In his poem “Strontium-90,” the great Turkish poet Nazım Hikmet says: “We are in a race with ourselves, my dear. Either we will take life to the dead stars, or death will descend upon our world.” Do you think that one day, in the distant future, humans will truly be able to reach other stars? Will they be able to bring human civilization there?
When it comes to astronomical scales, the human mind struggles to grasp the sheer vastness of the solar system, galaxies, and the entire universe. As Douglas Adams pointed out:
“Space is big. Really big. You just won’t believe how vastly, hugely, mind-bogglingly big it is.”
To put the enormity of space and the mind-boggling distances between stars into perspective: if the Sun were the size of a pea, the closest star to us (other than the Sun, of course), Proxima Centauri, which is about 4.243 light-years from Earth, would be approximately 202 kilometers (125 miles) away! At this scale, even a snail’s crawling speed would be vastly greater than the speed of light.
As of 2024, the Parker Solar Probe is the fastest human-made spacecraft. On September 27, 2023, it reached a speed of 176 km/s (approximately 635,000 km/h or 395,000 mph). Even at this staggering speed, it would take approximately 7,231 years to reach Proxima Centauri.
And let’s not forget that Proxima Centauri is the closest star to us. The other stars in the galaxy are much farther away. Will we truly ever reach a level of technology capable of overcoming these vast distances?
Going to the stars, but how?
There are only three possible ways to reach the stars. Though, “possible” might not be the right word here. If you keep reading, you’ll see why.
1. Going faster than light (FTL)
The first of these is to travel faster than light. That would be truly amazing. Imagine if we could travel at Warp 8 (approximately 512 times the speed of light) like the USS Enterprise (NCC-1701) in Star Trek, we could even reach stars hundreds of light-years away by embarking on a months-long journey, much like ancient sailors. Even at that speed, crossing the galaxy would be a lifetime endeavor, but for example, we could reach the nearest star, Proxima Centauri, in just a 3-day journey. The same amount of time it took Apollo astronauts to get to the Moon!
However, faster-than-light travel is impossible, contrary to the laws of physics, so the first “possible” way of reaching the stars is not feasible. This isn’t a technological barrier; it’s a physical one (or a mathematical one, if you prefer). Therefore, we shouldn’t dream about it.
2. Going at relativistic speeds
The second way is to build a spacecraft that can travel at a speed very close to the speed of light, even if not at the speed of light itself. If this spacecraft could travel at a relativistic speed (let’s say 99.99% of the speed of light), it could reach Proxima Centauri in a reasonable time frame, approximately 4.5 years, which could be considered acceptable, at least for interstellar travel.
When astronauts travel at 99.99% of the speed of light, they experience a phenomenon known as time dilation, which is predicted by Einstein’s theory of relativity. Time dilation means that time passes more slowly for the astronauts relative to someone who is stationary or moving at much lower speeds, such as an observer on Earth.
- From the astronauts’ perspective: Time would feel normal to them. Their clocks, biological processes, and activities would all proceed as they always do. If they looked at a clock, it would tick normally, and they wouldn’t notice anything unusual.
- From the perspective of someone on Earth: Time would appear to be passing much more slowly for the astronauts. For example, if they traveled for 1 year at 99.99% of the speed of light, significantly more time (many years, depending on the exact speed and duration of travel) would have passed on Earth.
If an astronaut is traveling at 99.99% of the speed of light, the time dilation factor, known as the Lorentz factor (γ), can be calculated using the formula:
γ = 1 / √(1-v2/c2)
Where:
- v = 0.9999c (99.99% of the speed of light)
- c is the speed of light.
This gives us:
γ ≈ 70.71
This means that for every 1 year the astronauts experience, 70.71 years would pass on Earth.
Although traveling at relativistic speeds seems like a promising possibility for interstellar journeys, in reality, it is far from being a “possibility.”
- According to Einstein’s famous equation, E=mc², as an object approaches the speed of light, its relativistic mass increases. This means that the energy required to continue accelerating it increases dramatically as it gets closer to the speed of light. To accelerate a spacecraft to 99.99% of the speed of light, you would need an astronomical amount of energy, far beyond what we can currently generate. For even a small spacecraft, the energy required would be equivalent to the total energy output of our civilization over many decades or even centuries.
- Our current propulsion systems (chemical rockets) or the nuclear propulsion systems we might build in the future are not even remotely capable of achieving such speeds. We would need entirely new types of propulsion, such as antimatter engines or some form of controlled fusion, but even these concepts are far from being feasible and still might not provide the necessary energy.
- Even if we had a way to generate the required energy, storing and managing fuel for such a journey would be a significant challenge. The amount of fuel required would likely be prohibitive.
- Moreover, at relativistic speeds, even tiny particles in space, like dust and hydrogen atoms, would pose a serious threat. A collision with a small particle at such speeds would release an enormous amount of energy, potentially destroying the spacecraft.
- The spacecraft would also encounter a massive increase in radiation due to the Doppler effect, where the frequency of radiation shifts to higher, more dangerous energies as the ship moves close to the speed of light.
- The spacecraft would also need to dissipate the heat generated by particle collisions and other sources, which would be very challenging.
- At near-light speeds, even the tiniest miscalculations in navigation could lead to catastrophic results. The time available to react to obstacles or changes in course would be severely limited.
- Communicating with Earth would become nearly impossible because of the time dilation effects and the vast distances involved. Real-time control or adjustments from Earth would not be feasible.
- The effects of time dilation on human psychology and society would also be significant. Astronauts on such a journey would return to a world that had aged significantly, which could have profound social and psychological consequences. A science fiction novel I love, Return from the Stars by Stanislaw Lem (1961), focuses precisely on this aspect of traveling at relativistic speeds. It recounts the story of a cosmonaut who returns to Earth after what has been over a century on Earth but only 10 years for him. He finds his homeworld utterly transformed, with many changes he disapproves of. The novel explores themes of social alienation, culture shock, and dystopia.
3. Long (very long) journeys
Since traveling at faster-than-light or relativistic speeds is impossible, only one option remains for reaching the stars: embarking on a journey that would last hundreds, or even thousands, of years.
Considering that hibernation, a method beloved by science fiction, is impossible, this journey could only be accomplished by building a “generation ship,” where many generations would be born and die aboard the same spacecraft.
What is a generation ship?
A generation ship is a theoretical spacecraft designed for interstellar travel that spans multiple generations of human life. These ships would sustain a self-contained population over centuries or even millennia as they travel between stars. Equipped with habitats, agricultural systems, and life support, a generation ship would function as a miniature society, where people are born, live, and die aboard the vessel. The concept addresses the immense distances between stars, requiring long-term planning to ensure the survival and stability of the crew across many generations.
Generation ship: a mini-world
We can think of a generation ship as a model of humanity – a micro-society in space. Just as our civilization on Earth includes facilities for people of all ages and health conditions, the same must be true for a generation ship: it must have schools, hospitals, elderly care homes, recreation and entertainment spaces, sports facilities, and more. There should also be security forces, and possibly even prisons. Building such a ship will require contributions not only from engineers and scientists but also from social scientists, to ensure a balanced and sustainable society.
The people embarking on this journey will likely be volunteers. However, even in this case, extensive psychological testing and perhaps years of training will be necessary to prevent “unsuitable” individuals from infiltrating the ship. At the very least, the first generation of crew members responsible for controlling and commanding the ship should undergo rigorous testing and training, similar to astronaut candidates.
Among the necessary plans is determining the size of the human population to be sent to the stars. To avoid a genetic bottleneck, a sufficiently large population must embark on the journey. But how large is “sufficiently large”? 10,000? 50,000? 100,000? In any case, the spacecraft to be built would need to be unimaginably large, far beyond what our current technology (or even that of 100 years from now) can conceive.
Given that many generations will be born and die on the ship during this journey, and that no generation except the first and the one at the destination will ever see anything outside the limited space of the ship, there must also be an educational plan to ensure that every generation has enough doctors, engineers, pilots, etc.
I’m also very curious about the psychological state of the intermediate generations. For example, what will the first generation born on the ship think when they grow up about the decision their parents made on their behalf? While not always, I’m sure there will be serious intergenerational conflicts – many children will blame their parents for this choice, perhaps even become disillusioned with “society,” and cease to be productive members of it. It’s entirely possible that within 1-2 generations, the ship’s population could succumb to such “degradation” and be destroyed.
Minimizing these kinds of societal problems is only possible by designing the ship’s population to be large enough (let’s say hundreds of thousands of people) to create a miniature world. However, this brings significant engineering challenges: the ship’s size becomes extremely large. Leaving aside the construction of such a massive spacecraft, even accelerating it to a certain speed would be a major challenge – and the same energy problem applies to decelerating at the possible destination.
Resource management and environmental stability
Unlike Earth, where resources can be replenished from external sources, a generation ship must be entirely self-sufficient. This requires advanced closed-loop systems for recycling water and air, as well as agricultural systems capable of producing enough food to support the population. Any failure in these systems could lead to resource shortages, threatening the survival of the crew. Moreover, the challenge is compounded by the need to maintain these systems across multiple generations, which demands constant monitoring, maintenance, and innovation to prevent depletion or contamination of vital resources.
Maintaining environmental stability on a generation ship is also crucial for the health and well-being of its inhabitants. The ship must provide a stable climate, with controlled temperature, humidity, and air quality, to mimic Earth-like conditions. This involves complex life support systems that regulate oxygen levels, remove carbon dioxide, and manage waste.
Additionally, the ship’s ecosystem, including plants and possibly animals, must remain balanced to prevent ecological collapse. Over generations, small imbalances could escalate, leading to catastrophic consequences.
The confined space also increases the risk of spreading diseases, so advanced medical systems and quarantine protocols are essential.
Severed Ties with Earth
Communication between the ship and Earth will become a serious problem, especially as distances increase. Eventually, it will become entirely impractical. For instance, when the ship is 1 light-year away, messages will take 2 years to travel back and forth, and this problem will only grow as the distance increases.
We know that cultures and languages change dramatically over just a few hundred years – even on the same planet, Earth! After many hundreds or thousands of years, the language on the ship, and therefore its culture, will undergo extreme changes (just think about how likely it is that you wouldn’t understand people born in your own country just a few hundred years ago – how many people today can understand the English spoken in the Middle Ages?).
The receivers (both those on the ship and those on Earth) may even no longer have any idea how to decipher the message. To prevent this, expert linguists on Earth should be employed to continuously monitor and understand the evolution of the language on the ship,
I also think that interesting cults might emerge during such a journey: for example, conspiracy theories suggesting that Earth never existed and that the ship has been wandering through space since the beginning of time, could arise and even gain followers.
There are other possibilities: the new generations born on the ship might “rebel” against Earth and the journey they were forced into without their consent. They could turn the ship back toward Earth or seek ways to sabotage the mission in other ways. Even if these things don’t happen, the new “civilization” established at the destination could harbor hostile attitudes toward Earth, driven by a sense of “revenge” for the journey. If they become powerful enough and reach a certain level of technological advancement, they might even attempt to invade or destroy Earth. This part is left to the imagination of science fiction writers…
Unpredictable Risks
An unforeseen event, such as a critical system failure, social collapse, or external threat like a micrometeorite impact, could lead to the mission’s failure long before reaching the destination. Considering the journey will take a very long time, the likelihood of such mishaps occurring over the centuries is quite high.
What Will Happen Upon Reaching the Destination?
Let’s say a journey is undertaken toward an exoplanet that appears suitable for human life, and after centuries, the ship arrives. But what if it turns out that the planet isn’t as suitable for life as it seemed? This realization could even occur during the journey, potentially leading to social collapse among the ship’s population.
In any case, even if the planet is suitable for life, it’s almost certain that it won’t be as compatible with human biology as Earth. Preparing the first generations to live on this planet will be a challenging process. Some degree of terraforming might even be necessary. Many people might prefer to continue living in the spaceship’s human-friendly ecosystem rather than face the harsh conditions of such a planet.
The Wait Calculation Problem: When Should Our Interstellar Journey Begin?
One of the key challenges in interstellar travel is the “wait calculation,” which seeks to determine the optimal time to embark on a journey by waiting for technological advancements that could significantly improve spaceship speeds.
In simpler terms: if you leave too early, your spaceship could be overtaken by future astronauts who depart Earth centuries or even millennia later, benefiting from superior technology.
American physicist and science fiction writer Robert L. Forward (August 15, 1932 – September 21, 2002) argued that if an interstellar mission cannot be completed within 50 years, it shouldn’t be started at all. Instead, resources should be directed toward developing better propulsion systems, assuming we are still on an upward trajectory in velocity advancements and have not yet reached technological limits.
In his 2006 study titled “Interstellar Travel – The Wait Calculation and the Incentive Trap of Progress,” Andrew Kennedy calculated that, given our current rate of progress, the earliest human civilization might reach Barnard’s Star, located 6 light-years away, would be about 784 years from 2006 (the year 2790). If you’d like to see the calculations behind this, refer to the section titled ‘Revisiting Andrew Kennedy’s calculations’ below.
Even this estimate may be optimistic, as Kennedy focused solely on speed. We must also address other significant challenges, such as surviving decades of interstellar radiation and avoiding collisions with interstellar material at very high speeds, and many other problems I listed above.
Revisiting Andrew Kennedy’s calculations – how to calculate wait time?
We can use the classic doubling equation to examine the improvement in spacecraft speed:
v = v0 x 2t/h [1]
Where:
- v: Target travel velocity. Let’s say our target velocity is one-twentieth of the speed of light, or c/20. At that speed, the journey to Bernard’s star would take 120 years (or if we consider 1g of acceleration and deceleration, about 120.1 years. Let’s say 120 years for simplicity.
- v0 = Current achieved max. spacecraft speed. In 2006, Kennedy considered this to be one two-thousandth of the speed of light, or c/2000. Remarkably, since then, it seems that the Parker Solar Probe has almost matched this speed: c/2000 is approximately 149.9 km/s. The Parker Solar Probe, as I mentioned earlier, has reached a speed of 176 km/s. Therefore, even for 2024, we can also consider c/2000 as a reasonable estimate for the current achieved max. spacecraft speed.
- t: Waiting time interval.
- h: The average time it takes for the maximum speed of spacecraft to double. Let’s take h = 100 years.
Re-arranging equation [1] gives:
t = h ((log v – log v0) / log 2)
t ≈ 664 years.
So, if we wait 664 years and add 120 years of journey time, we could reach Barnard’s Star in 784 years. Starting from 2006 (the year Kennedy published the study), the ship should be launched in the year 2670, and humanity would reach Barnard’s Star around the year 2790.
What if we waited 100 years more and launched the ship in 2770? Then we could potentially travel at c/10. At c/10, the journey would take about 60 years. Adding the 764 years of waiting time, the total would be 824 years, leading to an arrival year of 2830. Therefore, waiting longer doesn’t seem advantageous.
What if we launched the ship 100 years earlier, in 2570? The journey at that time might take about 240 years. Adding this to the 664 years of waiting time gives a total of 804 years, with an arrival year of 2810. In this case, the ship launched 100 years later (in 2670) would catch up to and overtake this earlier ship.
Of course, I’ve simplified the calculations here. For a more detailed analysis, you can refer to Kennedy’s study.
Going to the stars, but why?
Because, above all, one of the most important things that make us human is “curiosity” and the drive to challenge what seems impossible. We didn’t go to the Moon just to say we did it; our curiosity and desire to overcome challenges were as much a factor as the experience, scientific knowledge, and technological advancements we gained from the endeavor.
Moreover, our efforts to reach the Moon led to significant technological revolutions. Many of the technologies that make our lives easier today, from modern computers to GPS, owe their existence to our efforts to reach the Moon and space exploration in general. Our efforts to reach the stars will also pave the way for numerous scientific and technological revolutions.
While, as a chronic pessimist, I believe we may never reach the level of being able to travel to the stars, if we do somehow achieve the necessary technological capability, it will be worth it, even just to say that we made it to the stars.
Nazım Hikmet’s Poem Strontium-90
The weather has turned strange,
one moment sun, then rain, then snow.
They say it’s because of the atomic bomb tests.
Strontium-90 is falling
on the grass, the milk, the meat,
on hope, on freedom,
on the great longing, we knock on the door of.
We are in a race with ourselves, my dear.
Either we will take life to the dead stars,
Or death will descend upon our world.
Sources
- List of vehicle speed records on Wikipedia
- Wait/walk dilemma on Wikipedia
- Interstellar travel on Wikipedia
- Study: “Interstellar Travel – The Wait Calculation and the Incentive Trap of Progress”, A. Kennedy. Link
- Relativistic Speed on Wikipedia
- What is a Generation Ship? on the Universe Today blog
- Generation ship on Wikipedia
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