NOTE: This post contains plot spoilers
How hard would it be to send people to other star systems? What challenges would they face? Quite a few, it turns out. These are the questions author Kim Stanley Robinson addressed in his 2015 sci-fi novel Aurora. Rather than warp drives, worm holes, or some other means of faster-than-light travel that is standard fare for science fiction, the future he depicts is plausible, using technologies that we know are theoretically possible, even if we don't currently have the means of developing them.
Robinson's novel-length thought experiment on the viability of interstellar travel will leave readers wondering whether our destiny is truly among the stars. After all, there are hard limitations baked into the laws of physics, at least as we understand them now. The enormous distances involved in space travel make it questionable that we'll ever be able to break free of our own solar system.
Even if we do, the problems don't stop there. What will we find when we arrive? And will it be suitable for human life? Robinson makes a strong case that Earth is our only real home, and the one we evolved to thrive on. Therefore, maybe we should take care of this place rather than wasting our energies on a quixotic quest to colonize a galaxy that is profoundly hostile to human life.
To make his case, Robinson starts his tale near the end of a hypothetical trip to a neighboring star.
The vessel in the novel is a generation ship with 2,100 passengers traveling at ten percent the speed of light to the nearby Tau Ceti system. The goal is to settle on an Earth-analog moon named Aurora orbiting one of Tau Ceti's planets. The story picks up in the year 2704, or 159 years into the 170-year journey. Seven generations have lived and died since the ship's launch in 2545.
How did they do it? How did they survive for so long without running out of supplies?
A generation ship like this traveling for centuries would need to be self-sustainable, able to keep producing food and water for its passengers for decades or even centuries.
The generation ship is a marvel of technological achievement, a miniature version of Earth. It contains two rings, each 48 kilometers around and connected to a central access. Both rings have twelve self-contained sub-sections, each four kilometers long and one kilometer in diameter. These sub-sections replicate unique biomes from Earth (jungle, prairie, tundra, alpine, forest, etc.) and include various plants and wildlife.
As Robinson shows, even a starship designed with the parameters described above will still face massive challenges sustaining an ecological equilibrium over the long haul.
But first, let's take a look at the most obvious of challenges the travelers will face: distance.
The Sobering Reality of Interstellar Travel
In Robinson's version of the future, humanity has colonized the solar system and began sending out starships to neighboring systems. However, something that we have never developed in this future is an exotic technology that allows us to escape the tyranny of distance. There are no sci-fi cheats in Aurora like wormholes or warp drives to zip us across the galaxy in no time at all.
In Aurora, future physics has not changed the basic fact that nothing goes faster than light. This means that even reaching our galactic neighbors takes decades or even centuries. Interstellar arks in Aurora are "only" able to go at about ten percent of light speed. That may not sound like much, but reaching a speed like this in a massive starship and then decelerating later on would be a remarkable technological achievement.
One idea that Robinson does a great job conveying is the vast scale of distance in space. For example, the ship in Aurora is traveling to Tau Ceti, a star about 11.9 light-years away from Earth. On a galactic scale, Tau Ceti is one of our next-door neighbors. Yet, even at one-tenth the speed of light, the journey would still take 170 years.
To put this in terms easier to understand, if the distance from Earth to the Sun were reduced to one meter, Tau Ceti would still be 750 kilometers away. Using this scale, the Voyager I probe, one of the fastest vehicles ever designed, has only progressed 152 meters since its launch forty-three years ago. If Voyager were heading to Tau Ceti (it's not), it would arrive in about 225,000 years at its current speed. Again, we're talking about one of our closest neighbors in the Milky Way, which itself is a mind-boggling 120,000 light-years in diameter.
Another problem with a 170-year one-way trip is keeping everyone alive and sane. Robinson's starship replicates Earth as much as possible, with various microclimates and ecosystems, weather, and artificial sunlight. These allow the passengers to grow crops to feed themselves and create an illusion that they are not stuck on a spaceship.
One big problem is the second law of thermodynamics, or entropy (disorder). A starship spending decades in empty interstellar space is a closed system. There's no way to resupply anything lost, no asteroids to mine, and no gas from planetary atmospheres to take in for fuel. What you have when you begin the journey is all you will have until the end. And what you have will gradually begin breaking down.
The question becomes how quickly this entropy will take place. In the novel, Devi, the ship's most gifted engineer, spends all of her time dealing with crisis after crisis as the systems begin breaking down. A biologically-closed ecosystem like that of the ship was becoming harder to keep balanced. Crop yields gradually but steadily declined. Birth rates and IQs were dropping as well. The designers of the ark no-doubt understood this would be a problem but had hoped to slow the effects of entropy long enough to get the ship to its destination where it could find raw materials to resupply and restore itself.
Thanks to Devi's ingenuity, the ship makes it to the moon Aurora intact. Now that they have finally arrived, the colonists can begin to establish their new civilization.
Here, after describing the challenges of transporting a small human society to another star system, Robinson shows the reader that the transit between stars is only part of the problem. An equally daunting challenge will be finding worlds compatible with human life. This turns out to be much harder than expected.
The Challenge of Colonizing Exoplanets
So let's say we someday build giant starships able to cover the vast interstellar distances to reach other star systems. What then? Even if they survive the centuries-long journey, the problems are just beginning. Robinson sees several difficulties the settlers may encounter, depending on whether the planet is dead or alive.
Dead exo-worlds like our moon or Mars would have to be terraformed to sustain permanent human settlement over the long-term. They are by default hostile to human life. Maybe they have no atmosphere like our own moon, or perhaps only a thin atmosphere with no oxygen like Mars. Whichever, we can't create a self-sufficient society on a dead world, while terraforming would likely take centuries. This might be possible in our own solar system where Earth remains a nearby safe harbor for resupply and aid, but not in a distant star system that is completely on its own.
On the other hand, living worlds could be hostile to human life in another way. This is what happens in the novel.
The colonists' target is the ocean world of Aurora, a moon orbiting another rocky planet. By all appearances, Aurora seems perfect: the temperatures are mild, water and mineral resources are abundant, and there is no native life, or at least there doesn't seem to be. After arriving, they quickly set to work building the first settlement. Things look good; everyone's optimistic and ecstatic to finally be off the ship.
That is, until people started getting sick.
It turns out Aurora was not dead after all but host to a primitive but deadly alien pathogen that humans have zero immunity against.
Robinson never really details the pathogen's nature, noting that it is unfamiliar and 100% fatal because the settlers have no natural immunities. After the landing party gets sick and starts dying off, the attempt to colonize Aurora is abandoned.
Imagine the situation: the settlers have come all that way only to find the one suitable planet in the Tau Ceti system poisonous. What now? Fighting breaks out among the survivors as they struggle to find a new way forward. Half want to turn around and return to Earth; the other half want to settle down on a Mars-like moon called Iris. Here they would begin the multi-millennia process of terraforming it.
In the end, the colonists split up; half stay to colonize Iris, while the other half returns to Earth. Neither option is very appealing to anyone. Both factions understand they will spend the rest of their lives either on the ship returning to Earth or cooped up in modular structures on a dead moon that will remain dead for a very long time.
What Robinson is getting at here is the double dilemma of interstellar colonization. Worlds already teeming with native life could be poisonous to us, even if everything else like climate and atmosphere is Earth-like. Think about it: humanity co-evolved over millions of years with deadly bacteria and viruses, which still, as we've seen, remain a serious threat to our health. Nevertheless, we have developed natural immunities to most pathogens because of this evolutionary arms race.
An alien world with its own pre-existing life and unique evolutionary tree may contain deadly pathogens that completely overwhelm our immune systems. That's what happened on Aurora. For all the technological wizardry that got them to Tau Ceti, they were conquered by a humble microbe. So maybe H.G. Wells was on to some profound universal truth in War of the Worlds when his Martians were not defeated by humanity but by microscopic pathogens.
To compound the issue, those planets that are the most earth-like, with water and a mild climate, are the ones that are going to have the most favorable conditions for life to evolve. Maybe there are temperate water worlds where life never evolved, but I suspect they may be few and far between. If the building blocks for life are readily available, the conditions favorable, and given enough time, I believe life in some form will eventually emerge.
That means exo-planets which appear the most earth like will tend to be the most dangerous. Maybe intelligent life is extremely rare, but simple microbial biology could be ubiquitous throughout the cosmos, and perhaps is even more dangerous to our expansionary dreams than technologically advanced aliens in star destroyers.
On the other hand, dead worlds are just as challenging. They would have to be terraformed, a process that would probably take thousands of years. In the meantime, humans would live confined in artificial environments little different from the starship they traveled in on, except this time not for "only" seven generations, but perhaps for over 100. In Aurora, the group that stays behind to terraform Iris does so even though the best AI-generated models indicate their chances for success are around one and a thousand. Robinson doesn't tell us straight out about the Iris colony's fate but notes near the end of the novel that it is no longer communicating with Earth.
Why Leave Earth?
If there is a thesis to Aurora, it comes from the mouth of one its characters, Euan, as he lies dying alone on an alien seashore.
"Maybe that's why we've never heard a peep from anywhere. It's not that the universe is too big. Which it is. That's the main reason. But then also, life is a planetary thing. It begins on a planet and is part of that planet. It's something that water planets do, maybe. But it [life] develops to live where it is. So it can only live there, because it evolved to live there. That's its home. So, you know, Fermi's paradox has its answer, which is this: by the time life gets smart enough to leave the planet, it's too smart to want to go. because it knows it won't work. So it stays home. It enjoys its home. As why wouldn't you? It doesn't even bother to try to contact anyone else. Why would you? You'll never hear back....So, of course, every once in a while some particularly stupid form of life will try to break out and move away from its home star. I'm sure it happens. I mean, here we are. We did it ourselves. But it doesn't work, and the life left living learns this lesson."
This is something to ponder. Many assume that our destiny is the stars; it's only a matter of time. Soviet rocket scientist Konstantin Tsiolkovsky once famously wrote, "The Earth is the cradle of humanity, but mankind cannot stay in the cradle forever." Here metaphors get us in trouble. Making the Earth a cradle implies that it is too cramped and unsuitable for humanity's long-term prosperity. We must grow up and leave the cradle. We must progress! We will progress! It is inevitable!
But is that really true? Or is it just faith? These vast distances between stars make interstellar travel an extraordinarily costly and high-risk venture. And for what? To maybe scrape out a miserable existence trying to turn an alien world into something like Earth? Will people be psychologically able to live their entire lives on starships knowing they will never live to see the payout? What about you? Would you be willing to spend the rest of your life on a starship knowing that only your great-great-great-great-great-great grandchildren will reach the other side?
Maybe you would, but do you want to commit your children and descendants to that same mission? Is that fair? And consider, humans are terrible at long-term thinking. We don't plan our actions to benefit remote descendants seven generations removed. Travel between stars will require massive investments and sacrifices that I do not believe many will be want to make, not once they weigh the risks versus the rewards.
That's why solving climate change is such a wicked problem, by the way. We're asking people to sacrifice now to benefit people 100 years in the future. We're not hard-wired to think that way. Likewise, Euan may be right. We may foolishly try to ignore the unexciting wisdom of staying home. Only after much heartache and loss will we learn Earth is not a cradle to be crawled out of but a paradise to be reveled in. Or, put more simply, it's home. That's the better metaphor; Earth is our garden of Eden, which is a better one still.
No doubt we will search the galaxy for something better. Still, I'll wager we find nothing even remotely as perfect as Mother Earth. There's nothing like her in the known universe, and even if there were, it would probably be too far away. So, by all means, look up at the stars and dream and wonder and ponder of what could be. But don't forget to look around at the magnificent world we already inhabit and take care of what we already have.
There is no Planet B to fall back on.