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.
