After 200,000 years or so of human existence, climate change threatens to make swathes of our planet unlivable by the end of the century. If we do manage to adapt, on a long enough timeline the Earth will become uninhabitable for other reasons: chance events like a comet strike or supervolcano eruption, or ultimately — if we make it that long — the expansion of the sun into a red giant in around five billion years, engulfing the planet completely or at a minimum scorching away all forms of life. Planning for potential escape routes from Earth is, if not exactly pressing, then at least a necessary response to a plausible threat.
The most obvious destination is our neighbor Mars. We’ve already sent multiple probes there, and NASA is planning another moon landing in 2024 with the eventual plan of using it as a waypoint on a mission to Mars. Elon Musk’s Space X claims to be aiming for a crewed trip to Mars in the same year. But Mars is a desert planet, cold and barren, with no atmosphere save for a thin blanket of CO2. Sure, we could survive there, in protective suits and hermetically sealed structures, but it’s not a great place to truly live.
Some scientists have another favorite relocation candidate: Proxima b, a planet that orbits a star called Proxima Centauri, some 4.24 light years distant from our sun. Located in the triple-star Alpha Centauri solar system, Proxima b has a mass 1.3 times that of Earth and a temperature range that allows for liquid water on the surface, raising the possibility that it could support life.
The biggest challenge is getting there. Proxima b is almost unimaginably far away. There is a program underway, Breakthrough Starshot, to send a probe to Alpha Centauri with a journey time of just 20 years, but the entire craft will weigh only a few grams, being propelled by a 100-billion-watt laser fired at it from Earth rather than carrying any of its own fuel or, for that matter, human passengers. Even by generous estimates, traveling one light year in a vessel large enough to transport humans will take centuries; reaching a planet in the range of Proxima b would take a thousand years or more.
This means that no one cohort of crew members would be able to survive the journey from start to finish, so those on the craft for the launch would have to pass on the torch to the next generation, and the next, and the next, and the next.
While it might sound like science fiction, a small network of researchers is tackling the problem of multi-generation space travel in a serious way. “There’s no principal obstacle from a physics perspective,” Andreas Hein, executive director of the nonprofit Initiative for Interstellar Studies — an education and research institute focused on expediting travel to other stars — tells me in a call from Paris. “We know that people can live in isolated areas, like islands, for hundreds or thousands of years; we know that in principle people can live in an artificial ecosystem like Biosphere2. It’s a question of scaling things up. There are a lot of challenges, but no fundamental principle of physics is violated.”
As one might expect from such an undertaking, the difficulties are many and broad, spanning not just physics but biology, sociology, engineering, and more. They include conundrums like artificial gravity, hibernation, life support systems, propulsion, navigation, and many problems that are nowhere near to being solved. But even if we never make it to Proxima b, in the process of exploring the question of how to escape Earth, some of the scientists involved in the work may stumble upon solutions for surviving on our planet, as resources like energy and water become increasingly scarce.
When it comes to traveling beyond our solar system to colonize the planets of a nearby star, the most basic question is whether it’s possible at all on a biological level.
Frédéric Marin, an astrophysicist at the Université de Strasbourg and a global expert on the radiation created by black holes, decided to address this question in a series of research papers produced without funding and in his spare time.
He was inspired to look into the issue by the work of Nick Kanas, a professor of psychiatry who studied NASA crew members to understand the psychological effect of months spent in the International Space Station. Kanas has published many papers and books on the subject, assessing the impact on the human mind of confinement, stress, zero gravity and isolation from Earth. He describes his own work as a precursor to mounting long-duration space missions. This body of research posed questions about whether manned journeys to the outer planets of the solar system and beyond are feasible, and Marin realized that very few people had tried to seriously address the question from a biological and sociological point of view. He also realized that he had the skills to try.
As an astrophysicist, Marin was accustomed to building simulated models of particle interaction in space. He designed a simulation in which each unit would represent not a particle but a human in a closed environment, with a certain probability of living healthily, succumbing to disease, and finally passing on genetic material to the next generation. In turn, humans of the next generation were born with some random attributes, and others based on the “consanguinity” of their parents — how closely related they were. The guiding question was whether an initial crew of a given size would be sufficient to complete a 200-year journey without outgrowing the ship’s capacity, dying off en masse, or arriving with excessive inbreeding. “You can use data from biology, anthropometry, anthropology, mathematics, to compute it,” Marin says. “This is a theoretical step, but it’s the first step.”
In 2017 Marin published a paper unveiling a software system, dubbed HERITAGE, that could simulate the growth of an isolated human population over time to predict whether an initial crew of a given size would be sufficient to complete a journey over multiple generations, and arrive with enough genetic diversity to populate a new planet. In 2018 he and co-author Camille Beluffi, a physicist at scientific data startup CASC4DE, applied the same technique to calculate the crew size needed for travel to Proxima b, estimating that just 98 crew members at departure from Earth would be enough to successfully navigate a 6,300-year voyage. At least theoretically, Marin reasoned, this proved that it was not impossible for humans to sustain a healthy gene pool on the trip to Proxima b. “And after that,” he explains, “you ask, how can we do it?”
He estimated next how much space would be required to produce food. The trick, he surmised in a paper from this year, would be to farm vegetables through aeroponics — a highly efficient growing system where nutrient mists are sprayed onto the roots of hanging plants — and derive some additional protein from animals, which have greater space requirements. Using these techniques, the total space needed to feed a crew of 500 would be 0.45 km2, or 111 acres: the same area as Vatican City, or roughly an eighth the size of Central Park. This area would be distributed around a slowly rotating cylinder in order to produce artificial gravity, crucial for humans to retain muscle mass and normal bodily functions over a prolonged period in space, and span multiple floors too. One architecture plan Marin suggests is a cylinder just 25 metres tall but with a radius of 224 metres, not dissimilar to NASA’s iconic Stanford Torus concept.
What Frédéric Marin takes to be an indication of the viability of interstellar travel would seem to prove the exact opposite to others. While the Breakthrough Starshot project lists significant challenges to be overcome in order to reach Alpha Centauri with a probe weighing less than a nickel, Marin’s calculations describe a ship bigger than the U.S. Navy’s largest aircraft carrier. Surely this giant vessel would be too massive to move across the sky?
When I spoke with Avi Loeb, Frank B. Baird Jr. professor of science at Harvard University and chair of the advisory committee to the Breakthrough Starshot project, I’d expected that he would scoff at the idea of a 500-person vessel, given the difficulty of interstellar travel even for ultra-small ships. But he didn’t. Theoretically, he explains, there’s no problem moving a far bigger load with the same laser propulsion system that Starshot will use. But there’s another obstacle. “Once you go out of the protective womb of the magnetic field of Earth,” Loeb says, “you’re exposed to very energetic particles that, within a year, will damage a significant fraction of the cells in your brain... This is a risk for people who go to Mars, without even thinking about a journey that lasts hundreds of years.”
Even so, he agrees with Marin that we may need to figure out how to pull off a multi-generation space mission. “There is no doubt that our future is in space,” he told me. “One way or another we’ll have to leave the Earth... At some point there will be a risk from an asteroid that will hit us, or eventually the Sun will heat up to the point that it will boil off all the oceans on Earth. Ultimately, to survive we will need to relocate.”
This June, a group of researchers from around the world converged on the Erasmus Space Exhibition Centre in Noordwijk, the Netherlands, for the European Space Agency (ESA)’s first ever Interstellar Workshop. Under the high roof of the auditorium with spotlights facing the stage, an audience of more than a hundred sat in orderly rows to watch presentations on multi-generation space travel.
Scientists had shown up from numerous fields of research: architecture, astrophysics, linguistics, sociology, engineering, materials science, human and plant biology, and more. Many of them aimed to answer questions that come up only after you assume that — like Marin’s simulations suggest — we can actually build the ship, and keep humans healthy inside it for a millennium or more.
This was the theory advanced in “World Ships: Feasibility and Rationale,” a presentation given by aerospace engineer Andreas Hein that expounded on the trade-offs of different ship designs, as well as the assumption behind “Sociology of Interstellar Exploration: Annotations on Social Order, Authority, and Power Structures,” in which sociology professor Elke Hemminger theorized about the kind of social structure a world ship mission would require. It was in artist/biologist Angelo Vermeulen’s “Evolving Asteroid Starships: A Bio-Inspired Approach for Interstellar Space Systems,” and in theology lecturer Michael Waltemathe’s “Philosophical Aspects of Interstellar Exploration,” a presentation spanning mission ethics, anti-contamination principles in space, and Christianity’s response to aliens. (For the latter he cites the Vatican’s former chief astronomer José Gabriel Funes, who has argued that it must logically be possible for an all powerful God to have made extraterrestrial species — and that without original sin, they might even enjoy a closer relationship to their creator than humans.)
Others looked at what it means for the crew of the ship — not the first generation, who choose to leave Earth behind, but for the second, tenth, fiftieth, one hundredth, the people for whom our planet is just a myth; for whom there will be no other life but the journey.
Andrew McKenzie and Jeffrey Punske, linguists from the University of Kansas and the University of Southern Illinois, write that “[i]f a trip takes several generations to complete, the language may differ significantly at arrival from that of the passengers at departure.” More evocatively they suggest: “Even if the onboard schools rigorously maintained the teaching of ‘Earth English’ the children would develop their own Vessel English dialect, which would diverge from Earth English over time.” The problem would be compounded by the fact that this “Vessel English” — using English as just one example — would be unique to each ship, so that the crew of two ships arriving at the same planet would speak a different dialect, or even a different language altogether.
Ultimately, to survive we will need to relocate.
Separately, Neil Levy, a professor of philosophy at Macquarie University in Sydney and senior research fellow in ethics at Oxford University, considered the moral implications in an article for Aeon:
“A generation ship can work only if most of the children born aboard can be trained to become the next generation of crew,” he writes. “They will have little or no choice over what kind of project they pursue. At best, they will have a range of shipboard careers to choose between: chef, gardener, engineer, pilot, and so on.”
In other words, their life options will be extremely limited, as would be the range of experiences they can enjoy. Would it even be ethical to put them in this situation?
The conclusion depends on what we believe is justified to preserve our species, a reckoning Levy declines to make. Instead, he points to the subtext of the question: Life outcomes are already defined by accident of birth in the world as it is; the range of any child’s possible futures is constrained by poverty, nationality, religion, culture. This may be unjust, but we accept this as part of the human condition. “Asking about the permissibility of generation ships,” he writes, “might give us a fresh perspective on the permissibility of the constraints we impose now on human lives, here on the biggest generation ship of them all — our planet.”
There are more than just technological obstacles to colonizing our nearest star. For one, we can’t afford it.
In his research, Andreas Hein of the Initiative for Interstellar Studies estimates that the world economy, if it continues to grow at current rates, would be able to cover the cost of building a generation ship sometime between the year 2500 and 3000. And it’s not only a matter of time: We most likely couldn’t develop a big enough economy with the resources of Earth alone, so would need to expand in some way beyond our home planet. Colonizing space would be necessary for both the funds — say from mining asteroids — and to test the idea that it’s possible to live in a spaceship for hundreds of years.
For his part, Professor Avi Loeb, the chair of the Breakthrough Starshot project advisory board, considers space travel so dangerous that it’s not worth making such a trip, though he hasn’t given up on the idea of human life arriving in far off star systems. Instead, he sees other paths to establishing life elsewhere as more likely, like sending out an artificial intelligence system that could build biological cells from the raw materials it encountered, assembling life again from scratch that may or may not resemble our current human race.
Given that it could take a millennium for such a trip to actually materialize and that a colonized planet might not even resemble our current culture, it’s easy to see the efforts around multi-generational space travel, even those by serious scientists, as nothing more than a pipe dream.
Paul M. Sutter, an astrophysicist at Ohio State University and the Flatiron Institute in New York, has published op-eds on the difficulty of interstellar travel, particularly the Breakthrough Starshot program. Starshot is not a bad idea, he argues, “it’s just that interstellar travel is beyond ridiculously hard.” In a YouTube video, Sutter explains that the Starshot laser propulsion method — which would require as much power as the output of all the nuclear power stations in the United States combined — will transfer only a few pounds of thrust to the space probe. Asked about using the same method to drive a ship that can carry even a single human, Sutter is skeptical. “You’ll need either a million times more energy, or it takes a million times longer,” he says — and neither sounds like a viable option.
The prohibitive cost and difficulty of space exploration also means that progress is slow. “It’s been 50 years [since the moon landing] and we can’t do much more than we did in the ’60s,” says Sutter. “So follow that line of thinking to work out what we could do 50 years from now.”
But we may find value long before the trip itself, from ancillary benefits of the research.
Angelo Vermeulen, an artist and biologist by training who now works as a space systems researcher at Delft University of Technology in the Netherlands, specializes in applying principles from the natural world to artificial systems. He describes his work as “theoretical research into morphogenetic engineering,” an approach where complex design emerges from a small set of initial rules and properties — like the way termites build large, naturally cooled mounds to live in without any central control.
Some of his work integrates research from the MELiSSA program, a project led by ESA to develop a closed, circular life-support system that will recycle carbon dioxide and organic waste into food, oxygen, and water. While MELiSSA’s ultimate goal is to make long-duration space missions possible, it has also spun off a sister company charged with developing commercial, terrestrial applications of the technology — like a modular sanitation hub that can provide wastewater treatment in off-grid environments, or a nutrient rich bacterium that also reduces cholesterol.
In some form or another, the majority of researchers I spoke to about multi-generation space travel pointed out that it’s not possible to map out all of the applications of a technological or scientific breakthrough until it has been released to the public. We can’t start connecting the dots, and finding new routes and patterns, until those dots exist somewhere on the page; but with hindsight, patterns over the short- and long-term become more obvious, sometimes in unexpected ways.
At the end of our call, Vermeulen tells me a story: In 1901, at the Pan American Exposition in Buffalo, the star attraction was a ride that simulated a trip to the moon. For 50 cents passengers could board the “spaceship” Luna, a winged wooden craft that through an artful combination of pulleys, theater props, optical illusions, and even dwarf actors, gave the impression of leaving Earth behind and climbing into space for an alien encounter.
The ride was wildly successful, attracting 400,000 paying customers, including then-President William McKinley, Thomas Edison, and various Supreme Court justices. It was reported in news bulletins around the world.
It was also pure turn-of-the-century showmanship, a triumph of creativity that, like the pulp sci-fi movies of the 1960s or ’70s, showed a vision of the future still hopelessly bound to the ideas of the time. But its exact impact — its impact on the collective consciousness — is hard to quantify. Perhaps without the Luna there would be no NASA, no Apollo mission, no Mars rover today. Without these leaps of imagination, without speculating about what the future could be before we get there, we never arrive at anywhere different to the present. And maybe, just maybe, one day a man or woman on a distant planet will look back at this research, antiquated as it will seem, and say the same.