Centauri Dreams
Imagining and Planning Interstellar Exploration
Amazing Worlds: A Review
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)
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 Institute for Advanced Studies Austin, 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
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
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
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
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<
- 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.
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