'A super adventure to infinite space': How generation ships could bring us to stars

As of 2019, three of these missions made it to interstellar space (Voyager 1 and 2 and Pioneer 10), while the other two are on a trajectory that will take them there in the coming years. At their current velocities, these missions are expected to reach the nearest star system (Proxima Centauri) in about 16,700, 20,300, and 34,100 years (respectively). When it comes to crewed missions, the wait times become even longer!

Unlike robotic space probes, crewed spacecraft need to be large enough to accommodate living spaces. They also need to be able to carry enough supplies for the journey and have all the necessary facilities to meet the crew's needs. This results in larger spaceships that require massive amounts of propellant, massive propellant tanks, and powerful engines, which adds even more mass. It's a bit of a vicious circle.

There are essentially two ways to get around this. One approach is to create an interstellar spacecraft that is lightweight and optimized for speed. Examples include Breakthrough Starshot and Project Dragonfly, that call for a smartphone-sized spacecraft (or "wafer craft") and a laser-driven lightsail. These proposed spacecraft will be accelerated to up to 20% of the speed of light and will be capable of reaching the nearest star (Alpha Centauri) in 20 to 100 years.

The other approach is to forego rapid transit altogether and create ships that can sustain crews for the long haul. The concept goes by a few names but is more popularly known as a Generation Ship, a Worldship, or an Interstellar Ark. The concept calls for a massive ship that can accommodate multiple generations of people (hundreds or thousands) for centuries or more.

In a nutshell

From the many scientific studies and fictional representations made since the late 19th and early 20th centuries, a basic framework has emerged. A Generation Ship is a massive vessel more akin to a self-contained world than a spacecraft. A common feature is a cylindrical structure that rotates to simulate gravity, similar to an O'Neill Cylinder (but on a smaller scale).

Some proposals involve a hollowed-out asteroid, which can also be spun up to simulate gravity and provide natural protection against radiation and collisions during the voyage. The spacecraft's interior is designed to accommodate living quarters, green spaces, bodies of water, and farmlands to ensure a steady food supply and create a regenerative life-support system.

Another important aspect of the Generation Ship has to do with the state of the crew. In some cases, the proposals recommend keeping the passengers in suspended animation for the voyage. Another possibility is to have "nursery ships" that transport ova and sperm that can be grown upon arrival. These measures reduce the supplies that must be brought along for the voyage (and the food that needs to be grown).

Others involve a crew being awake for the voyage and producing most of the food they consume, as well as tools and products they'll need for daily living. This necessitates larger ships with more interior space and farmlands. A combined approach can also be used, with rotating crews awake for predetermined periods and others in suspended animation (with the option of bringing ova and sperm along as well).

Regarding propulsion, some Generation Ships are designed to be smaller and faster so they can make an interstellar voyage in just a few generations. These typically rely on advanced propulsion technology to accelerate the ship to a small fraction of the speed of light (relativistic speed), which can be accomplished by several possible means. Examples include conventional rockets, nuclear-pulse propulsion, deuterium fusion, directed energy, antimatter annihilations, etc. - alone or in combination.

In other cases, ships are designed to be self-sufficient indefinitely and make the voyage over the course of millennia rather than decades or centuries. These ships are larger, are likely to contain more complex biospheres, and can house larger crews (which ensures greater genetic diversity). This last aspect is essential, given that many more generations will be born during the voyage.

Background

Like all proposals for space exploration and settlement, the Generation Ship is time-honored and predates the Space Age by more than half a century. The earliest known example was proposed by engineer and science fiction author John Munro in his novel A Trip to Venus (1897). In this novel, he describes how humanity may become an interstellar species one day:

"[W]ith a vessel large enough to contain the necessaries of life, a select party of ladies and gentlemen might start for the Milky Way, and if all went right, their descendants would arrive there in the course of a few million years."

The first scientific proposal was made in 1918 by aeronautics engineer Robert H. Goddard, one of the "forefathers of rocketry" (and for whom NASA's Goddard Space Flight Center is named). In an essay titled "The Ultimate Migration," he explained how interstellar travel could be accomplished by harnessing "intra-atomic energy" or placing the crew in a state of suspended animation. As he described it:

"Will it be possible to reduce the protoplasm in the human body to the granular state, so that it can withstand the intense cold of interstellar space? It would probably be necessary to desiccate the body, more or less, before this state could be produced. Awakening may have to be done very slowly. It might be necessary to have people evolve, through a number of generations, for this purpose."

A pilot would be awakened at intervals to make course corrections and conduct maintenance. His essay also included a description of a nuclear-powered clock that would keep track of time on the voyage. He also indicated that the spacecraft could be fashioned from a large body, "such as an asteroid or a small moon." In place of nuclear power, Goddard claimed that the energy requirements could be met with a combination of hydrogen and oxygen fuel and solar energy.

Konstantin E. Tsiolkovsky, another of the "forefathers of rocketry," also proposed a multi-generational spaceship in the early 20th century. In his essay "The Future of Earth and Mankind" (1928), Tsiolkovsky described a self-sufficient "Noah's Ark" that could maintain crews for many generations. In his proposal, the crews would be awake for the journey, which would last thousands of years.

In 1941, famed science fiction author Robert A. Heinlein also explored the physical, psychological, and social effects of a generation ship in one of his earliest novels, Orphans of the Sky. The ship in this story (the Vanguard) is lost in space after all the piloting officers are killed in a mutiny. Generations later, the descendants have forgotten they are adrift in space and believe that the ship is their entire Universe.

In 1964, Dr. Robert Enzmann proposed a fusion-powered interstellar spacecraft that was the most detailed concept of a generation ship to date. Known as an "Enzmann Starship," his design consisted of a sphere measuring 1000 feet (305 m) in diameter (where the Deuterium fuel would be stored) and a long cylindrical habitat and propulsion section.

The entire spacecraft would measure 2000 feet (600 m) long and accommodate an initial crew of 200. In 1973, G. Harry Stine (a longtime space advocate) introduced the concept to wider audiences in an issue of Analog Science Fact & Science Fiction.

In 1973, famed author and science communicator Arthur C. Clarke released arguably the best-known example of a generational ship in science fiction. Unlike other fictional treatments, Rendezvous with Rama focuses on a generation ship that is extra-terrestrial in origin. As Clarke describes it, the vessel is a massive space cylinder that houses a self-contained world traveling from one side of the galaxy to the other.

The interior resembles a city, with structures and transportation infrastructure. A sea stretches around the midsection, and horizontal trenches act as windows. As the ship gets closer to the Sun, the machinery begins to come to life, and we learn that the structures are factories that process the sea (a mix of organic molecules) to create "Ramans." Ultimately, it is revealed that our Solar System is just a stopover, which is how the Ramans seed the galaxy with their species.

In 2002, famed science fiction author Ursula K. LeGuin released her take on the effects of inter-generational space travel, titled Paradises Lost. The story takes place aboard a ship called the Discovery that has been traveling through space for generations. Over time, a new religion emerges among the younger generation called "Bliss" that teaches that the spaceship is bound for eternity rather than another planet.

In 2015, noted science fiction author and naturalist Kim Stanley Robinson released Aurora, which takes place aboard an interstellar starship of the same name. This ship comprises a series of Earth-analog environments within two toruses that rotate to simulate gravity. The story also explores the many considerations and challenges associated with space travel, including propulsion, crew psychology, biological changes, and the difficulty of recreating habitable environments in space.

Getting there

Many options exist for sending a Generation Ship from our Solar System to another star. In his proposal paper, Goddard estimated that with a combination of hydrogen and oxygen fuel (and solar power), a generation ship could accelerate to a velocity of 3 to 10 miles per second (4.8 to 16 km/s) - 10,737 to 36,000 mph (17,280 km/h to 57,600 km/h). That's 0.000016% to 0.00005%, the speed of light.

At this rate, the spacecraft would take 85,000 years to reach Proxima Centauri, the closest star beyond the Solar System (~4.25 light years away). Such a spacecraft would need to be entirely self-sufficient and capable of maintaining a crew indefinitely, which would be sure to grow considerably along the way. Simply put, something more advanced is necessary. As Les Johnson, the famed physicist, author, and NASA technologist, described it to Interesting Engineering:

"When traveling to planets circling other stars, we need to think big, really big, and we need to be patient. Traversing the ~25 trillion miles between Earth and Proxima Centauri in a crewed spaceship will require propulsion systems harnessing energy at a scale far beyond what the entire human race generates, in sum, on planet Earth today and will almost certainly require centuries of travel time.

"Today's best propulsion systems would require tens of thousands of years to send even a small robotic spacecraft to the nearest star. To send people on the journey with a trip time of less than a thousand years is certainly possible, but will require advances in engineering that, from today's point of view, may seem like more science fiction than science fact. The important thing to keep in mind is that the laws of nature say it is possible. The question is, are we up to the challenge?"

There's also Nuclear Pulse Propulsion (NPP), originally proposed in 1946 by Polish-American mathematician Stanislaw Ulam (one of the contributors to the Manhattan Project). In 1955, NASA began exploring this idea as part of Project Orion for possible interplanetary and interstellar travel applications. The program was canceled in 1963 with the signing of the Limited Test Ban Treaty (which permanently banned nuclear testing in Earth orbit).

This method consists of nuclear devices being ejected from the rear of a spacecraft and detonated. The resulting shockwaves are absorbed by a rear-facing push plate that transforms the waves into forward momentum. According to various estimates, this method could accelerate the spacecraft up to 33.55 million mph (54 million km/hr), or 5% the speed of light. At this velocity, and accounting for acceleration/deceleration, it would take an Orion spacecraft about a century to reach Proxima Centauri.

During the 1970s, the British Interplanetary Society (BIS) conducted a feasibility study for interstellar travel known as Project Daedalus. This study called for the creation of a two-stage fusion-powered spacecraft that would be able to make the trip to Barnard's Star (5.9 light-years from Earth) in a single lifetime. While this concept was for an uncrewed spacecraft, the research would inform future ideas for crewed missions.

According to the Project's estimates, the mission would take 50 years to reach Barnard's Star, located (5.9 light-years from Earth). Adjusted for Proxima Centauri, the same craft could make the trip in 36 years. In 2009, a group of volunteer scientists (many of whom worked for NASA or the European Space Agency) founded Icarus Interstellar to revitalize the concept - now known as Project Icarus.

Studies have also been conducted that have considered antimatter as a propulsion method. This method would involve colliding hydrogen and antihydrogen atoms in a reaction chamber, which releases a tremendous amount of energy. Because the method also involves a very low-mass propellant, NASA's Institute for Advanced Concepts (NIAC) has been researching the technology as a method for interstellar travel for years.

According to varying estimates, a ship equipped with an antimatter engine could make the trip to Proxima Centauri in 8 to 40 years. However, the fuel needed in both cases would be prohibitive - up to 900,000 tons (815,000 metric tons), depending on the ship's mass. The huge mass budget for propellant has led to proposals for propulsion that could produce its fuel in-situ.

For example, physicist Robert W. Bussard proposed in 1960 how a spacecraft could be equipped with a massive funnel to "scoop" hydrogen directly from the interstellar medium (a Bussard Ramjet). This scoop would be paired with tremendously powerful magnetic fields to compress the hydrogen inside a reactor to the point where fusion occurs.

A similar idea was proposed in 2011 by Richard Obousy of Icarus Interstellar for an antimatter system that could produce its own fuel. This concept is known as the Vacuum to Antimatter Rocket Interstellar Explorer System (VARIES), which relies on large lasers (powered by giant solar arrays) to create antimatter particles from space.

Yet another possibility is a Direct Energy (DE) propulsion system, as proposed by Robert Forward in 1984. This concept resembles a solar sail, which uses a reflective surface to turn solar wind into momentum. In this case, a powerful laser array is fired at a spacecraft with huge "light sails." This imparts momentum, slowly accelerating the spacecraft up to considerable velocities.

The international nonprofit Breakthrough Initiatives is investigating this technology for its proposed Starshot spacecraft. The design consists of a gram-scale "wafer craft" accelerated to 20% the speed of light, thus making it to Alpha Centauri in just 20 years. With appropriate scaling, the same technology could propel a generation ship to a smaller fraction of the speed of light, which would be enough to make the same journey in a few centuries.

These systems, especially if used in combination, could shorten the journey to just a few generations. This would mean that the spacecraft could be built smaller and lighter and would not need to be completely self-sufficient.

Staying healthy

But of course, the most important aspect of an interstellar journey is the health and safety of the crew. As decades of space exploration and research aboard the International Space Station (ISS) have shown, long durations in space present numerous health hazards. On an interstellar journey, these issues become even more acute and demand that countermeasures be developed well in advance.

These hazards include solar radiation, which becomes a major concern beyond Earth's protective magnetosphere. But in the interstellar medium, there is the added danger posed by increased levels of cosmic rays - high-energy protons and atomic nuclei created and accelerated by supernovae to close to the speed of light. Long-term exposure to these radiation sources leads to elevated cancer risks, requiring that missions have adequate radiation shielding.

There are also the effects of long-term exposure to lower gravity, which have also been well-documented by experiments aboard the ISS. These include muscle atrophy, bone density loss, effects on cardiovascular health, eyesight, organ function, and even genetic changes. These can be mitigated with a robust regimen of exercise, medical check-ups, and artificial gravity.

Beyond these, there are also the unique health concerns that an interstellar voyage presents. As noted already, the ship and crew will need to be as self-sufficient as possible during the voyage in terms of air, water, food, and energy. Of course, there are also questions related to reproduction, like the effects of being born and raised in space and the need for enough people to ensure genetic diversity.

In 2002, anthropologist Dr. John Moore (currently the chair of the Human Genome Diversity Project in North America) conducted a study to determine the crew size needed to ensure genetic diversity during a 200-year-long interstellar journey. His paper became part of an anthology titled Interstellar Travel and Multi-generation Space Ships, released in 2003.

This was followed by fellow anthropologist Dr. Cameron Smith, who conducted a similar study in 2014 as part of Project Hyperion. Icarus Interstellar launched this preliminary study for a generation ship, which is now being pursued by the Institute for Interstellar Studies (i4is). Both of these studies found that some degree of inbreeding would occur during an interstellar voyage.

In 2017, Dr. Frederic Marin of the Astronomical Observatory of Strasbourg conducted a probabilistic study that calculated the crew needed to ensure genetic diversity while avoiding overcrowding. A second study followed in 2018, where Dr. Marin and colleagues found that mission success would be assured with an initial crew of 98 (eventually growing to 500) coupled with a cryogenic bank of sperm, eggs, and embryos.

In a third study, released in 2019, Dr. Marin and another team of researchers placed constraints on how large a generation ship would need to be to accommodate a crew of 500 and enough farmland to sustain them. They found that a cylindrical ship with artifical gravity would need to measure 1050 feet (320 m) in length, 735 ft (224 m) in radius, and contain at least ~4,850 ft² (450 m²) of agricultural land for food production, air and water recycling.

In 2021, Cornell soil scientist and Carl Sagan Institute Research Fellow Morgan Irons co-authored a study on the potential for bioregenerative life support systems (BLSS) in space. In this and other papers, Irons indicated how BLSS technology is essential to ensuring that long-duration space missions are sustainable. One of the key design features of a Generation Ship is the creation of Earth-analog environments in the ship's interior.

This is intended to provide crews with familiar and comforting surroundings and a steady supply of food, water, and oxygen over long periods of time (possibly indefinitely). As Irons explained to Interesting Engineering via email, the task of creating such environments is very complicated as it requires that the same careful balance found in nature be recreated:

"There is no easy answer to the question of how will bioregenerative life support systems be realized on spacecraft or on planetary surfaces. Ultimately, the more closely the system replicates a natural ecosphere or process from Earth, the more sustainable it will be. What does it mean to replicate? That is the fundamental question. A sustainable bioregenerative life support system should minimize reliance on supply chains from Earth, maximize ecological services beyond provisional services, utilize power from sustainable power sources, minimize entropic waste, and have no end of life."

Pros/Cons

There are many advantages to a generation ship. The most obvious is that it can be built using existing technologies (or those known to be feasible). Another is how it is specifically designed to work around the issues of rapid transit and propellant mass.

There's also the way generation ships could maintain a human population in space until it reaches a potentially habitable exoplanet. Given sufficient food production, energy, self-sufficiency, and crew size, the mission would be able to carry on indefinitely if their initial destination proved to be uninhabitable.

Last, a crew that numbers in the hundreds or thousands would dramatically increase the odds of successfully settling on another planet. The ship's volume also means that stores of cryogenically-frozen embryos could be brought along while crews could operate in shifts (alternating between waking states and suspended animation). This would reduce the psychological effects of long-duration spaceflight while ensuring genetic diversity among the population.

Unfortunately, these advantages also have accompanying downsides. The most obvious is the cost of building such a large spacecraft (and advanced propulsion methods). Even by modest standards, an interstellar ship would require a massive commitment of money and resources that would rival the GDP of the wealthiest nations in the world combined.

Also, interstellar travel may still present numerous health problems, even if radiation shielding and simulated gravity are involved. In addition, the effects of long-term cryogenic suspension (especially where multiple rounds of suspension and revitalization are involved) are not well understood. As one generation gives way to the next, genetic changes and diseases could accumulate, leading to an escalating probability of failure.

But perhaps the greatest disadvantage is the level of uncertainty involved. With generation ships, hundreds or even thousands of years would pass before people back in our Solar System could learn whether a mission was successful. At that point, Earth may no longer be habitable, and humanity may not still exist (or not be remotely human anymore). As we explored in another article, maintaining a galactic empire simply isn't practical (not unless we figure out warp fields or some other form of FTL).

What's more, there's the distinct possibility that subsequent technological advancement could lead to interstellar missions that are much faster. If these ships could overtake their predecessors (even though they left many decades sooner), the entire point of the mission would be rendered moot.

Nevertheless, generation ships remain one of the most feasible ideas for seeding the galaxy with human life. In addition, there's no need for the inhabitants of Earth to know if such a mission was successful. Ultimately, the point is to leave Earth and the Solar System in a way that maximizes the odds of survival. And there's the appeal of undertaking a monumental challenge and how it inspires us to reach farther:

As Mars City Design founder and CEO Vera Mulyani told Interesting Engineering:

"A super adventure to infinite space does not require superhumans. It requires regular humans who demonstrate super qualities, like courage, perseverance, and wisdom."

Ultimately, if the crews manage to reach their destination and settle on a new world, they could give rise to a new civilization. Similarly, multiple generational ships traveling to many nearby star systems could give rise to several new civilizations, and perhaps they might meet each other someday! As the late and great Konstantin Tsiolkovsky once said, "Earth is the cradle of humanity, but one cannot remain in the cradle forever."