Centauri Dreams
Imagining and Planning Interstellar Exploration
The Ones Who Look Outward
Thinking about a great filter through which a civilization must pass before its survival is assured usually leads to catastrophic scenarios, such as planetary suicide by nuclear war, or climate holocaust (assuming the filter lies ahead of us and not behind). But filters may be more subtle and tied in with epistemology. How does intelligence view its place in the universe and thereby engage with other beings? In today’s essay, Ian Brownlie looks at the matter from the standpoint of how knowledge is acquired and transmitted. Based in New Zealand (near Hawke’s Bay on the North Island), Ian is an Electrical Engineer who works with commissioning high voltage complex power systems. The survival and evolution of technology may be a harder step than we think.
by Ian Brownlie, BE(E&E)
Intelligence has appeared on Earth in many forms. Science-like inquiry has appeared more than once. But sustained, cumulative science appears to be different. It survives across generations, sharpens its own methods, and turns curiosity first into instruments, then into machines capable of leaving the world. So far as we know, that has happened only once. That asymmetry may matter. The night sky has always been there—indifferent and immense —but only one lineage seems to have kept turning that view into telescopes, rockets, and radiodishes. The outward gaze did not merely happen; it had to be sustained. And that fragile fact may tell us something unsettling about why the rest of the galaxy seems so quiet.
The Fermi paradox begins with a straightforward expectation: if life arises easily, and intelligence follows often enough, then the Milky Way should be full of civilizations that have spread, signalled, or at least left detectable traces. Yet we see nothing. The Great Filter hypothesis—a phrase the economist Robin Hanson coined in the 1990s—tries to explain this silence by proposing that somewhere along the path from chemistry to starships lies a step so improbable that almost nothing passes it.
Most discussions place that step either very early (life is rare) or very late (civilizations destroy themselves). But there is a transition hiding in plain sight—one usually folded into the broad category we call “intelligence.” It is not intelligence itself. It is not tool use, language, or even culture. Nor is it the first appearance of science-like inquiry. It is something stranger and more fragile: the sustained decision to look outward, to ask questions about things that may offer no immediate survival advantage, and to keep asking them long enough for the answers to accumulate into a durable scientific tradition.
This essay proposes that the successful preservation of outward-directed inquiry across generations—a self-sustaining relay that accumulates observations, models, instruments, and knowledge faster than they are lost —may be one of the rarest steps in the entire cosmic sequence.
The difference between intelligence and curiosity
Earth is full of cleverness. Corvids solve puzzles. Octopuses manipulate objects with eerie precision. Dolphins invent games. Intelligence, in the broad sense, has evolved repeatedly. But cumulative outward-directed inquiry—the kind that leads a species to build durable models of realities beyond its immediate niche—appears much rarer.
A raven can plan several steps ahead, but we have no evidence that it wonders what stars are. A dolphin can understand symbols, but it does not build instruments to examine worlds beyond its own. Intelligence, even sophisticated intelligence, is not enough.
The outward turn is a different kind of trait. It is the moment a lineage stops using the sky as a tool—for navigation, calendars, and omens—and begins interrogating it. It is the moment curiosity turns toward the remote and the abstract: costly, impractical, and directed at things that cannot feed you, shelter you, or help you survive.
Galileo’s telescope did not cause this shift. It revealed it. Grinding a lens to ask whether Jupiter has moons is not a survival behaviour. It is a declaration of intent: we want to know what is out there, even if knowing does nothing for us.
That kind of curiosity is not something natural selection can easily target directly. Evolution is myopic. It rewards traits that help organisms survive and reproduce in particular environments. Curiosity about distant objects with no obvious payoff is more likely to arise as a by-product of cognitive machinery built for mapping danger, opportunity, social life, and time. Such by-products need not be unique. But they are unreliable. They can appear without becoming central to a culture; they can flare without compounding.
A sample of one—and a pattern inside it
We have only one biosphere to study, and that makes every inference precarious. Still, within that single record, the pattern is suggestive. Across billions of species and several independent origins of complex cognition, we have evidence for many kinds of cleverness and for more than one human culture that practised early scientific or astronomical traditions. What appears singular is not inquiry itself, but an unbroken lineage of inquiry that became cumulative, instrument-building, self-correcting, and ultimately capable of leaving its planet.
That does not prove it is improbable. It might simply be suppressed: the first lineage to achieve it monopolizes the niche, preventing others from following. Or perhaps Earth has not had enough time for a second instance.
However, the cultural record tells a more nuanced story—one that shifts the argument from biology to history.
The spark that keeps failing to catch
Humanity did not look outward once. We looked outward many times—and each attempt depended on something more delicate than insight.
Long before formal science, people built temples aligned to the sky. Later, Babylonian astronomers tracked the heavens with astonishing precision. Greek thinkers built geometric models of the cosmos. Chinese imperial astronomers kept continuous records of comets and novae for centuries—though largely in service of the calendar and the court, with the sky read as an instrument of statecraft rather than interrogated for its own sake. Medieval Islamic scholars constructed observatories, critiqued inherited models, and developed mathematical tools that later reappeared in Europe.
These were not minor sparks. They were bright, sustained attempts to understand the sky. Nor were they sealed off from one another: knowledge moved through translation, trade, conquest, correspondence, and inheritance. The story of astronomy is not a set of isolated awakenings, but a long and uneven transmission.
That is the point. The outward turn is not a single step. It is a relay—a chain of observation, mathematics, instrument-making, patronage, teaching, criticism, and memory passed from one generation to the next. A chain can strengthen. It can also break.
Institutions collapse. Dogma intrudes. Political priorities shift. Economic margins shrink. Languages of scholarship are lost or become inaccessible. Instruments decay. A culture may look outward for a century and then turn inward again, leaving its questions unanswered, or answered only in forms that later generations cannot use.
The European scientific tradition of the 17th century was not the first outward turn. It was the one that became self-amplifying: mathematically precise, institutionally protected, technologically useful, and increasingly public. It ran long enough, and continuously enough, to accumulate the observational and technological scaffolding required to leave the planet.
This is where the argument touches decades of work on cumulative cultural evolution—the ratchet effect, a term the psychologist Michael Tomasello coined for the way human culture locks in improvements without slipping backward. Humans, unlike even our closest relatives, do not merely invent; we preserve, copy, correct, recombine, and lock in improvements so that each generation starts from the accumulated position of the last rather than from scratch. No other species on earth ratchets this way, which is why a raven’s cleverness never compounds and a physicist’s does—the capacity the anthropologist Joseph Henrich calls the secret of our success. Science is an extreme, formalized instance of that process: it requires not only minds capable of wonder, but also social systems capable of keeping wonder alive and passing it forward intact.
If this pattern is general, then the rarity may lie less in the spark than in the sustaining. Many worlds may produce sky-watchers. Some may even produce science. Fewer may produce scientific traditions that survive long enough to become physics, engineering, and eventually spaceflight.
The ladder that burns
If the outward turn is a relay rather than a single step, then the relay must survive not only ignorance, dogma, and institutional decay, but success itself. A sustained scientific tradition eventually produces technologies powerful enough to threaten the conditions that allowed that tradition to continue.
This is the ladder that burns. The ladder is not only fuel; it is the whole chain of conditions that lets curiosity become spaceflight—preserved knowledge, industrial capacity, and usable access to orbit, each of which must be sustained in turn. These are not separate filters, but successive tests of the same fragile achievement.
The first test is whether knowledge can survive long enough to become industry. Industrial civilization may require a burst of dense, easily exploited energy to extract materials before it has the wisdom to manage the consequences. On Earth, fossil carbon formed one rung of that ladder: a one-time inheritance accumulated over hundreds of millions of years. It is at least a live hypothesis that industrialization requires access to unusually concentrated energy resources before a civilization possesses the technology needed to exploit more durable alternatives. On Earth, that energy windfall opened a narrow window in which humanity bootstrapped itself from pre-industrial life to radio astronomy, rockets, and planetary-scale measurement.
But an energy rung can be consumed as it is climbed. The same energy source that accelerates industrial capacity can destabilize the climate, exhaust cheap reserves, intensify competition, or weaken the institutions that make cumulative science possible. The danger is that a civilization may acquire planetary power before it has built the habits of restraint, coordination, and long-range reasoning needed to preserve the relay that produced that power in the first place.
The second test is whether industry can survive long enough to support spaceflight. Practical early spaceflight requires concentrated energy: engines, propellants, exhaust, heat, discarded stages, and machines operating near failure. Each launch may leave artefacts behind. Satellites die. Upper stages fragment. Collisions create debris, and that debris raises the chance of still more collisions—the runaway cascade that Donald Kessler and Burton Cour-Palais modelled for NASA in 1978, now known as Kessler syndrome. A world can begin climbing toward space and, through the accumulated residue of its own first attempts, make access to orbit progressively more dangerous.
The third test is whether access to space can be sustained once it becomes possible. It is not enough to invent rockets; a species must preserve the relay long enough to improve them, the industry long enough to build them, and the orbital environment long enough to launch them at all. Each depends on the same cumulative capacities as before: memory, measurement, correction, restraint, and cooperation across generations. If low orbit becomes a minefield before a civilization learns those habits at planetary scale, the path outward may narrow just when it is first opening, as the energy window disappears.
In that sense, the outward turn may be essential not only for discovering the sky, but for surviving the consequences of having discovered it. The same habits that make astronomy possible—modelling unseen systems, trusting abstract evidence, coordinating across generations, and caring about consequences beyond the immediate horizon—are also the habits needed to survive industrial acceleration and keep space usable.
Humanity is inside this window now. We must keep the chain intact at all three levels: knowledge, industry, and orbit. The question is whether the outward gaze can become durable enough—not merely in imagination, but in institutions, energy systems, launch practices, and habits of care—to survive the phase in which space becomes a real destination.
What the silence might mean
The usual readings of the Fermi paradox are grim. Either life is rare, or intelligence destroys itself. But there is a third possibility—speculative, but consistent with the pattern we can actually inspect.
It might be objected — with David Brin’s Principle of Non-Exclusiveness, or what Jason Wright calls the monocultural fallacy — that a filter most civilizations fail explains nothing, since a single exception could colonize the galaxy alone. The objection is decisive against filters of choice, which assume every species settles on the same fate forever; uniform behaviour is not even universal among humans. But two things blunt it here. First, we have no demonstrated case of the turn being made and held — ourselves included, since we are still inside that test rather than through it — so the exception the objection requires may simply not yet exist. And second, more decisively, the window for spaceflight opened late: only after billions of years of stellar enrichment laid down the metals a technological species needs. Even a genuine exception would have to have arisen early enough in that window to have already crossed to us — and we may be not alone so much as early. The early always look out on a silent sky.
The Galaxy may be full of life. Full of intelligence. Full of brief outward glances, and perhaps even repeated beginnings of science. And quiet—because those beginnings almost never become durable civilizations of inquiry.
Civilizations may look outward for a while—a century, a millennium—and then turn inward again, or collapse, or lose the institutional continuity required to sustain the relay. Their astronomies may be bright but brief. Their questions may be asked but not accumulated. Their telescopes may be built but not improved.
From a distance, such civilizations would appear as little more than flickers: a thin smear of radio leakage, a brief industrial glow, and then silence. On that reading, we are not necessarily the one species that looked when others did not. We are simply the one spark, so far as we can see, that may be allowed to continue.
And the work of continuing it—of passing the question to the next mind before we go—may be the rarest and most fragile part of the entire cosmic sequence.
A closing thought
If the outward turn is a relay, not a step, then our task is not merely to keep looking. It is to keep the chain unbroken: the institutions that preserve knowledge, the habits that reward difficult questions, the energy transition that buys time, and the long view that makes any of it matter.
The spark may be common. The chain is not.
And right now, we may be running the stretch where the chain most often breaks.
A note on prior work
The argument of this essay draws on several established bodies of work, named here so that its debts are explicit and its own contribution is clear. The framing of the Great Silence in terms of a Great Filter is Robin Hanson’s (1998). The claim that human culture is distinguished by a cumulative “ratchet” that locks in improvements across generations, rather than slipping back, is Michael Tomasello’s; the broader case that this cumulative cultural transmission, and not raw individual intelligence, is the true source of human capability is Joseph Henrich’s. The runaway orbital-debris cascade invoked in the discussion of sustaining access to space is the syndrome modelled by Donald Kessler and Burton Cour-Palais for NASA in 1978.
What this essay adds to these is the proposal that the sustaining of outward-directed inquiry across generations — the unbroken relay, rather than the first spark — may itself be one of the rarest and most fragile steps in the cosmic sequence, and that the same habits which keep that relay intact are the ones a civilisation needs to survive the industrial and orbital hazards its own success creates.
References
Hanson, R. (1998). The Great Filter — Are We Almost Past It? Working paper, George Mason University.
Henrich, J. (2015). The Secret of Our Success: How Culture Is Driving Human Evolution, Domesticating Our Species, and Making Us Smarter. Princeton University Press.
Kessler, D. J. & Cour-Palais, B. G. (1978). Collision Frequency of Artificial Satellites: The Creation of a Debris Belt. Journal of Geophysical Research 83(A6), 2637–2646.
Tomasello, M. (1999). The Cultural Origins of Human Cognition. Harvard University Press.
Detection of a Habitable Zone Planet with an Atmosphere
If we couldn’t figure out what to call CD-35 2722B b, which I assume is the correct way to refer to the planet-sized object in this red dwarf / brown dwarf/ gas giant system, another planet has a bit of a definitional problem as well. LHS 1140 b is an interesting super-Earth orbiting a red dwarf in Cetus in its habitable zone. Some 49 light years out, this planet is almost 6 times Earth’s mass and boasts a radius 1.7 times larger. Receiving 42 percent of the stellar radiation that Earth does, its surface temperatures allow the presence of liquid water.
But exactly what kind of planet is this? Is it an airless, rocky world, an ocean planet, or even a mini-Neptune? We can probably rule out the latter because its mass would be low for that category, and we can also, contrary to some press reports, not consider it in any way, shape or form ‘Earth-like.’ But thanks to new work out of Harvard and the Carnegie Institution for Science, we can now declare that it does have an atmosphere.
This is a useful finding because rocky planet atmosphere detection has been conspicuously difficult. What you gain with studying M-dwarf planets is the low contrast between a relatively dim star and a planet’s light, but you’re also dealing with a category of star prone to violent flare activity. Every M-dwarf in the galaxy is a young star compared to its likely lifespan, given that these stars, perhaps 85 percent of the stellar population, have lifetimes that can reach well beyond a trillion years.
Pulling a signal from water or carbon dioxide out of spectrographic data is problematic because these volatiles species would be expected in the lower levels of a planet’s atmosphere, a tough catch for any observatory. But helium escape is another matter. In detecting helium at LHS 1140 b, the new work, demonstrates that an atmosphere is indeed present, whether this is a water world or not.
Young M-dwarfs are notorious for violent flare activity, a problem not only because flares can obscure the signal of a transiting planet, but also because they can potentially scrub nearby planets of their atmospheres entirely. In this case the flares have helped us: The escaping helium is thought to be driven by bombardment of the upper atmosphere from the star’s X-ray and extreme ultraviolet (XUV) activity.
Helium escape is especially valuable because it is observable from the ground, unlike the classic hydrogen (Lyman-alpha) escape signature. We’re seeing the residue of a process that can play out over gigayears: lighter hydrogen escapes preferentially over time — a loss we can’t easily observe directly — leaving behind a helium-enriched upper atmosphere that we can detect.
The paper may even tell us something about the ‘cosmic shoreline,’ the dividing line between an orbit where an atmosphere can be sustained and one where it cannot. Indeed, this work, conducted using the WINERED spectrograph at Las Campanas Observatory in Chile, shows that the inner world at LHS 1140 is evidently airless, with the ‘shoreline’ located between the two worlds.
From the paper:
The cosmic shoreline is a proposed boundary that separates airless rocky planets from those that retain atmospheres for billions of years. The two planets in the LHS 1140 system are on either side of the proposed cosmic shoreline. Our non-detection of helium absorption by LHS 1140 c is consistent with the previously measured dayside emission, which indicates that the planet has little to no atmosphere. Therefore, this system is consistent with the proposed position of the cosmic shoreline.
Moreover, we seem to be tracking a phenomenon that changes in short order. Co-author Shreyas Vissapragada (Carnegie Science Observatories) comments:
“After much careful analysis and consideration of the spectra, we determined that helium was escaping from LHS 1140 b’s atmosphere in 2024 due to heating from stellar X-rays and extreme ultraviolet radiation. However, our 2025 observations revealed no escaping helium, so the atmospheric escape appears to be variable. It is a rare privilege to witness the atmosphere of an extrasolar planet change on such short, human timescales!”
Image: In this artist’s rendering, the exoplanet LHS 1140 b is shown in the foreground, surrounded by a helium-rich atmosphere. Another nearby rocky planet orbits the same cool red dwarf star in the distance. This new study provides the strongest evidence yet that LHS 1140 b has retained an atmosphere. Credit: Carnegie Science.
Colin Cherubim (Harvard University), lead author of the paper in Science, points out that this is the first detection of an atmosphere on a rocky planet in the habitable zone of any star. Nice work, as Cherubim’s team had predicted precisely this mechanism in the LHS 1140 system. Escaping gases may indeed be a useful tool as we press on with more detailed investigations of planetary atmospheres on smaller, more Earth-like worlds.
The paper is Cherubim et al., “Helium escaping from the atmosphere of a nearby rocky exoplanet orbiting in a habitable zone,” Science 16 July 2026. Full text.
Moon or Planet? The Awkward Case of CD-35 2722B
Problems of definition will long be with us as we take ever closer looks at exoplanets. But they’re suddenly on everyone’s mind because of the detection of what some are calling an ‘exomoon’ in the system CD-35 2722, found in the constellation Columba. The primary in this system is an M-dwarf thought to be 50–200 million years old. I imagine there is no shortage of flare activity on this star, although the paper doesn’t get into that. The interesting finding in this work just published in Nature is expressed in its title: “Planetary-Mass Exosatellite Detected Around a Star’s Substellar Companion.”
Image: This illustration shows the system around the star CD-35 2722, with the newly found moon-like object at the centre. The star –– the point source to the left –– has about half the mass of our Sun, and it is orbited by a brown dwarf, the reddish-brown object seen here in the foreground (right). The brown dwarf has about 37 times the mass of Jupiter: too massive to be a planet, but not massive enough to have sustained nuclear fusion like stars. This brown dwarf is, in turn, orbited by a newly discovered object at least as massive as Jupiter, seen at the centre of this image. This new object, found with ESO’s Very Large Telescope (VLT), is difficult to label. It behaves like a moon in the sense that it orbits an object that orbits a star. But this ‘moon’ is massive enough to be a planet, and the object it orbits, a brown dwarf, is neither a planet nor a star. Credit: ESO.
So is this the first solid detection of a ‘exomoon’? I can’t describe it as that, and ‘exosatellite’ is an awkward coinage. What we have here is an M-dwarf orbited by a brown dwarf about 30 times the mass of Jupiter. It is the brown dwarf, not the star, that is being orbited by a third object, evidently a gas giant with a minimum mass of 0.743 Jupiter masses. How this arrangement drives orbital mechanics of any other objects in this system (none have yet been found) is well worth pondering. For now, we see again the problem of definition.
Kevin Hoy (Universidad Diego Portales, Chile) is lead author of the paper on this work:
“This system is somewhat hard to define using Solar-System-based words like ‘planet’ and ‘moon’. The exosatellite is clearly massive enough to be a planet, but it does not orbit a star, though it orbits an object that orbits a star. Being the third wheel in this system makes us want to call it a moon, even if it is nothing like the small, rocky moons we have in our system.”
I think this discovery is straightforward. The object around brown dwarf CD−35 2722B fully qualifies as a planet. Brown dwarfs cannot sustain stable hydrogen fusion in their core for any length of time, so that they’re basically ‘failed stars’ that are cooling down throughout their lifetimes. Even if we demand that the term ‘star’ be defined by hydrogen burning, then whatever category we create to include brown dwarfs is clearly one that can sustain planets around it.
I’m seeing a lot of chatter about this detection, but let’s leave ‘exomoon’ out of the discussion. Anyway, it’s also interesting to see that brown dwarf CD−35 2722B has been directly imaged, as reported in 2011 in The Astrophysical Journal (citation below). Directly imaged planets are still a rarity.
Image: The discovery image of CD-35 2722B, the brown dwarf in this intriguing system. Credit: Wahhaj et al. 2011, ApJ 729, 139.
So this is an intriguing find, and it also points to our continuing inability to locate what could indisputably be called an ‘exomoon,’ though the HD 206893 system continues to be interesting as a possibility for further research. Until we have a confirmed exomoon, though, this odd configuration will have to do. It reminds me as well that as our explorations continue, we still find how unusual our own Solar System is. Looking for commonality between different star systems, we find over and over again that any facile Copernican notion that we live in an ordinary stellar environment continues to be proven wrong.
Indeed, just what constitutes an ‘ordinary’ stellar system? The beauty of this work is that we seem to find surprises almost everywhere we turn.
The paper is Hoy et al., “Planetary-mass exosatellite detected around the substellar companion of a star,” Nature 16 July 2026. Full text. The discovery paper for CD-35 2722B is Wahhaj et al., “The Gemini NICI Planet-Finding Campaign: Discovery of a substellar L dwarf companion to the nearby young M dwarf CD−35 2722. The Astrophysical Journal 729(2), (2011), 139. Full text.
New Propulsion Systems for Deep Space Smallsats
Building the infrastructure we will need for interstellar exploration requires an imaginative look at today’s technologies as applied to distant targets. Indeed, we can leverage the scientific interest in, say, an orbit around each of the ice giants to explore launch capabilities through beamed energy, but we will also need to consider how we can use economical smallsats to provide stationkeeping nodes in such studies. Advances in miniaturization and the growing sophistication of CubeSats all point in the same direction. We can exploit our early flybys by fleshing out an observational smallsat matrix around targets at system’s edge to form a robust data and communications network.
The thing that is going to vitalize the development of small satellite packages is a new kind of propulsion system of the sort recently developed at MIT. The problem is easily stated: A small spacecraft – think suitcase size or less – has to maximize payload while ensuring flexibility in adjusting trajectory or tweaking orbital parameters. Chemical thrusters can give you rapid maneuvers and orbital insertion while electrical thrusters can produce low-thrust for long-haul cruise purposes or stationkeeping. Both systems are bulky, which has led to the kind of trade-off that is holding back smallsats for longer deep space missions.
The MIT work, discussed in a recent paper in the Journal of Propulsion and Power, describes a way to combine both types of thruster by extending existing ‘electrospray’ thrusters into the realm of chemical propulsion. Electrospray technology was used successfully on the LISA Pathfinder mission in 2015, which was essentially a demonstrator mission to study the detection of gravitational waves. The tiny thrusters could produce thrust in minute increments that allowed the spacecraft to maintain position without introducing vibrations. Observations that are ‘jitter-free’ become possible..
The basic electrostatic thruster can be the size of a small coin or a computer chip. An electric field acts on a propellant – a conductive liquid – which is being fed to each thruster, the latter sitting on top of a reservoir of the propellant.
Let’s pause for a moment on the conductive liquid, which the paper refers to as an ‘ionic liquid. ‘ I’m seeing these described as a liquid salt at room temperature. The liquid is made up of negative and positive ions without a neutral solvent, which makes the fluid electrically conductive. In other words, there is no need to ionize the propellant. The idea is simply to pull the ions out of the ionic liquid using an electrical field. A storage benefit also accrues: These liquids are non-evaporative and can be stored without pressurized tanks or seals. The electric field charges a specified amount of ions, which are then channeled out of the reservoir through the thruster tips as a spray.
Image: These four flight unit electrospray thrusters were delivered by MIT Space Propulsion Laboratory to NASA for the upcoming Green Propulsion Dual Mode (GPDM) mission. Insert: The emitter arrays. Image credit: Amelia Bruno.
So we know that electrospray thrusters work. Moreover, the method is scalable, allowing thrusters to work in tandem for larger missions. Being able to combine this technology with more powerful chemical methods dramatically extends the possibilities for missions to other planets without the thrust penalty of conventional electrical propulsion systems. Amelia Bruno (MIT Department of Aeronautics and Astronautics) is lead author of the new paper:
“If you can have chemical and electrical propulsion in one small package, it’s the best of both worlds. This opens the door for small satellites to do even more science, more observations, and more interesting missions, all on a smaller and cheaper platform.”
The step forward here is the discovery that a chemical propellant can work with the same system and deliver higher levels of thrust when needed. It’s two propulsion methods working off the same tank of propellant. MIT has introduced a ‘green’ monopropellant developed originally by the US Air Force for chemical propulsion. Called ASCENT (Advanced Spacecraft Energetic Non-Toxic), the ionic liquid propellant is free of the health hazards of hydrazine for those working with it – hence the ‘green’ in the above reference. In this case, green means lower overall costs with fewer environmental liabilities.
The flight demonstrator that grows out of this is called the Green Propulsion Dual Mode (GPDM) mission. Scheduled for launch in November of this year, the mission is to be the first in-space checkout of this form of propulsion, testing the switching from chemical combustion mode to electromagnetic acceleration mode. A single propellant tank will feed the chemical thruster as well as the array of four electrospray thrusters. A Falcon 9 launch vehicle will deploy the 6U CubeSat with the new propellant onboard as a secondary payload.
Recent tests have shown that electrospray thrusters fueled with ASCENT operate successfully over periods lasting up to 100 hours and although the propellant was originally designed for chemical propulsion, it turns out to be just as efficient as the various ionic liquids the team has experimented with in their electric thrusters. Thus a single tank of fuel aboard a CubeSat can be used to produce both chemical and electrical propulsion in a compact system. No previous satellite has ever been designed with a shared propellant tank.
Paulo Lozano is a professor of aeronautics and astronautics at MIT:
“We could send CubeSats to Mars, or the asteroid belt, where they could make the journey slowly, using electrospray thrusters. You could then use your chemical thrusters to quickly move to look at interesting features. You could have a lot more flexibility to do a lot more things.”
Marshall Space Flight Center leads the the Green Propulsion Dual Mode mission, with MIT supplying hardware and subsystems, along with Georgia Tech. The NASA Small Spacecraft & Distributed Systems (SSDS) program manages the GPDM project. Keep an eye on this program.
The paper is Bruno et al., “Performance Characterization of Electrospray Thrusters with Energetic Ionic Liquid Monopropellant,” Journal of Propulsion | Power published online 31 May 2026 (abstract). Also available is Tong et al., “Mission Architecture for the Green Propulsion Dual Mode Mission,” presented at the 38th Annual Small Satellite Conference and available here.
What the Comments Taught Me: A Reply on Self-Replicating Probes
Self-replicating probes continue to be a controversial subject, just as they were when Frank Tipler up the ante on Michael Hart by invoking them as a way of further tightening the tension of the Fermi Paradox. After all, Tipler had discovered an economic edge. Any civilization that wants to colonize the galaxy is going to expend vast resources, but if self-replication is available, that culture need only create the first probe, and let subsequent ones harvest resources as needed. Self-replication or not, the galaxy gets filled up in only a fraction of the current age of the Milky Way, but the economic stimulus provides yet another tightening of the Fermi knot. I hadn’t thought about all of this in connection with actual probe designs, but Peter Marinko’s article on the matter clearly touched a nerve, judging from the messages I’ve been getting about it. When Peter wrote recently with his thoughts on reader reactions, I asked him for permission to run it as a regular post rather than a comment, because I think this is a lively question and would like to see us continue to explore it.
by Peter Marinko
My previous post here, “A Metallurgist’s Doubts About Self-Replicating Probes,” argued that von Neumann probes are constrained less by physics than by process-chain closure and materials aging. The discussion that followed sharpened my thinking more than the original post did, and I want to begin by paying some of that debt.
On carbon: Adam Crowl supplied the numbers I should have had at hand: carbonaceous chondrites run some 3–5% organics, dormant comets are thought to be coated in hydrocarbon-rich asphalt, and Freitas’s study considered Titan — an organics-drenched world — alongside the Moon. Alex Tolley added the wider inventory: aromatics, CO, CO₂, CH₄, hydrogen-rich giant atmospheres, and tholins — the reddish-brown organic polymers, first named by Sagan, that form when ultraviolet light and charged particles work on simple molecules like methane and nitrogen, and that coat Titan, Pluto, and many cometary surfaces.
I accept the correction. Carbon is not scarce in the cosmos. But availability is not accessibility: 3–5% organics dispersed through a chondrite is a feedstock concentration problem, and concentration is exactly the step that has no gravity, no water, and no atmosphere to help it. What the correction really did was promote carbon from an afterthought to a criterion — as the reader will see below, three independent process chains now demand it.
On refractories: Alex Tolley and John both pointed to the same escape: induction heating with magnetic levitation, melting metal without touching a crucible at all. This is a genuinely good answer, and I concede it for the class of operations it covers — melting and casting nickel-iron, which asteroids supply free of charge. But it is not free, and the price is paid in the currency this study cares most about. A levitated melt has its entire surface exposed, and it radiates as T⁴. A crucible is not merely a container; it is insulation. Remove it and the induction coil must continuously replace radiative losses that a lined furnace would simply have prevented. In an exergy ledger, that is a permanent tax on every kilogram melted.
Nor does levitation cover reduction. Extracting metal from oxide requires a hot, chemically aggressive, contained environment, and containment is where linings live. Here I owe the discussion a nuance from my own field that I should have raised myself: melting is not the only route. Iron oxide can be reduced in the solid state, producing iron powder which is then pressed and sintered to finished shape — this is how Höganäs in Sweden has made metal powder for decades, and it is how tungsten-carbide drill inserts are produced. Powder metallurgy skips the melt entirely, which is a real advantage for a probe: no crucible, no tapping, no casting. But it does not escape the problem. Solid-state reduction needs long tunnel furnaces, a reducing atmosphere of hydrogen, carbon, or carbon monoxide, and sustained high temperature — and those furnaces need linings too. The refractory bootstrap survives every route I know how to draw.
Image: PG: I sometimes wonder what John von Neumann would say if he could see the length and depth of the debate over self-replicating interstellar probes. To my knowledge, he never considered self-replication in the context of star systems and certainly not colonizing an entire galaxy. We could use his insights today: What are we missing? Credit: Physics Today, although this old photo is widely available and I don’t know its origin.
Abelard Lindsey, writing from inside the industry, put the honest bound on it: difficult, but perhaps a decade or two of development. That is a fair estimate, and it belongs in the ledger as a research task rather than a wall.
On semiconductors: Lindsey and I are in violent agreement: this is the hard one, and he notes it must be solved on Earth as well before AI becomes cheap. “Building chips rather than carving them,” as he puts it, is the holy grail. I have no better idea, and I am not going to pretend otherwise.
On the mother-factory: Tolley’s most interesting move was to propose a way around closure rather than through it: a factory ship that travels star to star, spawning non-replicating probes from local resources, populating the galaxy more slowly but without ever needing full self-reproduction. I want to be clear that this is a good idea — and also that it concedes my central point. A mother-factory does not close the loop; it carries the fraction it cannot produce, and expands until that stock is exhausted. It converts an infinite-generation architecture into a finite one, and the number of generations it buys is precisely the vitamin inventory divided by the vitamins per copy. That is not a refutation of the closure argument. It is the closure argument, written as a mission design. The right question becomes: how many nodes does the stock buy? That number is computable, and Hephaistos is built to compute it.
On life: The deepest challenge came from Henry Cordova and Elisee Reclus, and it deserves more than a nod, because it attacks my framing rather than my arithmetic: robust self-replicating systems demonstrably exist, cover the Earth, and arose without design. If biology can do it, why not machines?
Here is my answer, and it is the reason I am not moving the goalposts. Life is not a counterexample to the closure problem; life is what closure looks like when you pay its actual price. A cell does not manufacture bearings to micron tolerance, does not need vacuum, does not require phase-pure silicon or reference metrology, and above all does not need to specify its output. It tolerates enormous error, discards most of its offspring, and lets selection curate the survivors — over billions of years, in a medium (liquid water, at moderate temperature, with an atmosphere and a gravity well) that supplies concentration and transport for free. Biology bought replication by abandoning precision, determinism, and speed, and by spending geological time as its currency.
A von Neumann probe cannot make that trade. It must arrive at a specified place, build a specified artifact to specified tolerances, and do so in decades. The moment we relax those requirements enough for a biological strategy to work — accept vast error, accept mostly-failed offspring, accept deep time — we no longer have an engineering project; we have seeded a biosphere and lost the ability to say what it will become. That may be a defensible thing to do. It is not the thing anyone is proposing when they invoke probes crossing the galaxy in a few hundred thousand years.
So I take the biological objection seriously, and my conclusion from it is not optimism but a sharper statement of the problem: self-replication is cheap if you can pay in error and time, and murderously expensive if you must pay in precision and schedule. The exergy ledger below is an attempt to price the second option honestly.
Where this goes next:
Skepticism is cheap. The honest next step, for a critic who spent a career in industrial process engineering, is to try to make the thing work — on paper, with real process chains and mass balances — and see exactly where it breaks.
That is what I am now attempting, in a study I am calling Project Hephaistos, after the god who forged automata for Olympus: a virtual self-replicating probe, audited line by line, where every assumption and every capitulation is logged in public. Two ledgers run through it. The Vitamin List records every component the probe cannot make for itself, with masses attached. The Exception Ledger records every problem I have deliberately set aside. I expect the second document to be the more valuable of the two.
In a future post I will set out the mission architecture, and the first trade study: whether it is better to send a probe slowly, with today’s technology, and let it fight fifty millennia of aging — or quickly, at a tenth of light speed, and let it fight the interstellar medium. Both, it turns out, are running the same race against the same opponent. Only the costume changes.
My thanks again to everyone who wrote in. Keep it coming — the ledger has room.
A Metallurgist’s Doubts About Self-Replicating Probes
Frank Tipler jolted the astrophysics community in 1980 when he introduced self-replicating interstellar probes into discussion of the Fermi Paradox. The mathematical model of self-replication came from John von Neumann, and was codified in 1966 (after von Neumann’s death) by Arthur Burks in Theory of Self-Reproducing Automata (1966). SF fans will also know of Fred Saberhagen’s berserker novels and short stories (the first appeared in 1963). I’ve found an even earlier SF reference but will leave that for a future post. Right now I want to introduce Peter Marinko, who today weighs in on self-replication and the problems therein. Based in Uppsala, Sweden Peter holds an M.Sc. in metallurgy and has a career background in industrial process engineering. He has studied SETI under Erik Zackrisson at Uppsala University, and his current work explores the thermodynamics of technological civilizations — including a manuscript on high-exergy technospheres and the longevity of detectable civilizations, currently under peer review at the International Journal of Astrobiology. A preprint is available on Zenodo.
by Peter Marinko
Discussions of von Neumann probes — here and elsewhere — tend to treat replication as a systems problem: the probe arrives, mines local material, and builds a copy of itself. The hard part is usually assumed to be propulsion, navigation, or AI. As someone who has spent a career in metallurgy and industrial process engineering, I would like to suggest that the hardest part is the one that gets a single sentence: “mines local material and builds a copy.”
Let me raise four concrete problem areas, in increasing order of difficulty.
1. Beneficiation without gravity, water, or atmosphere
“Asteroid mining” is a misleading phrase. Mining is the easy part; the problem is beneficiation — concentrating useful elements out of undifferentiated regolith. Every terrestrial concentration process relies on things an asteroid lacks: gravity-driven sedimentation, water-based flotation, density separation in fluids, atmospheric combustion. Electrostatic and magnetic separation in microgravity are conceivable in principle, but neither has been demonstrated at industrial scale, and both work poorly on the fine, cohesive, electrostatically charged dust that dominates regolith.
2. Reduction metallurgy without an industrial hinterland
All terrestrial metal production rests on an invisible foundation: carbon or hydrogen as reducing agents, fluxes, and — critically — refractory materials for the furnaces. Refractories are the forgotten enabling technology of civilization. A furnace lining must itself be manufactured, at high temperature, in a furnace. Bootstrapping this loop from raw regolith, with fully closed chemical cycles (no atmosphere to vent to, no water to waste), is a chicken-and-egg problem that no study I am aware of has worked through at the level of actual process flowsheets.
3. The closure problem, honestly accounted
The classic NASA study (Freitas et al., 1980) assumed ~90–96% “closure” — the fraction of its own components a system can reproduce — with the remainder supplied as “vitamins” from home. But the missing few percent are not marginal; they are precisely the hardest items: semiconductors, precision bearings, sensors, and insulation. Consider something as unglamorous as wire insulation. Virtually all electrical insulation on Earth is organic polymer, resting on a petrochemical industry, resting in turn on a biosphere that spent hundreds of millions of years concentrating carbon. Inorganic alternatives (glass fiber, ceramics, mica) exist but are brittle, heavy, and require entirely different process chains to apply to fine conductors. A modern semiconductor fab is arguably the most complex artifact humanity has built, drawing on tens of thousands of specialized inputs. Shrinking that into a 500 kg seed — or even Freitas’ original 100-ton seed — is not an engineering detail. It may be the entire problem.
4. Aging over interstellar timescales
Even a probe that could replicate must first arrive functional after a voyage of tens of thousands of years. We have essentially no empirical data on machine longevity beyond ~50 years (Voyager, surviving on redundancy and switched-off instruments). Over interstellar timescales, materials face cumulative radiation damage and lattice defects, embrittlement and transmutation; creep and solid-state diffusion (solder joints, thin films and interfaces are only kinetically frozen, not thermodynamically stable); tin and zinc whisker growth; outgassing and cold welding in vacuum. The repair systems age too. Replication must outrun degradation — and degradation never sleeps.
A thermodynamic framing
These four problems share a common structure. A self-replicating probe is, in effect, a miniaturized high-exergy technosphere that must rebuild its entire exergy cascade — from raw, unconcentrated feedstock to precision components — at every node, before its own irreversible degradation catches up. The feasibility question is then not “does physics forbid it? (it does not) but “can accessible exergy per node sustain full process closure faster than irreversible losses accumulate?
This is the same ratio, I would argue, that governs the longevity of detectable civilizations generally — a question I explore in a recent preprint on the thermodynamics of technological civilizations. But the probe case is a cleaner test, because the system boundary is sharp and the accounting is (in principle) tractable.
Questions for discussion
1. Has anyone attempted an actual process flowsheet — not a block diagram — for closing even a simple metallurgical loop (say, iron from chondritic material to finished machine parts) without terrestrial inputs?
2. Is there a credible inorganic-only pathway for electrical insulation and semiconductor packaging?
3. What is the realistic closure fraction if “vitamins” are disallowed — and does the seed mass then grow beyond anything launchable?
4. Are there materials strategies (amorphous metals? self-annealing designs?) that could plausibly survive 10,000+ years of transit?
My suspicion, as a practitioner, is that von Neumann probes are constrained not by the laws of physics but by process-chain closure and materials aging — both, at root, thermodynamic limits. If that is right, it bears directly on the Fermi paradox: the galaxy may be quiet not because nobody tried, but because replication is harder than arithmetic suggests.
I would be glad to be proven wrong on any specific point above — ideally with a flowsheet.
