Centauri Dreams
Imagining and Planning Interstellar Exploration
Can an Interstellar Generation Ship Maintain a Population on a 250-year Trip to a Habitable Exoplanet?
I grew up on generation ship stories. I’m thinking of tales like Heinlein’s Orphans of the Sky, Aldiss’ Non-Stop, and (from my collection of old science fiction magazines) Don Wilcox’s “The Voyage that Lasted 600 Years.” The latter, from 1940, may have been the first generation ship story ever written. The idea grows out of the realization that travel to the stars may take centuries, even millennia, and that one way to envision it is to take large crews who live out their lives on the journey, their descendants becoming the ones who will walk on a new world. The problems are immense, and as Alex Tolley reminds us in today’s essay, we may not be fully considering some of the most obvious issues, especially closed life support systems. Project Hyperion is a game attempt to design a generation ship, zestfully tangling with the issues involved. The Initiative for Interstellar Studies is pushing the limits with this one. Read on.
by Alex Tolley
“Space,” it says, “is big. Really big. You just won’t believe how vastly, hugely, mindbogglingly big it is. I mean, you may think it’s a long way down the road to the chemist, but that’s just peanuts to space..” The Hitchhikers Guide to the Galaxy – Douglas Adams.
Detail of book cover of Tau Zero. Artist: Manchu.
Introduction
Science fiction stories, and most notably TV and movies, avoid the fact of the physical universe being so vast by various methods to reduce travel time. Hyperspace (e.g. Asimov’s galaxy novels), warp speed (Star Trek), and wormholes compress travel time to be like that on a planet in the modern era. Relativity will compress time for people at the cost of external time (e.g. Poul Anderson’s Tau Zero). But the energy cost of high-speed travel has a preferred slow-speed method – the space ark. Giancarlo Genta classifies this method of human crewed travel: H1 [1]. a space-ark type, where travel to the stars will take centuries. Either the crew will be preserved with cryosleep on the journey (e.g. Star Trek TOS S01E22, “Space Seed”) or generations will live and die in their ship (e.g. Aldiss Non-Stop) [2].
This last is now the focus of the Project Hyperion competition by the Initiative for Interstellar Studies (i4is) where teams are invited to design a generation ship within various constraints. This is similar to the Mars Society’s Design a City State competition for a self-supporting Mars city for 1 million people, with prizes awarded to the winners at the society’s conference.
Prior design work for an interstellar ship was carried out between 1973 and 1978, by the British Interplanetary Society (BIS). It was for an uncrewed probe to fly by Barnard’s star 5.9 lightyears distant within 50 years – their Project Daedalus.[3] Their more recent attempt at a redesign, the ironically named Project Icarus, failed to achieve a complete design, although there was progress on some technologies [4] Project Hyperion is far more ambitious based on the project constraints including human crews and a greater flight duration .[5].
So what are the constraints or boundary conditions for the competition design? Seven are given:
1. The mission duration is 250 years. In a generation ship that means about 10 generations. [Modern people can barely understand what it was like to live a quarter of a millennium ago, yet the ship must maintain an educated crew that will maintain the ship in working order over that time – AT].
2. The destination is a rocky planet that will have been prepared for colonization by an earlier [robotic? – AT] ship or directed panspermia. Conditions will not require any biological modifications of the crew.
3. The habitat section will provide 1g by rotation.
4. The atmosphere must be similar to Earth’s. [Presumably, gas ratios and partial pressures must be similar too. There does not appear to be any restriction on altitude, so presumably, the atmospheric pressure on the Tibetan plateau is acceptable. – AT]
5. The ship must provide radiation protection from galactic cosmic rays (GCR).
6. The ship must provide protection from impacts.
7. The crew or passengers will number 1000 +/- 500.
The entering team must have at least:
- One architectural designer
- One engineer
- One social scientist (sociologist, anthropologist, etc.)
Designing such a ship is not trivial, especially as unlike a Lunar or Martian city, there is no rescue possible for a lone interstellar vehicle traveling for a quarter of a millennium at a speed of at least 1.5% of lightspeed to Proxima, faster for more distant stars. If the internal life support system fails and cannot be repaired, it is curtains for the crew. As the requirements are that at least 500 people arrive to live on the destination planet, any fewer survivors, perhaps indulging in cannibalism (c.f. Neal Stephenson’s Seveneves), means this design would be a failure.
Figure 1. Technology Readiness Levels. Source NASA.
The design competition constraints allow for technologies at the technology readiness level of 2 (TRL 2) which is in the early R&D stage. In other words, the technologies are unproven and may not prove workable. Therefore the designers can flex their imaginations somewhat, from propulsion (onboard or external) to shielding (mass and/or magnetic), to the all-important life support system.
Obviously, the life support system is critical. After Gerry O’Neil designed his space colonies with interiors that looked rather like semi-rural places on Earth, it was apparent to some that if these environments were stable, like managed, miniature biospheres, then they could become generation ships with added propulsion. [6]. The Hyperion project’s 500-1500 people constraint is a scaled-down version of such a ship. But given that life support is so critical, why are the teams not required to have members with biological expertise in the design? Moreover, the design evaluators also have no such member. Why would that be?
Firstly, perhaps such an Environmental Control and. Life Support System (ECLSS) is not anticipated to be a biosphere:
The requirements matrix states:
ECLSS: The habitat shall provide environmental control and life support: How are essential physical needs of the population provided? Food, water, air, waste recycling. How far is closure ensured?
Ecosystem: The ecosystem in which humans are living shall be defined at different levels: animals, plants, microbiomes.
This implies that there is no requirement for a fully natural, self-regulating, stable, biosphere, but rather an engineered solution that mixes technology with biology. The image gallery on the Hyperion website [5] seems to suggest that the ECLSS will be more technological in nature, with plants as decorative elements like that of a business lobby or shopping mall, providing the anticipated need for reminders of Earth and possibly an agricultural facility.
Another possibility is that prior work is now considered sufficient to allow it to be grafted into the design. It seems the the problem of ECLSS for interstellar worldships is almost taken for granted despite the very great number of unsolved problems.
As Soilleux [7] states
“To date, most of the thinking about interstellar worldships has focused, not unreasonably, on the still unsolved problems of developing suitably large and powerful propulsion systems. In contrast, far less effort has been given to the equally essential life support systems which are often assumed to be relatively straightforward to construct. In reality, the seductive assumption that it is possible to build an artificial ecosystem capable of sustaining a significant population for generations is beset with formidable obstacles.”
It should be noted that no actual ECLSS has proven to work for any length of time even for interplanetary flight, let alone centuries for interstellar flight. Gerry O’Neill did not make much effort beyond handwaving for the stability of the more biospheric ECLSS of his 1970s-era space colony. Prior work from Bios 3, Biosphere II, MeLISSA, and other experiments, has demonstrated very short-term recycling and sustainability, geared more towards longer duration interplanetary spaceflight and bases. Multi-year biospheres inevitably lose material due to imperfect recycling and life support imbalances that must be corrected via venting. The well-known Biosphere II project couldn’t maintain a stable biosphere to support an 8-person crew for 2 years, accompanied by a 10% per year atmosphere loss.
On a paper design basis, the British Interplanetary Society (BIS) recently worked on a more detailed design for a post-O’Neill space colony called Avalon. It supported a living interior for 10,000 people, the same as the O’Neill Island One design, providing a 1g level and separate food growing, carbon fixing, and oxygen (O2) generating areas. However, the authors did not suggest it would be suitable for interstellar flight as it did require some inputs and technological support from Earth. Therefore it remains an idea, and as with the hardware experiments to date, such ECLSSs remain at low TRL values. While the BIS articles are behind paywalls, there is a very nice review of the project in the i4is Principium magazine [7].
Table 1. Inputs to support a person in Space. Source: Second International Workshop on Closed Ecological Systems, Krasnoyarsk, Siberia, 1989 [8]
Baseline data indicates that a person requires 2 metric tonnes [MT] of consumables per year, of which food comprises 0.2 MT and oxygen (O2) 0.3 MT. For 1000 people over 250 years that is 500,000 MT. To put this mass in context, it is about the greatest mass of cargo that can be carried by ships today. Hence recycling to both save mass and to manage potentially unlimited time periods makes sense. Only very short-duration trips away from Earth allow for the consumables to be brought along and discarded after use, like the Apollo missions to the Moon. Orbiting space stations are regularly resupplied with consumables, although the ISS is using water recycling to save on the resupply of these consumables for drinking, washing, and bathing.
If an ECLSS could manage 100% recycling, then if the annual amount is the buffer to allow for annual crop growth, the ship could support the crew over the time period with just over 2000 MT, but of course with the added requirement for power and infrastructure and replacement parts to maintain the ECLSS.
Figure 2. Conceptual ECLSS process flow diagram for the BIS Avalon project.
A review of the Avalon project’s ECLSS [7] stated:
“A fully closed ECLSS is probably impossible. At a minimum, hydrogen, oxygen, nitrogen, and carbon lost from airlocks must be replaced as well as any nutrients lost to the ecosystem as products (eg soil, wood, fibres, chemicals). This presents a resource-management challenge for any fully autonomous spacecraft and an economic challenge even if only partially autonomous…Recycling does not provide a primary source of fixed nitrogen for new plant growth and significant quantities are lost to the air as N2 because of uncontrollable denitrification processes. The total loss per day for Avalon has been estimated at 43 kg to be replaced from the atmosphere.”
[That last loss figure would translate to 390 MT for the 1/10 scaled crewed Hyperion starship over 250 years.]
Hempsell evaluated the ECLSS technologies for star flight based on the Avalon project and found them unsuited to a long-duration voyage [9]. It was noted that the Avalon design still required inputs from Earth to be maintained as it was not fully recycled:
“So effective is this transport link that it is unlikely Earth orbiting colonies will have any need for the self-sufficiency assumed by the original thinking by O’Neill. It is argued that a colony’s ability to operate without support is likely to be comparable to the transport infrastructure’s journey times, which is three to four months. This is three orders of magnitude less than the requirements for a world ship. It is therefore concluded that in many critical respects the gap between colonies and world ships is much larger than previous work assumed.”
We can argue whether this will be solved eventually, but clearly, any failure is lethal for the crew.
One issue is not mentioned, despite the journey duration. Over the quarter millennium voyage, there will be evolution as the organisms adapt to the ship’s environment. Data from the ISS has shown that bacteria may mutate into more virulent pathogens. A population living in close quarters will encourage pandemics. Ionizing radiation from the sun and secondaries from the hull of a structure damages cells including their DNA. 250 years of exposure to residual GCR and secondaries will damage DNA of all life on the starship.
However, even without this direct effect on DNA, the conditions will result in organisms evolving as they adapt to the conditions on the starship, especially the small populations, increasing genetic drift. This evolution, even of complex life, can be quite fast, as the continuing monitoring of the Galápagos island finches observed by Darwin attests. Of particular concern is the creation of pathogens that will impact both humans and the food supply.
In the 1970s, the concept of a microbiome in humans, animals, and some plants was unknown, although bacteria were part of nutrient cycling. Now we know much more about the need for humans to maintain a microbiome, as well as some food crops. This could become a source of pathogens. While a space habitat can just flush out the agricultural infrastructure and replace it, no such possibility exists for the starship. Crops would need to be kept in isolated compartments to prevent a disease outbreak from destroying all the crops in the ECLSS.
If all this wasn’t difficult enough, the competition asks that the target generation population find a ready-made terrestrial habitat/terraformed environment to slip into on arrival. This presumably was prebuilt by a robotic system that arrived ahead of the crewed starship to build the infrastructure and create the environment ready for the human crew. It is the Mars agricultural problem writ large [10], with no supervision from humans to correct mistakes. If robots could do this on an exoplanet, couldn’t they make terrestrial habitats throughout the solar system?
Heppenheimer assumed that generation ships based on O’Neill habitats would not colonize planets, but rather use the resources of distant stars to build more space habitats, the environment the populations had adapted to [6]. This would maintain the logic of the idea of space habitats being the best place for humanity as well as avoiding all the problems of trying to adapt to living on a living, alien planet. Rather than building space habitats in other star systems, the Hyperion Project relies on the older idea of settling planets, and ensuring at least part of the planet is habitable by the human crew on arrival. This requires some other means of constructing an inhabitable home, whether a limited surface city or terraforming the planet.
Perhaps the main reason why both the teams and evaluators have put more emphasis on the design of the starship as a human social environment is that little work has looked at the problems of maintaining a small population in an artificial environment over 250 years. SciFi novels of generation ships tend to be dystopias. The population fails to maintain its technical skills and falls back to a simpler, almost pretechnological, lifestyle. However, this cannot be allowed in a ship that is not fully self-repairing as hardware will fail. Apart from simple technologies, no Industrial Revolution technology that continues to work has operated without the repair of parts. Our largely solid-state space probes like the Voyagers, now over 45 years old and in interstellar space, would not work reliably for 250 years, even if their failing RTGs were replaced. Mechanical moving parts fail even faster.
While thinking about how we might manage to send crewed colony ships to the stars with the envisaged projections of technologies that may be developed, it seems to me that it is rather premature given the likely start date of such a mission. Our technology will be very different even within decades, obsoleting ideas used in the design. The crews may not be human 1.0, requiring very different resources for the journey. At best, the design ideas for the technology may be like Leonardo Da Vinci’s ideas for flying machines. Da Vinci could not conceive of the technologies we use to avoid moving our meat bodies through space just to communicate with others around the planet.
Why is the population constraint 1000 +/- 500?
Gerry O’Neill’s Island One space colony was designed for 10,000 people, a small town. The BIS Avalon was designed for the same number, using a different configuration that provided the full 1g with lower Coriolis forces. The Hyperion starship constrains the population to 1/10th that number. What is the reason? It is based on the paper by Smith [14] on the genetics and estimated safe population size for a 5-generation starship [125 years at 25 years per generation?]. This was calculated on the expected lower limit of a small population’s genetic diversity with current social structures, potential diseases, etc. This is well below the number generally considered for long-term viable animal populations. However, the calculated size of the genetic bottleneck in our human past is about that size, suggesting that in extremis such a small population can recover, just as we escaped extinction. Therefore it should be sufficient for a multi-generation interstellar journey.
From Hein[14]:
While the smallest figures may work biologically, they are rather precarious for some generations before the population has been allowed to grow. We therefore currently suggest figures with Earth-departing (D1) figures on the order of 1,000 persons.
But do we need this population size? Without resorting to a full seed ship approach with the attendant issues of machine carers raising “tank babies”, can we postulate an approach without assuming future TRL9 ECLSS technology?
As James T Kirk might say: “I changed the conditions of the test competition!”
Suppose we only need a single woman who will bear a child and then age until death, perhaps before 100. The population then would be a newborn baby, the 25-year-old mother, the 50-year-old grandmother, the 75-year-old great-grandmother, and the deceased great-great-grandmother. 3 adults and a newborn. At 20, the child will count as an adult, with a 45-year-old mother, a 70-year-old grandmother, and possibly a 95-year-old great-grandmother, 4 adults. Genetic diversity would be solved from a frozen egg, sperm, and embryo bank of perhaps millions of individual selected genomes. The mother would give birth only to preselected, compatible females using implantation (or only female sperm) to gain pregnancy, to maintain the line of women.
This can easily be done today with minimal genetic engineering and a cell-sorting machine [11]. At the destination, population renewal can begin. The youngest mature woman could bear 10 infants, each separated by 2 years. The fertile female population would rapidly increase 10-fold each generation, creating a million women in 6 generations, about 150 years. Each child would be selected randomly from the stored genetic bank of female sperm or embryos. At some point, males can be phased in to restore the 50:50 ratio. Because we are only supporting 4 women, almost all the recycling for food and air can be discarded, just recycling water. Water recycling for urine and sweat has reached 98% on the ISS, so we can assume that this can be extended to the total water consumption. Using a conservative 98% recycle rate for water (sanitary, drinking, and metabolic), O2 replenishment from water by electrolysis (off-the-shelf kit) and an 80% recycle rate of metabolic carbon dioxide (CO2) to O2 and methane (CH4 by a Sabatier process), implies that the total water storage needs only be 5x the annual consumption. The water would be part of the hull as described by McConnell’s “Spacecoach” concept.
Based on the data in Table 1 above, the total life support would be about 125 MT. The methane could be vented or stored for the lander propulsion fuel. For safety through redundancy, the total number of women population might be raised 10x to 30-40, requiring only 1250 MT total consumables. All this does not require any new ECLSS technology, just enough self-repair, repairable, and recyclable machinery, to maintain the ship’s integrity and functioning. The food would be stored both as frozen or freeze-dried, with the low ambient temperature maintained by radiators exposed to interstellar space. This low mass requires no special nutrient recycling, just water recycling at the efficiency of current technology, O2 recycling using existing technology, and sufficient technology support to ensure any repairs and medical requirements can be handled by the small crew.
Neal Stephenson must have had something like this in mind for the early period of saving and rebuilding the human population in his novel Seveneves when the surviving female population in space was reduced to 7. Having said this, such a low population size even with carefully managed genetic diversity through sperm/ar embryo banks, genetic failure over the 10 generation flight may still occur. Once the population has the resources to increase, the genetic issues may be overcome by the proposed approach.
Table 2. ETHNOPOP simulation showing years to demographic extinction for closed human populations. Bands of people survive longer with larger starting sizes, but these closed populations all eventually become extinct due to demographic (age & sex structure) deficiencies. Source Hein [14]
From table 2 above, even a tiny, natural,human population may survive hundreds of years before extinction. Would the carefully managed starship population fare better, or worse?
Despite this maneuver, there is still a lot of handwavium about the creation of a terrestrial habitable environment at the destination. All the problems of building an ECLSS or biosphere in a zone on the planet or its entirety by terraforming, entirely by a non-human approach, are left to the imagination. As with humans, animals such as birds and mammals require parental nurturing, making any robotic seedship only partially viable for terraforming. If panspermia is the expected technology, this crewed journey may not start for many thousands of years.
But what about social stability?
I do applaud the Hyperion team for requiring a plan for maintaining a stable population for 250 years. The issue of damage to a space habitat in the O’Neill era was somewhat addressed by proponents, mainly through the scale of the habitat and repair times. Major malicious damage by groups was not addressed. There is no period in history that we know of that lasted this long without major wars. The US alone had a devastating Civil War just 164 years ago. The post-WWII peace in Europe guaranteed by the Pax Americana is perhaps the longest period of peace in Europe, albeit its guarantor nation was involved in serious wars in SE Asia, and the Middle East during this period. Humans seem disposed to violence between groups, even within families. I couldn’t say whether even a family of women would refrain from existential violence. Perhaps genetic modification or chemical inhibition in the water supply may be solutions to outbreaks of violence. This is speculation in a subject outside my field of expertise. I hope that the competition discovers a viable solution.
In summary, while it is worthwhile considering how we might tackle human star flight of such duration, there is no evidence that we are even close to solving the issues inherent in maintaining any sort of human star flight of such duration with the earlier-stage technologies and the constraints imposed on the designs by the competition. By the time we can contemplate such flights, technological change will likely have made all the constraints superfluous.
References and Further Reading
Genta, G Genta, “Interstellar Exploration: From Science Fiction to Actual Technology,”” Acta Astronautica Vol. 222 (September 2024), pp. 655-660 https://www.sciencedirect.com/science/article/pii/S0094576524003655?via%3Dihub
Gilster P, (2024) “Science Fiction and the Interstellar Imagination,” Web https://www.centauri-dreams.org/2024/07/17/science-fiction-and-the-interstellar-imagination/
BIS “Project Daedalus” https://en.wikipedia.org/wiki/Project_Daedalus
BIS “Project Icarus” https://en.wikipedia.org/wiki/Project_Icarus_(interstellar)
i4is “Project Hyperion Design Competition” https://www.projecthyperion.org/
Heppenheimer, T. A. (1977). Colonies in space: A Comprehensive and Factual Account of the Prospects for Human Colonization of Space. Harrisburg, PA: Stackpole Books, 1977
Soilleux, R. (2020) “Implications for an Interstellar World-Ship in findings from the BIS SPACE Project” – Principium #30 pp5-15 https://i4is.org/wp-content/uploads/2020/08/Principium30-print-2008311001opt.pdf
Nelson, M., Pechurkin, N. S., Allen, J. P., Somova, L. A., & Gitelson, J. I. (2010). “Closed ecological systems, space life support and biospherics.” In Humana Press eBooks (pp. 517–565). https://doi.org/10.1007/978-1-60327-140-0_11
Hempsell, M (2020), “Colonies and World Ships” JBIS, 73, pp.28-36 (abstract) https://www.researchgate.net/publication/377407466_Colonies_and_World_Ships
Tolley, A. (2023) “MaRMIE: The Martian Regolith Microbiome Inoculation Experiment.” Web https://www.centauri-dreams.org/?s=Marmie
Bio-Rad “S3e Cell Sorter” https://www.bio-rad.com/en-uk/category/cell-sorters?ID=OK1GR1KSY
Allen, J., Nelson, M., & Alling, A. (2003). “The legacy of biosphere 2 for the study of biospherics and closed ecological systems.” Advances in Space Research, 31(7), 1629–1639. https://doi.org/10.1016/s0273-1177(03)00103-0
Smith, C. M. (2014). “Estimation of a genetically viable population for multigenerational interstellar voyaging: Review and data for project Hyperion.” Acta Astronautica, 97, 16–29. https://www.sciencedirect.com/science/article/abs/pii/S0094576513004669
Hein A.M,, et al (2020). “World ships: Feasibility and Rationale” Acta Futura, 12, 75-104, 2020 Web https://arxiv.org/abs/2005.04100
The Ethics of Spreading Life in the Cosmos
We keep trying to extend our reach into the heavens, but the idea of panspermia is that the heavens are actually responsible for us. Which is to say, that at least the precursor materials that allow life to emerge came from elsewhere, and did not originate on Earth. Over a hundred years ago Swedish scientist Svante Arrhenius suggested that the pressure of starlight could push bacterial spores between planets and we can extend the notion to interstellar journeys of hardy microbes as well, blasted out of planetary surfaces by such things as meteor impacts and flung into outbound trajectories.
Panspermia notions inevitably get into the question of deep time given the distances involved. The German physician Hermann Richter (1808-1876) had something interesting to say about this, evidently motivated by his irritation with Charles Darwin, who had made no speculations on the origin of the life he studied. Richter believed in a universe that was eternal, and indeed thought that life itself shared this characteristic:
“We therefore also regard the existence of organic life in the universe as eternal; it has always existed and has propagated itself in uninterrupted succession. Omne vivum ab aeternitate e cellula!” [All life comes from cells throughout eternity].
Thus Richter supplied what Darwin did not, while accepting the notion of the evolution of life in the circumstances in which it found itself. By 1908 Arrhenius could write:
“Man used to speculate on the origin of matter, but gave that up when experience taught him that matter is indestructible and can only be transformed. For similar reasons we never inquire into the origin of the energy of motion. And we may become accustomed to the idea that life is eternal, and hence that it is useless to inquire into its origin.”
The origins of panspermia thinking go all the way back to the Greeks, and the literature is surprisingly full as we get into the 19th and early 20th Century, but I won’t linger any further on that because the paper I want to discuss today deals with a notion that came about only within the last 60 years or so. As described by Carl Sagan and Iosif Shklovskii in 1966 (in Intelligent Life in the Universe, it’s that panspermia is not only possible but might be something that humans might one day attempt.
Indeed, Michael Mautner and Greg Matloff proposed this in the 1970s (citation below), while digging into the potential risks and ethical problems associated with such a project. The idea remains controversial, to judge from the continuing flow of papers on various aspects of panspermia. We now have a study from Asher Soryl (University of Otago, NZ) and Anders Sandberg (MIMIR Centre for Long Term Futures Research, Stockholm) again sizing up guided panspermia ethics and potential pitfalls. What is new here is the exploration of the philosophy of the directed panspermia idea.
Image: Can life be spread by comets? Comet 2I/Borisov is only the second interstellar object known to have passed through our Solar System, but presumably there are vast numbers of such objects moving between the stars. In this image taken by the NASA/ESA Hubble Space Telescope, the comet appears in front of a distant background spiral galaxy (2MASX J10500165-0152029, also known as PGC 32442). The galaxy’s bright central core is smeared in the image because Hubble was tracking the comet. Borisov was approximately 326 million kilometres from Earth in this exposure. Its tail of ejected dust streaks off to the upper right. Credit: ESA/Hubble.
Spreading life is perhaps more feasible than we might imagine at first glance. We have achieved interstellar capabilities already, with the two Voyagers, Pioneers 10 and 11 and New Horizons on hyperbolic trajectories that will never return to the Solar System. Remember, time is flexible here because a directed panspermia effort would be long-term, seeding numerous stars over periods of tens of thousands of years. The payload need not be large, and Soryl and Sandberg consider a 1 kg container sufficient, one containing freeze-dried bacterial spores inside water-dissoluble UV protective sheaths. Such spores could survive millions of years in transit:
…desiccation and freezing makes D. radiodurans able to survive radiation doses of 140 kGy, equivalent to hundreds of millions of years of background radiation on Earth. A simple opening mechanism such as thermal expansion could release them randomly in a habitable zone without requiring the use of electronic components. Moreover, normal bacteria can be artificially evolved for extreme radiation tolerance, in addition to other traits that would increase their chances of surviving the journey intact. Further genetic modifications are also possible so that upon landing on suitable exoplanets, evolutionary processes could be accelerated by a factor of ∼1000 to facilitate terraforming, eventually resulting in Earth-like ecological diversity.
If the notion seems science fictional, remember that it’s also relatively inexpensive compared to instrumented payload packages or certainly manned interstellar missions. Right now when talking about getting instrumentation of any kind to another star, we’re looking at gram-scale payloads capable of being boosted to a substantial portion of lightspeed, but directed panspermia could even employ comet nuclei inoculated with life, all moving at far slower speeds. And we know of some microorganisms fully capable of surviving hypervelocity impacts, thus enabling natural panspermia.
So should we attempt such a thing, and if so, what would be our motivation? The idea of biocentrism is that life has intrinsic merit. I’ve seen it suggested that if we discover that life is not ubiquitous, we should take that as meaning we have an obligation to seed the galaxy. Another consideration, though, is whether life invariably produces sentience over time. It’s one thing to maximize life itself, but if our actions produce it on locations outside Earth, do we then have a responsibility for the potential suffering of sentient beings given we have no control over the conditions they will inhabit?
That latter point seems abstract in the extreme to me, but the authors note that ‘welfarism,’ which assesses the intrinsic value of well-being, is an ethical position that illuminates the all but God-like perspective of some directed panspermia thinking. We are, after all, talking about the creation of living systems that, over billions of years of evolution, could produce fully aware, intelligent beings, and thus we have to become philosophers, some would argue, as well as scientists, and moral philosophers at that:
While in some cases it might be worthwhile to bring sentient beings into existence, this cannot be assumed a priori in the same way that the creation of additional life is necessarily positive for proponents of life-maximising views; the desirability of a sentient being’s existence is instead contingent upon their living a good life.
Good grief… Now ponder the even more speculative cost of waiting to do directed panspermia. Every minute we wait to develop systems for directed panspermia, we lose prospective planets. After all, the universe, the authors point out, is expanding in an accelerated way (at least for now, as some recent studies have pointed out), and for every year in which we fail to attempt directed panspermia, three galaxies slip beyond our capability of ever reaching them. By the authors’ calculations, we lose on the order of one billion potentially habitable planets each year as a result of this expansion.
These are long-term thoughts indeed. What the authors are saying is reminiscent in some ways of the SETI/METI debate. Should we do something we have the capability of doing when we have no consensus on risk? In this case, we have only begun to explore what ‘risk’ even means. Is it risk of creating “astronomical levels of suffering” in created biospheres down the road? Soryl and Sandberg use the term, thinking directed panspermia should not be attempted until we have a better understanding of the issue of sentient welfare as well as technologies that can be fine-tuned to the task:
Until then, we propose a moratorium on the development of panspermia technologies – at least, until we have a clear strategy for their implementation without risking the creation of astronomical suffering. A moratorium should be seen as an opportunity for engaging in more dialogue about the ethical permissibility of directed panspermia so that it can’t happen without widespread agreement between people interested in the long-term future value of space. By accelerating discourse about it, we hope that existing normative and empirical uncertainties surrounding its implementation (at different timescales) can be resolved. Moreover, we hope to increase awareness about the possibility of S-risks resulting from space missions – not only limited to panspermia.
By S-risks, the authors refer to those risks of astronomical suffering. They assume we need to explore further what they call ‘the ethics of organised complexity.’ These are philosophical questions that are remote from ongoing space exploration, but building up a body of thought on the implications of new technologies cannot be a bad thing.
That said, is the idea of astronomical suffering viable? Life of any kind produces suffering, does it not, yet we choose it for ourselves as opposed to the alternative. I’m reminded of an online forum I once participated in when the question of existential risks to Earth by an errant asteroid came up. In the midst of asteroid mitigation questions, someone asked whether we should attempt to save Earth from a life-killer impact in the first place. Was our species worth saving given its history?
But of course it is, because we choose to live rather than die. Extending that, if we knew that we could create life that would evolve into intelligent beings, would we be responsible for their experience of life in the remote future? It’s hard to see this staying the hand of anyone seriously attempting directed panspermia. What would definitely put the brakes on it would be the discovery that life occurs widely around other stars, in which case we should leave these ecosystems to their own destiny. My suspicion is that this is exactly what our next generation telescopes and probes will discover.
The paper is Soryl & Sandberg, “To Seed or Not to Seed: Estimating the Ethical Value of Directed Panspermia,” Acta Astronautice 22 March 2025 (full text). The Mautner and Matloff paper is “Directed Panspermia: A Technical and Ethical Evaluation of Seeding the Universe,” JBIS, Vol. 32, pp. 419-423, 1979.
Why Do Super-Earths and Mini-Neptunes Form Where They Do?
Exactly how astrophysicists model entire stellar systems through computer simulations has always struck me as something akin to magic. Of course, the same thought applies to any computational methods that involve interactions between huge numbers of objects, from molecular dynamics to plasma physics. My amazement is the result of my own inability to work within any programming language whatsoever. The work I’m looking at this morning investigates planet formation within protoplanetary disks. It reminds me again just how complex so-called N-body simulations have to be.
Two scientists from Rice University – Sho Shibata and Andre Izidoro – have been investigating how super-Earths and mini-Neptunes form. That means simulating the formation of planets by studying the gravitational interactions of a vast number of objects. N-body simulations can predict the results of such interactions in systems as complex as protoplanetary disks, and can model formation scenarios from the collisions of planetesimals to planet accretion from pebbles and small rocks. All this, of course, has to be set in motion in processes occurring over millions of years.
Image: Super-Earths and mini-Neptunes abound. New work helps us see a possible mechanism for their formation. Credit: Stock image.
Throw in planetary migration forced by conditions within the disk and you are talking about complex scenarios indeed. The paper runs through the parameters set by the researchers as to disk viscosity and metallicity and uses hydrodynamical simulations to model the movement of gas within the disk. What is significant in this study is that the authors deduce planet formation within two rings at specific locations within the disk, instead of setting the disk model as a continuous and widespread distribution. Drawing on prior work he published in Nature Astronomy in 2021, Izidoro comments:
“Our results suggest that super-Earths and mini-Neptunes do not form from a continuous distribution of solid material but rather from rings that concentrate most of the mass in solids.“
Image: This is Figure 7 from the paper. Caption: Schematic view of where and how super-Earths and mini-Neptunes form. Planetesimal formation occurs at different locations in the disk, associated with sublimation and condensation lines of silicates and water. Planetesimals and pebbles in the inner and outer rings have different compositions, as indicated by the different color coding (a). In the inner ring, planetesimal accretion dominates over pebble accretion, while in the outer ring, pebble accretion is relatively more efficient than planetesimal accretion (b). As planetesimals grow, they migrate inward, forming resonant chains anchored at the disk’s inner edge. After gas disk dispersal, resonant chains are broken, leading to giant impacts that sculpt planetary atmospheres and orbital reconfiguration (c). Credit: Shibata & Izidoro.
Thus super-Earths and mini-Neptunes, known to be common in the galaxy, form at specific locations within the protoplanetary disk. Ranging in size from 1 to 4 times the size of Earth, such worlds emerge in two bands, one of them inside about 1.5 AU from the host star, and the other beyond 5 AU, near the water snowline. We learn that super-Earths form through planetesimal accretion in the inner disk. Mini-Neptunes, on the other hand, result from the accretion of pebbles beyond the 5 AU range.
A good theory needs to make predictions, and the work at Rice University turns out to replicate a number of features of known exoplanetary systems. That includes the ‘radius valley,’ which is the scarcity of planets about 1.8 times the size of Earth. What we observe is that exoplanets generally form at roughly 1.4 or 2.4 Earth size. This ‘valley’ implies, according to the researchers, that planets smaller than 1.8 times Earth radius would be rocky super-Earths. Larger worlds become gaseous mini-Neptunes.
And what of Earth-class planets in orbits within a star’s habitable zone? Let me quote the paper on this:
Our formation model predicts that most planets with orbital periods between 100 days < P < 400 days are icy planets (>10% water content). This is because when icy planets migrate inward from the outer ring, they scatter and accrete rocky planets around 1 au. However, in a small fraction of our simulations, rocky Earthlike planets form around 1 au (A. Izidoro et al. 2014)… While the essential conditions for planetary habitability are not yet fully understood, taking our planet at face value, it may be reasonable to define Earthlike planets as those at around 1 au with rocky-dominated compositions, protracted accretion, and relatively low water content. Our formation model suggests that such planets may also exist in systems of super-Earths and mini-Neptunes, although their overall occurrence rate seems to be reasonably low, about ∼1%.
That 1 percent is a figure to linger on. If the planet formation mechanisms studied by the authors are correct in assuming two rings of distinct growth, then we can account for the high number of super-Earths and mini-Neptunes astronomers continue to find. Such planets are currently thought to orbit about 30 percent of Sun-like stars, meaning that there would be no more than one Earth-like planet around every 300 such stars.
How seriously to take such results? Recognizing that the kind of computation in play here take us into realms we cannot verify through experiment, it’s important to remember that we have to use tools like N-body simulations to delve deeply into emergent phenomena like planets (or stars, or galaxies) in formation. The key is always to judge computational results against actual observation, so that insights can turn into hard data. Being a part of making that happen is what I can only call the joy of astrophysics.
The paper is Shibata & Izidoro, “Formation of Super-Earths and Mini-Neptunes from Rings of Planetesimals,” Astrophysical Journal Letters Vol. 979, No. 2 (21 January 2025), L23 (full text). The earlier paper by Izidoro and team is “Planetesimal rings as the cause of the Solar System’s planetary architecture,” Nature Astronomy Vol. 6 (30 December 2021), 357-366 (abstract).
Deep Space and the Art of the Map
NASA’s recently launched SPHEREx space telescope will map the entire celestial sky four times in two years, creating a 3D map of over 450 million galaxies. We can hope for new data on the composition of interstellar dust, among other things, so the mission has certain astrobiological implications. But today I’m focusing on the idea of maps of stars as created from our vantage here on Earth. How best to make maps of a 3D volume of stars? I’ve recently been in touch with Kevin Wall, who under the business name Astrocartics Lab has created the Astrocartics and Interstellar Surveyor maps he continues to refine. He explains the background of his interest in the subject in today’s essay while delving into the principles behind his work.
by Kevin Wall
The most imposing aspect of the universe we live in is that it is 3-dimensional. The star charts of the constellations that we are most familiar with show the 2-dimensional positions of stars fixed on the dome of the sky. We do not, however, live under a dome. In the actual cosmography of the universe some stars are farther and some are closer to us and nothing about their positions is flat in any way. Only the planets that reside in a mostly 2D plane around the Sun can be considered that way.
Now if your artistic inclinations make you want a nice map of the Solar System, you can find many for sale on Amazon with which you can decorate your wall. But if your space interests lie much farther than Pluto or Planet 9 (if it exists) and you want something that shows true cosmography then your choices are next to zero. This has been true since I became interested in making 3D maps of the stars in the mid-90s.
A map of the stars nearest the Sun is of particular interest to me because they are the easiest to observe and will be the first humanity visits. These maps are not really that hard to make and there are a few that you can find today. There is one I once saw on Etsy which is 3D but has a fairly plain design. There are some you can get from Winchell Chung on his 3D Star Maps site, but they are not visually 3D (a number beside the star is used for the Z distance). Also, there are many free star maps on the Internet, but none that really capture the imagination, the sort of thing one might want to display..
So why can’t star travel enthusiasts get a nice map of the nearest stars to Earth with which they can adorn their wall? With that question in mind, I started my interstellar mission to make a map that might be nice enough to make people more interested in star travel. Although I do not consider myself an artist. I thought a map done with more attention to detail and maybe a little artistry would be appreciated. My quest to map the sky has many antecedents, as the image of an ancient Chinese map below shows.
Image: An early star chart. This is the Suzhou Planisphere, created in the 12th Century and engraved in stone some decades later. It is an astronomical chart made as an educational tool for a Chinese prince. Its age is clear from the fact that the north celestial pole lies over 5° from Polaris’ position in the present night sky. Credit: History of Chinese Science and Culture Foundation / Ian Ridpath.
How to Make a 3D Star Chart
Before I go any farther, I need to explain that this article is about how to map objects in 3D space onto a flat 2D map surface. It is not about 3D models, which can be considered a sort of map but clearly involve different construction methods. It is also not quite about computer star map programs. I use the word “quite” because although a star map program is dynamic and seems different from a static wall map, the scene on your flat computer screen is still 2D. The projection your program is using is the same as ones for paper maps but you get to pick your view angle.
The first thing you need to do in making a 3D star map is get the 3D coordinates. This is not really hard but involves some trigonometry. I will not go into the details here because this article is about how to put 3D stars on the maps once you have the coordinates and data for them. I refer you to the 3D Star Maps site for trigonometry assistance.
The basic 3D star map is a 2D X-Y grid with Z coordinates printed next to the stars. The biggest thing going for it is efficient use of map area and, in fact, it is the only way to make a 3D star map with scores of stars on it. This map makes for a good reference; however, visualizing the depth of objects on it is hard. Although you can use graphics to give this type of map some 3D, you really need to use the projection systems invented for illustrations.
Much has been learned about how to show 3-dimensional images on a 2-dimensional surface and I cannot cover everything here. Therefore, I will summarize the methods I have seen used for most 3D star maps. They engage the visual cues that we use to see depth in real life to make all 3 dimensions easy for your map users to read. There are basically 2 ways of projection to visually show the 3D dimensions of something on a flat piece of paper: Parallel and Perspective.
The basic 3D star map uses a parallel projection. In parallel projection, all lines of sight from objects on the map to the viewer are parallel to each. So, unlike in human vision where things get smaller as they get farther away, everything in this sort of projection stays the same size into infinity. Because of this and the fact that such a star map has an X-Y plane that is parallel to the viewing plane, the Z coordinates are perpendicular to the viewing plane so that you can’t see a line connecting the X-Y plane to a star. However, if you tilt the XY plane so that it is no longer parallel, you can see the z dimension as it also is no longer perpendicular to the viewing plane.
Rotating the axes like this is called axonometric projection. It is used for most of the 3D star maps I have seen and is easy to create. It also gives a good illusion of depth. The scale of distances on the map changes depending on the angles an object makes with the map origin point. Although this is not a problem for viewers who only need estimates between objects, if you want to calculate the real distances you will need to do some arithmetic.
Perspective projection simulates the natural way humans see depth. That is, things that are farther from us look smaller. In perspective drawings, lines are drawn toward one or more points on the 2D surface. They determine the progressively smaller proportions of objects in the drawing that are farther from the viewer where the point(s) are considered to be at infinity. Star maps that use this will have distances that scale smaller the farther they are away from the reader. This makes for inconsistent distances, but it allows for maps to have extremely deep objects included on them.
My first map was large and had a simple design. It had a lot of stars (20 LY radius) and used an axonometric projection but it was not much different from the free maps that were available. Looking at it, I saw the problems a map reader would have in using it. These were things my latest map tries to correct, as I elaborate on later.
The main thing I didn’t like about it, however, was that it was not good art and not good star map art as well. It just wasn’t achieving my goal of being something a lot of people would consider putting on their wall. I have already admitted that being an artist is not one of my strong points. However, there are star maps out there that look good. So, I tried to see what the nicest ones had in common and tried to make something different as I moved on to my next star map iteration.
The Art in the Science
A map is like a scientific hypothesis. It attempts to model a physical aspect of the natural world and then it is tested through observation and measurement. This means that map designs have to be based on logical and testable methods. But unlike other sciences, cartography can use art to elaborate on those methods. Therefore, things like color and graphic design and the artistic beauty of the map are important factors.
The art in star maps can go beyond just the graphical enhancements that were intended. Looking at the night sky gives people a sense of wonder. Star maps, if done right, can capture that sense of wonder. Most maps have minimalist art designs that seem to do that best – circles and lines amidst white dots on a dark background. Fantasy star map art goes a bit farther in design and creativity but such maps are always recognized as having to do with that wonder. However, I think the more minimalist ones show the mystery better. Here is a minimalist fantasy star map without any 3D aspect to it that I made that has some of the aesthetics that I like.
Another map that has captured this quality of mystery is called ‘Star Chart for Northern Skies.’ Published by Rand McNally in 1915, it is a sky map of the northern hemisphere, one that interior designer Thomas O’Brien referenced in the magazine Elle Décor and later discussed in one of his books. It is a real map intended for reference and maybe to decorate classroom walls. It is one of the maps that have a simple style that nevertheless gives that sense of wonder. Although having a star map in an astronomy magazine is great, Elle Décor magazine reaches a much wider audience of people who would never have given this subject a single thought. This minimalist style of star map art is what I wanted for my own map.
Image: The Rand McNally star chart as seen displayed in Thomas O’Brien’s home. Credit: Thomas O’Brien.
To make a minimalist art 3D star map demands a projection system that has some visual way to show depth without needing a lot of lines drawn on it. The axonometric and perspective projections seemed to me to require too much background graphics. So, I had to come up with something a bit different than what has been used before.
Eventually I thought of what was to be the 3D depth graphics of the map I call the Astrocartica (shown below). The graphics show visually the Z dimension of the star systems on it, but it is not as intuitively obvious as the other projections I have mentioned. You can use the map to find accurate distances between star systems with a ruler and without resorting to a calculator or even doing arithmetical estimating in your head.
I have found since making the Astrocartica that more than a few people have difficulty seeing how the graphics work to show the third dimension. Possibly this is because the system I use is unfamiliar to most people and my illustrations and efforts to explain it do not seem to be sufficient. So, I decided to make another star chart that was more user friendly.
Image: The Astrocartica Interstellar Chart
The Science in the Art
A book called Envisioning Information by Edward Tufte (Graphics PR, 1990) has greatly influenced my star map designs. Tufte is an expert in his field of behavioral science and his book covers many aspects of data presentation such as layering and color. I had read the book while doing the Astrocartica star map and what I took away was just how art and science need to work together for good information display. I can’t say I succeeded completely in applying every principle in the book, but Tufte did make me aware of what a good design for readability needs to accomplish.
In the Astrocartica map, I emphasized the art. But, in my next chart I wanted to emphasize the readability of a star map because of my experience with my other maps. Since the depth projection was the chief issue with the Astrocartica, I considered the proven ways to illustrate 3 dimensions on a flat surface. The parallel and perspective projections were designed to work with the use of the visual cues that humans use and they have been used for centuries. So, I decided to use one of these. I felt perspective is more appropriate for pictures and painting and that, for a map, the scaling for objects in a perspective illustration makes it hard to measure accurate distances without doing arithmetic. I chose axonometric because scales were not as inconsistent as perspective. Also, parallel projection is often used for scenes in computer games and is familiar to people as a way to illustrate 3D on a flat computer screen.
My first map had a single plane. This put some stars that were far from the plane on the end of long extenders that made reading their X-Y position awkward. Also, having all the extenders on the one plane made it crowded and hard to differentiate them. So, I tried maps with several planes and settled on a design with three.
I also put several visual cues on the new map, called Interstellar Surveyor, to show the differences in the local space cosmography. First, the grid lines that are close to the stars are a lighter color that stands out compared to the lines that are farther that are darker. This shows areas of local space that are relatively empty. Also, the edges of the parts of the planes that have stars are thicker than the parts that do not.
One thing that I took from the Astrocartica was the star system graphics. It shows distances in star systems by increasing scale by a magnitude of 10x as you go away from the primary. I tilted the system plane to match the main ones for the sake of the art.
I think, overall, the Interstellar Surveyor map is more user-friendly than the Astrocartica or the first map I attempted.
Image: The Interstellar Surveyor Chart.
Astrocartics
I think the cartography of outer space is a long neglected field of study. Cartography has a unique connection with the human understanding of the world around us. It seems to serve both our rationalizing and aesthetic senses at the same time. That alone makes it important to develop and expand. But, while cartography has been engaged in studying the Earth, it really hasn’t explored the rest of the universe like some other sciences have even though it can.
In every field of study that is based on science there are logical principles. General cartography principles divide map projections of the Earth into how they transform areas and distances. Maps of outer space also have differences and patterns that seem to be able to be put into a categorical system. Here is a possible system to categorize and (more importantly) understand maps of outer space. It attempts to put the different projection systems into categories and is based on how the X-Y plane of a map is rotated in 3-space.
Type I – Planes are parallel to the viewing plane – these would be flat maps with little 3D relief.
Type II – Planes are at an angle between parallel and perpendicular – these would use mostly axonometric projections for 3D relief.
Type III – Planes are perpendicular – for deep field maps and art.
There is one other thing – the name of the field. The term “stellar cartography” would be the first choice. But it is very deeply rooted in sci-fi and is too often pictured as the map room of a starship. That is not necessarily a bad thing but another problem is that there is more to mapping outer space than just stars – there are nebulas and planets with moons and perhaps other kinds of space terrains. Outer space mapping is more than just “stellar”. The word “astrocartography” is another logical choice. However, it has been used as a label for a branch of astrology for many decades now. Also, the word is trademarked. The word “astrography” is astro-photography.
The word “astrocartics” means charts of outer space. It comes from the word “astro” which means “star” but can also mean “outer space” and the word “carte” which is an archaic form of the word “chart” or “map”. So this is my proposal for a name. Also, I think it sounds cool.
Well, I think I have said all that I can about astrocartics. I am going to be making new maps. There will be an Astrocartica 2.0, a solar system map with unique graphics and a map that might be considered astro-political in essence. I will also be adding a blog to my store in a few months. I am willing to talk to anyone who wishes about space maps.
Are Supernovae Implicated in Mass Extinctions?
As we’ve been examining the connections between nearby stars lately and the possibility of their exchanging materials like comets and asteroids with their neighbors, the effects of more distant events seem a natural segue. A new paper in Monthly Notices of the Royal Astronomical Society makes the case that at least two mass extinction events in our planet’s history were forced by nearby supernova explosions. Yet another science fiction foray turned into an astrophysical investigation.
One SF treatment of the idea is Richard Cowper’s Twilight of Briareus a central theme of which is the transformation of Earth through just such an explosion. Published by Gollancz in 1974, the novel is a wild tale of alien intervention in Earth’s affairs triggered by the explosion of the star Briareus Delta, some 130 light years out, and it holds up well today. Cowper is the pseudonym for John Middleton Murry Jr., an author I’ve tracked since this novel came out and whose work I occasionally reread.
Image: New research suggests at least two mass extinction events in Earth’s history were caused by a nearby supernova. Pictured is an example of one of these stellar explosions, Supernova 1987a (centre), within a neighbouring galaxy to our Milky Way called the Large Magellanic Cloud. Credit: NASA, ESA, R. Kirshner (Harvard-Smithsonian Center for Astrophysics and Gordon and Betty Moore Foundation), and M. Mutchler and R. Avila (STScI).
Nothing quite so exotic is suggested by the new paper, whose lead author, Alexis Quintana (University of Alicante, Spain) points out that supernova explosions seed the interstellar medium with heavy chemical elements – useful indeed – but can also have devastating effects on stellar systems located a bit too close to them. Co-author Nick Wright (Keele University, UK) puts the matter more bluntly: “If a massive star were to explode as a supernova close to the Earth, the results would be devastating for life on Earth. This research suggests that this may have already happened.”
The conclusion grows out of the team’s analysis of a spherical volume some thousand parsecs in radius around the Sun. That would be about 3260 light years, a spacious volume indeed, within which the authors catalogued 24,706 O- and B-type stars. These are massive and short-lived stars often found in what are known as OB associations, clusters of young, unbound objects that have proven useful to astronomers in tracing star formation at the galactic level. The massive O type stars are considered to be supernova progenitors, while B stars range more widely in mass. Even so, larger B stars also end their lives as supernovae.
Making a census of OB objects has allowed the authors to pursue the primary reason for their paper, a calculation of the rate at which supernovae occur within the galaxy at large. That work has implications for gravitational wave studies, since supernova remnants like black holes and neutron stars and their interactions are clearly germane to such waves. The distribution of stars within 1 kiloparsec of the Sun shows stellar densities that are consistent with what we find in other associations of such stars, and thus we can extrapolate on the basis of their behavior to understand what Earth may have experienced in its own past.
Earlier work by other researchers points to supernovae that have occurred within 20 parsecs of the Sun – about 65 light years. A supernova explosion some 2-3 million years ago lines up with marine extinction at the Pliocene-Pleistocene boundary. Another may have occurred some 7 million years ago, as evidenced by the amount of interstellar iron-60 (60Fe), a radioactive isotope detected in samples from the Apollo lunar missions. Statistically, it appears that one OB association (known as the Scorpius–Centaurus association) has produced on the order of 20 supernova explosions in the recent past (astronomically speaking), and HIPPARCOS data show that the association’s position was near the Sun’s some 5 to 7 million years ago. These events, indeed, are thought by some to have produced the so-called Local Bubble, the low density cavity in the interstellar medium within which the Solar System currently resides.
Here’s a bit from the paper on this:
An updated analysis with Gaia data from Zucker et al. (2022) supports this scenario, with the supernovae starting to form the bubble at a slightly older time of ∼14 Myr ago. Measured outflows from Sco-Cen are also consistent with a relatively recent SN explosion occurring in the solar neighbourhood (Piecka, Hutschenreuter & Alves 2024). Moreover, there is kinematic evidence of families of nearby clusters related to the Local Bubble, as well as with the GSH 238+00 + 09 supershell, suggesting that they produced over 200 SNe [supernovae] within the last 30 Myr (Swiggum et al. 2024).
But the authors of this paper focus on much earlier events. Earth has experienced five mass extinctions, and there is coincidental evidence for the effects of a supernova in the late Devonian and Ordovician extinction events (372 million and 445 million years ago respectively). Both are linked with ozone depletion and mass glaciation.
A supernova going off within a few hundred parsecs of Earth would have atmospheric effects but little of significance. But the authors point out that if we close the range to 20 parsecs, things get more deadly. The destruction of the Earth’s ozone layer would be likely, with resultant mass extinctions a probable result:
Two extinction events have been specifically linked to periods of intense glaciation, potentially driven by dramatic reductions in the levels of atmospheric ozone that could have been caused by a near-Earth supernova (Fields et al. 2020), specifically the late Devonian and late Ordovician extinction events, 372 and 445 Myr ago, respectively (Bond & Grasby 2017). Our near Earth ccSN [core-collapse supernova] rate of ∼2.5 per Gyr is consistent with one or both of these extinction events being caused by a nearby SN.
So this is interesting but highly speculative. The purpose of this paper is to examine a specific volume of space large enough to draw conclusions about a type of star whose fate can tell us much about supernova remnants. This information is clearly useful for gravitational wave studies. The supernova speculation in regard to extinction events on Earth is a highly publicized suggestion that grows out of this larger analysis. In other words, it’s a small part of a solid paper that is highly useful in broader galactic studies.
Supernovae get our attention, and of course such discussions force the question of what happens in the event of a future supernova near Earth. Only two stars – Antares and Betelgeuse – are likely to become a supernova within the next million years. As both are more than 500 light years away, the risk to Earth is minimal, although the visual effects should make for quite a show. And we now have a satisfyingly large distance between our system and the nearest OB association likely to produce any danger. So much for The Twilight of Briareus. Great book, though.
The paper is Van Bemmel et al., “A census of OB stars within 1 kpc and the star formation and core collapse supernova rates of the Milky Way,” accepted at Monthly Notices of the Royal Astronomical Society (preprint).
Quantifying the Centauri Stream
The timescales we talk about on Centauri Dreams always catch up with me in amusing ways. As in a new paper out of Western University (London, Ontario), in which astrophysicists Cole Gregg and Paul Wiegert discuss the movement of materials from Alpha Centauri into interstellar space (and thence to our system) in ‘the near term,’ by which they mean the last 100 million years. Well, it helps to keep our perspective, and astronomy certainly demands that. Time is deep indeed (geologists, of course, know this too).
I always note Paul Wiegert’s work because he and Matt Holman (now at the Harvard-Smithsonian Center for Astrophysics) caught my eye back in the 1990s with seminal studies of Alpha Centauri and the stable orbits that could occur there around Centauri A and B (citation below). That, in fact, was the first time that I realized that a rocky planet could actually be in the habitable zone around each of those stars, something I had previously thought impossible. And in turn, that triggered deeper research, and also led ultimately to the Centauri Dreams book and this site.
Image: (L to R) Physics and astronomy professor Paul Wiegert and PhD candidate Cole Gregg developed a computer model to study the possibility that interstellar material discovered in our solar system originates from the stellar system next door, Alpha Centauri. Credit: Jeff Renaud/Western Communications.
Let’s reflect a moment on the significance of that finding when their paper ran in 1997. Wiegert and Holman showed that stable orbits can exist within 3 AU of both Alpha Centauri A and B, and they calculated a habitable zone around Centauri A of 1.2 to 1.3 AU, with a zone around Centauri B of 0.73 to 0.74 AU. Planets at Jupiter-like distances seemed to be ruled out around Centauri because of the disruptive effects of the two primary stars; after all, Centauri A and B sometimes close to within 10 AU, roughly the distance of Saturn from the Sun. The red dwarf Proxima Centauri, meanwhile, is far enough away from both (13,000 AU) so as not to affect these calculations significantly.
But while that and subsequent work homed in on orbits in the habitable zone, the Wiegert and Gregg paper examines the gravitational effects of all three stars on possible comets and meteors in the system. The scientists ask whether the Alpha Centauri system could be ejecting material, analyze the mechanisms for its ejection, and ponder how much of it might be expected to enter our own system. I first discussed their earlier work on this concept in 2024 in An Incoming Stream from Alpha Centauri. A key factor is that this triple system is in motion towards us (it’s easy to forget this). Indeed, the system approaches Sol at 22 kilometers per second, and in about 28,000 years will be within 200,000 AU, moving in from its current 268,000 AU position.
This motion means the amount of material delivered into our system should be increasing over time. As the paper notes:
…any material currently leaving that system at a low speed would be heading more-or-less toward the solar system. Broadly speaking, if material is ejected at speeds relative to its source that are much lower than its source system’s galactic orbital speed, the material follows a galactic orbit much like that of its parent, but disperses along that path due to the effects of orbital shear (W. Dehnen & Hasanuddin 2018; S. Torres et al. 2019; S. Portegies Zwart 2021). This behavior is analogous to the formation of cometary meteoroid streams within our solar system, and which can produce meteor showers at the Earth.
The effect would surely be heightened by the fact that we’re dealing not with a single star but with a system consisting of multiple stars and planets (most of the latter doubtless waiting to be discovered). Thus the gravitational scattering we can expect increases, pumping a number of asteroids and comets into the interstellar badlands. The connectivity between nearby stars is something Gregg highlights:
“We know from our own solar system that giant planets bring a little bit of chaos to space. They can perturb orbits and give a little bit of extra boost to the velocities of objects, which is all they need to leave the gravitational pull of the sun. For this model, we assumed Alpha Centauri acts similarly to our solar system. We simulated various ejection velocity scenarios to estimate how many comets and asteroids might be leaving the Alpha Centauri system.”
Image: In a wide-field image obtained with an Hasselblad 2000 FC camera by Claus Madsen (ESO), Alpha Centauri appears as a single bright yellowish star at the middle left, one of the “pointers” to the star at the top of the Southern Cross. Credit: ESO, Claus Madsen.
This material is going to be difficult to detect, to be sure. But the simulations the authors used, developed by Gregg and exhaustively presented in the paper, produce interesting results. Material from Alpha Centauri should be found inside our system, with the peak intensity of arrival showing up after Alpha Centauri’s closest approach in 28,000 years. Assuming that the system ejects comets at a rate like our own system’s, something on the order of 106 macroscopic Alpha Centauri particles should be currently within the Oort Cloud. But the chance of one of these being detectable within 10 AU of the Sun is, the authors calculate, no more than one in a million.
There should, however, be a meteor flux at Earth, with perhaps (at first approximation) 10 detectable meteors per year entering our atmosphere, most no more than 100 micrometers in size. That rate should increase by a factor of 10 in the next 28,000 years.
Thus far we have just two interstellar objects known to be from sources outside our own system, the odd 1I/’Oumuamua and the comet 2I/Borisov. But bear in mind that dust detectors on spacecraft (Cassini, Ulysses, and Galileo) have detected interstellar particles, even if detections of interstellar meteors are controversial. The authors note that this is because the only indicator of the interstellar nature of a particle is its hyperbolic excess velocity, which turns out to be very sensitive to measurement error.
We always think of the vast distances between stellar systems, but this work reminds us that there is a connectedness that we have only begun to investigate, an exchange of materials that should be common across the galaxy, and of course much more common in the galaxy’s inner regions. All this has implications, as the authors note:
…the details of the travel of interstellar material as well as its original sources remain unknown. Understanding the transfer of interstellar material carries significant implications as such material could seed the formation of planets in newly forming planetary systems (E. Grishin et al. 2019; A. Moro-Martín & C. Norman 2022), while serving as a medium for the exchange of chemical elements, organic molecules, and potentially life’s precursors between star systems—panspermia (E. Grishin et al. 2019; F. C. Adams & K. J. Napier 2022; Z. N. Osmanov 2024; H. B. Smith & L. Sinapayen 2024).
The paper is Gregg & Wiegert, “A Case Study of Interstellar Material Delivery: α Centauri,” Planetary Science Journal Vol. 6, No. 3 (6 March 2025), 56 (full text). The Wiegert and Holman paper, a key reference in Alpha Centauri studies, is “The Stability of Planets in the Alpha Centauri System,” Astronomical Journal 113 (1997), 1445–1450 (abstract).
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