Ark ships, also known as generation ships or world ships, are large space craft designed to carry a significant crew of humans on extremely long journeys across space, typically between different solar systems. Humans probably have the technology to build a generation ship now, but there are significant challenges outside of those encountered in the construction and engineering of such a craft, such as the psychological impact of living out one’s life on a space ship and never seeing Earth again.

History of the Interstellar Ark

One of the first known arks was that built by Noah in the Book of Genesis. Approximately 150m long, Noah was instructed by God to take on board samples of all the animals and birds, so that they would survive the deluge along with Noah’s family.

More than 2000 years later, the British Interplanetary Society conducted one of the first engineering studies into the difficulties of interstellar travel, Project Daedalus. The study concluded that a "1000-year ark" or world ship, in which a kilometer-dimension ship with a closed ecosystem carries its population for many generations between neighboring stars, was in fact the only possibility for a manned interstellar mission.

The concept of an Ark is of some kind of vessel designed to preserve against the dangers of a falling, or fallen, world.² Returning to the question of whether humans would ever overcome the psychological barrier to sending a generation ship out, probably never to return, we examine possible scenarios in which the building of such a ship is the only way the human race could survive - much like in the deluge mythology of ancient times.

• Impact by a NEO - a large enough comet or asteroid (the size threshold is calculated at anything more than 1-2km in diameter) hitting the Earth would have a catastrophic effect. Dinosaur extinction theories include this scenario, showing that it has happened, and it is devastating to life on Earth.
• Nuclear war - the specture of "mutual assured destruction" has never quite left us since the end of the Cold War. Other nations than the traditional nuclear powers continue to research and build nuclear weapons, and the existing powers still maintain more than 20,000 intact warheads³ between them.
• Alien invasion - the concept of intelligent alien life is certainly not discounted by the scientific community. Varients of the Drake Equation predict with near certainty that there are at least some alien civilizations out there somewhere, the only question being how many. While the notion of an invasion by little green men may at first seem silly and easily dismissed, the truth is we simply do not know if such a threat is real or imagined. The discovery of large numbers of exosolar planets at least would tip the balance towards the possibility of intelligent life in our galactic neighborhood. Whether they would be hostile towards us is a difficult question, but is a possibility we should not exclude.

There remain plenty of other scenarios in which Earth is destroyed and the prospect of building an ark ship becomes a lot more attractive. In the next section, we will examine the technological and psychological challenges to building such a craft, and possible destinations.

Building an Ark Ship

The Daedalus Project considered two propulsion options for carrying a generation ship between the stars: fusion pulse, and solar sails.

Fusion pulse propulsion

Fusion pulse technology considers producing thrust by the reaction of a thermonuclear bomb, or an electron beam imploded inertial fusion micropellet. There are a number of possible reactants, with advantages and disadvantages such as being easy to ignite but producing large amounts of neutron radiation; or relatively clean, but difficult to ignite. (Refer to the article on nuclear fusion for more information.) A fusion pulse ark ship might be able to travel between two nearby stars in 500-1000 years.

An advantage of the high relativistic velocities (0.01 - 0.03c) attainable using pulsed nuclear propulsion is the possibility of using a ramscoop to collect additional fuel from the interstellar medium. There are significant problems with this concept however, the strongest objection being the relative lack of useful hydrogen isotopes in the interstellar medium.

Fusion pulse technology is also one step beyond the fissile propulsion methods currently being considered by NASA⁴ in their Prometheus Nuclear Systems and Technology program; research and development into nuclear pulsed propulsion is currently receiving significant funding from the US Congress.

Hyperthin solar sails

A highly reflective sheet sail, probably manufactured in space with a thickness measuring only in nanometers and attached to the ark ship with diamond strength cable, could be used as a solar sail to provide propulsion between the stars. Solar sails utilize solar radiation pressure to propel their payload out of the solar system, and the destination star’s sunlight to decelerate.

In ¹, calculations indicate for a small interstellar ark massing a few million kilograms, the fully unfurled solar sail would be about 100km in dimensions, yielding a peak acceleration of a few g (between 10 and 30 meters per second squared acceleration). With this acceleration, one-way voyage times to the nearest stars, in the region of 5 light years away (9 x 10¹⁵m, or 9 trillion kilometers) would take 1000 years at the very least.

Methods to reduce the travel time have been proposed, such as using more complex pre-perihelion trajectories or by increasing the solar luminosity (increasing the propulsion system’s capabilities if it were used to escape a dying star, whose physical size and luminosity both increase as it leaves the main sequence).

Life support and habitation

The notion of living aboard a space ship with anything close to the cramped quarters of the Mier or International Space Stations is a nightmare scenario. Human beings, although social animals, also need their space. This is a psychological imperative for a mentally healthy crew, especially one whose individual members will live out their life and die aboard the generation ship.

Artificial gravity could be provided by a rotating cylindrical, or toiroidal, hull. Air and water would have to be scrupulously conserved and recycled, and hydroponic farms could be used to produce food. In the interstellar gulf, little energy would be available from solar radiation, so conservation of power would also be important. Given these constraints, it is difficult to imagine how a crew might live out their lives and maintain moral.

One alternative to keeping the crew awake is sending them into some form of suspended animation, making the ark a sleeper ship similar to those in the Alien movies.

While cryogenic suspension (using extremely low temperatures to freeze organisms) has implicit hazards in the resuscitation process, a recent breakthrough by a US team in Seattle offers an alternative means to keeping a sleeper ship crew under. The study reported mice put in a chamber filled with air mixed with hydrogen sulphide (used to regulate body temperature and metabolic activity) stopped breathing and moving almost entirely. Their breathing rate dropped from 120 breaths per minute to less than 10, their metabolic rates dropped by 90% and their body temperature fell from 37C to as low as 11C. After six hours’ exposure, the mice were given fresh air and reportedly recovered, suffering no ill effects.

Scientists at the European Space Agency have also investigated the possibility of inducing hibernation-like states in astronauts for journeys to the outer planets, drastically reducing the required mass of the spacecraft used for the mission as savings in food, water and psychological space requirements are realized.

Possible destinations

As our capability to detect and observe exosolar planets increases, the likelihood of finding a habitable planet suitable for human settlement becomes less a science fiction staple and more a reality. Suggested destinations in ¹ include Procyon, an F5 star 11.3 light years distant; Beta Hydri, 21.3 light years away, a G2 like our sun; and Pollux, 35 light years away, a KO red giant.

Generation ships in science fiction

There are many interesting insights into the possibility of generation ships found in science fiction. In Frank Herbert’s Destination Void, a skeleton crew perform shifts to maintain a sleeper ship’s navigation and operational systems while the majority of its colonists remain in suspended animation. Heritage of the Star by Sylvia Engdahl explores the legacy of a generation ship sent by a dying Earth millenia previously in the form of its "child" civilization. The Battlestar Galactica series takes a more short-term look at preserving the last remnants of civilization aboard a small fleet of ships in an interstellar odyssey in search of Earth.

References

1. National Institute for Discovery Science: The Reenchantment of the Solar System
2. Glossary of Terms, Names and Concepts in Blake
3. Global Nuclear Stockpiles, 1945-2002; Bulletin of the Atomic Scientists
4. NASA - Exploration Systems - Prometheus Nuclear Systems and Technology
5. BBC News - Mice put in ’suspended animation’