Interstellar Expansion WITHOUT Faster Than Light Travel (Script) (Patreon)

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 ođwallđf 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 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 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 will 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 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 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. 

There are other studies that try to find the starting number needed to repopulate our species—the so-called minimum viable population—and some of those studies conclude that the same ballpark of 100-ish individuals is needed. However there are also studies that claim this number should be much higher—up to 10s of 1000s for a founding population. That’s particularly true if you want to capture a good amount of the genetic richness of humanity, important both for genetic robustness and because we are, afterall, trying to save humanity in all its glorious diversity here. Ships that carry such numbers have been conceived—some call them world ships—but we only have 30 years to build this thing. 

For our 6300 year journey it’s going to have to carry 500. If we achieve our fusion drive we can get away with a smaller crew because they’ll soon be able to expand to much larger numbers when they make landfall. However in that case we’ll still need to support multiple generations of travelers en route—a few hundred people at least. For simplicity let’s take 500 as our maximum crew size in both cases.

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. NASA reports that for every month in space, astronauts can lose up to 1% of their bone density. This can be slowed, for example with special exercises, but some density loss will still occur. This is going to be a huge problem for someone spending their entire life in zero gravity, and who knows what the effect would be for a baby carried to term in zero-g. 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. Because most molecules break down over time, we can’t just pack the ship with multivitamins, supplements and dried food. Fortunately, we know of some miraculous molecular factories capable of producing everything the human body needs. I’m talking about nature itself. To put it another way, we’re going to have to grow our food. 

There have been extensive studies on the viability of growing plants in space, and luckily it doesn’t seem too hard. NASA’s “veggie” experiment has already shown much promise; growing various types of lettuce, Chinese cabbage, mizuna mustard, red Russian kale and even zinnia flowers in an LED based garden on the International Space Station

To support our crew of 500 we’ll need more than a corner of the ISS. 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. 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. 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 these 50,000 tons—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. If that’s as good as we get for our “quick” mission we need a 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 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 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. Anything resembling real-time communication is only possible near the beginning. For the slow boat, with its 6300 year travel time, the crew that launches with the ship will be able to maintain some semblance of meaningful contact—after 50 years the two-way lag will be 23 days. Not conducive to lively conversation, but not completely hopeless. The fast boat reaches the same lag time after only one year. After that, and for later generations, contact with Earth will be less and less interactive. 

Thankfully, 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 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 is 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. 

This cultural and social know-how also needs to be stable over generations—perhaps many generations. And we also need to preserve the know-how needed to run the mission. But more than that: the crew that eventually lands is going to need the technical, social, and cultural knowledge to start a new civilization. Overall, the transmission of knowledge and wisdom and our core humanity across generations is going to be a huge priority. I’m not sure we’ve mastered this process after hundreds of thousands of years on spaceship Earth. But maybe the effort we put into figuring this out for the sake of this mission will save us from whatever impending disaster necessitates the mission in the first place.

The final question is deciding who to send. And I won’t say more on this besides that we want healthy and capable people who are going to get along with each other over a very long and challenging journey, and will raise their kids to do so too.

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.