by Paul Gilster | Feb 16, 2024 | Sail Concepts |
How long before we can send humans to another star system? Ask people active in the interstellar community and you’ll get answers ranging from ‘at least a century’ to ‘never.’ I’m inclined toward a view nudging into the ‘never’ camp but not quite getting there. In other words, I think the advantages of highly intelligent instrumented payloads will always be apparent for missions of this duration, but I know human nature well enough to believe that somehow, sometime, a few hardy adventurers will find a way to make the journey. I do doubt that it will ever become commonplace.
You may well disagree, and I hope you’re right, as the scenarios open to humans with a galaxy stuffed with planets to experience are stunning. Having come into the field steeped in the papers and books of Robert Forward, I’ve always been partial to sail technologies and love the brazen, crazy extrapolation of Forward’s “Flight of the Dragonfly,” which appeared in Analog in 1982 and which would later be turned into the novel Rocheworld (Baen, 1990). This is the novel where Forward not only finds a bizarre way to keep a human crew sane through a multi-decade journey but also posits a segmented lightsail to get the crew home.
Image: The extraordinary Robert Forward, whose first edition of Flight of the Dragonfly was expanded a bit from the magazine serial and offered in book form in 1984. The book would later be revised and expanded further into the 1990 Baen title Rocheworld. The publishing history of this volume is almost as complex as the methods Forward used to get his crew back from Barnard’s Star!
Forward was a treasure. Like Freeman Dyson, his imagination was boundless. Whether we would ever choose to build the vast Fresnel lens he posited in the outer Solar System as a way of collimating a laser beam from near-Sol orbit, and whether we could ever use that beam to reflect off detached segments of the sail upon arrival to slow it down are matters that challenge all boundaries of engineering. I can hear Forward chuckling. Here’s the basic idea, as drawn from his original paper on the concept.
Image: Forward’s separable sail concept used for deceleration, from his paper “Roundtrip Interstellar Travel Using Laser-Pushed Lightsails,” Journal of Spacecraft and Rockets 21 (1984), pp. 187-195. The ‘paralens’ in the image is a huge Fresnel lens made of concentric rings of lightweight, transparent material, with free space between the rings and spars to hold the vast structure together, all of this located between the orbits of Saturn and Uranus. Study the diagram and you’ll see that the sail has three ring segments, each of them separating to provide a separate source of braking or acceleration for the arrival, respectively, and departure of the crew. Imagine the laser targeting this would require. Credit: Robert Forward.
I tend to think that Les Johnson is right about sails as they fit into the interstellar picture. In a recent interview with a publication called The National, Johnson (NASA MSFC) made the case that we might well reach another star with a sail driven by a laser. Breakthrough Starshot, indeed, continues to study exactly that concept, using a robotic payload miniaturized for the journey and sent in swarms of relatively small sails driven by an Earth-based laser. But when it comes to human missions to even nearby stars, Johnson is more circumspect. Let me quote him on this from the article:
“As for humans, that’s a lot more complicated because it takes a lot of mass to keep a group of humans alive for a decade-to centuries-long space journey and that means a massive ship. For a human crewed ship, we will need fusion propulsion at a minimum and antimatter as the ideal. While we know these are physically possible, the technology level needed for interstellar travel seems very far away – perhaps 100 to 200 years in the future.”
Johnson’s background in sail technologies for both near and deep space at Marshall Space Flight Center is extensive. Indeed, there was a time when his business card described him as ‘Manager of Interstellar Propulsion Technology Research’ (he once told me it was “the coolest business card ever”). He has also authored (with Gregory Matloff and Giovanni Vulpetti), books like Solar Sails: A Novel Approach to Interstellar Travel (Springer, 2014) and A Traveler’s Guide to the Stars (Princeton University Press, 2022), as well as editing the recent Interstellar Travel: Purpose and Motivations (Elsevier, 2023). In addition to that, his science fiction novels have explored numerous deep space scenarios.
Image: NASA’s Les Johnson, a prolific author and specialist in sail technologies. Credit: NASA.
So there’s a much more optimistic take on the human interstellar guideline than the one I gave in my first paragraph, and of course I hope it’s on target. We’re probably not going to be going to what is sometimes called an “Earth 2.0,” in Johnson’s view, because he doubts there are any such reasonably close to us. That’s something we’ll be learning a great deal more about as future space instrumentation comes online, but we can bear in mind that the explorers who tackled the Pacific in the great era of sail didn’t set out thinking they were going to find another Europe, either. The point is to explore and to learn what you can, with all the unexpected benefits that brings.
Johnson’s early interest in sails, by the way, was fired not so much by Forward’s Rocheworld as by Larry Niven and Jerry Pournelle’s novel The Mote in God’s Eye (Simon & Schuster, 1974), where an incoming laser sail from another civilization is detected. The realization that unusual astronomical observations point to a technology, and a laser-beaming one at that, is an exciting part of the book. Here the authors’ human starship crew describes the detection of a strange light emanating from a smaller star (the Mote) in front of a much larger supergiant (the Eye):
“…I checked with Commander Sinclair. He says his grandfather told him the Mote was once brighter than Murcheson’s Eye, and bright green. And the way Gavin’s describing that holo – well, sir, stars don’t radiate all one color. So -”
“All the more reason to think the holo was retouched. But it is funny, with that intruder coming straight out of the Mote…”
“Light,” Potter said firmly.
“Light sail!” Rod shouted in sudden realization…”
For more on all this, see my Our View of a Decelerating Magsail in these pages. It’s not surprising that Niven and Pournelle ran their lightsail concept past Robert Forward at a time when the idea was just gaining traction. We all have career-changing literary experiences. I can remember how a childhood reading of Poul Anderson’s The Enemy Stars (J.B. Lippincott, 1959) utterly fired my imagination toward the idea of leaving the Solar System entirely. It was a finalist for the Hugo Award that year following serialization in Astounding, though I didn’t encounter it until later.
Johnson’s work at Marshall Space Flight Center takes in the deployment of a large solar sail quadrant for the Solar Cruiser mission that was first unfurled in 2022 to demonstrate TRL 5 capability, and has just been deployed at contractor Redwire Corp.’s facility in Longmont, Colorado to demonstrate TRL 6. In NASA’s terms, that means going from “Component or breadboard validation in relevant environment” (TRL 5) to “System or subsystem model or prototype demonstration in a relevant environment (ground or space).” In other words, this is progress. At TRL 6, a system is considered “a fully functional prototype or representational model.” Says Johnson in a recent email:
“25 years ago, when I first met Dr. Forward, he inspired me to plan a development program for solar sails that would eventually lead us to the stars. With Bob’s help, I laid out a milestone driven roadmap that began with the space flight of a 10 m² solar sail, which we did in 2010 with NanoSail-D.
“Next on the plan was the development of something an order of magnitude larger. This was achieved with the development and launch of the 86 m² Near Earth Asteroid Scout solar sail in 2022 and the soon to be launched ACS-3 sail. The Solar Cruiser sail is an order of magnitude larger still at 1653 Square meters. The next step is 10,000!”
Image: NASA and industry partners used two 100-foot lightweight composite booms to unfurl the 4,300-square-foot sail quadrant for the first time Oct. 13, 2022, at Marshall Space Flight Center, making it the largest solar sail quadrant ever deployed at the time. On Jan. 30, 2024, NASA cleared a key technology milestone, demonstrating TRL6 capability at Redwire’s new facility in Longmont, Colorado, with the successful deployment of one of four identical solar sail quadrants. Credit: NASA (although I’ve edited the caption slightly to reflect the TRL level reached).
Solar sails are becoming viable choices for space missions, and the Breakthrough Starshot investigations remind us that sails driven not just by sunlight but by lasers are within the bounds of physics. A key question that will be informed by our experience with solar sails is how laser-driven techniques scale. Theoretically, they seem to scale quite well. Are the huge structures Forward once wrote about remotely feasible (perhaps via nanotech construction methods), or is Johnson right that fusion and one day antimatter may be necessary for craft large enough (and fast enough) to carry human crews?
by Paul Gilster | Feb 2, 2024 | Astrobiology and SETI |
We live in a world that is increasingly at ease with the concept of intelligent extraterrestrial life. The evidence for this is all around us, but I’ll cite what Louis Friedman says in his new book Alone But Not Lonely: Exploring for Extraterrestrial Life (University of Arizona Press, 2023). When it polled in the United States on the question in 2020, CBS News found that fully two-thirds of the citizenry believe not only that life exists on other planets, but that it is intelligent. That this number is surging is shown by the fact that in polling 10 years ago, the result was below 50 percent.
Friedman travels enough that I’ll take him at his word that this sentiment is shared globally, although the poll was US-only. I’ll also agree that there is a certain optimism that influences this belief. In my experience, people want a universe filled with civilizations. They do not want to contemplate the loneliness of a cosmos where there is no one else to talk to, much less one where valuable lessons about how a society survives cannot be learned because there are no other beings to teach us. Popular culture takes many angles into ETI ranging from alien invasion to benevolent galactic clubs, but on the whole people seem unafraid of learning who aliens actually are.
Image: Louis Friedman, Co-Founder and Executive Director Emeritus, The Planetary Society. Credit: Caltech.
The silence of the universe in terms of intelligent signals is thus disappointing. That’s certainly my sentiment. I wrote my first article on SETI back in the early 1980s for The Review of International Broadcasting, rather confident that by the end of the 20th Century we would have more than one signal to decipher from another civilization. Today, each new report from our active SETI efforts at various wavelengths and in varying modes creates a sense of wonder that a galaxy as vast as ours has yet to reveal a single extraterrestrial.
It’s interesting to see how Friedman approaches the Drake equation, which calculates the number of civilizations that should be out there by setting values on factors like star and planet formation and the fraction of life-bearing planets where life emerges. I won’t go through the equation in detail here, as we’ve done that many times on Centauri Dreams. It’s sufficient to note that when Friedman addresses Drake, he cites the estimates for each factor in the current scientific literature and also gives a column with his own guess as to what each of these items might be.
Image: This is Table 1 from Friedman’s book. Credit: Louis Friedman / University of Arizona Press.
This gets intriguing. Friedman comes up with 1.08 civilizations in the Milky Way – that would be us. But he also makes the point that if we just take the first four terms in the Drake equation and multiply them by the time that Earth life has been in existence, we get on the order of two billion planets that should have extraterrestrial life. Thus a point of view I find consistent with my own evolving idea on the matter: Life is all over the place, but intelligent life is vanishingly rare.
Along the way Friedman dismisses the ‘cosmic zoo’ hypothesis that we looked at recently as being perhaps the only realistic way to support the idea that intelligent life proliferates in the Milky Way. Ian Crawford and Dirk Schulze-Makuch see a lot wrong with the zoo hypothesis as well, but argue that the idea we are being observed but not interacted with is stronger than any other explanation for what David Brin and others have called ‘the Great Silence.’ I’ll direct you to Milan M. Ćirković’s The Great Silence: Science and Philosophy of Fermi’s Paradox for a rich explanation both cultural and scientific of our response to the ‘Where are they?’ question.
Before reading Alone But Not Lonely, my own thinking about extraterrestrial intelligence has increasingly focused on deep time. It’s impossible to run through even a cursory study of Earth’s geological history without realizing how tiny a slice our own species inhabits. The awe induced by these numbers tends to put a chill up the spine. The ‘snowball Earth’ episode seems to have lasted, for example, about 85 million years in its entirety. Even if we break it into two periods (accounting for the most severe conditions and excluding periods of lesser ice penetration), we still get two individual eras of global glaciation, each lasting ten million years.
These are matters that are still in vigorous debate among scientists, of course, so I don’t lean too heavily on the precise numbers. The point is simply to cast something as evidently evanescent as our human culture against the inexorable backdrop of geological time. And to contrast even that with a galaxy that is over 13 billion years old, where processes like these presumably occurred in multitudes of stellar systems. What are the odds that, if intelligence is rare, two civilizations would emerge at the same time and live long enough to become aware of each other? And does the lack of hard evidence for extraterrestrial civilizations not make this point emphatic?
But let me quote Friedman on this:
Let’s return to that huge difference between the time scales associated with the start of life on Earth and its evolution to intelligence. The former number was 3.5 to 3.8 billion years ago, a “mere” 0.75 to 1 billion years after Earth formed. Is that just a happenstance, or is that typical of planets everywhere? I noted earlier that intelligence (including the creation of technology) has only been around for 1/2,000,000 of that time—just the last couple thousand years. Life has been on Earth for about 85 percent of its existence; intelligence has been on Earth for about 0.0005 percent of that time. Optimists might want to argue that intelligence is only at its beginning, and after a million years or so those numbers will drastically change, perhaps with intelligence occupying a greater portion of Earth’s history. But that is a lot of optimism, especially in the absence of any other evidence about intelligence in the universe.
Friedman argues that the very fact we can envision numerous ways for humanity to end – nuclear war, runaway climate effects, deadly pandemics – points to how likely such an outcome is. It’s a good point, for technology may well contain within its nature the seeds of its own destruction. What scientists like Frank Tipler and Michael Hart began pointing out decades ago is that it only takes one civilization to overcome such factors and populate the galaxy, but that means we should be seeing some evidence of this. SETI continues the search as it should and we fine-tune our methods of detecting objects like Dyson spheres, but shouldn’t we be seeing something by now?
The reason for the ‘but not lonely’ clause in Friedman’s title is that ongoing research is making it clear how vast a canvas we have to analyze for life in all its guises. Thus the image below, which I swipe from the book because it’s a NASA image in the public domain. What I find supremely exciting when looking at an actual image of an exoplanet is that this has been taken by our latest telescope, which is itself in a line of technological evolution leading to completely feasible designs that will one day be able to sample the atmospheres of nearby exoplanets to search for biosignatures.
Image: This image shows the exoplanet HIP 65426 b in different bands of infrared light, as seen from the James Webb Space Telescope: purple shows the NIRCam instrument’s view at 3.00 microns, blue shows the NIRCam instrument’s view at 4.44 microns, yellow shows the MIRI instrument’s view at 11.4 microns, and red shows the MIRI instrument’s view at 15.5 microns. These images look different because of the ways that the different Webb instruments capture light. A set of masks within each instrument, called a coronagraph, blocks out the host star’s light so that the planet can be seen. The small white star in each image marks the location of the host star HIP 65426, which has been subtracted using the coronagraphs and image processing. The bar shapes in the NIRCam images are artifacts of the telescope’s optics, not objects in the scene. Credit: NASA, ESA, CSA, Alyssa Pagan (STScI).
Bear in mind the author’s background. He is of course a co-founder (with Carl Sagan and Bruce Murray) of The Planetary Society. At the Jet Propulsion Laboratory in the 1970s, Friedman was not only involved in missions ranging from Voyager to Magellan, but was part of the audacious design of a solar ‘heliogyro’ that was proposed as a solution for reaching Halley’s Comet. That particular sail proved to be what he now calls ‘a bridge too far,’ in that it was enormous (fifteen kilometers in diameter) and well beyond our capabilities in manufacture, packaging and deployment at the time, but the concept led him to a short book on solar sails and has now taken him all the way into the current JPL effort (led by Slava Turyshev) to place a payload at the solar gravitational lens distance from the Sun. Doing this would allow extraordinary magnifications and data return from exoplanets we may or may not one day visit.
Friedman is of the belief that interstellar flight is simply too daunting to be a path forward for human crews, noting instead the power of unmanned payloads, an idea that fits with his current work with Breakthrough Starshot. I won’t go into all the reasons for his pessimism on this – as the book makes clear, he’s well aware of all the concepts that have been floated to make fast interstellar travel possible, but skeptical they can be adapted for humans. Rather than Star Trek, he thinks in terms of robotic exploration. And even there, the idea of a flyby does not satisfy, even if it demonstrates that some kind of interstellar payload can be delivered. What he’s angling for beyond physical payloads is a virtual (VR) model in which AI techniques like tensor holography can be wrapped around data to construct 3D holograms that can be explored immersively even if remotely. Thus the beauty of the SGL mission:
We can get data using Nature’s telescope, the solar gravity lens, to image exoplanets identified from Earth-based and Earth-orbit telescopes as the most promising to harbor life. It also would use modern information technology to create immersive and participatory methods for scientists to explore the data—with the same definition of exploration I used at the beginning of this book: an opportunity for adventure and discovery. The ability to observe multiple interesting exoplanets for long times, with high-resolution imaging and spectroscopy with one hundred billion times magnification, and then immerse oneself in those observations is “real” exploration. VR with real data should allow us to use all our senses to experience the conditions on exoplanets—maybe not instantly, but a lot more quickly than we could ever get to one.
The idea of loneliness being liberating, which Friedman draws from E. O. Wilson, is a statement that a galaxy in which intelligence is rare is also one which is entirely open to our examination, one which in our uniqueness we have an obligation to explore. He lists factors such as interplanetary smallsats and advanced sail technologies as critical for a mission to the solar gravitational lens, not to mention the deconvolution of images that such a mission would require, though he only hints at what I consider the most innovative of the Turyshev team’s proposals, that of creating ‘self-assembling’ payloads through smallsat rendezvous en-route. In any case, all of these are incremental steps forward, each yielding new scientific discoveries from entirely plausible hardware.
Such virtual exploration does not, of course, rule out SETI itself, including the search for other forms of technosignature than radio or optical emissions. Even if intelligence ultimately tends toward machine incarnation, evidence for its existence might well turn up in the work of a mission to the gravitational lens. So I don’t think a SETI optimist will find much to argue with in this book, because its author makes clear how willing he is to continue to learn from the universe even when it challenges his own conceptions.
Or let’s put that another way. Let’s think as Friedman does of a program of exploration that stretches out for centuries, with not one but numerous missions exploring through ever refined technologies the images that the bending of spacetime near the Sun creates. We keep hunting, in other words, for both life and intelligence, for we know that the cosmos seems to have embedded within it the factor of surprise. A statement sometimes attributed to Asimov comes to mind: “The most exciting phrase to hear in science, the one that heralds new discoveries, is not “Eureka!” (I found it!) but “That’s funny…” The history of astronomy is replete with such moments. There will be more.
The book is Friedman, Alone but Not Lonely: Exploring for Extraterrestrial Life, University of Arizona Press, 2023.
by Paul Gilster | Jan 30, 2024 | Astrobiology and SETI |
Globular clusters, those vast ‘cities of stars’ that orbit our galaxy, get a certain amount of traction in SETI circles because of their age, dating back as they do to the earliest days of the Milky Way. But as Henry Cordova explains below, they’re a less promising target in many ways than the younger, looser open clusters which are often home to star formation. Because it turns out that there are a number of open clusters that likewise show considerable age. A Centauri Dreams regular, Henry is a retired map maker and geographer now living in southeastern Florida and an active amateur astronomer. Here he surveys the landscape and points to reasons why older open clusters are possible homes to life and technologies. Yet they’ve received relatively short shrift in the literature exploring SETI possibilities. Is it time for a new look at open clusters?
by Henry Cordova
If you’re looking for signs of extra-terrestrial intelligence in the cosmos, whether it be radio signals or optical beacons or technological residues, doesn’t it make sense to observe an area of sky where large numbers of potential candidates (particularly stars) are concentrated? Galaxies, of course, are large concentrations of stars, but they are so remote that it is doubtful we would be able to detect any artifacts at those distances. Star clusters are concentrations of stars gathered together in a small area of the celestial sphere easily within the field of view of a telescope or radio antenna. These objects also have the advantage that all their members are at the same distance, and of the same age,
Ask any amateur astronomer; “How many kinds of star cluster are there?” and he will answer; “Two, Open Clusters (OCs) and Globular Clusters (GCs)”. The terms “Globular” and “Open” refer to both their general morphology as well as their appearance through the eyepiece. It’s important to keep in mind that both are collections of stars presumably born at the same time and place (and hence, from the same material) but they are nevertheless very different kinds of objects. There does not seem to be a clearly defined transitional or intermediate state between the two. One type does not evolve into the other. Incidentally, the term ‘Galactic Cluster’ is often encountered when researching this field. It is an obsolete term for an OC and should be abandoned. It is too easily misunderstood as meaning a ‘cluster of galaxies’ and can lead to confusion.
GCs are in fact globular. They are collections of thousands, if not hundreds of thousands, of stars forming spheroidal aggregates much more densely packed towards their centers. OCs are amorphous and irregular in shape, random clumps of several hundred to several thousand stars resembling clouds of buckshot flying through space. Their distribution throughout the galaxy is different as well. GCs orbit the galactic center in highly elliptical orbits scattered randomly through space. They are, for the most part, located at great distances from us. OCs, on the other hand, appear to be restricted to mostly circular orbits in the plane of the Milky Way. Due to the obscuring effects of interstellar dust in the plane of the galaxy, most are seen relatively near Earth. although they are scattered liberally throughout the spiral arms.
Image: The NASA/ESA Hubble Space Telescope has captured the best ever image of the globular cluster Messier 15, a gathering of very old stars that orbits the center of the Milky Way. This glittering cluster contains over 100 000 stars, and could also hide a rare type of black hole at its center. The cluster is located some 35 000 light-years away in the constellation of Pegasus (The Winged Horse). It is one of the oldest globular clusters known, with an age of around 12 billion years. Very hot blue stars and cooler golden stars are seen swarming together in this image, becoming more concentrated towards the cluster’s bright center. Messier 15 is also one of the densest globular clusters known, with most of its mass concentrated at its core. Credit: NASA, ESA.
Studies of both types of clusters in nearby galaxies confirm these patterns are general, not a consequence of our Milky Way’s history and architecture, but a feature of galactic structure everywhere. Other galaxies are surrounded by clouds of GCs, and swarms of OCs circle the disks of nearby spirals. It appears that the Milky Way hosts several hundred GCs and several thousand OCs. It is now clear that not only is the distribution and morphology of star clusters divided into two distinct classes but their populations are as well. OCs are often associated with clouds of gas and dust, and are sometimes active regions of star formation. Their stellar populations are often dominated by massive bright, hot stars evolving rapidly to an early death. GCs, on the other hand, are relatively dust and gas free, and the stars there are mostly fainter and cooler, but long-lived. Any massive stars in GCs evolved into supernovae, planetary nebulae or white dwarfs long ago.
It appears that the globulars are very old. They were created during the earliest stages of the galaxy’s evolution. Conditions must have been very different back then; indeed, globulars may be almost as old as the universe itself. GC stars formed during a time when the interstellar medium was predominantly hydrogen and helium and their spectra now reveal large concentrations of heavy elements (“metals”, in astrophysical jargon). The metals have been carried up from the stellar cores by convective processes late in the stars’ life. Any planets formed around this early generation of stars would likely be gas giants, composed primarily of H and He—not the rocky Earth-type worlds we tend to associate with life.
Open Clusters, on the other hand, are relatively new objects. Many of them we can see are still in the process of formation, condensing from molecular clouds well enriched by metals from previous cycles of nucleogenesis and star formation. These clouds have been seeded by supernovae, solar winds and planetary nebulae with fusion products so that subsequent generations of stars will have the higher elements to incorporate in their own retinue of planets.
Image; Some of our galaxy’s most massive, luminous stars burn 8,000 light-years away in the open cluster Trumpler 14. Credit: NASA, ESA, and J. Maíz Apellániz (Institute of Astrophysics of Andalusia, Spain); Acknowledgment: N. Smith (University of Arizona).
Older OCs may have broken up due to galactic tidal stresses but new ones seem to be forming all the time, and there appears to be sufficient material in the galactic plane to ensure a continuous supply of new OCs for the foreseeable future. In general, GCs are extremely old and stable, but not chemically enriched enough to be suitable for life. OCs are young, several million years old, and they usually don’t survive long for life to evolve there. Any intelligent life would probably evolve after the cluster broke up and its stars dispersed. BUT…there are exceptions.
The most important parameter that determines a star’s history is its initial mass. All stars start off as gravitationally collapsing masses of gas, glowing from the release of gravitational potential energy. Eventually, temperatures and pressures in the stars’ cores rise to the point where nuclear fusion reactions start producing light and heat. This energy counteracts gravity and the star settles down to a long period of stability, the main sequence. The terminology arises from a line of stars in the color-magnitude diagram of a star cluster. Main sequence stars stay on this line until they run out of fuel and wander off the main sequence.
All stars follow the same evolutionary pattern, but where on the main sequence they wind up, and how long they stay there, depend on their initial mass. Massive stars evolve quickly, lighter ones tend to stay on the main sequence a long time. Our Sun has been a main sequence star for about 4.6 billion years, and it will remain on the main sequence for about another 5 billion years. When it runs out of nuclear fuel it will wander off the main sequence, getting brighter and cooler as it evolves.
All stars evolve in a similar way, but the amount of time they spend in that stable main sequence state is highly dependent on their mass at birth. Studying the point on the color-magnitude diagram of a cluster’s main sequence where stars start to “peel-off” from the MS allows astrophysicists to determine the age of the cluster. It is not necessary to know the absolute brightness, or distance, of the stars since, by definition, all the stars in a cluster are at the same distance. The color-magnitude (or Hertzprung-Russell) diagram is as important to astronomy as the periodic table is to chemistry. It allows us to visualize stellar evolution using a simple graphic model to interpret the data. It is one of the triumphs of 20th century science.
It is this ability to determine the age of a cluster that allows us to select a set of OCs that meet the criterion of great age needed for biological evolution to take place. Although open clusters tend to quickly lose their stars through gravitational interactions with molecular clouds in the disc of the galaxy, a surprising number seem to have survived long enough for biological, and possibly technologically advanced, species to evolve. Although less massive stars, such as main sequence red dwarfs, tend to be preferentially ejected from OCs due to gravitational tides, more massive F, G, and K stars are more likely to remain.
Sky Catalog 2000.0 (1) lists 32 OCs of ages greater than 1.0 Gyr. A more up-to-date reference, the Wikipedia entry (2), lists others. No doubt, a thorough search of the literature will reveal still more. A few of these OCs are comparable in age to the globulars. They are relics of an ancient time. But many others are comparable to our Sun in age (indeed, our own star, like many others, was born in an open cluster).
Regardless of the observing technique or wavelength utilized, an OC provides the opportunity to examine a large number of stars simultaneously, stars which have been pre-selected as being of a suitable age to support life or a technically advanced civilization. It will also be assured that, as members of an OC, all the stars sampled were formed in a metal-rich environment, and that any planets formed about those stars may be rocky or otherwise Earthlike.
If a technical civilization has arisen on any of those stars, it is possible that they have explored or colonized other stars in the cluster and we have the opportunity to eavesdrop on intra-cluster communications. And from the purely practical point of view, when acquiring scarce funding or telescope time for such a project, it will be possible to piggy-back a SETI program onto non-SETI cluster research. Other than SETI, there are very good reasons to study OCs. They provide a useful laboratory for investigations into stellar evolution.
References
1) Sky Catalog 2000.0, Vol II, Sky Publishing Corp, 1985.
2) https://en.wikipedia.org/wiki/List_of_open_clusters
Suggestions for Additional reading
1. H. Cordova, The SETI Potential of Open Star Clusters, SETIQuest, Vol I No 4, 1995
2. R. De La Fuente Marcos, C. De La Fuente Marcos, SETI in Star Clusters: A Theoretical Approach, Astrophysics and Space Science 284: 1087-1096, 2003
3. M.C. Turnbull, J.C. Tarter, Target Selection for SETI II: Tycho-2 Dwarfs, Old Open Clusters, And the Nearest 100 Stars, ApJ Supp. Series 149: 423-436, 2003
by Paul Gilster | Jan 10, 2024 | Astrobiology and SETI |
Speculating about the diffusion of intelligent species through the galaxy, as we’ve been doing these past few posts, is always jarring. I go back to the concept of ‘deep time,’ which is forced on us when we confront years in their billions. I can’t speak for anyone else, but for me thinking on this level is closer to mathematics than philosophy. I can accept a number like 13.4 × 10⁹ years (the estimate for the age of globular cluster NGC 6397 and a pointer to the Milky Way’s age) without truly comprehending how vast it is. As biological beings, a century pushes us to the limit. What exactly is an aeon?
NGC 6397 and other globular clusters are relevant because these ancient stellar metropolises are the oldest large-scale populations in the Milky Way. But I’m reminded that even talking about the Milky Way can peg me as insufferably parochial. David Kipping takes me entirely out of this comparatively ‘short-term’ mindset by pushing the limits of chronological speculation into a future so remote that elementary particles themselves have begun to break down. Not only that – the Columbia University astrophysicist finds a way for human intelligence to witness this.
You absolutely have to see how he does this in Outlasting the Universe, a presentation on his Cool Worlds YouTube channel. Now Cool Worlds is a regular stop here because Kipping is a natural at rendering high-level science into thoughtful explanations that even the mathematically challenged like me can understand. Outlasting the Universe begins with Kipping the narrator saying “We are in what you would call the future…the deep future” and takes human evolution through the end of its biological era and into a computer-borne existence in which a consciousness can long outlive a galaxy.
Image: Astrophysicist, author and indeed philosopher David Kipping. Credit: Columbia University.
Along the way we remember (and visit in simulation) Freeman Dyson, who once speculated that to become (almost) immortal, a culture could slow down the perceived rate of time. “Like Zeno’s arrow,” says Kipping, “we keep dialing down the speed.” The visuals here are cannily chosen, the script crisp and elegant, imbued with the ‘sense of wonder’ that brought so many of us to science fiction. Outlasting the Universe is indeed science fiction of the ‘hard SF’ variety as Kipping draws out the consequences of deep time and human consciousness in ways that make raw physics ravishing. I envy this man’s students.
With scenarios like this to play with, where do we stand with the ‘zoo hypothesis?’ It must, after all, reckon with years by the billions and the spread of intelligence. Science fiction writer James Cambias responded to my Life Elsewhere? Relaxing the Copernican Principle post with a tight analysis of the notion that we may be under observation from a civilization whose principles forbid contact with species they study. This is of course Star Trek’s Prime Directive exemplified (although the lineage of the hypothesis dates back decades), and it brings up Jim’s work because he has been so persistent a critic of the idea of shielding a population from ETI contact.
Jim’s doubts about the zoo hypothesis go back to his first novel. A Darkling Sea posits an Europa-like exoplanet being studied by a star-faring species called the Sholen, who are employing a hands-off policy toward local intelligence even as they demand that human scientists on the world’s sea bottom do the same. Not long after publication of the novel (Tor, 2014), he told John Scalzi that he saw Prime Directives and such as “ …a mix of outrageous arrogance and equally overblown self-loathing, a toxic brew masked by pure and noble rhetoric.” The arrogance comes from ignoring the desires of the species under study and denying them a choice in the matter.
In a current blog post called The Zoo Hypothesis: Objections, Jim lays this out in rousing fashion:
…we deduce that you can’t hide a star system which contains a civilization capable of large-scale interstellar operations, which the Zookeepers are by definition. They’re going to be emitting heat, EM radiation, laser light, all the spoor of a Kardashev Type I or higher civilization. And the farther away they are, the more they’re going to be emitting because they need to be bigger and more energy-rich in order to have greater reach.
This gives us one important lesson: if the goal of a Zoo is to keep the civilizations inside from even knowing of the existence of other civilizations, the whole thing is impossible. You can’t have a Zoo without Zookeepers, and the inhabitants of the Zoo will detect them.
Jim’s points are well-taken, and he extends the visibility issue by noting that we need to address time, which must be deep indeed. For a civilization maintaining all the apparatus of a protected area around a given star has to do so on time frames that are practically geological in length. Here we can argue a bit, for a ‘zoo’ set up for reasons we don’t understand in the first place might well come into existence only when the species being studied has reached the capability of detecting its observers.
I referenced Amri Wandel (Hebrew University of Jerusalem) on this the other day. Wandel argues that our own industrial lifespan is currently on the order of a few centuries, and who knows what level of technological sophistication a ‘zoo-keeping’ observer culture might want us to reach before it decides it can initiate contact? That would drop the geological timeframe down to a more manageable span, although the detectability problem still remains. So does the issue of interaction with other star-faring species who might conceivably need to be warned off entering the zoo. Cambias again:
If Captain Kirk or whoever shows up on your planet and says “I’m from another planet. Let’s talk and maybe exchange genetic material — or not, if you want me to leave just say so,” that’s an infinitely more reasonable and moral act than for Captain Kirk to sneak around watching you without revealing his own existence. The first is an interaction between equals, the second is the attitude of a scientist watching bacteria. Is that really a moral thing to do? Why does having cooler toys than someone else give you the right to treat them like bacteria?
This is lively stuff, and speculation of this order is why many people begin reading and writing science fiction in the first place. A hard SF writer, a ‘world builder,’ will make sure that he or she has thought through implications for every action he attributes not only to his characters but the non-human intelligences they may interact with. One thing that had never occurred to me was the issue of visibility when translated to the broader galaxy. Because a zoo needs to be clearly marked. Here’s Jim’s view:
If you’re going to exclude other civilizations from a particular region of the Galaxy, you have to let them know. Shooting relativistic projectiles or giant laser beams at incoming starships is a very ham-fisted way of communicating “keep out!” — and it runs the risk of convincing the grabby civilization that you’re shooting at to start shooting back. And if they’re grabby and control a lot of star systems, that’s going to be a lot of shooting.
Jim’s points are telling, and the comments on my recent Centauri Dreams posts also reflect readers’ issues with the zoo hypothesis. My partiality to it takes these issues into account. If the zoo hypothesis is the best of the solutions to the Fermi question, then the likelihood that other intelligent species are in our neighborhood is vanishingly small. Which lets me circle back to the paper by Ian Crawford and Dirk Schulze-Makuch that set off this entire discussion. It asked, you’ll recall, whether the zoo hypothesis wasn’t the last standing alternative to the idea that technological civilizations are, at the least, rare. It’s not a good alternative, but there it is.
In other words, I’d like the zoo hypothesis to have some traction, because it’s the only way I can find to imagine a galaxy in which intelligent civilizations are common.
Consider the thinking of Crawford and Schulze-Makuch on other hypotheses. Interstellar flight might be impossible for reasons of distance and energy, but this seems a non-starter given that we know of ways within known physics to send a payload to another star even in this century. A slow exploration front moving at Voyager speeds could do the trick in a fraction of the time available given the age of the Milky Way. The lack of SETI detections likewise points to technologies that are physically feasible (various kinds of technosignatures) but are not yet observed.
Is the answer that civilizations don’t live very long, and the chances of any two existing at the same brief time in the galaxy are remote? The nagging issue here is that we would have to assume that all civilizations are temporally limited. It takes only one to find a way through whatever ‘great filter’ is out there and survive into a star-faring maturity to get the galaxy effectively visited and perhaps colonized by now. Crawford and Schulze-Makuch reject models that result in volumes of the galactic disk being unvisited during the four billion years of Earth’s existence, considering them valid mathematically but implausible as solutions to the larger Fermi puzzle.
Many of the hypotheses to explain the Great Silence go even further into the unknowable. What, for example, do we make of attempts to parse out an alien psychology, which inevitably is seen, wittingly or not, as reflecting our own human instincts and passions? Monkish cultures that choose not to expand for philosophical reasons will remain unknowable to us, for example, as will societies that self-destruct before they achieve interstellar flight. We can still draw a few conclusions, though, as Crawford and Schulze-Makuch do, all pointing at least to intelligence being rare.
Although we know nothing of alien sociology, it seems inevitable that the propensity for self-destruction, interstellar colonization and so on must be governed by probability distributions of some kind. The greater the number of ETIs that have existed over the history of the Galaxy, the more populated will be the non-self-destructed and/or pro-colonization wings of these distributions, and it is these ETIs that we do not observe. On the other hand, if the numbers of ETIs have always been small, these distributions will have been sparsely populated and the non-observation of ETIs in their expansionist wings follows naturally.
Image: Are ancient ruins the only thing we may expect to find if we reach other star systems? Are civilizations always going to destroy themselves? The imposing remains of Angkor Wat. Credit: @viajerosaladeriva.
Likewise, we still face the problem that, as Stapledon long ago noted, different cultures will choose different priorities. Why assume that in a galaxy perhaps stuffed with aliens adopting Trappist-like vows of silence there will not be a few societies that do want to broadcast to the universe, a METI-prone minority perhaps, but observable in theory. We have no paradox in the Fermi question if we assume that aliens are rare, but if they are as common as early science fiction implied, the paradox is only reinforced.
So Crawford and Schulze-Makuch have boiled this down to the zoo hypothesis or nothing, with the strong implication that technological life must indeed be rare. I rather like my “one to ten” answer to the question of how many technological species are in the galaxy, because I think it squares with their conclusions. And while we can currently only speculate on reasons for this, it’s clear that we’re on a path to draw conclusions about the prevalence of abiogenesis probably in this century. How often technologies emerge after unicellular life covers a planet is a question that may have to wait for the detection of a technosignature. And as is all too clear, it’s possible this will never come.
The paper is Crawford & Schulze-Makuch, “Is the apparent absence of extraterrestrial technological civilizations down to the zoo hypothesis or nothing?” Published online in Nature Astronomy 28 December 2023 (abstract). James Cambias’ fine A Darkling Sea (Tor, 2014) is only the first of his novels, the most recent of which is The Scarab Mission (Baen, 2023), part of his ‘billion worlds’ series. Modesty almost, but not quite, forbids me from mentioning my essay “Ancient Ruins” which ran in Aeon a few years back.
by Paul Gilster | Jan 3, 2024 | Astrobiology and SETI |
Most people I know are enthusiastic about the idea that other intelligent races exist in the galaxy. Contact is assumed to be an inevitable and probably profoundly good thing, with the exchange of knowledge possibly leading to serious advances in our own culture. This can lead to a weighting of the discourse in favor of our not being alone. The ever popular Copernican principle swings in: We can’t be unique, can we? And thus every search that comes up empty is seen as an incentive to try still other searches.
I’m going to leave the METI controversy out of this, as it’s not my intent to question how we should handle actual contact with ETI. I want to step back further from the question. What should we do if we find no trace of extraterrestrials after not just decades but centuries? I have no particular favorite in this race. To me, a universe teeming with life is fascinating, but a universe in which we are alone is equally provocative. Louis Friedman’s new book Alone But Not Lonely (University of Arizona Press, 2023) gets into these questions, and I’ll have more to say about it soon.
I’ve thought for years that we’re likely to find the galaxy stuffed with living worlds, while the number of technological civilizations is tiny, somewhere between 1 and 10. The numbers are completely arbitrary and, frankly, a way I spur (outraged) discussion when I give talks on these matters. I’m struck by how many people simply demand a galaxy that is alive with intelligence. They want to hear ‘between 10,000 and a million civilizations,’ or something of that order. More power to them, but it’s striking that such a lively collection of technological races would not have become apparent by now. I realize that the search space is far vaster than our efforts so far, but still…
Image: The gorgeous M81, 12 million light years away in Ursa Major, and seen here in a composite Spitzer/Hubble/Galaxy Evolution Explorer view. Blue is ultraviolet light captured by the Galaxy Evolution Explorer; yellowish white is visible light seen by Hubble; and red is infrared light detected by Spitzer. The blue areas show the hottest, youngest stars, while the reddish-pink denotes lanes of dust that line the spiral arms. The orange center is made up of older stars. Should we assume there is life here? Intelligence? Credit: NASA/JPL.
So when Ian Crawford (Birkbeck, University of London) was kind enough to send me a copy of his most recent paper, written with Dirk Schulze-Makuch (Technische Universität Berlin), I was glad to see the focus on an answer to the Fermi question that resonates with me, the so-called ‘zoo hypothesis.’ A variety of proposed resolutions to the ‘where are they’ question exist, but this one is still my favorite, a way we can save all those teeming alien civilizations, and a sound reason for their non-appearance.
As far as I know, Olaf Stapledon first suggested that intelligent races might keep hands off civilizations while they observed them, in his ever compelling novel Star Maker (1937). But it appears that credit for the actual term ‘zoo hypothesis’ belongs to John Ball, in a 1973 paper in Icarus. From Ball’s abstract:
Extraterrestrial intelligent life may be almost ubiquitous. The apparent failure of such life to interact with us may be understood in terms of the hypothesis that they have set us aside as part of a wilderness area or zoo.
That’s comforting for those who want a galaxy stuffed with intelligence. I want to get into this paper in the next post, but for now, I want to note that Crawford and Schulze-Makuch remind us that what is usually styled the Fermi ‘paradox’ is in fact no paradox at all if intelligent races beyond our own do not exist. We have a paradox because we are uneasy with the idea that we are somehow special in being here. Yet a universe devoid of technologies other than ours will look pretty much like what we see.
The angst this provokes comes back to our comfort with the ‘Copernican principle,’ which is frequently cited, especially when we use it to validate what we want to find. Just as the Sun is not the center of the Solar System, so the Solar System is not the center of the galaxy, etc. We are, in other words, nothing special, which makes it more likely that there are other civilizations out there because we are here. If we can build radio telescopes and explore space, so can they, because by virtue of our very mediocrity, we represent what the universe doubtless continues to offer up.
But let’s consider some implications, because the Copernican principle doesn’t always work. It was Hermann Bondi, for example, who came up with the notion that we could apply the principle to the cosmos at large, noting that the universe was not only homogeneous but isotropic, and going on to add that it would show the exact same traits for any observer not just at any place but at any time. The collapse of the Steady State theory put an end to that speculation as we pondered an evolving universe where time’s vantage counted critically in terms of what we would see.
Our position in time matters. So, for that matter, does our position in the galaxy.
But physics seems to work no matter where we look, and the assumption of widespread physical principles is essential for us to do astronomy. So as generalizations go, this Copernican notion isn’t bad, and we’d better hang on to it. Kepler figured out that planetary orbits weren’t circular, and as Caleb Scharf points out in his book The Copernicus Complex: Our Cosmic Significance in a Universe of Planets and Probabilities (Farrar, Straus and Giroux, 2014), this was a real break from the immutable universe of Aristotle. So too was Newton’s realization that the Sun itself orbits around a variable point close to its surface and well offset from its core.
So even the Sun isn’t the center of the Solar System in any absolute sense. As we move from Ptolemy to Copernicus, from Tycho Brahe to Kepler, we see a continuing exploration that pushes humanity out of any special position and any fixed notions that are the result of our preconceptions. I think the problem comes when we make this movement a hard principle, when we say that no ‘special places’ can exist. We can’t assume from a facile Copernican model that each time we apply the principle of mediocrity, we’ve solved a mystery about things we haven’t yet proven.
Consider: We’ve learned how unusual our own Solar System appears to be; indeed, how unusual so many stellar systems are as they deviate hugely from any ‘model’ of system development that existed before we started actually finding exoplanets. This is why the first ‘hot Jupiters’ were such a surprise, completely unexpected to most astronomers.
Is the Sun really just another average star lost in the teeming billions that accompany it in its 236 million year orbit of the galaxy? There are many G-class stars, to be sure, but if we were orbiting a more average star, we would have a red dwarf in the sky. These account for 75 percent, and probably more, of the stars in the Milky Way. We’re not average on that score, not when G-class stars amount to a paltry 7 percent of the total. Better to say that we’re only average, or mediocre, up to a point. If we want to take this to its logical limit, we can back our view out to the scale of the cosmos. Says Scharf::
The fact that we are so manifestly located in a specific place in the universe — around a star, in an outer region of a galaxy, not isolated in the intergalactic void, and at just this time in cosmic history — is simply inconsistent with ‘perfect’ mediocrity.
And what about life itself? Let me quote Scharf again (italics mine). Here he works in the anthropic idea that our observations of the universe are not truly random but are demanded by the fact that the universe can produce life in the first place:
…a Copernican worldview at best suggests that the universe should be teeming with life like that on Earth, and at worst doesn’t really tell us one way or the other. The alternative — anthropic arguments — require only a single instance of life in the universe, which would be us. At best, some fine-tuning studies suggest that the universe could be marginally suitable for heavy-element-based-life-forms, rather than being especially fertile. Neither view reveals much about the actual abundance of life to be expected in our universe, or much about our own more parochial significance or insignificance.
So when we speculate about the Fermi question, we need to be frank about our assumptions and, indeed, our personal inclinations. If we relax our Copernican orthodoxy, we have to admit that because we are here does not demand that they are there. Let’s just keep accumulating data to begin answering these questions.
And as we’ll discuss in the next post, Crawford and Schulze-Makuch point out that we’re already entering the era when meaningful data about these questions can be gathered. One key issue is abiogenesis. How likely is life to emerge even under the best of conditions? We may have some hard answers within decades, and they may come from discoveries in our own system or in biosignatures from a distant exoplanet.
If abiogenesis turns out to be common (and I would bet good money that it is), we still have no knowledge of how often it evolves into technological societies. An Encyclopedia Galactica could still exist. Could John Ball be right that other civilizations may be ubiquitous, but hidden from us because we have been sequestered into ‘nature preserves’ or the like? Are we an example of Star Trek’s ‘Prime Directive’ at work? There are reasons to think that the zoo hypothesis, out of all the Fermi ‘solutions’ that have been suggested, may be the most likely answer to the ‘where are they’ question other than the stark view that the galaxy is devoid of other technological societies. We’ll examine Crawford and Schulze-Makuch’s view on this next time.
Caleb Scharf’s The Copernicus Complex: Our Cosmic Significance in a Universe of Planets and Probabilities is a superb read, highly recommended. The Ball paper is “The Zoo Hypothesis,” Icarus Volume 19, Issue 3 (July 1973), pp. 347-349 (abstract). The Crawford & Schulze-Makuch paper we’ll look at next time is “Is the apparent absence of extraterrestrial technological civilizations down to the zoo hypothesis or nothing?” Nature Astronomy 28 December, 2023 (abstract).
