plans where followed that this would be the main reason that a lunar base would have been built. I would suspect that even the Russians would have been involved with a cooperative program using their N1 super booster. The early sixties had studies about asteroid impacts by the then think tanks, and the vision of a large scale master plan was lost with the Kennedy assignations…

      • Alex Tolley February 20, 2021, 11:52

        Season 2 of “For All Mankind” has started, an alt-hist of the space program. At least there are a lot more astronauts on the Moon to do some science.

        As you suggest, the Moon is a preserved repository of solar system events sitting right on our doorstep within easy reach. What discoveries await us with much more extensive exploration.

    • Michael Fidler February 19, 2021, 23:07

      Alvarez hypothesis. Wiki

      Main article: Timeline of Cretaceous-Paleogene extinction event research.
      “In 1980, a team of researchers led by Nobel prize-winning physicist Luis Alvarez, his son, geologist Walter Alvarez, and chemists Frank Asaro and Helen Vaughn Michel discovered that sedimentary layers found all over the world at the Cretaceous–Paleogene boundary (K–Pg boundary, formerly called Cretaceous–Tertiary or K–T boundary) contain a concentration of iridium hundreds of times greater than normal.[4]

      Previously, in a 1953 publication, geologists Allan O. Kelly and Frank Dachille analyzed global geological evidence suggesting that one or more giant asteroids impacted the Earth, causing an angular shift in its axis, global floods, fire, atmospheric occlusion, and the extinction of the dinosaurs.[5][6] There were other earlier speculations on the possibility of an impact event, but without strong confirming evidence.[7]”

      Timeline of Cretaceous–Paleogene extinction event research.

      Petroleos Mexicanos, also known as PEMEX, discovered an unusual subsurface circular structure in the Yucatan Peninsula of Mexico.[24]

      E. Stechow proposed that the extinction of the dinosaurs may be attributable to solar flares that destroyed the ozone layer, allowing ultraviolet radiation to shower the planet.[21]

      M. W. de Laubenfels hypothesized that at the end of the Cretaceous, a bolide entered Earth’s atmosphere, “[f]lash heating” it and incinerating the dinosaurs.[25]

      PEMEX began drilling into the unusual ring-like structure under the Yucatan and extracting rock cores in search of oil.[24]


      How much did the oil industry know then and how much do they know now because of information from drilling and seismic soundings? There best interest is to keep this data private due to competition from competitors.

    • Bruce D Mayfield February 19, 2021, 23:11

      The “smoking gun” indicating that the KT extinction event was from a massive asteroid and not form a comet is the presence of Iridium in the KT boundary layer. Iridium is about the densest/heaviest element there is, so it would be one of the least likely elements to have been brought by comet impact.

      • Michael Fidler February 20, 2021, 10:08

        Now just a minute, what happened to Uranium? Comets have to form around something and that something will be the denser Iron/Nickel rocks in space with Iridium in them. Large Comets will have a core and Iridium, it is just rare on earth because the aliens mined it all! ;-}

  • Alex Tolley February 20, 2021, 11:47

    Absolutely. This is why I am skeptical of Loeb’s latest controversial theory. Briefly, Loeb’s logic seems to be:

    1. The frequency of large bodies capable of creating the Chicxulub crater is too low to account for its geologically relative recent creation.
    2. We need a mechanism to create more larger bodies of the needed size.
    3. Comets can break up near perihelion.
    4. Therefore comets provide the mechanism to create the needed fragment frequency.
    5. The crater was a comet fragment.