Interstellar Expansion WITHOUT Faster Than Light Travel

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.