Centauri Dreams — Imagining and Planning Interstellar Exploration View RSS

Building an Interstellar Philosophy 5:36 AM (6 hours ago)

As the AI surge continues, it’s natural to speculate on the broader effects of machine intelligence on deep space missions. Will interstellar flight ever involve human crews? The question is reasonable given the difficulties in propulsion and, just as challenging, closed loop life support that missions lasting for decades or longer naturally invoke. The idea of starfaring as the province of silicon astronauts already made a lot of sense. Thinkers like Martin Rees, after all, think non-biological life is the most likely intelligence we’re likely to find.

But is this really an either/or proposition? Perhaps not. We can reach the Kuiper Belt right now, though we lack the ability to send human crews there and will for some time. But I see no contradiction in the belief that steadily advancing expertise in spacefaring will eventually find us incorporating highly autonomous tools whose discoveries will enable and nurture human-crewed missions. In this thought, robots and artificial intelligence invariably are first into any new terrain, but perhaps with their help one day humans do get to Proxima Centauri.

An interesting article in the online journal NOĒMA prompts these reflections. Robin Wordsworth is a professor of Environmental Science and Engineering as well as Earth and Planetary Sciences at Harvard. His musings invariably bring to mind a wonderful conversation I had with NASA’s Adrian Hooke about twenty years ago at the Jet Propulsion Laboratory. We had been talking about the ISS and its insatiable appetite for funding, with Hooke pointing out that for a fraction of what we were spending on the space station, we could be putting orbiters around each planet and some of their moons.

Image credit: Manchu.

It’s hard to argue with the numbers, as Wordsworth points out that the ISS has so far cost many times more than Hubble or the James Webb Space Telescope. It is, in fact, the most expensive object ever constructed by human beings, amounting thus far to something in the range of $150 billion (the final cost of ITER, by contrast, is projected at a modest $24 billion). Hooke, an aerospace engineer, was co-founder of the Consultative Committee for Space Data Systems (CCSDS) and was deeply involved in the Apollo project. He wasn’t worried about sending humans into deep space but simply about maximizing what we were getting out of the dollars we did spend. Wordsworth differs.

In fact, sketching the linkages between technologies and the rest of the biosphere is what his essay is about. He sees a human future in space as essential. His perspective moves backward and forward in time and probes human growth as elemental to space exploration. He puts it this way:

Extending life beyond Earth will transform it, just as surely as it did in the distant past when plants first emerged on land. Along the way, we will need to overcome many technical challenges and balance growth and development with fair use of resources and environmental stewardship. But done properly, this process will reframe the search for life elsewhere and give us a deeper understanding of how to protect our own planet.

That’s a perspective I’ve rarely encountered at this level of intensity. A transformation achieved because we go off planet that reflects something as fundamental as the emergence of plants on land? We’re entering the domain of 19th Century philosophy here. There is precedent in, for example, the Cosmism created by Nikolai Fyodorov in the 19th Century, which saw interstellar flight as a simple necessity that would allow human immortality. Konstantin Tsiolkovsky embraced these ideas but welded them into a theosophy that saw human control over nature as an almost divine right. As Wordsworth notes, here the emphasis was entirely on humans and not any broader biosphere (and some of Tsiolkovsky’s writings on what humans should do to nature are unsettling}.

But getting large numbers of humans off planet is proving a lot harder than the optimists and dreamers imagined. The contrast between Gerard O’Neill’s orbiting arcologies and the ISS is only one way to make the point. As we’ve discussed here at various times, human experiments with closed loop biological systems have been plagued with problems. Wordsworth points to the concept of the ‘ecological footprint,’ which makes estimates of how much land is required to sustain a given number of human beings. The numbers are daunting:

Per-person ecological footprints vary widely according to income level and culture, but typical values in industrialized countries range from 3 to 10 hectares, or about 4 to 14 soccer fields. This dwarfs the area available per astronaut on the International Space Station, which has roughly the same internal volume as a Boeing 747. Incidentally, the total global human ecological footprint, according to the nonprofit Global Footprint Network, was estimated in 2014 to be about 1.7 times the Earth’s entire surface area — a succinct reminder that our current relationship with the rest of the biosphere is not sustainable.

As I interpret this essay, I’m hearing optimism that these challenges can be surmounted. Indeed, the degree to which our Solar System offers natural resources is astonishing, both in terms of bulk materials as well as energy. The trick is to maintain the human population exploiting these resources, and here the machines are far ahead of us. We can think of this not simply as turning space over to machinery but rather learning through machinery what we need to do to make a human presence there possible in longer timeframes.

As for biological folk like ourselves, moving human-sustaining environments into space for long-term occupation seems a distinct possibility, at least in the Solar System and perhaps farther. Wordsworth comments:

…the eventual extension of the entire biosphere beyond Earth, rather than either just robots or humans surrounded by mechanical life-support systems, seems like the most interesting and inspiring future possibility. Initially, this could take the form of enclosed habitats capable of supporting closed-loop ecosystems, on the moon, Mars or water-rich asteroids, in the mold of Biosphere 2. Habitats would be manufactured industrially or grown organically from locally available materials. Over time, technological advances and adaptation, whether natural or guided, would allow the spread of life to an increasingly wide range of locations in the solar system.

Creating machines that are capable of interstellar flight from propulsion to research at the target and data return to Earth pushes all our limits. While Wordsworth doesn’t address travel between stars, he does point out that the simplest bacterium is capable of growth. Not so the mechanical tools we are so far capable of constructing. A von Neumann probe is a hypothetical constructor that can make copies of itself, but it is far beyond our capabilities. The distance between that bacterium and current technologies, as embodied for example in our Mars rovers, is vast. But machine evolution surely moves to regeneration and self-assembly, and ultimately to internally guided self-improvement. Such ‘descendants’ challenge all our preconceptions.

What I see developing from this in interstellar terms is the eventual production of a star-voyaging craft that is completely autonomous, carrying our ‘descendants’ in the form of machine intellects to begin humanity’s expansion beyond our system. Here the cultural snag is the lack of vicarious identification. A good novel lets you see things through human eyes, the various characters serving as proxies for yourself. Our capacity for empathizing with the artilects we send to the stars is severely tested because they would be non-biological. Thus part of the necessary evolution of the starship involves making our payloads as close to human as possible, because an exploring species wants a stake in the game it has chosen to play.

We will need machine crewmembers so advanced that we have learned to accept their kind as a new species, a non-biological offshoot of our own. We’re going to learn whether empathy with such beings is possible. A sea-change in how we perceive robotics is inevitable if we want to push this paradigm out beyond the Solar System. In that sense, interstellar flight will demand an extension of moral philosophy as much as a series of engineering breakthroughs.

The October 27 issue of The New Yorker contains Adam Kirsch’s review of a new book on Immanuel Kant by Marcus Willaschek, considered a leading expert on Kant’s era and philosophy. Kant believed that humans were the only animals capable of free thought and hence free will. Kirsch adds this:

…the advance of A.I. technology may soon put an end to our species’ monopoly on mind. If computers can think, does that mean that they are also free moral agents, worthy of dignity and rights? Or does it mean, on the contrary, that human minds were never as free as Kant believed—that we are just biological machines that flatter ourselves by thinking we are something more? And if fundamental features of the world like time and space are creations of the human mind, as Kant argued, could artificial minds inhabit entirely different realities, built on different principles, that we will never fully understand?

My thought is that if Wordsworth is right that we are seeing a kind of co-evolution at work – human and machine evolution accelerated by expansion into this new environment – then our relationship with the silicon beings we need will demand acceptance of the fact that consciousness may never be fully measured. We have yet to arrive at an accepted understanding of what consciousness is. Most people I talk to see that as a barrier. I’m going to see it as a challenge, because our natures make us explorers. And if we’re going to continue the explorations that seem part of our DNA, we’re now facing a frontier that’s going to demand consensual work with beings we create.

Will we ever know if they are truly conscious? I don’t think it matters. If I’m right, we’re pushing moral philosophy deeply into the realm of the non-biological. The philosophical challenge is immense, and generative.

The article is Wordsworth, “The Future of Space is More Than Human,” in the online journal NOĒMA, published by the Berggruen Institute and available here.

Jupiter’s Impact on the Habitable Zone 29 Oct 10:34 AM (7 days ago)

I’ve been thinking about how useful objects in our own Solar System are when we compare them to other stellar systems. Our situation has its idiosyncrasies and certainly does not represent a standard way for planetary systems to form. But we can learn a lot about what is happening at places like Beta Pictoris by studying what we can work out about the Sun’s protoplanetary disk and the factors that shaped it. Illumination can come about in both directions.

Think about that famous Voyager photograph of Earth, now the subject of an interesting new book by Jon Willis called The Pale Blue Data Point (Princeton, 2025). I’m working on this one and am not yet ready to review it, but when I do I’ll surely be discussing how the best we can do at studying a living terrestrial planet at a considerable distance is our own planet from 6 billion kilometers. We’ll use studies of the pale blue dot to inform our work with new instrumentation as we begin to resolve planets of the terrestrial kind.

But let’s look much further out, and a great deal further back in time. A 2003 detection at Beta Pictoris led eventually to confirmation of a planet in the early stages of formation there. Probing how exoplanets form is an ongoing task stuffed with questions and sparkling with new observations. As with every other aspect of exoplanet research, things are moving quickly in this area. Perhaps 25 million years old, this system offers information about the mechanisms involved in the early days of our own. Here on Earth, we also get the benefit of meteorites delivering ancient material for our inspection.

The role of Jupiter in shaping the protoplanetary disk is hard to miss. We’re beginning to learn that planetesimals, which are considered the building blocks of planets, did not form simultaneously around the Sun, and the mechanisms now coming into view affect any budding planetary system. In new work out of Rice University, senior author André Izidoro and graduate student Baibhav Srivastava have gone to work on dust evolution and planet formation using computer simulations that analyze the isotopic variation among meteorites as clues to a process that may be partially preserved in carbonaceous chondrites.

Image: Enhanced image of Jupiter by Kevin M. Gill (CC-BY) based on images provided courtesy of NASA/JPL-Caltech/SwRI/MSSS (Credit: NASA).

The authors posit that dense bands of planetesimals, created by the gravitational effects of the early-forming Jupiter, were but the second generation of such objects in the system’s history. The earlier generation, whose survivors are noncarbonaceous (NC) magmatic iron meteorites, seems to have formed within the first million years. Some two to three million years would pass before the chondrites formed, containing within themselves calcium-aluminum–rich inclusions from that earlier time. The rounded grains called ‘chrondules’ contain once molten silicates that help to preserve that era.

The key fact: Meteorites from objects that formed during the first generation of planetesimal formation melted and differentiated, making retrieval of their original composition problematic. Chondrites, which formed later, better preserve dust from the early Solar System and also contain distinctive ‘chondrules,’ which solidified after going through an early molten state. But the very presence of this isotopic variation demands explanation. From the paper:

…the late accretion of a planetesimal population does not appear readily compatible with a key feature of the Solar System: its isotopic dichotomy. This dichotomy—between NC and carbonaceous (CC) meteorites —is typically attributed to an early and persistent separation between inner and outer disk reservoirs, established by the formation of Jupiter or a pressure bump. In this framework, Jupiter (or a pressure bump) acts as a barrier that prevents the inward drift of pebbles from the outer disk and mixing, preserving isotopic distinctiveness.

But this ‘barrier’ would also seem to prevent small solids moving inwards to the inner disk, so the question becomes, how did enough material remain to allow the formation of early planetesimals at the later era of the chondrites? What is needed is a way to ‘re-stock’ this reservoir of material. Hence this paper. The authors hypothesize a ‘replenished reservoir’ of inner disk materials gravitationally gathered in the gaps in the disk opened up by Jupiter. The accretion of the chondrites and the locations where the terrestrial planets formed are interconnected as the early disk is shaped by the gas giant.

André Izidoro (Rice University) is senior author of the paper:

“Chondrites are like time capsules from the dawn of the solar system. They have fallen to Earth over billions of years, where scientists collect and study them to unlock clues about our cosmic origins. The mystery has always been: Why did some of these meteorites form so late, 2 to 3 million years after the first solids? Our results show that Jupiter itself created the conditions for their delayed birth.”

Image: This is Figure 1 from the paper. Caption: Schematic illustration of the proposed evolutionary scenario for the early inner Solar System over the first ~3 Myr. (A) At early times (t ~ 0.1 Myr), radial drift and turbulent mixing transport dust grains across the disk. (B) Around ≲ 0.t to 1 Myr, primordial planetesimal formation occurs in rings. (C) By ~1.5 Myr, growing planetary embryos start to migrate inward under the influence of the gaseous protoplanetary disk, whereas Jupiter’s core enters rapid gas accretion phase. (D) Around ~2 Myr, Jupiter’s gravitational perturbations excite spiral density waves, inducing pressure bumps in the inner disk. Giant impacts among migrating embryos generate additional debris. Pressure bumps act as dust traps, halting inward drift of small solids and leading to dust accumulation. (E) Between ~2 and 3 Myr, dust accumulation at pressure bumps leads to the formation of a second generation of planetesimals. Rapid gas depletion in the inner disk, combined with the presence of these traps, limits the inward migration of growing embryos. (F) By ~3 Myr, the inner gas disk is largely dissipated, resulting in a system composed of terrestrial embryos and a second generation of planetesimals—potentially the parent bodies of ordinary and enstatite chondrites—whereas the inner disk evolves into a gas-depleted cavity.

A separation between material from the outer Solar System and the inner regions preserved the distinctive isotopic signatures in the two populations. Opening up this gap, according to the authors, enabled regions where new planetesimals could grow into rocky worlds. Meanwhile, the presence of the gas giant also prevented the flow of gaseous materials toward the inner system, suppressing what might have been migration of young planets like ours toward the Sun. These are helpful simulations in that they sketch a way for planetesimals to form without being drawn into our star, but there are broad issues that remain unanswered here, as the paper acknowledges:

Our simulations demonstrate that Jupiter’s induced rapid inner gaseous disk depletion, gaps, and rings are broadly consistent with both the birthplaces of the terrestrial planets and the accretion ages of the parent bodies of NC chondrites. Our results suggest that Jupiter formed early, within ~1.5 to 2 Myr of the Solar System’s onset, and strongly influenced the inner disk evolution….

And here is reason for caution:

…we…neglect the effects of Jupiter’s gas-driven migration. This simplification is motivated by the fact that, once Jupiter opens a deep gap in a low-viscosity disk, its migration is expected to be fairly slow, particularly as the inner disk becomes depleted. Simulations show that, in low-viscosity disks, migration can be halted or reversed depending on the local disk structure. In reality, Jupiter probably formed beyond the initial position assumed in our simulations and first migrated via type I migration and eventually entered in the type II regime… but its exact migration history is difficult to constrain.

The authors thus guide the direction of future research into further consideration of Jupiter’s migration and its effects upon disk dynamics. Continuing study of young disks like that afforded by the Atacama Large Millimeter/submillimeter Array (ALMA) and other telescopes will help to clarify the ways in which disks can spawn first gas giants and then rocky worlds.

The paper is Srivastava & Izidoro, “The late formation of chondrites as a consequence of Jupiter-induced gaps and rings,” Science Advances Vol. 11, No. 43 (22 October 2025). Full text.

Interstellar Mission to a Black Hole 23 Oct 6:10 AM (13 days ago)

We normally think of interstellar flight in terms of reaching a single target. The usual destination is one of the Alpha Centauri stars, and because we know of a terrestrial-mass planet there, Proxima Centauri emerges as the best candidate. I don’t recall Proxima ever being named as the destination Breakthrough Starshot officially had in mind, but there is such a distance between it (4.2 light years) and the next target, Barnard’s Star at some 5.96 light years, that it seems evident we will give the nod to Proxima. If, that is, we decide to go interstellar.

Let’s not forget, though, that if we build a beaming infrastructure either on Earth or in space that can accelerate a sail to a significant percentage of lightspeed, we can use it again and again. That means many possible targets. I like the idea of exploring other possibilities, which is why Cosimo Bambi’s ideas on black holes interest me. Associated with Fudan University in Shanghai as well as New Uzbekistan University in Tashkent, Bambi has been thinking about the proliferation of black holes in the galaxy, and the nearest one to us. I’ve been pondering his notions ever since reading about them last August.

Black holes are obviously hard to find as we scale down to solar mass objects, and right now the closest one to us is GAIA-BH1, some 1560 light years out. But reading Bambi’s most recent paper, I see that one estimate of the number of stellar mass black holes in our galaxy is 1.4 X 10⁹. Bambi uses this number, but as we might expect, estimates vary widely, from 10 million to 1 billion. These numbers are extrapolated from the population of massive stars and to a very limited extent on clues from observational astronomy.

Image: The first image of Sagittarius A*, or Sgr A*, the supermassive black hole at the center of our galaxy. Given how hard it was to achieve this image, can we find ways to locate far smaller solar-mass black holes, and possibly send a mission to one? Credit: Event Horizon Telescope Collaboration.

Bambi calculates a population of 1 black hole and 10 white dwarfs for every 100 stars in the general population. If he’s anywhere close to right, a black hole might well exist within 20 to 25 light years, conceivably detected in future observations by its effects upon the orbital motion of a companion star, assuming we are so lucky as to find a black hole in a binary system. The aforementioned GAIA-BH1 is in such a system, orbiting a companion star.

Most black holes, though, are thought to be isolated. One black hole (OGLE-2011-BLG-0462) has been detected through microlensing, and perhaps LIGO A+, the upgrade to the two LIGO facilities in Hanford, Washington, and Livingston, Louisiana, can help us find more as we increase our skills at detecting gravitational waves. There are other options as well, as Bambi notes:

Murchikova & Sahu (2025) proposed to use observational facilities like the Square Kilometer Array (SKA), the Atacama Large Millimiter/Submillimiter Array (ALMA), and James Webb Space Telescope (JWST). Isolated black holes moving through the interstellar medium can accrete from the interstellar medium itself and such an accretion process produces electromagnetic radiation. Murchikova & Sahu (2025) showed that current observational facilities can already detect the radiation from isolated black holes in the warm medium of the Local Interstellar Cloud within 50 pc of Earth, but their identification as accreting black holes is challenging and requires multi-telescope observations.

If we do find a black hole out there at, say, 10 light years, we now have a target for future beamed sailcraft that offers an entirely different mission concept. We’re now probing not simply an unknown planet, but an astrophysical object so bizarre that observing its effects on spacetime will be a primary task. Sending two nanocraft, one could observe the other as it approaches the black hole. A signal sent from one to the other will be affected by the spacetime metric – the ‘geometry’ of spacetime – which would give us information about the Kerr solution to the phenomenon. The latter assumes a rotating black hole, whereas other solutions, like that of Schwarzschild, describe a non-rotating black hole.

Also intriguing is Bambi’s notion of testing fundamental constants. Does atomic physics change in gravitational fields this strong? There have been some papers exploring possible variations in fundamental constants over time, but little by way of observation studying gravitational fields much stronger than white dwarf surfaces. Two nanocraft in the vicinity of a black hole may offer a way to emit photons whose energies can probe the nature of the fine structure constant. The latter sets the interactions between elementary charged particles.

For that matter, is a black hole inevitably possessed of an event horizon, or is it best described as an ‘horizonless compact object’ (Bambi’s term)?

In the presence of an event horizon, the signal from nanocraft B should be more and more redshifted (formally without disappearing, as an observer should never see a test-particle crossing the event horizon in a finite time, but, in practice, at some point the signal leaves the sensitivity band of the receiver on nanocraft A). If the compact object is a Kerr black hole, we can make clear predictions on the temporal evolution of the signal emitted by nanocraft B. If the compact object is a fuzzball [a bound state without event horizon], the temporal evolution of the signal should be different and presumably stop instantly when nanocraft B is converted into fuzzball degrees of freedom.

There are so many things to learn about black holes that it is difficult to know where to begin, and I suspect that if many of our space probes have returned surprising results (think of the remarkable ‘heart’ on Pluto), a mission to a black hole would uncover mysteries and pose questions we have yet to ask. What an intriguing idea, and to my knowledge, no one else has made the point that if we ever reach the level of launching a mission to Proxima Centauri, we should be capable of engineering the same sort of flyby of a nearby black hole.

And on the matter of small black holes, be aware of a just released paper examining the role of dark matter in their formation. This one considers black holes on a much smaller scale, possibly making the chances of finding a nearby one that much greater. Let me quote the abstract (the italics are mine). The citation is below:

Exoplanets, with their large volumes and low temperatures, are ideal celestial detectors for probing dark matter (DM) interactions. DM particles can lose energy through scattering with the planetary interior and become gravitationally captured if their interaction with the visible sector is sufficiently strong. In the absence of annihilation, the captured DM thermalizes and accumulates at the planet’s center, eventually collapsing into black holes (BHs). Using gaseous exoplanets as an example, we demonstrate that BH formation can occur within an observable timescale for superheavy DM with masses greater than 10⁶ GeV and nuclear scattering cross sections. The BHs may either accrete the planetary medium or evaporate via Hawking radiation, depending on the mass of the DM that formed them. We explore the possibility of periodic BH formation within the unconstrained DM parameter space and discuss potential detection methods, including observations of planetary-mass objects, pulsed high-energy cosmic rays, and variations in exoplanet temperatures. Our findings suggest that future extensive exoplanet observations could provide complementary opportunities to terrestrial and cosmological searches for superheavy DM.

The paper is Bambi, “An interstellar mission to test astrophysical black holes,” iScience Volume 28, Issue 8113142 (August 15, 2025). Full text. The paper on black holes and dark matter is Phoroutan-Mehr & Fetherolf, “Probing superheavy dark matter with exoplanets,” Physical Review D Vol. 112 (20 August 2025), 036012 (full text).

Teegarden’s Star b: A Habitable Red Dwarf Planet? 18 Oct 6:34 AM (18 days ago)

I have a number of things to say about Teegarden’s Star and its three interesting planets, but I want to start with the discovery of the star itself.
Here we have a case of a star just 0.08 percent as massive as the Sun, an object which is all but in brown dwarf range and thus housing temperatures low enough to explain why, despite its proximity, it took until 2003 to find it.

Moreover, conventional telescopes were not the tools of discovery but archival data. Bonnard Teegarden (NASA GSFC) dug into archival data from the Near-Earth Asteroid Tracking program, surmising that there ought to be more small stars near us than we were currently seeing. The data mining paid off, and then paid off again when the team looked at the Palomar Sky Survey of 1951. This was a team working without professional astronomers and telescopes.

Image: Teegarden’s Star was subsequently identified in astronomical images taken more than 50 years ago. Credit: Palomar Sky Survey / SolStation.com.

That it took until 2019 to announce evidence of planets reflects the fact that for some time, we had trouble even coming up with a workable parallax reading, one that finally produced a distance of 12.497 light years. There are no transits at Teegarden’s Star, so the discovery data on the first two planets came through radial velocity studies conducted with the CARMENES instrument at the Calar Alto Observatory in Spain. In 2024 a third planet was found, likewise by RV.

Teegarden’s Star compels attention because of those planets, and in particular Teegarden’s Star b, which orbits just inside the habitable zone with an orbit lasting about five days. Its minimum mass, calculated from the radial velocity data, is 1.05 times that of Earth, but recall that because of the limitations of the RV method, we can’t know the orbital inclination of a world that is likely larger. The primary is relatively quiescent by red dwarf standards, and indeed produces flares that would not be problematic if planet b retained an atmosphere.

That last note is important, because the whole question of how long the planets of young red dwarfs retain an atmosphere is crucial. Teegarden’s Star is about eight billion years old and, as I mention, comparatively calm. But young red dwarfs can spit out enormous flares and present serious problems for atmospheric gases, to the point that the atmospheres may be depleted or destroyed altogether. We need to come to grips with the possibility that red dwarfs may simply be inhospitable to life, a notion skillfully dissected by David Kipping in a recent paper that I plan to address in the near future.

For now, though, let’s take a brief look at a new paper from Ryan Boukrouche and Rodrigo Caballero (Stockholm University) and Neil Lewis (University of Exeter). The authors tackle climate and potential habitability of Teegarden’s Star b, noting that its proximity puts it in the catalog for study by next-generation observatories. The team uses a three-dimentional global climate model (GCM) to study habitability, including position in the habitable zone and the question of surface climate if there is an Earth-like atmosphere.

The interest in Teegarden’s Star draws not only from its Earth-mass planets – which Boukrouche and colleagues see as prime fodder for future telescopes like the ESO’s Extremely Large Telescope and its Planetary Camera and Spectrograph – but the effect of flare activity on atmospheres. The authors assume that Teegarden’s Star b is tidally locked given its tight orbit (4.9 days) around the primary. They consider values for surface albedo which approximate first an ocean-dominated surface and then a surface dominated by land masses. Here the discussion reaches this conclusion: “Although the sensitivity to surface albedo is… relatively small, it illustrates that the habitable zone does depend on factors intrinsic to the planet, not just on orbital parameters.”

The results indicate that this intriguing planet may be habitable under these atmosphere assumptions, but if so, it is still close to the inner edge of the habitable zone. Indeed, of two different orbital distances chosen from earlier papers investigating this planet, one produces a runaway greenhouse effect that would prevent the presence of liquid water on the surface. Fittingly, the authors are sensitive to the fact that the habitability question hinges upon the configuration of their models. Citing studies of TRAPPIST-1, they note this:

…different GCMs configured to simulate the same planet can produce a range of climates and circulation regimes (presumably owing to differences between the parameterizations included in each model). For example, models capable of consistently simulating non-dilute atmospheres may explore the possibility that under a range of instellation values, the planet’s atmosphere might be in a moist greenhouse state (Kasting et al. 1984) instead of a runaway, where water builds up enough that the stratosphere becomes moist, driving photodissociation and loss of water to space.

Installation is critical. The classic problem: We need more data, which can only be supplied by future telescopes. Teegarden’s Star remains highly interesting, but if we have yet to nail down the orbital distance of a planet so close to the inner edge of the habitable zone, we can’t yet make the call on how much light and heat this planet receives. And the authors themselves point out that the origins of nitrogen on Earth are not completely understood, which makes guesses at its abundance in other worlds’ atmospheres problematic.

Image: Even in our local ‘neighborhood,’ the data on Earth-mass planets is paltry, and our investigations rely heavily on simulations with values plugged in to gauge the possibilities. New instrumentation will help but it’s sobering to realize how far we are from making the definitive call on such basic issues as whether a given rocky world even has an atmosphere. Image credit: Inductiveload – self-made, Mathematica, Inkscape. Via Wikimedia Commons.

Let me just suggest that this is where we are right now when it comes to key questions about life around nearby stars. Plugging in the necessary data on any system takes time and, when it comes to Earth-mass planets around red dwarfs, the kind of instrumentation that is still on the drawing boards or in some cases under construction. Given all that, we’re going to have quite a few years ahead of us in which we’re constructing theories to explain what we see without the solid data that will help us choose among myriad alternatives.

The paper is Boukrouche, Caballero & Lewis, “Near the Runaway: The Climate and Habitability of Teegarden’s Star b,” accepted at The Astrophysical Journal Letters. Preprint available.

A Dark Object or ‘Dark Matter’? 14 Oct 9:29 AM (22 days ago)

We are fortunate enough to be living in the greatest era of discovery in the history of our species. Astronomical observations through ever more sensitive instruments are deepening our view of the cosmos, and just as satisfyingly, forcing questions about its past and uncertain future. I’d much rather live in a universe with puzzling signs of accelerated expansion (still subject to robust debate) and evidence of matter that does not interact with the electromagnetic force (dark matter) than in one I could completely explain.

Thus the sheer enjoyment of mystery, a delight accented this morning as I contemplate the detection of a so-called ‘dark object’ of unusually low mass. Presented in both Nature Astronomy and Monthly Notices of the Royal Astronomical Society, the papers describe an object that could only be detected through gravitational lensing, a familiar exoplanet detection tool that reshapes light passing near it. With proper analysis, the nature of the distortion can produce a solid estimate of the amount of matter involved.

Image: The black ring and central dot show an infrared image of a distant galaxy distorted by a gravitation lens. Orange/red shows radio waves from the same object. The inset shows a pinch caused by another, much smaller, dark gravitational lens (white blob). Credit: Devon Powell, Max Planck Institute for Astrophysics.

We wouldn’t have any notion of dark matter were it not for the fact that while we cannot see it via photons, it does interact with gravity, and was indeed first hypothesized because of the anomalous rotation of distant galaxies. Fritz Zwicky was making conjectures about the Coma Cluster of galaxies way back in the 1930s, while Jan Oort pondered mass and observed motion of the Milky Way’s stars in the same period. It would be Vera Rubin in the 1970s who reawakened the study of dark matter, with her observations of stellar rotation around galactic centers, which proved to be too fast to be explained without additional mass, meaning mass that we currently couldn’t see.

The present work involves the Green Bank Telescope in West Virginia, the Very Long Baseline Array in Hawaii and the European Very Long Baseline Interferometric Network, which creates a virtual telescope the size of Earth. Heavy-hitter instrumentation for sure, and all of it necessary to spot the infinitesimal signals of the gravitational lensing created by this object.

Devon Powell (Max Planck Institute for Astrophysics, Germany is lead author of the paper in Nature Astronomy:

“Given the sensitivity of our data, we were expecting to find at least one dark object, so our discovery is consistent with the so-called ‘cold dark matter theory’ on which much of our understanding of how galaxies form is based. Having found one, the question now is whether we can find more and whether the numbers will still agree with the models.”

An interesting question indeed. It raises the question of whether dark matter can exist in regions without any stars, and offers at least a tentative answer. Or will we subsequently learn that this object is something a bit more prosaic, a compact and inactive dwarf galaxy from the very early universe? The authors point out that this is the lowest mass object ever found through gravitational lensing, which points to the likelihood that future searches will uncover other examples. We’re clearly at the beginning of the study of dark matter and remain ignorant of its makeup, so we can expect this work to continue. New lens-modeling techniques and datasets taken at high angular resolution provide the tools needed to make images more detailed than any before taken of the high-redshift universe and gravitationally lensed objects.

From the Powell et al. paper:

Strong gravitational lensing offers a powerful alternative pathway for studying low-mass objects with little to no EM luminosity. A spatially extended source in a galaxy-scale strong lens system acts as a backlight for the gravitational landscape of its lens galaxy, revealing low-mass perturbers through their gravitational effects alone. Furthermore, lens galaxies typically lie in the redshift range 0.2 ≲ z ≲ 1.5, which means that low-mass, low-luminosity objects can be detected and studied across cosmic time. To date, observations of galaxy-scale lenses with resolved arcs have been used to detect three low-mass perturbers: discovered by Hubble, Keck and ALMA…[E]xpanding the mass range that we can robustly probe necessitates that we use strong lens observations at the highest possible angular resolution.

The first paper is Powell et al., “A million-solar-mass object detected at a cosmological distance using gravitational imaging,” Nature Astronomy 4 March 2025. Full text. The second paper is McKean et al., “An extended and extremely thin gravitational arc from a lensed compact symmetric object at redshift of 2.059.” Monthly Notices of the Royal Astronomical Society Vol. 544, Issue 1 (November, 2025), L24-30. Full text.

Solar Sails for Space Weather 9 Oct 5:34 AM (27 days ago)

A new paper dealing with solar phenomena catches my eye this morning. Based on work performed at the University of Michigan, it applies computer modeling to delve into what we can call ‘structures’ in the solar wind, which basically means large-scale phenomena like coronal mass ejections (CMEs) and powerful magnetic flux ‘ropes’ that are spawned by the interaction of a CME and solar wind plasma. What particularly intrigues me is a mission concept that the authors put to work here, creating virtual probes to show how our questions about these structures can be resolved if the mission is eventually funded.

More on that paper in a minute, but first let me dig into the mission’s background. It has been dubbed Space Weather Investigation Frontier, or SWIFT. Originally proposed in 2023 in Frontiers in Astronomy and Space Sciences and with a follow-up in 2025 in Acta Astronautica (citations below), the mission is the work of Mojtaba Akhavan-Tafti and collaborators at the University of Michigan, Les Johnson (NASA MSFC) and Adam Szabo (NASA GSFC). The latter is a co-author on today’s paper., which studies the use of the SWIFT probes to study large-scale solar activity.

What SWIFT would offer is the ability to monitor solar activity at a new level of detail through multiple space weather stations. A solar sail is crucial to the concept, for one payload must be placed closer to the Sun than the L1 Lagrange point, where gravitational equilibrium keeps a satellite in a fixed position relative to Sun and Earth. Operating closer to the Sun than L1 requires a solar sail that can exactly balance the momentum of solar photons and the gravitational force pulling it inward. If this ‘statite’ idea sounds familiar, it’s because the work grows out of NASA’s Solar Cruiser sail, a quadrant of which was successfully deployed last year on Earth. We’ve discussed Solar Cruiser in relation to the study of the Sun’s high latitudes using non-Keplerian orbits.

Image: An artist’s rendering of the spacecraft in the SWIFT constellation stationed in a triangular pyramid formation between the sun and Earth. A solar sail allows the spacecraft at the pyramid’s tip to hold station beyond L1 without conventional fuel. Credit: Steve Alvey, University of Michigan.

The SWIFT mission, unlike Solar Cruiser, would consist of four satellites – one of them using a large sail, and the other three equipped for chemical propulsion. Think of a triangular pyramid, with the sail-equipped probe at the top and the other three probes at each corner of the base in a plane around L1. Most satellites tracking Solar activity are either in low-Earth orbit or geosynchronous orbit, while current assets at the L1 point (WIND, ACE, DSCOVR, and IMAP) can offer up to 40 minutes of advance warning for dangerous solar events.

SWIFT would buy us more time, which could be used to raise satellite orbits that would be compromised by the increased drag caused by atmospheric heating during a geomagnetic storm. Astronauts likewise would receive earlier warning to take cover. But the benefits of this mission design go beyond small increases in alert time. With an approximate separation of up to 1 solar radius, the SWIFT probes would be able to extract data from four distinct vantage points.

Designed to fit the parameters of a Medium-Class Explorer mission, SWIFT is described in the Johnson et al. paper in Acta Astronautica as a ‘hub’ spacecraft (sail-equipped) with three ‘node’ spacecraft, all to be launched aboard a Falcon 9. The craft will fly “in an optimized tetrahedron constellation, covering scales between 10 and 100s of Earth radii.” A bit more from the paper:

This viewing geometry will enable scientists to distinguish between local and global processes driving space weather by revealing the spatial characteristics, temporal evolution, and geo-effectiveness of small-to meso-scale solar wind structures and substructures of macro-scale structures, such as interplanetary coronal mass ejections (ICMEs) and stream interaction regions (SIRs). In addition, real time measurements of earth-bound heliospheric structures from sub-L1 will improve our current forecasting lead-times by up to 35 percent.

Looking now at the paper “High-resolution Simulation of Coronal Mass Ejection–Corotating Interaction Region Interactions: Mesoscale Solar Wind Structure Formation Observable by the SWIFT Constellation,” with lead author W. B. Manchester, the scale of the potential SWIFT contribution becomes clear::

The radial and longitudinal spacecraft separations afforded by the SWIFT constellation enable analyses of the magnetic coherence and dynamics of meso- to large-scale solar wind structures… The main advantage of the tetrahedral constellation is its ability to distinguish between a structure’s spatial and temporal variations, as well as their orientations.

Coronal mass ejections are huge outbursts of plasma whose injections into the solar wind form mesoscale structures with profound implications for Earth. Magnetic field interactions can produce geomagnetic storms that play havoc with communications and navigation systems. The effects show up in unusual places. Sudden changes in Earth’s atmosphere can affect satellite orbits. On the ground, this particular study, out of the University of Michigan, cites a 2024 geomagnetic event that created large financial losses in agricultural areas in the US Midwest by crippling navigation systems on farm-belt tractors.

Mojtaba Akhavan-Tafti, co-author of the paper, notes the need for more precise detection of this phenomenon:

“If there are hazards forming out in space between the sun and Earth, we can’t just look at the sun. This is a matter of national security. We need to proactively find structures like these Earth-bound flux ropes and predict what they will look like at Earth to make reliable space weather warnings for electric grid planners, airline dispatchers and farmers.”

Flux ropes, between 3,000 and 6 million miles wide, are hard to recreate in current CME simulations and prove too large for existing modeling of magnetic field interactions with plasma. The new study produces a simulation that probes the phenomenon, showing that these ‘ropes’ are produced as the coronal mass ejection moves outward through solar wind plasma, and can form vortices interacting with different streams of solar wind. The authors compare them to tornadoes, noting that existing space weather-monitoring spacecraft are not always able to detect their formation. SWIFT will be able to spot them.

Image: A computer-generated image shows where rotating magnetic fields form at the edges of a coronal mass ejection 15 hours after a solar eruption. The coronal mass ejection is the large bubble extending from the sun at the left edge of the image. Two streams of plasma extend from the edge of the coronal mass ejection as it hits neighboring streams of fast and slow solar wind. Shades of red and yellow depict the strength and orientation of the plasma’s magnetic field (labeled “Bz” in the figure legend). Shades of red represent plasma that could trigger geomagnetic storms if it hits Earth, while shades of yellow represent plasma with a strong, positive orientation. The red-brown circle around the sun shows the area not covered by the simulation, about ten million miles wide. ILLUSTRATION: Chip Manchester, University of Michigan.

This is fascinating research into a topic with near-term ramifications, a matter of planetary defense that we can take steps to resolve soon, although as always we await funding decisions. In the larger perspective, here is another way sails can be employed for purposes no other propulsion method could achieve. Future sails could use these methods to ‘hover’ over the Sun’s polar regions. Moreover, close observation of space weather changes near the Earth has much to tell us about future mission concepts that would harness solar wind plasma to drive a magsail or enable an electric sail to reach high velocities.

The 2023 paper on SWIFT is Akhavan-Tafti et al., “Space weather investigation Frontier (SWIFT),” Frontiers in Astronomy and Space Sciences Vol 10 (2023), 1185603 (abstract). The 2025 paper is Johnson et al., “Space Weather Investigation Frontier (SWIFT) mission concept: Continuous, distributed observations of heliospheric structures from the vantage points of Sun-Earth L1 and sub-L1,” Acta Astronautica Vol. 236 (November 2025), 684=691 (abstract). The paper on solar activity as a target for SWIFT is Manchester et al., “High-resolution simulation of CME-CIR interactions: small- to mesoscale solar wind structure formation observable by the SWIFT constellation,” The Astrophysical Journal Vol. 992 No. 1 (2025), 51 (full text).

Rogue Planets: A Stellar Infancy? 2 Oct 6:03 AM (last month)

How exoplanets emerge from circumstellar disks has always intrigued me, and many open questions remain, including the precise mechanisms behind the fast growth of gas giants. When the topic swings to so-called ‘rogue’ planets, formation issues seem to be the same, since we’ve assumed most such worlds have been ejected from a host system through gravitational interactions. But is there another formation path? We are learning that rogue planets are capable of feats not seen in conventional star/planet systems.

Research out of the National Institute for Astrophysics (INAF) in Italy is provocative. Using data from the European Southern Observatory’s Very Large Telescope (VLT) as well as the James Webb Space Telescope, Víctor Almendros-Abad (Astronomical Observatory of Palermo) and an international team of astronomers have found a large rogue planet (five to ten times as massive as Jupiter) that continues to form, accreting gas and dust from a surrounding cloud. No circumstellar disk required here. Growth comes in waves, now about eight times faster than just a few months before. We’re talking about accreting six billion tonnes per second, and a growth phenomenon hitherto restricted to young stars not yet on the Main Sequence.

Given this, we might consider Almendros-Abad’s comment an understatement:

“People may think of planets as quiet and stable worlds, but with this discovery we see that planetary-mass objects freely floating in space can be exciting places.”

Image: Astronomers have identified an enormous ‘growth spurt’ in a so-called rogue planet. Unlike the planets in our Solar System, these objects do not orbit stars, free-floating on their own instead. The new observations, made with the European Southern Observatory’s Very Large Telescope (ESO’s VLT), reveal that this free-floating planet is eating up gas and dust from its surroundings at a rate of six billion tonnes a second. This is the strongest growth rate ever recorded for a rogue planet, or a planet of any kind, providing valuable insights into how they form and grow. Credit: ESO.

The rogue planet in question is dubbed Cha 1107-7626, some 620 light years out in the direction of the constellation Chamaeleon. This is one of those deep southern sky constellations announced in the early 17th Century by Dutch navigators. How they ever found a chamaeleon shape in its dim stars is beyond me, but this niche of the sky holds, interestingly enough, one of the closest star-forming regions to the Sun. It is now home to what is considered the strongest accretion event on record for an object with the mass of a planet.

So we have an instance of a rogue planet that behaves in some respects like a star, with the current burst of accretion mediated by magnetic activity. Strong hydrogen alpha (Hα) emission picked up in the spectroscopic data is considered “a hallmark for channeled, magnetospheric accretion,” and the size of the change in the hydrogen lines is what flags the dramatic increase in accretion rate. Finding it in a planet-mass object is highly unusual. A major tool for astronomers, the hydrogen alpha line is emitted when an electron moves from the third lowest to the second lowest energy state in the hydrogen atom.

Looking into the paper (just published in The Astrophysical Journal Letters, I learned that bursts like this are well studied in young stars:

In particular, such events can have a significant effect on chemical and physical evolution of the disk (P. Ábrahám et al. 2009; S. A. Smith et al. 2025), and potentially on the early stages of planet formation. Our target is the lowest mass object observed thus far that is going through an accretion burst, and by far the lowest in the EXor category [young stars pre-Main Sequence]. Detailed studies of accretion variability have in the past helped to illuminate the interactions between young stellar objects and their disks, including the role of magnetic fields. Similarly, the observations presented here provide a glimpse into the nature of accretion in planetary-mass objects.

Image: This visible-light image, part of the Digitized Sky Survey 2, shows the position in the sky of the rogue planet Cha 1107-7626. The planet (not visible here) is located exactly at the centre of the frame. Credit: ESO/ Digitized Sky Survey 2.

The belief that rogue planets are invariably the result of ejections from a planetary system is challenged by these findings. From my reading of this paper, we seem to be looking at a ‘star-like’ formation of a gas giant through accretion, hinting at a variety of formation scenarios in free-floating worlds. Surely we will find more, but for now I can see why the authors call Cha1107-7626 “the poster child for disk accretion in the planetary-mass domain.”

The paper is Almendros-Abad et al., “Discovery of an Accretion Burst in a Free-Floating Planetary-Mass Object,” Volume 992, Number 1 (2 October 2025), L2 (full text).

A Potential Martian Biosignature 26 Sep 10:30 AM (last month)

I’ve long maintained that we’ll find compelling biosignatures on an exoplanet sooner than we’ll find them in our own Solar System. But I’d love to be proven wrong. The recent flurry of news over the interesting findings from the Perseverance rover on Mars is somewhat reminiscent of the Clinton-era enthusiasm for the Martian meteorite ALH8001. Now there are signs, as Alex Tolley explains below, that this new work will prove just as controversial. Biosignatures will likely be suggestive rather than definitive, but Mars is a place we can get to, as our rovers prove. Will Perseverance compel the sample return mission that may be necessary to make the definitive call on life?

by Alex Tolley

Overview of jezero Crater and sample site in article. Credit NASA/MSSS/USGS.

On September 10, 2025, Nature published an article that got wide attention. The authors claimed that they had discovered a possible biosignature on Mars. If confirmed, they would have won the race to find the first extraterrestrial biosignature. Exciting!

One major advantage of detecting a biosignature in our system is that we can access samples and therefore glean far more information than we can using spectroscopic data from an exoplanet. This will also reduce the ambiguity of simpler atmospheric gas analyses that are all we can do with our telescopes at present.

Figure 1. Perseverance’s path through Neretva Vallis and views of the Bright Angel formation. a, Orbital context image with the rover traverse overlain in white. White line and arrows show the direction of the rover traverse from the southern contact between the Margin Unit and Neretva Vallis to the Bright Angel outcrop area and then to the Masonic Temple outcrop area. Labelled orange triangles show the locations of proximity science targets discussed in the text. b, Mastcam-Z 360° image mosaic looking at the contact between the light-toned Bright Angel Formation (foreground) and the topographically higher-standing Margin Unit from within the Neretva Vallis channel. This mosaic was collected on sol 1178 from the location of the Walhalla Glades target before abrasion. Upslope, about 110 m distant, the approximate location of the Beaver Falls workspace (containing the targets Cheyava Falls, Apollo Temple and Steamboat Mountain and the Sapphire Canyon sample) is shown. Downslope, about 50 m distant, the approximate location of the target Grapevine Canyon is also shown. In the distance, at the southern side of Neretva Vallis, the Masonic Temple outcrop area is just visible. Mastcam-Z enhanced colour RGB cylindrical projection mosaic from sol 1178, sequence IDs zcam09219 and zcam09220, acquired at 63-mm focal length. A flyover of this area is available at https://www.youtube.com/watch?v=5FAYABW-c_Q. Scale bars (white), 100 m (a), 50 m (b, top) and 50 cm (b, bottom left). Credit: NASA/JPL-Caltech/ASU/MSSS.

Let’s back up for context. The various rover missions to Mars have proceeded to determine the history of Mars. From the Pathfinder mission starting in 1996, the first mission since the two 1976 Viking landers, the various rovers from Soujourner (1997), Spirit & Opportunity (2004), Curiosity (2012), and now Perseverance (2021), have increased the scope of their travels and instrument capabilities. NASA’s Perseverance rover was designed to characterize environments and look for signs of life in Jezero Crater, a site that was expected to be a likely place for life to have existed during the early, wet phase of a young Mars. The crater was believed to be a lake, fed by water running into it from what is now Neretva Vallis, and signs of a delta where the ancient river fed into the crater lake are clear from the high-resolution orbital images. Perseverance has been taking a scenic tour of the crater, making stops at various points of interest and taking samples. If there were life on Mars, this site would have both flowing water and a lake, with sediments that create a variety of habitats suitable for prokaryotic life, like the contemporary Earth.

Perseverance had taken images and samples of a sedimentary rock formation, which they called Bright Angel. The work involved using the Scanning Habitable Environments with Raman and Luminescence for Organics and Chemicals (SHERLOC) instrument to obtain a Raman UV spectrum of rock material from several samples. The authors claimed that they had detected 2 reduced iron minerals, greigite and vivianite, and organic carbon. The claim is that these have been observed in alkaline environments on Earth due to bacteria, and therefore prove to be a biosignature of fossil life. The images showed spots (figure 2) which could possibly be the minerals formed by the metabolism of anaerobic bacteria, reducing sulfur and iron for energy. The organic carbon in the mudstone rock matrix is the fossil remains of the bacteria living in the sediments.

Exciting, no? Possible proof that life once existed on Mars. The authors submitted a paper to Nature with the title, “Detection of a Potential Biosignature by the Perseverance Rover on Mars“. The title was clearly meant to catch the scientific and popular attention. At last, NASA’s “Follow the Water” strategy and exploration with their last rover equipped to detect biosignatures had found evidence of fossil life on Mars. It might also be a welcome boost for NASA’s science missions, currently under funding pressure from Congress.

Then the peer review started, and the story seemed less strong. Just as 30 years ago, when the announcement from the White House by the US president, Bill Clinton claimed that a Martian meteorite retrieved from the Antarctic, ALH8001, was evidence of life on Mars proved very controversial. Notably, slices of that meteorite viewed under an electron microscope showed images of what might have been some forms of bacteria. These images were seen around the world and were much discussed. The consensus was that the evidence was not unambiguous, with even the apparent “fossil bacteria” being explained as natural mineral structures.

Well, the new paper created one of the longest peer review documents I have ever read. Every claimed measurement and analysis was questioned, including the interpretation. The result was that the paper was published as the much drier “Redox-driven mineral and organic associations in Jezero Crater, Mars”. There are just 3 uses of the term biosignatures, each prefaced with the term “potential”, and the null hypothesis of abiotic origin emphasized as well. One of the three peer reviewers even wanted Nature to reject the paper, based on what might be another ALH8001 fiasco. A demand, too far.

What were the important potential biosignature findings?

Organisms extract energy from molecules via electron transfer. This often results in the compounds becoming more reduced. For example, sulfur-reducing bacteria convert sulfates (SO4) to sulfide (S). Iron may be reduced from its ferric (Fe3+) state to its ferrous (Fe2+) state. Two minerals that are often found reduced as a result of bacterial energy extraction are greigite Fe2+Fe3+2S4] and vivianite [Fe2+3(PO4)2·8H2O]. On Earth, these are regarded as biosignatures. In addition, unidentified carbon compounds were associated with these 2 minerals. The minerals were noticed as spots on the outcrop and identified with the Planetary Instrument for X-ray Lithochemistry (PIXL), which can identify elements via X-ray spectroscopy. The SHERLOC instrument identified the presence of carbon in association with these minerals.

Figure 2. An image of the rock named “Cheyava Falls” in the “Bright Angel formation” in Jezero crater, Mars, collected by the WATSON camera onboard the Mars 2020 Perseverance rover. The image shows a rust-colored, organic matter in the sedimentary mudstone sandwiched between bright white layers of another composition. The small dark blue/green to black colored nodules and ring-shaped reaction fronts that have dark rims and bleached interiors are proposed to be potential biosignatures. Credit: NASA/JPL-Caltech/MSSS.

To determine whether the carbon associated with the greigite and vivianite was organic or inorganic, the material was subjected to ultraviolet rays. Organic carbon bonds, especially carbon-carbon bonds, will respond to specific wavelengths by vibrating, like sound frequencies can resonate and break wine glasses. Raman spectroscopy is the technique used to detect the resonant vibrations of types of carbon bonds, particularly specific arrangements of the atoms and their bonds that are common in organic carbon. The spectroscopic data indicated that the carbon material was organic, and therefore possibly from decayed organisms. This would tie together the findings of the carbon and the 2 minerals as a composite biosignature. However, the reviewers also questioned the interpretation of the Raman spectrum.. The sp2 carbon bonds (120 degrees) seen in aromatic 6-carbon rings, in graphene, graphite, and commonly in biotic compounds, should show both a G-band (around 1600 cm-1) and a D-band (around 2700 cm-1), yet the spectrum only clearly showed the G-band. Did this imply that the organic carbon may not have been found? The reviewers also questioned why the biological explanation was favored over an abiotic one. No one questioned the greigite and vivianite findings, other than that they are not exclusively associated with anaerobic bacterial metabolism.

Figure 3 – Raman spectrum with interpolated curves to highlight the G-band in the 4 samples taken at the location.

So what to make of this? Clearly, the authors backed down on their more positive interpretation of their findings as a biosignature.

What analyses would we want to do on Earth?

Assuming the samples from Perseverance are eventually retrieved and returned to Earth, what further analysis would we want to do to increase our belief that a biosignature was discovered?

A key analysis would be to analyze the carbon deposits. The Raman UV spectra indicate that the carbon is organic, which is almost a given. You may recall that the private MorningStar mission to Venus will do a similar analysis but use a laser-induced fluorescence that detects aromatic rings [1]. Neither of these techniques can distinguish between abiotic and biogenic carbon. The carbon may even be in the form of common polycyclic aromatic hydrocarbons (PAH), a form that is ubiquitous and is easily formed, especially with heat.

One useful approach to distinguish the source of the carbon is to measure the isotopic ratios of the 2 stable carbon isotopes, carbon-12 and carbon-13. Living organisms favor the lighter carbon-12, and therefore, the C13/C12 ratio is reduced when the carbon is from living organisms. This must be compared to known abiotic carbon to confirm its source. This analysis requires a mass spectrometer, which was not included with the Perseverance instrument pack.

The second approach is to analyze the carbon compounds. Gas chromatography followed by infrared spectroscopy is used to characterize the compounds. Life restricts the variety of compounds compared to random reactions, and can be compared to expectations based on Assembly Theory [2], although exposure to UV and particle radiation for billions of years may make the composition of the carbon more random.

Lastly, if the carbon were once protein or nucleotide macromolecules, any chirality might distinguish its source as biotic.

Isotopic analysis can also be made on the sulfur compounds in the greigite. As with carbon, life will preferentially use lighter isotopes. Bacteria reduce sulfate to sulfide for energy, and the iron sulfide mineral, greigite, is a waste product of this metabolism. Of the 2 stable sulfur isotopes, sulfur-32 and sulfur-34, if the S34/S32 ratio is reduced, then this hints that the greigite was formed biotically.

Lastly, opening up the samples and inspecting them with an electron microscope, there may be physical signs of bacteria. However, any physical features will need to be identified unambiguously to avoid the ALH8001 controversy.

Unfortunately for these proposed analyses, the Mars Sample Return (MSR) mission has been cut with the much-reduced NASA budget. When, or whether, we get these samples for analyses on Earth is currently unknown.

My view on the findings

If the findings and the interpretation of their compositions is correct, then this would probably be the most convincing, but still not unambiguous biosignature to date. If the samples are returned to Earth and the findings are extended with other analyses, then we probably would have detected fossil life on Mars. In my opinion, that would validate the idea that Martian life existed, and further exploration is warranted. We would then want to know if that life was similar or different from terrestrial life to shed light on abiogenesis or panspermia between Earth and Mars. As the formation of the Moon would have been very destructive, if life emerged on Mars and was spread to Earth, this might provide more time for living cells to evolve compared to the conditions on Earth. It would also stimulate the search for subsurface life on Mars, where interior heat and water between rock grains would support such a niche habitat as it does on Earth.

It seems a pity that without an MSR, we may have the evidence for Martian fossil life, packaged for analysis, but kept frustratingly remote and unavailable, mere millions of kilometers distant.

The paper is Hurowitz, J.A., Tice, M.M., Allwood, A.C. et al. “Redox-driven mineral and organic associations in Jezero Crater, Mars.” Nature 645, 332–340 (2025). https://doi.org/10.1038/s41586-025-09413-0

Other readings

Tolley A, (2022) Venus Life Finder: Scooping Big Science web: https://www.centauri-dreams.org/2022/06/03/venus-life-finder-scooping-big-science/

Walker, S. I., Mathis, C., Marshall, S., & Cronin, L. (2024). “Experimentally measured assembly indices are required to determine the threshold for life.” Journal of the Royal Society Interface, 21(220). https://doi.org/10.1098/rsif.2024.0367

Exoplanets: Refining the Target List 24 Sep 10:59 AM (last month)

I wasn’t surprised to learn that the number of confirmed exoplanets had finally topped 6,000, a fact recently announced by NASA. After all, new worlds keep being added to NASA’s Exoplanet Science Institute at Caltech on a steady basis, all of them fodder for a site like this. But I have to admit to being startled by the fact that fully 8000 candidate planets are in queue. Remember that it usually takes a second detection method finding the candidate world for it to move into the confirmed ranks. That 8000 figure shows how much the velocity of discovery continues to increase.

The common theme behind much of the research is often cited as the need to find out if we are alone in the universe. Thus NASA’s Dawn Gelino, head of the agency’s Exoplanet Exploration Program (ExEP) at JPL:

“Each of the different types of planets we discover gives us information about the conditions under which planets can form and, ultimately, how common planets like Earth might be, and where we should be looking for them. If we want to find out if we’re alone in the universe, all of this knowledge is essential.”

I sometimes think, though, that the emphasis on an Earth 2.0 is over-stated. The search for other life is fascinating, but deepening our scientific knowledge of the cosmos is worthwhile even if we learn we are alone in the galaxy. What nature creates in bewildering variety merits our curiosity and deep study even on barren worlds. With ESA’s Gaia and NASA’s Roman Space Telescope in the mix, exoplanet detections will escalate dramatically. And a little further down the road is the Habitable Worlds Observatory, assuming we have the good sense to green-light the project and build it.

Bear in mind that almost all the known exoplanets are within a few thousand light years of Sol. We are truly awash in immensity. If there will ever be a complete catalog of the Milky Way’s planets, it will likely be from a Kardashev Type III civilization immensely older than ourselves. It’s fascinating to think that such a catalog might already exist somewhere. But it’s also fascinating to consider that we may be alone, which raises all kinds of questions about abiogenesis and the possible lifetime of civilizations.

Image: Scientists have found thousands of exoplanets (planets outside our solar system) throughout the galaxy. Most can be studied only indirectly, but scientists know they vary widely, as depicted in this artist’s concept, from small, rocky worlds and gas giants to water-rich planets and those as hot as stars. Credit: NASA’s Goddard Space Flight Center.

The idea of a large and growing catalog of exoplanets is the kind of thing I used to dream about as a kid reading science fiction magazines. Now we’re on the cusp of biosignature detection capabilities via the deep study of exoplanet atmospheres. Fewer than a hundred exoplanets have been directly imaged, a number that is likewise expected to rise with the help of the Roman instrument’s coronagraph. With new tools to better block out the overwhelming glare of the host star, we’ll be seeing gas giants in Jupiter-like orbits. That in itself is interesting – how many exoplanet systems have gas giants in such positions? Thus far, the Solar System pattern is rarely replicated.

The Fortunes of K2-18b

The sheer variety of planetary systems brings even more zest to this work. Consider the planet K2-18b, so recently in the news as the home of a possible global ocean, and one with prospects for life given all the parameters studied by a team at the University of Cambridge. It’s a fabulous scenario, but now we have a new study that questions whether sub-Neptunes like this are actually dominated by water. Caroline Dorn (ETH Zurich), co-author of the paper appearing in The Astrophysical Journal, believes that water on sub-Neptunes is far more limited than we have been thinking.

Here’s another demonstration of how our Solar System is so unlike what we’re finding elsewhere. Lacking a sub-Neptune among our own planets, we’re learning now that such worlds – larger than Earth but smaller than Neptune and cloaked in a thick atmosphere abundant in hydrogen and helium – are relatively common in our galaxy as, for that matter, are higher density but smaller ‘super-Earths.’ A global ocean seems to make sense if a sub-Neptune formed well beyond the snowline and brought a robust inventory of ice with it as it migrated into the warmer inner system. Indeed, the term Hycean (pronounced HY-shun) has been proposed to label a sub-Neptune planet with a deep ocean under an atmosphere rich in hydrogen.

The new paper examines the chemical coupling between the planet’s atmosphere and interior, with the authors assuming an early stage of formation in which sub-Neptunes go through a period dominated by a magma ocean. The hydrogen atmosphere helps to maintain this phase for millions of years. And the problem is that magma oceans have implications for the water content available. Using computer simulations to model silicates and metals in the magma, the team studied the chemical interactions that ensue. Most H20 water molecules are destroyed, with hydrogen and oxygen bonding into metallic compounds and disappearing deep into the planet’s interior.

From the paper:

Our results, which focus on the initial (birth) population of sub-Neptunes with magma oceans, suggest that their water mass fractions are not primarily set by the accretion of icy pebbles during formation but by chemical equilibration between the primordial atmosphere and the molten interior. None of the planets in our model, regardless of their initial H2O content, retain more than 1.5 wt% water after chemical equilibration. This excludes the high water mass fractions (10–90 wt%) invoked by Hycean-world scenarios (N. Madhusudhan et al. 2021), even for planets that initially accreted up to 30% H2O by mass. These findings are consistent with recent studies suggesting that only a small amount of water can be produced or retained endogenously in sub-Neptunes and super-Earths.

The work analyzes 19 chemical reactions and 26 components across the range of metals, silicates and gases, with the core composed of both metal and silicate phases. A computer model known as New Generation Planetary Population Synthesis (NGPPS) combines planetary formation and evolution and meshes with code developed for global thermodynamics. Thus a population of sub-Neptunes with magma oceans is generated, and consistently primordial water is destroyed by chemical interactions.

Image: This is Figure 3 from the paper. Caption: Envelope H2O mass fraction as a function of semimajor axis… The left panel shows planets that predominantly formed inside the water ice line; the right panel shows those that formed outside. Classification is based on the accreted H2O mass fraction, with a threshold set at 5% of the total planetary mass. The colorbar indicates the molar bulk C/O ratio. Planets formed inside the ice line are systematically depleted in carbon due to the lack of volatile ice accretion and exhibit higher envelope H2O mass fractions. In contrast, planets formed beyond the ice line retain lower H2O content despite higher bulk volatile abundances. Each pie chart shows the mean mass fraction of hydrogen in H2 (gas), H (metal), H2 (silicate), H2O (gas), and H2O (silicate), normalized to the total mean hydrogen inventory for each population. Only components contributing more than 5% are labeled. Planets that formed beyond the ice line store most hydrogen as H2 (gas) and H (metal), while those that formed inside the ice line retain a larger share of hydrogen in H (metal), H2 (silicate), and H2O (gas + silicate).. Credit: Werlen et al.

This is pretty stark reading if you’re fascinated with deep ocean scenarios. Here’s Dorn’s assessment:

“In the current study, we analysed how much water there is in total on these sub-Neptunes. According to the calculations, there are no distant worlds with massive layers of water where water makes up around 50 percent of the planet’s mass, as was previously thought. Hycean worlds with 10-90 percent water are therefore very unlikely.”

The paper is suggesting that we re-think what had seemed an obvious connection between planet formation beyond the snowline and water in the atmosphere. Instead, the interplay of magma ocean and atmosphere may deliver the verdict on the makeup of a planet. By this modeling, planets with atmospheres rich in water are more likely to have formed within the snowline. That leaves rocky worlds like Earth in the mix, but raises serious doubts about the viability of water in the sub-Neptune environment.

From the paper:

Counterintuitively, the planets with the most water-rich atmospheres are not those that accreted the most ice, but those that are depleted in hydrogen and carbon. These planets typically form inside the ice line and accrete less volatile-rich material. While some retain significant atmospheric H2O, the high-temperature miscibility of water and hydrogen likely prevents the presence of surface liquid water—even on these comparatively water-rich worlds.

This has broad implications for theories of planet formation and volatile evolution, as well as for interpreting exoplanet atmospheres in the era of the James Webb Space Telescope (JWST), the Extremely Large Telescope (ELT), ARIEL, the Habitable World Observatory (HWO), and the Large Interferometer for Exoplanets (LIFE). It also informs atmospheric composition priors in interior characterization of transiting planets observed by Kepler, TESS, CHEOPS, and PLATO with radial velocity (RV) or transit timing variation (TTV) constraints.

K2-18b will obviously receive continued deep study. But with an aggregate 14,000 exoplanets confirmed or listed as candidates, consider how overwhelmed our instruments are by sheer numbers. To do a deep dive into any one world demands the shrewdest calculations to find which exoplanets are most likely to reward the telescope time. This aspect of target selection will only get more critical as we proceed.

The paper is Werlen et al., “Sub-Neptunes Are Drier than They Seem: Rethinking the Origins of Water-rich Worlds,” Tje Astrophysical Journal Letters Vol. 991, No. 1 (18 September 2025), L16 (full text).

Beaming and Bandwidth: A New Note on the Wow! Signal 22 Sep 6:19 AM (last month)

James Benford (president of Microwave Sciences, Lafayette CA) has just published a note in the Journal of the British Interplanetary Society that has relevance to our ongoing discussion of the Wow! Signal. My recent article was in the context of new work at Arecibo, where Abel Mendez and the Arecibo Wow! research effort have refined several parameters of the signal, detected in 1977 at Ohio State’s Big Ear Observatory. Let me slip in a quick look at Benford’s note before we move on from the Wow! Signal.

Benford has suggested both here and in other venues that the Wow! event can be explained as the result of an interstellar power beam intercepting our planet by sheer chance. Imagine if you will the kind of interstellar probe we’ve often discussed in these pages, one driven by a power beam to relativistic velocities. Just as our own high-powered radars scan the sky to detect nearby asteroids, a beam like this might sweep across a given planet and never recur in its sky.

But it’s quite interesting that in terms of the signal’s duration, bandwidth, frequency and power density, an interstellar power beam would be visible from another star system if this were to occur. All the observed features of such a beam are found in the Wow! Signal, which does not prove its nature, but suggests an explanation that corresponds with what data we have.

An interesting sidenote to this is, as Benford has discussed in these pages before, that if the Wow! Signal were an attempt to communicate, it should at some point repeat. Whereas a power beam from a source doing some kind of dedicated work in its system would never recur. We have had a number of attempts to find the Wow! Signal through the years but none have been successful.

Image: The Ohio State University Radio Observatory in Delaware, Ohio, known as the Big Ear. Credit: By Иван Роква – Own work, CC BY-SA 4.0, via Wikimedia Commons.

The note in JBIS comes out of one of the Breakthrough Discuss meetings, where Michael Garrett (Jodrell Bank) asked Benford why, if a power beam explanation were the answer to the Wow!, a technical civilization would limit their beam to a narrow band of less than 10 kHz. It turns out there is an advantage in this, and that as Benford explains, narrow bandwidth is a requirement for high power-beaming systems in the first place.

Here I need to quote the text:

High power systems involving multiple sources are usually built using amplifiers, not oscillators, for several technical reasons. For example, the Breakthrough Starshot system concept has multiple laser amplifiers driven by a master oscillator, a so-called master oscillator-power amplifier (MOPA) configuration. Amplifiers are themselves characterized by the product of amplifier gain (power out divided by power in) and bandwidth, which is fixed for a given type of device, their ‘gain-bandwidth product.’ This product is due to phase and frequency desynchronization between the beam and electromagnetic field outside the frequency bandwidth.

Now we come to the crux. Power beaming to power up, say, an interstellar sail demands high power delivered to the target. To produce high power, each of the amplifiers involved must have small bandwidth, with the number of amplifiers used being determined by the power required. Benford puts it this way: “…you get narrow bandwidth by using very high-gain amplifiers to essentially ‘eat up’ the gain-bandwidth product.’

Thus we have a bandwidth limit for amplifiers, one that would apply both to beacons and power beams, which by their nature would be built to project high power levels. Small bandwidth is the physics-dictated result. None of this proves the nature of the Wow! Signal, but it offers an explanation that resonates with the fact that the four Wow! parameters are consistent with power beaming.

SETI Odds and Ends 15 Sep 6:39 AM (last month)

I’m catching up with a lot of papers in my backlog, prompted by a rereading yesterday of David Kipping’s 2022 paper on the Wow! Signal, the intriguing, one-off reception at the Big Ear radio telescope in Ohio back in 1977 (Kipping citation below). I had just finished checking Abel Mendez’ work at Arecibo, where the Arecibo Wow! project has announced a new analysis based on study of previously unpublished observations using updated signal analysis techniques. No huge surprises here, but both Kipping’s work and Arecibo Wow! are evidence of our continuing fascination with what Kipping calls “the most compelling candidate for an alien radio transmission we have ever received.”

They also remind us that no matter how many times this intriguing event has been looked at, there are still new ways to approach it. I give the citation for the Mendez paper, written with a team of collaborators (one of whom is Kipping) below. Let me just pull this from Mendez’ statement on the Arecibo Wow! site, showing how the new work has refined the original Wow! Signal’s properties:

Location: Two adjacent sky fields, centered at right ascensions 19h 25m 02s ± 3s or 19h 27m 55s ± 3s, and declination –26° 57′ ± 20′ (J2000). This is both more precise and slightly shifted from earlier estimates.

Intensity: A peak flux density exceeding 250 Janskys, more than four times higher than the commonly cited value.

Frequency: 1420.726 MHz, placing it firmly in the hydrogen line but with a greater radial velocity than previously assumed.

Leading Mendez (University of Puerto Rico at Arecibo) to comment:

“Our results don’t solve the mystery of the Wow! Signal. “But they give us the clearest picture yet of what it was and where it came from. This new precision allows us to target future observations more effectively than ever before…. This study doesn’t close the case,” Méndez said. “It reopens it, but now with a much sharper map in hand.”

Image: Comparison of the previously estimated locations of the Wow! Signal (gray boxes) with the refined positions from the Arecibo Wow! Project (yellow boxes). The signal’s source is presumed to lie within one of these boxes and beyond the foreground Galactic hydrogen clouds shown in bright red. Credit: PHL @ UPR Arecibo.

Homing in on interesting anomalies is of course one way for SETI to proceed, although the host of later one-off detections from other locations (none evidently as powerful a signal as Wow!) doesn’t yield optimism that one of these will eventually repeat. A sweeping beam that by sheer chance swept across Earth from some kind of ETI installation? Works for me, if only we had repeating evidence. In the absence of it, we continue to dig into existing data using new techniques.

We can also proceed with targeted searches of nearby stars of interest both because of their proximity as well as the presence of unusual planetary configurations. The TRAPPIST-1 system isn’t remotely like ours, with its seven Earth-sized planets crammed into tight orbit around an M8V red dwarf star, but the fact that each of these transits makes the system of huge value. Now a team led by Guang-Yuan Song (Dezhou University, China) has used the FAST instrument (Five hundred meter Aperture Spherical Telescope) to search for SETI signals, delving into the frequency range 1.05–1.45GHz with a spectral resolution of ~7.5Hz. No signals detected, but the scientists plan to continue to search other nearby systems and do not rule out a return to this one. Citation below.

The Factors Leading to Technology

The frustration over lack of success at finding an extraterrestrial civilization is understandable, so it’s no surprise that theoretical work explaining it from an entirely opposite direction continues to appear. As witness a new study just presented at the EPSC-DPS Joint Meeting 2025 in Helsinki. Here we’re asking what factors go into making a technological society possible, assuming an evolutionary history something like our own, and probing whether changes to any of the parameters that helped us emerge would have made us impossible.

All this goes back to the 1960s and the original Drake Equation, which makes a loose attempt at sizing up the possibilities. Manuel Scherf and Helmut Lammer of the Space Research Institute at the Austrian Academy of Sciences in Graz paint a distressing picture for those intent on plucking a signal from ETI out of the ether. Their work has focused on plate tectonics and its relationship with the critical gas carbon dioxide, which governs the carbon-silicate cycle.

CO2 is a huge factor in sustaining photosynthesis, but too much of it creates greenhouse effects that can likewise spell the end of life on a planet like ours. The fine-tuning that goes on through the carbon-silicate cycle ensures that CO2 gets released back into the atmosphere through plate tectonics and volcanic emissions, recycling it back out of the rock in which it had been previously locked. Scherf pointed out to the EPSC-DPS gathering that if we wait somewhere between 200 million and one billion years, loss of atmospheric CO2 will bring an end to photosynthesis. The Sun may have another five billion years of life ahead, but the environment that sustains us won’t last nearly as long.

Indeed, the researchers argue, surface partial pressures and mixing ratios of CO2, O2, and N2 likewise affect such things as combustion, needed for the smelting of metals that underpins the growth of a technological civilization. Imagine a planet with 10 percent of its atmosphere taken up by CO2 (as opposed to the 0.042 percent now found on Earth). This world produces a biosphere that can sustain itself against a runaway greenhouse if further away from its star than we are from the Sun, but it would also require no less than 18 percent oxygen (Earth now has 21 percent) to ensure that combustion can occur.

If we do away with plate tectonics, so critical to the carbon-silicate cycle, we likewise limit habitable conditions at the surface. So we need this as well as enough oxygen to provide combustion to make technology possible. In other words, we have astrophysical, geophysical, and biochemical criteria that have to be met even when a planet is in the habitable zone if we are hoping to find lifeforms that have survived long enough to create technology. Rare Earth?

Scherf and Lammer weigh these factors against the amount of time it takes technology to emerge, assuming that the longer an ETI civilization exists, the more likely we are to observe it. Here I don’t have a paper to work with, so I can only report the conclusions presented at the EPSC-DPS conference, which are stated bluntly by Scherf:

“For 10 civilizations to exist at the same time as ours, the average lifetime must be above 10 million years. The numbers of ETIs are pretty low and depend strongly upon the lifetime of a civilization.”

I can also fall back on a 2024 paper from the same team discussing these matters, which delves not only into the question of the perhaps rare combination of circumstances which allows for technological civilizations to emerge but also our use of the Copernican Principle in framing the issues:

…our study is agnostic about life originating on hypothetical habitats other than EHs. Any more exotic habitats (e.g., subsurface ocean worlds) could significantly outnumber planets with Earthlike atmospheres, at least in principle. Finally, we argue that the Copernican Principle of Mediocrity cannot be valid in the sense of the Earth and consequently complex life being common in the Galaxy. Certain requirements must be met to allow for the existence of EHs and only a small fraction of planets indeed meet such criteria. It is therefore unscientific to deduce complex aerobic life to be common in the Universe, at least based on the Copernican Principle. Instead, we argue, at maximum, for a combined Anthropic-Copernican Principle stating that life as we know it may be common, as long as certain criteria are met to allow for its existence. Extremophiles, anaerobic and simple aerobic lifeforms, however, could be more common.

Image: An artist’s impression of our Milky Way Galaxy, showing the location of the Sun. Our Solar System is about 27,000 light years from the centre of the galaxy. The nearest technological species could be 33,000 light years away. Credit: NASA/JPL–Caltech/R. Hurt (SSC–Caltech).

All of which illuminates the paucity of data. We could say that it took four and a half billion years for technology to emerge on Earth, but that is our only reference point. We also have no data on how long technological societies exist. It is clear, though, that the longer the survival period, the more likely we are to be present in the cosmos at the same time they are. For there to be even one technological civilization in the galaxy coinciding with our existence, ETI would have to have survived in a technological phase for at least 280,000 years. I think that matches up with the case Brian Lacki has been making for some time now, which emphasizes the ‘windows’ of time within which we view the cosmos.

But Scherf adds this:

“Although ETIs might be rare there is only one way to really find out and that is by searching for it. If these searches find nothing, it makes our theory more likely, and if SETI does find something, then it will be one of the biggest scientific breakthroughs ever achieved as we would know that we are not alone in the Universe.”

Image: This graph shows the maximum number of ETIs presently existing in the Milky Way. The solid orange line describes the scenario of planets with nitrogen–oxygen atmospheres with 10 per cent carbon dioxide. In this case the average lifetime of a civilization must be at least 280,000 years for a second civilization to exist in the Milky Way. Changing the amount of atmospheric carbon dioxide produces different results. Credit: Manuel Scherf and Helmut Lammer.

I’ll note in passing that Adam Frank (Rochester Institute of Technology) and Amedeo Balbi (University of Rome Tor Vergata) have analyzed the question of an ‘oxygen bottleneck’ for the emergence of technology in a recent paper in Nature Astronomy. The memorable thought that if there are no other civilizations in the galaxy, it’s a tremendous waste of space sounds reasonable only if we have fully worked out how likely any planet is to be habitable. This new direction of astrobiological research tells me we have a long way to go.

The Mendez paper is Mendez et al., “Arecibo Wow! II: Revised Properties of the Wow! Signal from Archival Ohio SETI Data,” currently available as a preprint but submitted to The Astrophysical Journal. The Kipping paper from 2022 is Kipping & Gray, “Could the ‘Wow’ signal have originated from a stochastic repeating beacon?” Monthly Notices of the Royal Astronomical Society, Volume 515, Issue 1 (September 2022), pp.1122-1129 (full text). Thanks to my friend Antonio Tavani for the heads-up on the Mendez paper. The paper on the FAST search of TRAPPIST-1 is Guang-Yuan Song et al., “A Deep SETI Search for Technosignatures in the TRAPPIST-1 System with FAST,” submitted to The Astrophysical Journal and available as a preprint.

The Scherf and Lammer presentation is titled “How common are biological ETIs in the Galaxy?” EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–12 Sep 2025, EPSC-DPS2025-1512. The abstract is available here. The same team’s 2024 paper on these matters is “Scherf et al. “Eta-Earth Revisited II: Deriving a Maximum Number of Earth-Like Habitats in the Galactic Disk,” Astrobiology 24 (2023), e916 (full text). The paper from Adam Frank is Frank & Balbi, “The oxygen bottleneck for technospheres,” Nature Astronomy 8 (2024), pp. 39–43 (abstract / preprint).

And I want to be sure to mention Robert Gray, Kipping’s co-author on his 2022 paper, who devoted years to the study of the Wow! Signal and was kind enough to write about his quest in these pages. If you’re not familiar with Gray’s work, I hope you’ll read my An Appreciation of SETI’s Robert Gray (1948-2021). I only wish I had gotten to know him better. His death was a loss to the entire community. David Kipping’s fine video covering his work with Gray is available at Cool Worlds. Keith Cooper also explicates this paper in One Man’s Quest to Investigate the Mysterious “Wow!” Signal.

Stitching the Stars: Graphene’s Fractal Leap Toward a Space Elevator 10 Sep 7:47 AM (last month)

The advantages of a space elevator have been percolating through the aerospace community for quite some time, particularly boosted by Arthur C. Clarke’s novel The Fountains of Paradise (1979). The challenge is to create the kind of material that could make such a structure possible. Today, long-time Centauri Dreams reader Adam Kiil tackles the question with his analysis of a new concept in producing graphene, one which could allow us to create the extraordinarily strong cables needed. Adam is a satellite image analyst located in Perth, Australia. While he has nursed a long-time interest in advanced materials and their applications, he also describes himself as a passionate advocate for space exploration and an amateur astronomer. Today he invites readers to imagine a new era of space travel enabled by technologies that literally reach from Earth to the sky.

by Adam Kiil

In the quiet predawn hours, a spider spins its web, threading together a marvel of biological engineering: strands that are lightweight, elastic, and capable of absorbing tremendous energy before failing. This isn’t just nature’s artistry; it’s a lesson in hierarchical design, where proteins self-assemble into beta-pleated sheets and amorphous regions, creating a material tougher than Kevlar — able to dissipate impacts like a shock absorber — while outperforming steel in strength-to-weight ratio, though falling short of Kevlar’s raw tensile strength.

As we gaze upward toward the stars, dreaming of bridges to orbit, such bio-inspired ingenuity beckons. Could we mimic this to construct a space elevator tether, a ribbon stretching 100,000 kilometers from Earth’s equator to geostationary orbit and beyond? The demands are staggering: a material with a specific strength exceeding 50 GPa·cm³/g to support its own weight against gravity’s pull, all while withstanding radiation, micrometeorites, and immense tensile stresses. [GPa is a reference to gigapascals, the units used to measure tensile strength at high pressures and stresses. Thus GPa·cm³/g represents the ratio of strength to density].

Image: A space elevator is a revolutionary transportation system designed to connect Earth’s surface to geostationary orbit and beyond, utilizing a strong, lightweight cable – potentially made of graphene due to its extraordinary tensile strength and low density—anchored to an equatorial base station and extending tens of thousands of kilometers to a counterweight in space. This megastructure would enable low-cost, efficient transport of payloads and people into orbit, leveraging a climber mechanism that ascends the cable, potentially transforming space access by reducing reliance on traditional rocket launches. Credit: Pat Rawlings/NASA.

Enter a recent breakthrough in graphene production from professor Chris Sorensen at Kansas State University and Vancouver-based HydroGraph Clean Power, whose detonation synthesis yields pristine, fractal, and reactive graphene — potentially a key ingredient in weaving this cosmic thread.

But this alone may not suffice; we must think from first principles, exploring uncharted solutions to assemble nanoscale wonders into macroscale might.

Graphene’s Promise and Perils: The Historical Context

Graphene, that atomic-thin honeycomb of carbon, tantalizes with its theoretical tensile strength of 130 GPa and density of 2.2 g/cm³, yielding a specific strength around 59 GPa·cm³/g—right on the cusp of space elevator viability.

Yet, production has long been the bottleneck. Chemical vapor deposition churns out high-quality but limited sheets; mechanical exfoliation delivers impure, aggregated flakes. These yield composites where graphene platelets, bound weakly by van der Waals forces (mere 0.1-1 GPa), slip under strain, like loose pages in a book. For a tether, we need seamless load transfer, hierarchical reinforcement, and defect minimization—echoing the energy-dissipating nanocrystals in spider silk’s protein matrix.

Sorensen’s Detonation Concept: Fractal and Reactive Graphene

Chris Sorensen’s innovation at HydroGraph Clean Power flips the script. Using a controlled detonation of acetylene and oxygen in a sealed chamber, his team produces graphene with over 99.8% purity, fractal morphology, and tunable reactivity—all at scale, with zero waste and low emissions.

The fractal form — branched, snowflake-like platelets with 200 m²/g surface area — enhances interlocking, outperforming traditional graphene by 10-100 times in composites, but crucially, these gains shine at ultra-low loadings (0.001%) and under modest stresses, not yet the gigapascal realms of a space elevator.

Reactive variants add edge functional groups like carboxylic acids (COOH), enabling covalent bonding—yet, note that simple condensation reactions here yield strengths akin to polymer chains (1-5 GPa), not the in-plane prowess of graphene’s sp² lattice.This fractal graphene could form a foundational scaffold, reconfigurable into aligned structures that mimic bone’s porosity or silk’s hierarchy. Earthly spin-offs abound: tougher concrete, sensitive sensors, efficient batteries. But for the stars, we must bridge the gap from nanoplatelets to kilometer-long cables.

Image: Conceptual view of Hydrographs’ turbostratic, 50nm nanoplatelets, 99.8% pure carbon, sp2 bonded graphene. Credit: Adam Kiil.

From First Principles: Many Paths to a Cosmic Thread

To transcend these limits, let’s reason from fundamentals. A space elevator tether must maximize tensile strength while minimizing density and defects, distributing stress across scales like spider silk’s beta-sheets (crystalline strength) embedded in an extensible amorphous matrix.

Graphene’s strength derives from its delocalized electrons in a defect-free lattice; any assembly must preserve this while forging inter-platelet bonds rivaling intra-platelet ones. Current methods fall short, so here are myriad speculative solutions, drawn from physics, chemistry, and biology—some extant, others nascent or hypothetical, demanding innovation:

• Edge-Fusion via Plasma or Laser Annealing: Functionalize edges with hydrogen or halogens, then use plasma arcs or femtosecond lasers to fuse platelets into seamless, extended sheets or ribbons, healing defects to approach single-crystal continuity. This could yield tensile strengths nearing 100 GPa by eliminating weak interfaces.
• Supramolecular Self-Assembly in Liquid Crystals: Disperse fractal graphene in nematic solvents, applying shear or electric fields to align platelets into helical fibrils, stabilized by pi-pi stacking and hydrogen bonding. Inspired by silk’s pH-induced assembly, this bottom-up approach might create defect-tolerant bundles with built-in energy dissipation.
• Bio-Templating with Engineered Proteins: Design peptides (via AI like AlphaFold) that bind graphene edges, mimicking silk spidroins’ repetitive motifs to fold platelets into hierarchical nanocrystals. Extrude through microfluidic spinnerets, acidifying to trigger beta-sheet formation, embedding graphene in a tough, elastic matrix.
• Covalent Cross-Linking with Boron or Nitrogen Dopants: Introduce boron atoms during detonation to create sp³ bridges between platelets, forming diamond-like nodes in a graphene network. This could boost shear strength to 10-20 GPa without sacrificing tensile properties, verified by molecular dynamics.
• Electrospinning with Magnetic Alignment: Mix reactive graphene in a polymer dope, electrospin under magnetic fields to orient platelets, then pyrolyze the polymer, leaving aligned, sintered graphene fibers. Enhancements: Add ultrasonic waves for dynamic packing, targeting <1 defect per 100 nm².
• Hierarchical Bundling via 3D Printing: Nanoscale print graphene inks layer-by-layer, using click chemistry (e.g., thiol-ene) for instant cross-links. Scale up to micro-bundles, then macro-cables, tapering density like a tree trunk to root.
• Dynamic Compression and Sintering: Apply gigapascal pressures in a diamond anvil cell, combined with heat, to induce partial sp²-to-sp³ transitions at overlaps, creating hybrid structures akin to lonsdaleite—ultra-hard yet flexible.
• Biomineralization Analogs: Introduce calcium or silica ions to reactive groups, mineralizing interfaces like nacre, adding compressive strength and crack deflection.
• AI-Optimized Hybrid Composites: Simulate (via quantum computing) blends of fractal graphene with silk-mimetic polymers or boron nitride, optimizing ratios for 90% tensile efficiency. Fabricate via wet-spinning, testing at centimeter scales.

These aren’t exhaustive; hybrids abound—e.g., combining bio-templating with laser fusion. Each target’s aim: moving beyond low-load enhancements and polymer-like bonds to harness graphene’s full lattice strength.

Weaving and Laminating: Practical Steps Forward

Drawing from these, a viable process might start with a high-solids dispersion of reactive fractal graphene, extruded via wet-spinning into aligned fibers, where optimized cross-linkers (not mere condensations) ensure graphene-dominant strength. Stack into nacre-like laminates, using hot isostatic pressing (5-20 GPa) to forge sp³ bonds, elevating shear (and thus overall tensile) resilience to 10-20 GPa. Taper the structure: thick at the base for 7 GPa stresses, thinning upward.

Scaling leverages HydroGraph’s modular reactors, producing tonnage graphene for kilometer segments.

Join via overlap lamination, braid for redundancy, deploy from orbit. Prototypes must demonstrate cohesive failure, >90% load transfer, via nanoindentation.

A Bridge to the Cosmos

Sorensen’s detonation-born graphene, fractal and reactive, ignites possibility. Yet, as spider silk teaches, true mastery lies in hierarchy and adaptation.

Success means a tether with inter-platelet bond strength nearing single-crystal graphene (>100 GPa), verified by nanoindentation or pull-out tests, with >90% tensile transfer efficiency. Centimetre-scale prototypes should show minimal defects (<1 per 100 nm² via TEM), failing cohesively, not delaminating, like a spider’s web holding under a gale. The full tether, massing under 500 tonnes, could be deployed from orbit, a lifeline to the cosmos. This graphene tether embodies our ‘sea-longing,’ a bridge to the stars woven from carbon’s hexagons, inspired by nature’s spinners and builders.

By innovating from first principles—fusing, assembling, templating—we edge closer to stitching the stars. This isn’t mere materials science; it’s the warp and weft of humanity’s interstellar tapestry, a web to catch the dreams of Centauri and beyond.

3I/ATLAS: The Case for an Encounter 6 Sep 5:57 AM (2 months ago)

The science of interstellar objects is moving swiftly. Now that we have the third ‘interloper’ into our Solar System (3I/ATLAS), we can consider how many more such visitors we’re going to find with new instruments like the Vera Rubin Observatory, with its full-sky images from Cerro Pachón in Chile. As many as 10,000 interstellar objects may pass inside Neptune’s orbit in any given year, according to information from the Southwest Research Institute (SwRI).

The Gemini South Observatory, likewise at Cerro Pachón, has used its Gemini Multi-Object Spectrograph (GMOS) to produce new images of 3I/ATLAS. The image below was captured during a public outreach session organized by the National Science Foundation’s NOIRLab and the Shadow the Scientists initiative that seeks to connect citizen scientists with high-end observatories.

Image: Astronomers and students working together through a unique educational initiative have obtained a striking new image of the growing tail of interstellar Comet 3I/ATLAS. The observations reveal a prominent tail and glowing coma from this celestial visitor, while also providing new scientific measurements of its colors and composition. Credit: Gemini Observatory/NSF NOIRlab.

Immediately obvious is the growing size of the coma, the cloud of dust and gas enveloping the nucleus as 3I/ATLAS moves closer to the Sun and continues to warm. Analyzing spectroscopic data will allow scientists to understand more about the object’s chemistry. So far we’re seeing cometary dust and ice not dissimilar to comets in our own system. We won’t have this object long, as its orbit is hyperbolic, taking it inside the orbit of Mars and then off again into interstellar space. Perihelion should occur at the end of October. It’s interesting to consider, as Marshall Eubanks and colleagues do in a new paper, whether we already have spacecraft that can learn something further about this particular visitor.

Note this from the paper (citation below):

Terrestrial observations from Earth will be difficult or impossible roughly from early October through the first week of November, 2025… [T]he observational burden during this period will, to the extent that they can observe, largely fall on the Psyche and Juice spacecraft and the armada of spacecraft on and orbiting Mars. Our recommendation is that attempts should be made to acquire imagery from encounter spacecraft during the entire period of the passage of 3I through the inner solar system, and in particular from the period in October and November of 2025, when observations from Earth and the space telescopes will be limited by 3I’s passage behind the Sun from those vantage points.

As we consider future interstellar encounters, flybys begin to look possible. Such was the conclusion of an internal research study performed at SwRI, which examined costs and design possibilities for a mission that may become a proposal to NASA. SwRI was working with software that could create a large number of simulated interstellar objects, while at the same time calculating a trajectory from Earth to each. Matthew Freeman is project manager for the study. It turns out that the new visitor is itself within the study’s purview:

“The trajectory of 3I/ATLAS is within the interceptable range of the mission we designed, and the scientific observations made during such a flyby would be groundbreaking. The proposed mission would be a high-speed, head-on flyby that would collect a large amount of valuable data and could also serve as a model for future missions to other ISCs [interstellar comets].”

Image: Upper left panel: Comet 3I/ATLAS as observed soon after its discovery. Upper right panel: Halley’s comet’s solid body as viewed up close by ESA’s Giotto spacecraft. Lower panel: The path of comet 3I/Atlas relative to the planets Mercury through Saturn and the SwRI mission interceptor study trajectory if the mission were to be launched this year. The red arc in the bottom panel is the mission trajectory from Earth to interstellar comet 3I/ATLAS. Courtesy of NASA/ESA/UCLA/MPS.

So we’re beginning to undertake the study of actual objects from other stellar systems, and considering the ways that probes on fast flyby missions could reach them. 3I/ATLAS thus makes the case for further studies of flyby missions. SwRI’s Mark Tapley, an expert in orbital mechanics, is optimistic indeed:

“The very encouraging thing about the appearance of 3I/ATLAS is that it further strengthens the case that our study for an ISC mission made. We demonstrated that it doesn’t take anything harder than the technologies and launch performance like missions that NASA has already flown to encounter these interstellar comets.”

The paper on a fast flyby mission to an interstellar object is Eubanks et al., “3I/ATLAS (C/2025 N1): Direct Spacecraft Exploration of a Possible Relic of Planetary Formation at “Cosmic Noon,” available as a preprint.

Ancient Life on Ceres? 3 Sep 10:51 AM (2 months ago)

We keep going through revolutions in the way science fiction writers handle asteroids. Discovered in 1801, Ceres and later Pallas (1802) spawned the notion that there once existed a planet where what came to be thought of as the asteroid belt now exists. Heinrich Olbers was thinking of the Titius-Bode law when he suggested this, pointing to the mathematical consistency of planetary orbits implicit in the now discredited theory. Robert Cromie wrote a novel called Crack of Doom in 1895 that imagined a fifth planet blown apart by futuristic warfare, a notion picked up by many early science fiction writers.

Nowadays, that notion seems quaint, and asteroids more commonly appear in later SF either as resource stockpiles or terraformed habitats, perhaps hollowed out to become starships. Nonetheless, there was a flurry of interest in asteroids as home to extraterrestrial life in the 1930s (thus Clark Ashton Smith’s “Master of the Asteroid”), and actually none other than Konstantin Tsiolkovsky wrote an even earlier work called “On Vesta” (1896), where he imagines a technological civilization emerging on the asteroid. Before someone writes to scold me for leaving it out, I have to add that Antoine de Saint-Exupéry’s The Little Prince (1943) assumes an asteroid as the character’s home world.

The Dawn mission that did so much to reveal the surface of Ceres made it clear how cold and seemingly inimical to life this largest of all asteroids seems to be. But even here, there is new speculation about a warmer past and the possibility of at least primitive life emerging under the frigid surface. The reservoir of interior salty water whose surface residue can be seen in Ceres’ reflective regions now appears, via Dawn data, to contain carbon molecules. So we have water, carbon molecules and a likely source of chemical energy here.

Image: Dwarf planet Ceres is shown in these enhanced-color renderings that use images from NASA’s Dawn mission. New thermal and chemical models that rely on the mission’s data indicate Ceres may have long ago had conditions suitable for life. Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA.

The new work from lead author Samuel Courville (Arizona State University) and team is built around computer models of Ceres, looking at its chemistry and thermal characteristics since the early days of the Solar System. The evidence shows that as late as 2.5 billion years ago, the asteroid’s internal ocean was still being heated by radioactive decay of elements deep in the interior, providing dissolved gases carried by upwelling hot water exposed to the rocks in Ceres’ core.

That’s a significant marker, because we know that here on Earth, mixing hot water, metamorphic rock and ocean provides everything life needs. Demonstrating that Ceres once had hydrothermal fluid feeding into its ocean would imply that microbes could have formed there, even if today they are long gone. Today heat from radioactive decay should be insufficient to keep Ceres’ water from turning into a concentrated brine, but the situation several billion years ago was similar to that of many Solar System objects which lack any source of tidal heating because they do not orbit a large planet.

Image: This illustration depicts the interior of dwarf planet Ceres, including the transfer of water and gases from the rocky core to a reservoir of salty water. Carbon dioxide and methane are among the molecules carrying chemical energy beneath Ceres’ surface. Credit: NASA/JPL-Caltech.

From the paper:

Since Ceres is not subject to as many complex evolutionary factors (e.g., tidal heating) as are many other candidate ocean worlds that orbit gas giants, it is an ideal body to study evolutionary pathways relevant to candidate ocean worlds in the ~500- to 1000-km radius range. Being in large numbers, these bodies might represent the most abundant type of habitable environment in the early solar system.

An interesting thought! We can even speculate on how long this window of astrobiological opportunity might have remained open:

From a chemical energy perspective, the most habitable periods for these objects were when the rocky interiors underwent thermal metamorphism. Metamorphism leads to an influx of fluids into the ocean. These fluids could provide a steady source of chemical disequilibrium for several hundred million years. In the case of Ceres, the metamorphic period between ~0.5 and 2 Gyr would have created a potentially habitable environment at the seafloor if the rocky mantle reached temperatures greater than ~700 K. The decreasing temperature of Ceres’s interior over the past ~1 Gyr would likely render it thermodynamically inactive at present.

A sample return mission to Ceres? Such is recommended as a New Frontiers mission in the decadal survey Origins, worlds, and life: a decadal strategy for planetary science and astrobiology 2023–2032 (The National Academies Press, 2022). The document cites the need for samples “…collected from young carbonate salt deposits, typified by those identified by the Dawn mission at Occator crater, as well as some of Ceres’s typical dark materials.”

The paper is Courville et al., “Core metamorphism controls the dynamic habitability of mid-sized ocean worlds—The case of Ceres,” Science Advances Vol. 11 Issue 34 (20 August 2025). Full text.

Amazing Worlds: A Review 29 Aug 8:34 AM (2 months ago)

I hardly ever watch a film version of a book I love because my mental images from the book get mangled by the film maker’s vision. There’s also the problem of changes to the plot, since film and novels are entirely different kinds of media. The outliers, though, are interesting (and I sure did love Bladerunner). And when I heard that AppleTV would do Asimov’s Foundation books, I resolved to watch because I was satisfied there was no way on Earth my book images would conflict with what a filmmaker might do. How could anyone possibly produce a film version of these books?

Judging from the comments I see online, a lot of people realize how remote the AppleTV series is from the source. But here we get into something interesting about the nature of science fiction, and it’s something I have been thinking about since reading Keith Cooper’s book Amazing Worlds of Science Fiction and Science Fact. For the streaming variant of Foundation is visually gorgeous, and it pulls a lot of taut issues out of what I can only describe as the shell or scaffolding of the Asimov titles. Good science fiction is organic, and can grow into productive new directions.

A case in point: The ‘moonshrikes.’ They may not be in the books, but what a marvelous addition to the story. These winged creatures the size of elephants take advantage of another science fictional setting, a ‘double planet,’ two worlds so tightly bound that their atmospheres mix. You may remember Robert Forward playing around with this idea in his novel Rocheworld (1990), and to my knowledge that is the fictional origin of what appears to be a configuration well within the laws of physics. In the streaming Foundation, a scene where Hari Seldon watches moonshrikes leaping off cliffs to soar into the sky and graze on the sister world is pure magic.

Keith Cooper is all about explaining how this kind of magic works, and he goes at the task in both literary and filmed science fiction. Because the topic is the connection between real worlds and imagined ones, he dwells on that variant of science fiction called ‘hard SF’ to distinguish it from fantasy. As we’ve seen recently in talking about neutron stars and possible life forms there, the key is to imagine something that seems fantastic and demonstrate that it is inherently plausible. Asimov could do this, as could Clarke, as could Heinlein, and of course the genre continues into Benford, Baxter, Vinge, Reynolds, Niven and so on.

Image: The moonshrikes take wing. Credit: AppleTV / Art of VFX.

It’s hard to know where to stop with lists like that (and yes, I should mention Brin and Bear and many more), but the point is, this is the major thrust of science fiction, and while AppleTV’s Foundation takes off on explorations far from the novels, its lush filmography contains within it concepts that have been shrewdly imagined and presented with lavish attention to detail. Other worlds, as Keith Cooper will remind us in his fine book, are inescapably alien, yet they can be (at least to our imaginations, since we can’t directly see most of them yet) astonishingly beautiful. Cooper’s intuitive eye gets all that.

The rich history of science fiction, from the pulp era through to today’s multimedia extravaganzas, gets plenty of attention. I’m pleased to report that Cooper’s knowledge of SF history is deep and he moves with ease through its various eras. His method is to interview and quote numerous writers on the science behind their work, and numerous scientists on the origins of their interest. Thus Alison Sinclair, whose 1996 novel Blueheart takes place on an ocean world. Sinclair, a biochemist with a strong background in neuroscience, knows about the interplay between the real and the imagined.

Sinclair talks about how Blueheart’s ocean, being warmer and less salty than Earth’s oceans on average, is therefore less dense and floats atop a deeper layer of denser water. The aquatic life on Blueheart lives in that top layer, but when that life dies its remains, along with the nutrients those remains contain, would sink right to the bottom of the dense layer. She raises an additional point that on Earth, deep water is mixed with surface water by winds that drive surface water away from coasts, allowing deep water to well up, but with no continents on Blueheart there are no coasts, and with barely any land there’s no source of nutrients to replenish those that have sunk to the bottom.

Here is the science fictional crux, the hinge where an extrapolated problem is resolved through imaginative science. Sinclair, with an assist from author Tad Williams, will come up with a ‘false bottom,’ a layer of floating forests with a root system dense enough to act as a nutrient trap. It’s an ingenious solution if we don’t look too hard, because the question of how these floating thickets form in the first place when nutrients are in the oceanic deep still persists, but the extent to which writers trace their planet building backwards remains highly variable. It’s no small matter imagining an entire ecosystem over time.

The sheer variety of exoplanets we have thus far found and continue to hypothesize points to science fiction’s role in explaining research to the public. Thus Cooper delves deeply into desert worlds including the ultimate dry place, Frank Herbert’s Arrakis, from the universe he created in Dune (1965) and subsequent novels. Here he taps climatologists from the University of Bristol, where Alexander Farnsworth and team have modeled Arrakis, with Farnsworth noting that world-building creates huge ‘blue-sky’ questions. As he puts it, SF “…asks questions that probably wouldn’t be asked scientifically by anyone else.”

Solid point. Large, predatory creatures don’t work on desert worlds like Arrakis (there go the sand worms), but Arrakis does force us to consider how adaptation to extremely dry environments plays out. Added into the team’s simulations were author Herbert’s own maps of Arrakis, with seas of dunes at the equator and highlands in the mid-latitudes and polar regions, and the composition of its atmosphere. Herbert posits high levels of ozone, much of it produced by sand worms. Huge storms of the kind found in the novel do fit the Bristol model and lead Cooper into a discussion of Martian dust storms, factoring in surface heating and differences in albedo. All told, Dune is an example of a science fiction novel tat compels study because of the effort that went into its world building, and recent work helps us see when its details go awry.

Image: Judging from the comments of many scientists I’ve known, Frank Herbert’s Dune inspired more than a few careers that have led to exoplanet research. The publishing history is lengthy, but here’s the first appearance of the planet Arrakis in “Dune World,” the first half of the original novel, as serialized beginning with the December, 1963 issue of Analog.

The explosion of data on exoplanets, of which there were close to 6000 confirmed as Cooper was wrapping up his manuscript, has induced subtle shifts in science fiction that are acknowledged by writers as well as scientists (and the two not infrequently overlap). I think Cooper is on target as he points out that in the pre-exoplanet discovery era, Earth-like worlds were a bit easier to imagine and use as settings. But we still search for a true Earth analogue in vain.

…it’s probably fair to say that SF before the exoplanet discoveries of the 1990s was biased towards imagining worlds that were like something much closer to home. Alas, comfortably habitable worlds like Earth are, so far, in short supply. Instead, at best, we might be looking at habitable niches rather than whole welcoming worlds. Increasingly, more modern SF reflects this; think of the yin-yang world of unbearable heat and deathly cold from Charlie Jane Anders’s Locus award-winning 2019 novel The City in the Middle of the Night or the dark, cloud-smothered moon LV-426 in Alien (1979) and Aliens (1986) that has to be terraformed to be rendered habitable (although that example actually pre-dates the discovery of exoplanets).

Changes in the background ‘universe’ of a science fiction tale are hardly new. It was in 1928 that Edward E. ‘Doc’ Smith published The Skylark of Space, an award-winning tale which broached the idea that science fiction need not be confined to the Solar System. In the TV era, Star Trek reminded us of this when we suddenly had a show where the Earth was seldom mentioned. Both had some precursors, but the point is that SF adapts to known science but then can make startling imaginative jumps.

Thus novelist Stephen Baxter, a prolific writer with a background in mathematics and engineering:

’Now that we know planets are out there, it’s different because as a writer you’re exploring something that’s already defined to some extent scientifically, but it’s still very interesting…You know the science and might have some data, so you can use all that as opposed to either deriving it or just imagining it.”

What a terrific nexus for discovery and imagination. If you’re been reading science fiction for as long as I have, you’ll enjoy how famous fictional worlds map up against the discoveries we’re making with TESS and JWST. I found particular satisfaction in Cooper’s explorations of Larry Niven’s work, which clearly delights any number of scientists because of its imaginative forays within known physics and the sheer range of planetary settings he deploys.

No wonder fellow SF writers like Alastair Reynolds and Paul MacAuley cite him within these pages as an influence on their subsequent work. Niven, as McAuley points out, can meld Earth-like features with profound differences that breed utterly exotic locales. This is a man who has, after all, written (like Clement and Forward) about extreme environments for astrobiology (think of his The Integral Trees, for example, with hot Jupiters and neutron star life).

And then there’s Ringworld, with its star-encircling band of technology, and the race known as Pierson’s Puppeteers, developed across a range of stories and novels, who engineer a ‘Klemperer Rosette’ out of five worlds, one of them their home star. Each is at the point of a pentagon and all orbit a point with a common angular momentum. Their home world, Hearth, is an ‘ecumenopolis,’ a world-spanning city on the order of Asimov’s Trantor. Here again the fiction pushes the science to come up with explanations. Exoplanet scientist and blogger Alex Howe (NASA GSFC) explains his own interest:

“The Puppeteer’s Hearth is one of the things that keyed me in to the waste heat problem,” says Howe, who is a big fan of Niven: “I describe Larry Niven as re-inventing hard science fiction… not as SF that conforms strictly to known physics, but as SF that invents new physics or perhaps extrapolates from what we currently know, but applies it rigorously.”

Howe is an interesting example of the involvement of scientists with science fiction. A writer himself, he maintains his own blog devoted to the subject and has been working his way through all the classic work in the field. I’ve focused on SF in this review, but need to point out that Cooper’s work is equally strong coming in the non-fictional direction, with productive interviews with leading exoplanetologists. For now that we’re actually studying real planets around other stars, worlds like TOI-1452b, a habitable zone super-Earth around a binary, point to how fictional some of these actual planets seem.

So with known planets as a steadily growing database, we can compare and contrast the two approaches. Thus we meet Amaury Triaud (University of Birmingham), a co-discoverer of the exotic TRAPPIST-1 system and its seven small, rocky worlds. The scientist worked with Nature to coax Swiss SF writer Laurence Suhner into setting a story in that system.

Says Triaud: “If you were in your back garden with a telescope on one of these planets, you’d be able to actually see a city on one of the other planets.” Similarly, the snowball planet Gethen from Ursula le Guin’s The Left Hand of Darkness (1969) is put through analysis by planetary scientist Adiv Paradise (University of Toronto). Thus we nudge into studies of Earth’s own history extrapolated into fictional planets that invoke entirely new questions.

Here’s Paradise on snowball planets and their fate. Must they one day thaw?

“If you have a planet that doesn’t have plate tectonics, and doesn’t have much volcanism, can the carbon dioxide still escape from the outside?… You might end up with a planet where all the carbon dioxide gets locked into the mantle, and volcanism shuts off and you end up with a runaway snowball that might suppress volcanism – we don’t fully understand the feedback between surface temperature and volcanism all that well. In that case, the snowball would become permanent, at least until the star becomes brighter and melts it.”

Cooper’s prose is supple, and it allows him to explain complicated concepts in terms that newcomers to the field will appreciate. Beyond the ‘snowball’ process, the carbonate-silicate cycle so critical to maintaining planetary climates gets a thorough workout, as does the significance of plate tectonics and the consequences if a world does not have this process. Through desert worlds to water worlds to star-hugging M-dwarf planets, we learn about how atmospheres evolve and the methods scientists are using to parse out their composition.

Image: NASA’s playful poster of the TRAPPIST-1 system as a travel destination. Credit: NASA.

Each world is its own story. I hope I’ve suggested the scope of this book and the excitement it conveys even to someone who has been immersed in both science fiction and exoplanetary science for decades. Amazing Worlds of Science Fiction and Science Fact would make a great primer for anyone looking to brush up on knowledge of this or that aspect of exoplanet discovery, and a useful entry point for those just wanting to explore where we are right now.

I also chuckle at the title. Amazing Stories was by consensus the first true science fiction magazine (1926). Analog, once Astounding with its various subtitles, used ‘Science Fiction – Science Fact’ on its cover (I remember taking heat from my brother in law about this, as he didn’t see much ‘fact’ in what I was reading. But then, he wasn’t an SF fan). As a collector of old science fiction magazines, I appreciate Keith Cooper’s nod in their direction.

Claudio Maccone (1948-2025) 22 Aug 7:18 AM (2 months ago)

In all too many ways, I wasn’t really surprised to learn that Claudio Maccone had passed away. I had heard the physicist and mathematician had been in ill health, and because he was a poor correspondent in even the best of times, I was left to more or less assume the worst. His death, though, seems to have been the result of an accident (I’m reminded of the fall that took Freeman Dyson’s life). Claudio and I spent many hours together, mostly at various conferences, where we would have lengthy meals discussing his recent work.

Image: I took this photo of Claudio in Austin, TX in 2009. More on that gathering below.

With degrees in both physics and mathematics from the University of Turin, Claudio received his PhD at King’s College London in 1980. His work on spacecraft design began in 1985, when he joined the Space Systems Group of Alenia Spazio, now Thales Alenia Space Italia, which is where he began to develop ideas ranging from scientific uses for the lunar farside, SETI detections and signal processing, space missions involving sail concepts and, most significantly, a mission to the solar gravitational lens, which is how he and I first connected in 2003.

Coming into the community of deep space scientists as an outsider, a writer whose academic expertise was in far different subjects, I always appreciated the help I received early on from people who were exploring how we might overcome the vast distances between the stars. I had written about Claudio’s ideas on gravitational lensing and the kind of mission that might use it for observation of exoplanets, but was startled to find him waiting at breakfast one morning in Princeton, where Greg Matloff had invited me up for a conference.

Image: At one of the Breakthrough Starshot meetings not long after the project was announced. I took this in the lobby of our hotel in Palo Alto.

Ever the gentleman, Claudio wanted to thank me for my discussion of his work in my Centauri Dreams book, and that breakfast with Greg, his wife C, and Claudio remains a bright memory. As is the conference, chaired by Ed Belbruno, where I made many contacts talking to scientists about their work. I was already writing this site, which began in 2004, and over the years that followed, I would run into Claudio again and again, and not always at major conferences. He appeared, for example, at a founding session of what would grow into the Interstellar Research Group in Oak Ridge, quite a hike from Italy, but when it came to interstellar ideas, Claudio always wanted to be there.

One memorable trip was at Aosta in the Italian Alps, a meeting I particularly cherish because of our meals discussing local history as much as spaceflight. Several of the participants at the Aosta conference had brought their families, and one young boy was fascinated with something Claudio said one night at dinner about Italian history. I’ll never forget his asking Claudio if he could explain what had happened in the Thirty Years War. Claudio didn’t miss a beat. He began talking and in about fifteen minutes had laid out the causes of the conflict between Protestants and Catholics in 17th Century Europe within the context of the Holy Roman Empire, complete with names, dates and details.

I asked him afterwards if he had ever considered a career in history, and it turned out that we both shared an interest especially in Greek and Latin, and that yes, the subject appealed to him, whether it was 17th Century political evolution or the fine points of Pericles’ Funeral Oration. But here he voiced a caution. “You can’t do everything. You just can’t. You have to give so many things up to do your work.” True, of course, and yet somehow his knowledge was that of a polymath. He schooled me on Leibniz over wine and Beef Wellington one night in Dallas, a conversation that went on until late in the evening. In the Italian Alps, he took me into the history of Anselm of Canterbury, who had been born in Aosta, and explained the significance of his philosophy.

Mostly, of course, we talked about space. His fascination with SETI was obvious, and his work on the mathematics of first contact brought the Kosambi–Karhunen–Loève (KLT) theorem into play as a signal processing solution. His work on gravitational lensing and the mission he called FOCAL put that concept in the spotlight, paving the way for later approaches that could solve some of the problems he had identified. A look through the archives here will reveal dozens of articles I’ve written on this and other aspects of his work.

Version 1.0.0

I won’t go into the technical details here – this is a time for recollection more than analysis. But I need to mention that Claudio served as Technical Director for Scientific Space Exploration at the International Academy of Astronautics, a post of which he was rightly proud. Over 100 scientific papers bear his name, as do a number of books, the most influential being Mathematical SETI (2012) and Deep Space Flight and Communications (2009). His ongoing efforts to preserve the lunar farside for astronomical observations remind us of how precious this resource should be in our thinking. Toward the end of his life, he put his mathematical skills to work on the evolution of cultures on Earth and asked whether what he called Evo-SETI could be developed as a way to predict the likelihood of civilizations including our own surviving.

Claudio’s death caused a ripple of comment from people who had worked with him over the years. Greg Matloff, a close friend and colleague, wrote this:

I met Claudio during a Milan solar sail symposium in 1990. We collaborated for years on sails to the solar gravity focus. He was instrumental in my guest professorship in Siena in summer 1994 and my election to IAA. I knew that his health had become a challenge. Apparently, a book shelf had collapsed and he was pinned overnight until his student found him the following morning. I will miss Claudio forever but am glad that I saw him in Luxembourg last December.

I only wish I could have seen Claudio one more time, but it was not to be. I will always honor this good man and thank him for his generosity of spirit, his engaging humor and his willingness to bring me up to speed on concepts that, early on, I found quite a stretch. Now that the concept of a gravitational lensing mission is widespread in the literature and superb work continues at the Jet Propulsion Laboratory on completely new technologies to make this happen, I think Claudio’s place in the history of the interstellar idea is guaranteed.

Image: Claudio Maccone speaking before the United Nations Committee on the Peaceful Uses of Outer Space in Vienna 2010. He was proposing a radio-quiet zone on the farside that will guarantee radio astronomy and SETI a defined area in which human radio interference is impossible. It’s an idea with a pedigree, going back to 1994, when the French radio astronomer Jean Heidmann first proposed a SETI observatory in the farside Saha Crater with a link to the nearside Mare Smythii plain and thence to Earth. Credit: COPUOS.

A final happy memory. Claudio and I had met in Austin Texas and were headed to the Austin Institute for Advanced Studies, to meet with Marc Millis, Eric Davis and Hal Puthoff, among others. Claudio had just bought a new laptop so powerful that it could handle the kind of equations he threw at it. On the way, he bragged about it and showed it to me with delight all over his face.

But the joke was that with facial recognition, it refused to recognize him despite his protracted training of the gadget. I can still hear his normally calm voice gradually growing in volume (the meeting had already started and everyone was taking notes), until finally he burst out with “It’s me, damn you!” We all guffawed, and a few seconds later, amazingly, the computer let him in.

Claudio loved Latin, so here’s a bit from Seneca:

Non est ad astra mollis e terris via.

Which means “There is no easy way from the earth to the stars.”

He certainly would have agreed with that statement, but Claudio Maccone did everything in his power to tackle the problem and show us the possibilities that lay beyond our current technologies. His work was truly a gift to our future.

Generation Ships and their Consequences 15 Aug 10:44 AM (2 months ago)

Our ongoing discussion of the Project Hyperion generation ship contest continues to spark a wide range of ideas. For my part, the interest in this concept is deeply rooted, as Brian Aldiss’ Non-Stop (1958 in Britain, and then 1959 in the U.S. under the title Starship), was an early foray into science fiction at the novel length for me. Before that, I had been reading the science fiction magazines, mostly short stories with the occasional serial, and I can remember being captivated by the cover of a Starship paperback in a Chicago bookstore’s science fiction section.

Of course, what was striking about Criterion Books’ re-naming of the novel is that it immediately gave away the central idea, which readers would otherwise have had to piece together as they absorbed Aldiss’ plot twists. Yes, this was a starship, and indeed a craft where entire generations would play out their lives. Alex Tolley and I were kicking the Chrysalis concept around and I was reminded how, having been raised in Britain, Alex had been surprised to learn of the American renaming of the book. But in a recent email, he reminded me of something else, and I’ll pass that along to further seed the discussion.

What follows is from Alex, with an occasional interjection by me. I’ll label my contributions and set them in italics to avoid confusion. Alex begins:

I should mention that in Aldiss’ novel Non-Stop, the twist was that the starship was no longer in transit, but was in Earth’s orbit. The crew could not be removed from the ship as it slowly degenerated. The Earthers were the ‘giants’ visiting the ship to monitor it and study the occupants.

PG: Exactly so. To recapitulate, the starship had traveled to a planet around Procyon, and in a previous generation had experienced a pandemic evidently caused by human incompatibility with the amino acids found in its water. On the return trip, order breaks down and the crew loses knowledge of their circumstances, although we learn that there are other beings who sometimes appear and interact in mysterious ways with the crew. The twenty-three generations that have passed are far more than was needed to reach their destination, but now, in Earth orbit, their mutated biology causes scrutiny from scientists who restrict their movement while continuing to study them.

PG: The generation ship always raises questions like this, not to mention creating questions about the ethics of controlling populations for the good of the whole. I commented to Alex about the Chrysalis plan to have multiple generations of prospective crew members live in Antarctica to ensure their suitability for an interstellar voyage and its myriad social and ethical demands. He mentions J.G. Ballard’s story “Thirteen to Centaurus” below, a short story discussed at some length in these pages by Christopher Phoenix in 2016.

Image: The original appearance of “Thirteen to Centaurus,” in The July, 1962 issue of Amazing Fact and Science Fiction Stories. Rather than having to scan this out of my collection, I’m thankful to the Classics of Science Fiction site for having done the scanning for me.

I missed the multiple generations in Antarctica bit, probably because I knew the UK placed Antarctic hopefuls in a similar environment for at least several weeks to evaluate suitability. The 500-day Martian voyage simulation would be like a prison sentence for the very motivated. But several generations in some enclosed environment would perhaps be like the simulated starship in “Thirteen to Centaurus” or the 2014 US TV series Ascension. Note that Antarctica is just a way of suggesting an isolated environment, which the authors indicate is TBD. Like the 500-day Mars simulation, all the authors want is a way to test for psychological suitability.

To do this over a span of multiple generations seems very unethical, to say the least. How are they going to weed out the “unsuitable”, especially after the first generation? I also think that there is a flaw in the reasoning. Genetics is not deterministic, especially as the authors expect normal human partnering on the ship. The sexual reproduction of the genes will constantly create genetically different children. This implies that the nurture component of socialization will be very important. How will that be maintained in the simulation, let alone the starship? Will the simulation inhabitants have to resolve all problems and any anti-social behavior by themselves? What if it becomes a “Lord of the Flies” situation? Is the simulation ended and a new one started when a breakdown occurs? It is a pity that the starship cannot be composed of an isolated tribe that has presumably already managed to maintain multi-generational stability.

If we’re going to simulate an interstellar voyage, we could build the starship, park it in an orbit within the solar system, and monitor it for the needed time. This would test everything for reliability and stability, yet ensure that the population could be rescued if it all goes pear-shaped. The ethics are still an issue, but if the accommodation is very attractive, it is perhaps not too different from living on a small island in the early industrial period, isolated from the world. The Hebrides until the mid-20th century might be an example, although the adventurous could leave, which is not a possibility on the starship.

Ethics aside, I suspect that the Antarctica idea is more hopeful than viable. In my view, it will take a very different kind of society to maintain a 100+ year simulation. But there are advantages to doing this in Earth orbit. It could be that the crew becomes a separate basket of eggs to repopulate the Earth after a devastating war, as Moon or Mars colonies are sometimes depicted.

PG: I’ve always thought that rather than building a generation ship, such vessels would evolve naturally. As we learn how to exploit the resources of the Solar System, we’ll surely become adept at creating large habitats for scientists and workers. A natural progression would be for some crew, no longer particularly interested in living on a planet, to ‘cast off’ and set off on a generational journey.

Slow boating to star systems will probably require something larger, more like an O’Neill Island 3 design. Such colonies will be mature, and the remaining issue of propulsion “solved” by strapping on whatever is the most appropriate – fusion, antimatter, etc. The ethics problem is presumably moot in such colonies, as long as the colony votes to leave the solar system, and anyone preferring to stay is allowed to leave.

This is certainly what Heppenheimer and O’Leary were advocating when the space colony idea was new and shiny. On the other hand, maybe the energy is best used to propel a much smaller ship at high fractional c to achieve time dilation. If it fails, only the first-generation explorer crew dies. In extremis, this is Anderson’s Tau Zero situation.

PG: With your background in biology, Alex, what’s your take on food production in a generation ship? I realize that we have to get past the huge question of closed loop life support first, but if we do manage that, what is the most efficient way to produce the food the crew will need?

I think that by the time a Chrysalis ship can be built, they won’t be farming field crops as we do today. The time allocated to agricultural activities might be better spent on some other activity. Food production will be whatever passes for vertical farms and food factory culture, with 3-D printing of foods for variety.

The only value I can see for traditional crop farming is that it may be the only way to expand the population on the destination planet, and that means maintaining basic farming skills. The Chrysalis design did not allow animal husbandry, which means that the crew would be Vegan or Vegetarian only. In that future, that may even be the norm, and eating animal flesh a repellant idea.

In any case, space colonies should be the first to develop the technology for very long-duration missions, then generation starships if that is the only way to reach the stars, and assuming it is deemed a worthwhile idea. That techbro, Peter Thiel, cannot get seasteading going. I do wonder whether human crewed starships for colonization make much sense.

But multi-year exploration ships evoking the golden age of exploration in sailing ships might be a viable idea. Exciting opportunities to travel, discover new worlds (“new life, and new civilizations…”), yet returning to the solar system after the tour is over. It would need fast ships or some sort of suspended animation to reduce the subjective time during the long cruise phase, so that most of the subjective time would be the exploration of each world.

PG: I’ll add to that the idea that crews on generation-class ships and their counterparts on this kind of faster mission may well represent the beginning of an evolutionary fork in our species. Plenty of interesting science fiction to be written playing with the idea that there is a segment of any population that would prefer to experience life within a huge, living habitat, and thus eventually become untethered to planting colonies or exploiting a planetary surface for anything more than scientific data-gathering.

Like the university-crewed, habitat-based starship in Vonda McIntyre’s Starfarers tetralogy. The ship is based on O’Neill’s space colony technology, but it can travel at FTL velocities and is mostly about exploring new worlds. It is very Star Trek in vibes, but more exploratory, fewer phasers and photon torpedoes.

PG: So the wave of outward expansion could consist of the fast ships Alex mentions followed by a much slower and different kind of expansion through ships like Chrysalis. I’ll bring this exchange to a close here, but we’ll keep pondering interstellar expansion in coming months, including the elephant-in-the-room question Alex mentioned above. Will we come to assume that crewed starships are a worthwhile idea? Is the future outbound population most likely to consist of machine intelligence?

Chrysalis: Designing a Generation Ship 12 Aug 11:06 AM (2 months ago)

If you want to explore the history of generation ships in science fiction, you might start with a story by Don Wilcox. Writing in 1940 for Amazing Stories, Wilcox conceived a slick plot device in his “The Voyage that Lasted 600 Years,” a single individual who comes out of hibernation once every century to see how the rest of the initial crew of 33 is handling their job of keeping the species going. Only room for one hibernation chamber, and this means our man becomes a window into social change aboard the craft. The breakdown he witnesses forces him into drastic action to save the mission.

In a plot twist that anticipates A. E. van Vogt’s far superior “Far Centaurus,” Wilcox has his ragged band finally arrive after many generations at destination, only to find that a faster technology has long ago planted a colony there. Granted, Konstantin Tsiolkovsky had written about generation ships before Wilcox, and in a far more learned way. Fictional precedents like Laurence Manning’s “The Living Galaxy” (Wonder Stories, 1934) and Olaf Stapledon’s Star Maker (1937) imagined entire worlds as stellar wanderers, but we can give Wilcox a nod for getting the concept of generations living and dying aboard a constructed craft in front of the public. Heinlein’s “Universe” wouldn’t appear until 1941, and the generation ship was soon to become a science fiction trope.

We can hope that recent winners of the generation ship contest for Project Hyperion have produced designs that avoid the decadence and forgetfulness that accompany so many SF depictions. We do, after all, want a crew to reach destination aware of their history and eager to add to the store of human knowledge. And we have some good people working these issues, scientists such as Andreas Hein, who has been plucky enough to have led Project Hyperion since 2011. Working with the Initiative for Interstellar Studies, Hyperion has announced a contest winner that leverages current technologies and speculates in the best science fiction tradition about how they can be extended.

Hein is an energetic visionary, a man who understands that imaginative forays can help us define key issues and sketch out solutions. The winning design is reminiscent of the kind of space habitats Gerard O’Neill advocated, a 58-kilometer multi-layered cylinder dubbed Chrysalis that offers space enough for Earth-like amenities such as grasslands and parks, art galleries and libraries. The notion includes animals, though only as a token of biodiversity in a culinary scene where vegetarianism is the order of the day.

Interstellar Necessities

What intrigues me about the Chrysalis design is that the need for cultural as well as physical survival in a society utterly closed off for centuries is emphasized. Thus Chrysalis offers habitable conditions for 1,000 people plus or minus 500, with care to ensure the handing off of experience and knowledge to future generations, critical both for societal health as well as the maintenance of the ship’s own technologies. This presumes, after all, the kind of closed-loop life support we have yet to prove we can create here on Earth (more on that in a minute). Gravity is provided through rotation of the craft.

Chrysalis is designed around a journey to Proxima Centauri, with the goal of entering into orbit around Proxima b in some 400 years. And here we hit an immediate caveat. Absent any practical means of propelling something of this magnitude to another star at present (much less of building it in the first place), the generation ship designers have no choice but to fall back on extrapolation. As in the tradition of hard science fiction, the idea is to stick rigorously within the realm of known physics while speculating on technologies that could one day prove feasible. This is not intended as a criticism; it’s just a reminder of how speculative the Chrysalis design is given that I keep seeing that 400 year figure mentioned in press coverage of the contest. We might well have said 600. Or 4,000. Or 40,000.

Image: Chrysalis, the Project Hyperion winner. Credit: Project Hyperion/i4IS.

Like the British Interplanetary Society’s Daedalus starship, Chrysalis is envisioned as using deuterium and helium-3 to power up its fusion engines, with onboard power also fed by fusion generators within the ship. The goal is 0.01C with 0.1G acceleration during the acceleration phase and deceleration phase. As to cruise, we learn this about the fusion power sources that will prove crucial:

All Chrysalis power generators consist of toroidal nuclear fusion reactors housed in the hull frame structure and the habitat axial frame structure separating the various stages. The multiple redundancy of the generators for each shell and each stage guarantees a high tolerance to failure in the event of the failure of one or more reactors. The D and He3 liquid propellant is contained in the propellant tank units located in the forward and after interface propellant bays of the habitat module…

Inside Chrysalis

What would it be like to live aboard a generation starship? The Chrysalis report is stuffed with images and ideas. I like the concept of structures designed around capturing what the team calls ‘generational memories.’ These appear to be tall, massive cylinders designed around what can only be called the aesthetics of worldship travel. Thus:

Each treelike structure hosts multi-story and multi-purpose environments [such] as halls, meeting rooms, and other kinds of infrastructure used by all the inhabitants as collective spaces. There are enough of these public environments to have redundant spaces and also to allow each generation to leave a mark on creation (paintings, sculptures, decorations, etc) for future generations…

The Chrysalis slide show makes it tricky to capture the extensive interior design in a blog format like this, but I advocate paging through it so you can blow the imagery up for a closer look at the included text. As with some of the O’Neill concepts, there is an almost idyllic feel to some of these vistas. Chrysalis is divided into five sections, and within each section there are levels that rotate to provide artificial gravity. The report refers to Chrysalis as a ‘biome ark,’ saying that within each stage there are two shells for dedicated biomes and one for agricultural food production.

Here, of course, we run into a key problem (and readers of Kim Stanley Robinson’s novel Aurora (2015) certainly get a taste of this conundrum). Let me quote the Chrysalis report, which describes ‘controlled ecological bio-regenerative life support systems (CEBLSS)’:

Through a controlled ecological BLSS all chemicals are recycled and reused in a closed loop ecosystem together with a circular bio-economy system in which all organic wastes from the living environments are reintroduced and composted in the agricultural soils.

The acronym nudges the idea into credibility, for we tend to use acronyms on things we’ve pinned down and specified. But the fact is that closed-loop life support is as big a problem as propulsion when it comes to crafting a ship made to sustain human beings for perhaps thousands of years. The Soviet BIOS-1 and subsequent BIOS projects made extensive experiments with human crews, succeeding with full closure for up to 180 days in one run at Krasnoyarsk, while in the U.S., Biosphere 2 ran into serious problems in CO2 and food production. As far as I know, the Chinese Yuegong-1 experiments produced a solid year of closed ecological life support, although I haven’t been able to verify whether this system was 100 percent closed.

Daily Life Between the Stars

So I think we’re making progress, and the Chrysalis report certainly lays out how we might put closed-loop life support to work on the millennial scale. But all this does make me reflect on the fact that we’ve spent most of our energies in interstellar studies trying to work out propulsion, when we’re still in the early days when it comes to onboard ecologies, no matter how beautifully designed. In the same way, we know how to get a payload to Mars, but how to get a healthy crew to the Red Planet and back is still opaque. We need a dedicated orbital facility studying both near and long-term human physiology in space.

The Chrysalis living spaces are made to order as science fiction settings. Interior walls can be functional screens producing panoramic views from Earth environments to overcome the spatial (and psychiatric) limitations of the craft. The inhabitants are given the capability of continually engineering their own living spaces through customizable 3D printing technologies so that the starship itself can be seen as evolving as the crew generations play out their lives. Individuals are provided with parks and gardens to enhance privacy, no small consideration in such a ship. The authors’ slide show goes into considerable detail on ecology and sustainability, social organization and mental health.

In a lovely touch, the team envisions a ‘Cosmos Dome,’ a giant glassy structure where the plenary council for the mission would transact its business. One gets a goose bump or two here, reminiscent as all this is of, say, the control room in Heinlein’s Orphans of the Sky. Burst in there and you suddenly are reminded of just where you are, with Sol behind and Alpha Centauri ahead.

How exactly to select and train a crew, or maybe I should say ‘initial passenger list,’ for such a mission? The Hyperion team’s forays into sociology are curious and almost seem totalitarian. Consider their Antarctic strategy: Three or four generations of crew will live in experimental biospheres in Antarctica…

…to select and monitor all the characteristics that an interstellar population should have. In addition, the creation of a strong group identity and an almost tribal sense of cooperation among the generations of inhabitants is intended to enhance the inter-generational cooperative attitude of the future Chrysalis starship population.

If I’m reading this correctly, it presupposes people who are willing to consign their entire lives to living in Antarctica so that their descendants several generations along can get a berth on Chrysalis. That’s a pretty tough sell, but it emphasizes how critical the suppression of conflict in a tiny population can be. I’m reminded of John Brunner’s “Lungfish,” which ran in the British SF magazine Science Fantasy in 1957 (thanks to Elizabeth Stanway, whose “Journey of (more than) a Lifetime” covers generation ship fictional history well). Here the descendants have no interest at all in life on a planet. As Brunner says:

These had been children like any other children: noisy, inquisitive, foolhardy, disobedient…. And yet they had grown up into these frighteningly self-reliant people who could run the ship better than the earthborn any time they put their minds to it, and still refused to take the initiative.

Definitely an outcome to be avoided!

Language and Stability

The Chrysalis team describes their crew’s mental stability as being enhanced by many reminders of their home:

Chrysalings will also be able to take walks within the different terrestrial biomes of Shell 1 to be in contact with natural elements and plants of the terrestrial biosphere. In Shell 2 there will be opportunities to do concerts, experience theater activities, access ancient Earth materials (books, art objects, etc.), make crafts and other handmade hobby activities. Shell 2 is the real beating heart of the society, where people come together and can freely co-create new cultures and ideas. Thanks to the use of recyclable materials with which the buildings were constructed, residents can also decide to recreate new architectural forms with different shapes and spaces more suitable to their cultural style.

I think the linguistic notion here is quite a reach, for the team says that to avoid language problems, everyone on board the spacecraft will speak a common initial artificial language “used and improved by the Antarctic generations in order to render it a natural language.” And a nod to Star Trek’s holodeck:

The inhabitants may also occasionally decide to meet in simulated metaverses through a deep integration system for cyberspace…to transcend the physical barriers of the starship and experience through their own twin-avatar new worlds or simulations of life on Earth.

Image: The people behind Chrysalis. Left to right: Giacomo Infelise (architect/designer), Veronica Magli (economist/social innovator), Guido Sbrogio (astrophysicist), Nevenka Martinello (environmental engineer/artist), Federica Chiara Serpe (psychologist). Team Chrysalis.

Anyone developing a science fiction story involving generation ships will want to work through the Chrysalis slide show, as the authors leave few aspects of such a journey untouched. I’ve simply been cherry-picking items that caught my eye out of this extensively developed presentation. If we ever become capable of sending humans and not just instruments to nearby stars, we’ll have to have goals and aspirations firmly fixed, and compelling reasons for sending out an expedition that will have no chance of ever returning. Just defining those issues alone is subject for investigations scientific, medical, biological and philosophical, not to mention the intricate social issues that humans pose in closed environments. Chrysalis pushes the discussion into high relief. Nice work!

A Candidate Gas Giant at Alpha Centauri A 8 Aug 6:02 AM (2 months ago)

Early next week I’ll be discussing the winning entry in Project Hyperion’s design contest to build a generation ship. But I want to sneak in the just announced planet candidate at Alpha Centauri A today, a good fit with the Hyperion work given that the winning entry at Hyperion is designed around a crewed expedition to nearby Proxima Centauri. Any news we get about this triple star system rises immediately to the top, given that it’s almost certainly going to be the first destination to which we dispatch instrumented unmanned probes.

And one day, perhaps, manned ships, if designs like Hyperion’s ‘Chrysalis’ come to fruition. More on that soon, but for today, be aware that the James Webb Space Telescope is now giving us evidence for a gas giant orbiting Centauri A, the G-class star intriguingly similar to the Sun, which is part of the close binary that includes Centauri B, both orbited by the far more distant Proxima.

Image: This artist’s concept shows what the gas giant orbiting Alpha Centauri A could look like. Observations of the triple star system Alpha Centauri using the NASA/ESA/CSA James Webb Space Telescope indicate the potential gas giant, about the mass of Saturn, orbiting the star by about two times the distance between the Sun and Earth. In this concept, Alpha Centauri A is depicted at the upper left of the planet, while the other Sun-like star in the system, Alpha Centauri B, is at the upper right. Our Sun is shown as a small dot of light between those two stars. Credit: NASA, ESA, CSA, STScI, R. Hurt (Caltech/IPAC).

JWST’s Mid-Infrared Instrument (MIRI) once again proves its worth, as revealed in two papers in process at The Astrophysical Journal Letters. If this can be confirmed as a planet, its orbit appears to be eccentric (e ≈ 0.4) and significantly inclined with respect to the orbital plane of Centauri A and B. But we have a lot of work ahead to turn this candidate, considered ‘robust’ by the team working on it, into a solid detection.

The proximity of the central binary stars at Alpha Centauri makes this kind of work extremely difficult, one reason why a system so close to our own is only gradually revealing its secrets. Bear in mind that MIRI was able to subtract the light from both stars to reveal an object 10,00 times fainter than Centauri A. The Webb instrument took observations beginning in August of 2024 that posed a subsequent problem, for two additional observation periods in the spring of this year failed to find the object. Interestingly, computer simulations have clarified what may have happened, according to PhD student Aniket Sanghi (Caltech), co-first author of one of the two papers describing this work:

“We are faced with the case of a disappearing planet! To investigate this mystery, we used computer models to simulate millions of potential orbits, incorporating the knowledge gained when we saw the planet, as well as when we did not,.. We found that in half of the possible orbits simulated, the planet moved too close to the star and wouldn’t have been visible to Webb in both February and April 2025.”

Image: This 3-panel image captures the NASA/ESA/CSA James Webb Space Telescope’s observational search for a planet around the nearest Sun-like star, Alpha Centauri A. The initial image shows the bright glare of Alpha Centauri A and Alpha Centauri B, then the middle panel shows the system with a coronagraphic mask placed over Alpha Centauri A to block its bright glare. However, the way the light bends around the edges of the coronagraph creates ripples of light in the surrounding space. The telescope’s optics (its mirrors and support structures) cause some light to interfere with itself, producing circular and spoke-like patterns. These complex light patterns, along with light from the nearby Alpha Centauri B, make it incredibly difficult to spot faint planets. In the panel at the right, astronomers have subtracted the known patterns (using reference images and algorithms) to clean up the image and reveal faint sources like the candidate planet. Credit: NASA, ESA, CSA, STScI, DSS, A. Sanghi (Caltech), C. Beichman (JPL), D. Mawet (Caltech), J. DePasquale (STScI).

The combination of observations and orbital simulations indicates that a gas giant of about Saturn mass moving in an elliptical orbit within Centauri A’s habitable zone remains a viable option. Also fed into the mix were the parameters of a 2019 observation of Centauri A and B from the European Southern Observatory’s Very Large Telescope. It is clear that the point source referred to as S1 is not a background object like a galaxy or a foreground asteroid moving between JWST and the star. Its orbital parameters would make it quite interesting given the tight separation between Centauri A and B.

The second of the two papers clarifies the significance of such a find and the need to confirm it. The temperature calculated below is based on the photometry and orbital properties of the candidate object, with 200–350 K originally expected for a planet heated by Centauri A at 1.3 AU:

A confirmation of the S1 candidate as a gas giant planet orbiting our closest solar-type star,α Cen A, would present an exciting new opportunity for exoplanet research. Such an object would be the nearest (1.33 pc), coldest (∼225 K), oldest (∼5 Gyr), shortest period (∼2–3 years), and lowest mass (≲ 200 M⊕) planet imaged in orbit around a solar-type star, to date. Its extremely cold temperature would make it more analogous to our own gas giant planets and an important target for atmospheric characterization studies. Its very existence would challenge our understanding of the formation and subsequent dynamical evolution of planets in complex hierarchical systems. Future observations will confirm or reject its existence and then refine its mass and orbital properties, while multi-filter photometric and, eventually, spectroscopic observations will probe its physical nature.

The papers are Beichman et al., “Worlds Next Door: A Candidate Giant Planet Imaged in the Habitable Zone of α Cen A. I. Observations, Orbital and Physical Properties, and Exozodi Upper Limits,” accepted at Astrophysical Journal Letters (preprint); and Sanghi, et al., “Worlds Next Door: A Candidate Giant Planet Imaged in the Habitable Zone of α Cen A. II. Binary Star Modeling, Planet and Exozodi Search, and Sensitivity Analysis,” accepted at ApJL (preprint). The paper on the 2019 observation is Wagner at al., “Imaging low-mass planets within the habitable zone of α Centauri,” Nature Communications 10 February 2021 (full text).

A Rotating Probe Launcher Alternative to TARS 5 Aug 6:43 AM (3 months ago)

Shortly before publishing my article on David Kipping’s TARS concept (Torqued Accelerator using Radiation from the Sun, I received an email from Centauri Dreams associate editor Alex Tolley. Alex had come across TARS and offered his thoughts on how to improve the concept for greater efficiency. The publication of my original piece has launched a number of comments that have also probed some of these areas, so I want to go ahead and present Alex’s original post, which was written before my essay got into print. All told, I’m pleased to see the continuing contribution of the community at taking an idea apart and pondering alternative solutions. It’s the kind of thing that gives me confidence that the interstellar effort is robust and continuing.

by Alex Tolley

Dr. Kipping’s TARS proposed system for accelerating probes to high velocity is both simple and elegant. With no moving parts other than any tether deployment and probe release, if it works, there is little that can fail during the spin-up period. There are improvements to the basic idea that increase performance, although this essay will suggest a more complex, but possibly more flexible and performant approach using the basic rotating tether concept.

First, a small design change of TARS to increase the rate of spin-up. The TARS design is like a Crookes radiometer, but working in reverse, with the mirror face of the sail experiencing a greater force than the obverse dark, emissive face. As the tethers rotate, the reflective face increases the spin rate, whilst the emissive face swinging back towards the sun acts as a retarding force. An easy improvement, at the cost of a moving part, is to have the sail reorient itself to be edge-on to the sun as it returns. This is illustrated in Figure 1 below. The rotation can be any mechanism that sequentially rotates the sail by 90 degrees after the tether is aligned with the sun, or other electromagnetic radiation source.

Figure 1. The simplified TARS system with the sail rotating around the tether to reduce the retarding force in the rotation phase.

There are other possibilities to tweak the performance, but at a cost of complexity and added mass.

However, I want to offer an alternative approach that solves some of the limitations of the proposed TARS system.

These limitations include:

• The propulsive force is very phase-dependent as the tether rotates.
• The rotation rate is dependent on the sail aerial density and size&lt
• The sails add mass to the tether and therefore increase the tether tension, requiring an increased taper
• The TARS rotation must be aligned with the radiation source, limiting the direction it can throw the payloads. This means that a target on an inclined plane to the planets, such as a comet or exoplanet, requires the TARS to take on an inclined orbit, limiting its flexibility.
• The asymmetric forces on TARS change its orbit.

These limitations can be alleviated by eliminating the sails and replacing the rotation with an electric motor, powered by a solar panel. The basic design is shown in Figure 2.

Figure 2. Basic design of a rotating probe launcher using motor-driven tethers.

The tether is powered by an electric motor that requires a counter-rotating wheel or tether (see later) to prevent the system from rotating. This is similar to the power equipment astronauts use in space. The tether is attached to the solar panel by a 3-axis joint to allow full control of the rotational plane of the tether. As the only loads on the tether are its own mass and the releasable probes, the amount of taper should be less than TARS, allowing longer tethers of the same material. The tethers can be flexible or stiff, depending on deployment preferences. Figure 2 shows a preferred arrangement where the tethers form a square, with cable stays to increase rigidity and offset bending during spin-up.

The tether would have 2 releasable probes and 2 small ballasts to maintain tension, or 4 probes. The probes can be released simultaneously in opposite directions, or in the same direction from 1-10 milliseconds apart, depending on the rotation rate. If released in the same direction, the system will tend to be pushed in the opposite direction as the probes released in the same direction would act as propellant, generating thrust in the opposite direction.

A variant would allow for 2 contra-rotating tethers. Because they are mechanically coupled to the same motor, this guarantees that they rotate in synchrony and eliminate the gyroscopic action of a single tether. This removes the need for a counter-rotating disc for the motor, but more importantly, for multiple payloads allows the rotation plane to be changed between payload releases, allowing for different target destinations for the probes to travel in. This would be ideal for a standby to target comets and objects coming from different orbital inclinations, as well as more detailed mapping of the solar system’s heliosphere.

Because the rotation is controlled by a motor, this provides more precise timing of the payload releases. Once the maximum rotation rate is reached, the motor can idle, and the system continue its orbit until the optimum probe[s] release position is achieved, for example when Mars is in opposition. This avoids the continual rotation rate increase of TARS that must release its probe[s] before the tethers snap.

So what sort of rotational speed can a motor provide? The maximum speed for a small motor is 100,000 rpm, or 1667 rps. A much lower speed is achieved by hard disk drives at 7200 rpm or 120 rps.

This translates to:

Because the rotation rate is so fast, any probe release must be timed with very high precision to ensure it travels on the correct flight path towards its destination. While not critical for some missions, encounters with small bodies such as interstellar objects (ISO) like 2I/Borisov will require very high precision releases.

Unlike TARS, the tethers can also be spun down, making the system reusable to reload the payloads. If multiple payloads can be released sequentially like a Pez dispenser, then these can be reloaded periodically when the payloads have been exhausted. With extra complexity, these cartridges of probes could be carried on the system, and attached to the tethers after the rotation has been reduced to zero, making the device relatively autonomous for long periods.

Lastly, because the rate of rotation acceleration is dependent on the motor and power available, the power can be increased with a larger solar array, and the motor torque increased with a larger motor. These are independent of the tether design, making any desired upgrades simpler, or like CubeSats, configurable on manufacture before launch.