Before we get started I wanted to let you know,
today’s episode is brought to you by
Space: The Longest Goodbye. Available now on the PBS App and YouTube. So check out the link below.
We are the middle children of history. Born too late to explore Earth, and born too early
to explore the universe —- to partially quote someone on the internet whose wisdom is only
matched by their anonymity. In the far future we may have advanced propulsion technologies
like matter-antimatter engines and compact fusion drives that allow humans to travel
to other stars on timescales shorter than their own lives. But what if those technologies never
materialize? Are we imprisoned by the vastness of space—doomed to remain in the solar system
of our origin? Perhaps not. A possible path to a contemporary cosmic dream may just be to build a
ship which can support human life for several generations; a so-called generation ship.
Faster than light travel is almost certainly impossible—so says Einstein’s special theory
of relativity—and we rarely win when we bet against Einstein. That sounds like bad news
for the galactic future of humanity given that the Milky Way is 100,000 lightyears across,
and there are relatively few stars within what most would consider to be a reasonable commute.
But that doesn’t mean we can’t reach for the stars. If we can build spacecraft capable of
reaching 80, 90, even 99% the speed of light then relativistic time dilation would slow the
clock of the spacecraft relative to Earth’s. At these speeds a single crew could reach an
interstellar destination up to 100s of light years away within their own lifetimes. But
such speeds would require some pretty out-there propulsion methods like matter-antimatter engines,
compact fusion reactors, or even black hole drives. And even if we eventually
do build such devices, there are a whole range of dangers that uniquely arise when
traveling through the cosmos at such high speeds, as we’ve discussed previously.
So, what if it turns out we have to travel the slow road? What if it proves impossible to send
humans any faster than a tiny fraction of the speed of light? Or what if we decide we
really really need to leave Earth ASAP using technology that we at least understand today.
OK, Here’s the scenario: Something is coming. It could be a comet impact or catastrophic climate
collapse or the Tri-Solarian fleet. Whatever it is, there’s enough of an existential threat
that we decide to insure the future of our species by trying to settle another world.
Quite naturally, NASA tasks PBS Space Time with planning a mission to settle
Proxima Centauri B in the Alpha-Centauri system. This is the closest known exoplanet to Earth at a
mere 4.2 light years away. To keep things simple, let’s pretend that we discovered that Proxima-B
is already habitable so all we need to do is get some people there in good condition.
We only have a few decades to make this happen, so ultra-advanced propulsion is out of the question.
We launch whatever we can throw together in around 30 years. The fastest ship we could conceivably
hope to build might reach speeds of 10% that of light. That’s a 42 year journey—launch a crew in
their 20s and they’ll arrive at retirement age. More likely our craft will travel much slower,
so that no crew that starts the journey will live to see its end. Assuming that cryogenics won’t
be 100% reliable within decades—which is pretty fair—it sounds like we need to plan for a mission
in which multiple generations of humans are born, live, and die en route, and that landfall
is made by descendents of the launch crew. It sounds like we need to plan a generation ship.
There are lots of decisions to make in how we do this, but remember our constraint:
it has to be something we can plausibly launch in 30 years. We’re going to need
to choose a propulsion system, a crew size and composition, life support systems, and
finally we need to ensure the mental, social and cultural wellbeing and stability of this group.
Starting with the propulsion method; this determines the speed we can travel, the potential
size of the ship, and so the size and number of generations of the population we need to sustain.
The fastest vehicle ever built by humans is the Parker Solar Probe, which accelerates by
blasting a propellant—hydrazine in this case—using electrical power. Although it
was really more the gravitational assists that enabled the Parker to reach 700,000 kilometers
per hour. If we could scale up this tech to something large enough to carry lots of humans then
at this speed we could get our crew to Proxima-B in … 6,300 years. That’s like 200 generations,
and roughly the length of recorded human history. It’s difficult to imagine that
nothing would go wrong in that much time. But we’re also pretty sure we can get a ship to this speed,
so we should see if this timescale is at least feasible. Also, this is the speed
assumption made by French scientists Frédéric Marin and Camille Beluffi in a series of
studies, and we’ll be coming back to their conclusions regarding a trip of this length.
We’ll also consider a much faster craft—one propelled by nuclear
fusion—smashing light elements together to form heavier elements plus lots of energy,
just like the Sun does. We haven’t yet managed to build a commercially viable fusion reactor,
let alone the sort of compact reactor we’d need for a spacecraft. But there IS a fusion
technology that we’ve thoroughly mastered—and that’s the thermonuclear explosion. There are
various concepts for spacecraft that accelerate under a series of fusion
pulses—aka explosions—rather than sustained fusion reactions. These vary in sophistication
from the more advanced internal confinement engine of Project Daedalus to more achievable,
if scarier proposals where you literally detonate thermonuclear explosions behind the craft,
like in Project Orion or the Enzmann starship, or into a forward sail like in the Medusa design.
Top speeds for some of these have been estimated at 30% lightspeed, but that’s highly optimistic.
A little under 10% is more realistic for a mature version of this technology. For us,
with our limited timeline, we’re going to assume we can get to 3% lightspeed. That’s around 50
times faster than our conventional drive, so gets us to Proxima-B in a mere 140 years—just
four or five generations. So, today we’re going to plan towards these two travel times—140 years
if fusion pans out and 6300 years if not. We’ll have teams working on both, and you can think of
these as representing the extreme boundaries of what we can achieve in the little time we have.
The next decision will influence all of the choices that follow. How many people are we
sending? This determines the size of the ship or ships and the resources we need to bring.
Perhaps the most important factor determining population size on a generation ship is the
issue of genetic diversity. There are two aspects to this: how many people are needed to ensure a
healthy multigenerational crew during the journey, and how many are needed to healthily populate a new planet.
A 6300 year journey means 200 generations give or take. If the genetic diversity of
the starting population isn’t sufficient there will be genetic health issues en route. Marin and
Beluffi explore this question in a 2018 paper. They use Monte Carlo simulations to calculate
the minimum number of humans that would be needed to avoid many of the potential genetic pitfalls,
also accounting for various forms of misfortune such as a random
disaster eliminating a third of the population, different infertility rates, and even an overall
“chaotic factor” intrinsic to any human exploration. From all of this they came
up with the minimum numbers needed to achieve a sustainable population during
the journey. They conclude that we need to launch with a crew of at least 100, who will
multiply to a population of 500—and that’s the level to support for most of the journey.
How big a ship does it take to comfortably carry 500? Well,
SpaceX’s Starship is supposed to be able to carry 100. So,
the equivalent of 5 of those at least? However that doesn’t include the space needed for
systems to support 500 lives long term. For that we’re definitely going to need a bigger boat.
Missions around the solar system don’t need to be luxurious. But centuries or
millenia long trips to Proxima-B will need some home comforts. Like gravity.
Living in zero gravity or microgravity has clear negative effects on health,
with the most well documented being on bone density. To avoid our travelers reaching
Proxima-B as Wall-E-esque gelatinous blobs, we need artificial gravity.
We’ve discussed previously how this could be done. There’s only one way, and fortunately
it’s not that complicated. The ship’s habitats need to be spun in a circle to
give 1-g of centrifugal acceleration, perfectly mimicking Earth’s surface
gravity. There are lots of designs for centrifugal artificial gravity,
but the simplest might be a rotating ring habitat. A 100m radius ring would need to rotate 3 times
per minute to replicate Earth gravity. That seems not completely crazy, so let’s move on.
The next step is to feed our crew Another study led by the French team finds that we’d need 0.45
km^2 for an omnivorous and balanced diet. Our 5 Starships have a surface area of about 1%
of that. So we either send 500 starships just to feed our crew, or find a way to
produce food more efficiently. That 0.45 km^2 is dominated by the space for raising livestock,
so burger night is the first thing we’ll have to cut. It’s possible to get the required area
down 0.015 km^2 if we grow nutrition-dense crops like sweet potatoes using our best hydroponic or
aeroponic systems. That’s just 30 starships worth of farm, so we’re back in the realm of the sane.
The crew is also going to need protein. Now maybe we can get the quantity and variety
from an efficient veggie source, especially with a little genetic tinkering. But if not
there are plausible meat options. Now lab-grown meat technology is a bit speculative at the moment,
but there’s a very well established carnivorous option suited to the less squeamish interstellar
traveler. I’m talking about insects. For example, mealworms can be farmed at high densities and
provide extreme protein richness. One to a few Starships worth of mealworm might do the trick.
Overall, we’re going to need something like 6 to 10 times our crew’s living
space for food production. And that’s for a pretty boring and slightly crawly diet.
But maybe there are some gourmet yam and grub recipes just waiting to be discovered.
A bigger challenge than food is the water, which our travelers need in order to grow that food,
and also in order to just live. An adult human needs around 2 liters of water per day,
give or take. 500 humans need 1000 liters per day—that’s a cubic meter weighing a
metric ton. Our 140 year journey may be able to haul the required 50,000 tons of water —just
barely—but forget about it for our 6300 year slog. In either case we’re going to want very
good water recycling. Just recently, the ISS reached a new milestone of 98% water recycling
efficiency. Now if that’s as good as we get for our “fast” mission we need a more reasonable 500 ton
supply of reserve water—perhaps one Starship worth of water storage in terms of volume.
For our 6-millenia-slog we need 50 times that. So our generation ship just doubled in size just to
haul enough water. And remember that we haven’t even considered water used and lost growing food.
Maybe add as much water again for 100 starships in water. In order for the long trip to be plausible,
we may need to focus on improving our water recycling—get it to at least 99.5% efficiency,
which brings the reserve storage requirement down a factor of four to a similar scale as our farm requirement.
There is perhaps one upside to needing to store all this water,
and that’s that water can double as radiation shielding. About one meter depth of water
surrounding habitats is enough to stop most dangerous space radiation. This is
a solution that’s being considered for trips to Mars, but would work well for
a non-relativistic interstellar trip. By the way, this is an upside of traveling relatively
slowly—relatively minimal shielding is sufficient and bumping into a single dust grain doesn’t kill us.
The last ingredient to add to our ship's biosphere is breathable air. Just as with water, recycling
is critical here. The ISS currently uses a system designed by the European Space Agency called the
Advanced Closed Loop System, which recycles carbon dioxide back into breathable oxygen,
with around 50% efficiency. That’s not nearly enough for a generation
ship because huge supplies of fresh oxygen would be needed to replenish the losses. Instead,
we’d probably need to rely heavily on our natural CO2 recyclers—the plants we are growing for
food. There have been various efforts to build self-contained biospheres capable of sustaining
a breathable atmosphere. Maybe the most famous is the Biosphere 2 project, which did OK, all things
considered. Yes they had to install artificial CO2 scrubbers to help the plants, but the
project at least demonstrated that a combination of natural and artificial systems could maintain
a breathable atmosphere for some time. We have a few decades to perfect this, so there’s a good
chance we can come up with an air recycling system that will work over long timescales.
So maybe we can keep our crew alive and physically healthy for centuries,
or even millenia. But will they be happy? And will they stay sane? The
sense of isolation on such a long voyage will likely be a major challenge for
maintaining the mental health of the crew. We need them to feel connected to Earth,
to be part of something grander than their janky little spacecraft on its lonely journey.
The first generation in particular will want to stay connected to their loved ones. But
the two-way light travel time between the ship and the Earth will increase over the journey,
ultimately reaching a lag of nearly 8.5 years near the end
NASA has done some tests to mitigate the dread that could follow from such separation
from our home world. One solution could be the use of virtual reality. Crew members could find
solace in digital 3D models of comforting and beautiful Earth environments, and in the case
of generation one, their homes and loved ones. As the time lag increased, messages from friends
and family and well-wishers could be recorded on Earth, beamed to the ship, and played back in VR.
On our cramped and sterile spaceship, it may be important to grant our travelers
certain experiences that we on Earth take for granted. By improving the immersion and
interactivity of our VR technology we may be able to provide convincing visual experiences
of mountains and sunsets, and auditory and even tactile experiences of wind and rain,
and the olfactory joys of a forest or freshly cut grass. We can’t build a StarTrek holodeck,
but we can certainly push VR a lot further in the time we have before launch.
Of course, the humans on the ship will still be humans. Arguments will happen,
relationships will experience strain, and sensitivity and frustration levels may be
heightened due to the isolation and confined spaces. And yet a high level of synergy and
teamwork is needed for this mission to succeed. Sometimes a stressed human needs another human.
But maybe, when tensions rise and trust wanes, it would be helpful to have a trusted third
party to give advice, confide in, and to overall receive encouragement from. One that remembers
and learns from the problems of past generations. Maybe we need
an AI therapist. NASA has already piloted such a tool, namely Cimon 2.0 the therapy AI robot.
Preliminary testing seems promising and it is generally agreed upon that some tool or
AI of this form will be incredibly important for the success of a long term space mission.
Our plan so far will hopefully get our crew to Proxima-B in good health, genetically,
physically, and mentally. But how do we make sure that the mission of the launch
crew is still the mission of the landing crew? How do we ensure that the knowledge
and skills needed to complete the mission are passed across generations? Or that we
preserve the wealth of cultural knowledge and tradition of these once-Earthlings?
This is where things get more speculative as there isn’t much research to go on. We just know that
this stuff is going to be very important and probably very tricky. The ship-bound
society is going to need a culture and social structure that balances different needs.
That structure needs to enable efficient operation of the mission—which may mean
clear hierarchies in each operational area. But the culture also needs to promote crew
happiness—otherwise we have a revolution in a generation or two. So, an efficient
and stable social structure that somehow also promotes mutual respect, individual freedoms,
and all the various values that we want this new branch of humanity to carry forward.
Overall, it seems at least possible to build a generation ship that can reach Proxima B,
to launch in the not-to-distant future. There are so many things that we know could go wrong—and
no doubt many more unknown fail points. And the longer the mission, the more risk of unexpected
disaster, so maybe we should really focus on getting fusion on track. But it’s encouraging
to think that this sort of sci-fi endeavor is at least within our grasp if existential
need or our adventurous spirit compels us. We are the middle children of history,
but perhaps we’re ready to grow up. Perhaps soon our generation ships will slip the bonds
of gravity and distance to explore the new frontier of interstellar spacetime.
Hey Everyone. If you enjoyed today’s episode then I highly recommend you check out the new
feature length documentary “Space: The Longest Goodbye.” The film explores the realities that
NASA’s goal to send astronauts to Mars would require a three-year absence from Earth,
which would be twice as long as the current record for consecutive time in space. Bridging the gap
between the astronauts who dream of space travel and the psychologists whose job is
to keep astronauts mentally stable in outer space, the documentary vividly displays how
those who dream of space travel must balance their dream of reaching new frontiers and the
harsh psychological realities of space. The film is now available on YouTube so
check out the link below. After June 4th, it will be available exclusively on the PBS app.