Relocating the human race to a more hospitable planet would mean that multiple generations would be born in-transit
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| Illustration: Janet Mac |
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 nearest 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.
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| Credit: NASA Ames Research Center |
hat
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.
Corin Faife
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