PBS Space Time

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 spacedoomed 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 impossibleso says Einsteins special theory of relativityand 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 doesnt mean we cant 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 Earths.

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 weve 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, Heres the scenario: Something is coming.

It could be a comet impact or catastrophic climate collapse or the Tri-Solarian fleet.

Whatever it is, theres 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, lets 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.

Thats a 42 year journeylaunch a crew in their 20s and theyll 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 wont be 100% reliable within decadeswhich is pretty fairit 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.

Were 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 propellanthydrazine in this caseusing 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.

Thats like 200 generations, and roughly the length of recorded human history.

Its difficult to imagine that nothing would go wrong in that much time.

But were 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 Frdric Marin and Camille Beluffi in a series of studies, and well be coming back to their conclusions regarding a trip of this length.

Well also consider a much faster craftone propelled by nuclear fusionsmashing light elements together to form heavier elements plus lots of energy, just like the Sun does.

We havent yet managed to build a commercially viable fusion reactor, let alone the sort of compact reactor wed need for a spacecraft.

But there IS a fusion technology that weve thoroughly masteredand thats the thermonuclear explosion.

There are various concepts for spacecraft that accelerate under a series of fusion pulsesaka explosionsrather 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 thats highly optimistic.

A little under 10% is more realistic for a mature version of this technology.

For us, with our limited timeline, were going to assume we can get to 3% lightspeed.

Thats around 50 times faster than our conventional drive, so gets us to Proxima-B in a mere 140 yearsjust four or five generations.

So, today were going to plan towards these two travel times140 years if fusion pans out and 6300 years if not.

Well 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 isnt 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 500and thats the level to support for most of the journey.

How big a ship does it take to comfortably carry 500?

Well, SpaceXs Starship is supposed to be able to carry 100.

So, the equivalent of 5 of those at least?

However that doesnt include the space needed for systems to support 500 lives long term.

For that were definitely going to need a bigger boat.

Missions around the solar system dont 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.

Weve discussed previously how this could be done.

Theres only one way, and fortunately its not that complicated.

The ships habitats need to be spun in a circle to give 1-g of centrifugal acceleration, perfectly mimicking Earths 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 lets move on.

The next step is to feed our crew Another study led by the French team finds that wed 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 well have to cut.

Its 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.

Thats just 30 starships worth of farm, so were 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 theres a very well established carnivorous option suited to the less squeamish interstellar traveler.

Im 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, were going to need something like 6 to 10 times our crews living space for food production.

And thats 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 daythats a cubic meter weighing a metric ton.

Our 140 year journey may be able to haul the required 50,000 tons of water just barelybut forget about it for our 6300 year slog.

In either case were going to want very good water recycling.

Just recently, the ISS reached a new milestone of 98% water recycling efficiency.

Now if thats as good as we get for our fast mission we need a more reasonable 500 ton supply of reserve waterperhaps 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 havent 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 recyclingget 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 thats 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 thats 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 slowlyrelatively minimal shielding is sufficient and bumping into a single dust grain doesnt 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.

Thats not nearly enough for a generation ship because huge supplies of fresh oxygen would be needed to replenish the losses.

Instead, wed probably need to rely heavily on our natural CO2 recyclersthe 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 theres 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 cant 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 isnt 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 missionwhich may mean clear hierarchies in each operational area.

But the culture also needs to promote crew happinessotherwise 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 wrongand 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 its 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 were 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.