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.