The generation ship Ark, from the 70’s TV series The Starlost.
O誰eil-Style Generation Ship
Tech Level: 13
Hollowed Asteroid Generation Ship
Tech Level: 14
Converted Moon Generation Ship
Tech Level: 16
Converted Planet Generation Ship
Tech Level: 20
Mobile Megastructure Generation Ship
Tech level: 25
Generation ships are also called space arks or world ships.

The idea of a generation ship has been around since the golden age of science fiction, and has seen many an incarnation down through the decades. The two most famous examples are the short story, “Orphans of the Sky” by Robert Heinlein and the original Star Trek episode, “For the World is Hollow and I Have Touched the Sky.” The cheesy 1970s series “The Starlost” was set on board a generation, ship, and a generation ship that used time dilation to reduce a five million year journey into a mere subjective 1000 years was seen in the novel “Ring” by Stephen Baxter. The very first Science Fiction RPG, “Metamorphosis Alpha,” was also set aboard a gigantic space ark. “Rendezvous with Rama”, by Arthur C. Clarke, features an interstellar craft built very similarly to the vision of an O誰eil-style generation ship.

A generation ship is one of several “low C” options for exploring interstellar space. Low C options assume that achieving significant fractions of lightspeed are impossible or highly impractical for one reason or another, and astronauts must resign themselves to interstellar voyages of centuries or even millennia.

A generation ship is the most straightforward means of dealing with century-long journeys from one star system to the next. The ship is a miniature, self-sustaining world in and of itself, much like an O誰eil colony, and the crew of the ship will live out their natural lifetimes on board, working, playing, building, breeding, etc. Their children and grandchildren and so on will be brought up on board ship as it coasts between the stars, and become the operating crew in their turn.

Generations ships are usually touted as a type of interstellar ship that could be built today, but the truth is many significant advances would still have to be made in propulsion, recycling, and artificial habitat technology to make them possible. The most efficient rocket engine now available to us is the ion drive, and even using a vast array of such devices, it would still take over 10,000 years to reach even the nearby stars. Building an enormously complex, completely autonomous habitat with a self-renewing ecology that could last even a fraction of that time is beyond our current understanding. As structures such as arcologies and space colonies are built and perfected this may change, but for now even this “simple” means of interstellar travel is beyond our means.

Nuclear Pulse Drives, Fusion Rockets, Plasma Rockets and simple Antimatter Rockets could reduce travel time from many millennia to a “mere” few centuries, making a generation ship much more feasible. However, any one of these propulsion options are still decades away.

Extremely efficient resource management would be absolutely essential to the success of a generation ship. Recycling systems would have to work at nearly 100% efficiency to ensure that the ship and its human population will survive the many decades needed to reach its destination. The generation ship could also pick up additional resources by mining comets or asteroids that may be along its way with subcraft, or the world where the ship was launched may have sent unmanned resupply ships ahead of the main spacecraft. The ship would have to rendezvous with these resupply craft en route in the depths of interstellar space.

One of the perils of generation ships that has served as the gristle for a great many science fiction stories is that human societies are dynamic, not static, and the culture on board a generation ship is bound to change in the decades or centuries the vessel will be en route. Sometimes, in these stories, the society which starts the voyage will be torn down in a revolution and something wholly other has taken its place by the time the ship reaches its destination. Democratic societies are replaced by ruthless dictatorships, or carefully engineered but rigid social structures are ripped apart by an unknown element such as a disaster or unacceptable thoughts and ideas. Or, in the most cliched type of societal breakdown aboard a generation ship, the population somehow loses its high-tech knowledge and forgets that it is on a vehicle. It comes to view the ship as the entire universe as generations go by.

These scenarios may not seem very likely, but they do underscore the necessity of taking into account the inevitable changing tides in a human society over the long period of time the ship will be en route. One way of dealing with this may be to start the ship with only a small seed population confined to one small area of the space ark痴 artificial habitat, which will then grow and “settle” the rest of the ship痴 living space as the decades or centuries go by. This way, all the extra space can serve as a “safety valve” for relieving societal stress by giving disgruntled sections of the culture places they can claim as their own. This can only work as long as there is fallow habitat to settle, but if planned properly the ship should reach its destination long before the population runs out of new living space.

The exact minimum crew needed at the start of a voyage is a matter of some debate. The fewer inhabitants in the beginning, the fewer resources they will consume and the slower the population will grow throughout the voyage. However, if you have too few people, the crew risks inbreeding problems that are sure to arise in succeeding generations. Numbers as large as 10,000 and as small as 25 have all been proposed depending on different sociological and biological assumptions. However, a compromise range of several hundred to a thousand or so initial individuals would probably be a good median strategy.

Tech Level: 13 The interior of Rama, an O’Neil-Style generation ship. From the novel Rendezvous With Rama, by Arthur C. Clarke.
This is basically a gigantic rotating cylinder or disk, very similar to the vision of an O誰eil colony, with an interstellar propulsion system attached. The cylinder may be from several hundred meters to several hundred kilometers in diameter, and rotated on its long axis to provide artificial gravity along its inner surface. The interior is sculpted and pressurized to provide an Earth-like environment, complete with forests, hills, streams, lakes, and so on. The inner environment is usually envisioned as being large enough to generate its own weather, supplemented and/or controlled by the ship痴 systems. On an orbiting O誰eil colony, light is provided by gimbaled mirrors and enormous transparent sections of the hull. On an interstellar generation ship, illumination would have to be provided by large strips or nodes of lighting equipment recessed into the inner surface.

An O誰eil style generation ship could ultimately hold from 10,000 to several hundred thousand inhabitants, depending on its exact size and design.

Tech Level: 14

A hollowed asteroid generation ship basically converts an entire asteroid a mile or more in size into an interstellar craft. The center is hollowed out into either a spherical or cylindrical chamber, and the asteroid itself is set spinning to provide artificial gravity along its inner surface. Usually the inner habitat takes up only a small fraction of the overall volume of the asteroid. However, these are more difficult to build, as gigatons of rock would have to be extracted from the center of the asteroid in addition to building the habitat and drive system.

The hollowed asteroid scheme has two advantages over the O誰eil style space arks. First and foremost, the inner habitat is protected by a thick shell of rock that, depending on the original size of the asteroid, could be kilometers thick. If the ship is expected to pass through hazardous conditions–such as areas of possible meteoroid impact or high radiation–such armor could prove very fortuitous. Secondly, the rock shell can provide megatons additional mineral resources the crew can mine during the long voyage, readily adding to whatever stores they may have packed along at the beginning. A hollowed asteroid generation ship is featured in the novel Eon, by Greg Bear.

A variation on the hollowed asteroid is the hollowed comet. This functions very similarly to an asteroid space ark, except that the outer shell would be comprised of water and methane ice. This could prove advantageous as the crew will have a readily available source of gigatons of water and hydrogen to use for consumables. A variation of the hollowed-comet ship was contemplated in the novel, Heart of the Comet by David Brin and Gregory Benford.

Tech Level: 16

This is basically the Hollowed Asteroid scheme scaled up enormously to include objects 50 miles or more in diameter. In this case, a gas giant moon is broken out of its orbit from its primary and converted into an enormous generation ship. Larger asteroids such as Vesta or Ceres can also be considered for use, as can large Kuiper Belt objects such as Chiron.

Depending on its exact size and whether it can generate sufficient gravity naturally, the moon may or may not be hollowed out and spun to create an artificial environment. Instead its surface and interior may be interwoven with many interconnected bases and habitats. Like with the converted asteroid, the inhabitants can harvest an enormous amount of mineral resources en route from the interior of the moon.

Propulsion for such an enormous object can be accomplished by several means. Nuclear explosives can be detonated in succession in one hemisphere, the crater they create becoming a sort of crude “nozzle” that can help direct succeeding blasts. The nuclear demolitions would have to carefully calibrated to “nudge” the moon with doing minimal damage to its infrastructure as a whole.

An alternative is a large array of mass drivers, which would continually fling small portions of mass gathered from the moon itself. These would provide only a very gentle nudge for acceleration, but it can build up over time, and the moon has a lot of mass it can work with. Fusion, fission, or simple antimatter rockets could also be used in large numbers, if the moon has enough of the appropriate fuel available.

Tech Level: 20

Perhaps the most desirable form of generation ship, this scheme contemplates moving an entire habitable, life-bearing planet from one solar system to the next. Propulsion by necessity would have to be some advanced form of artificial gravity technology, to avoid undo stress on the planet痴 fragile surface structure and biosystem. Orbiting electromagnetic radiators would provide heat and warmth.

In the Ringworld novels, an alien race called the Puppeteers moved their homeworld plus four “support” planets via this method to escape from a galactic core explosion.

Tech Level: 25 The Magog Worldship, an intergalactic generation ship, from the TV series Andromeda. (c)Tribune Entertainment
Perhaps the ultimate space ark, a megastructure such as a Mech Planet, Ringworld, Dyson Sphere, or Godwheel, can be made mobile by some unfathomably advanced means and sent on a journey across the interstellar depths. These “ships” would give the initial seed population of the ship near-unlimited room to grow and expand en route. While the other types of generation ships discussed in this section would be optimized for voyages of several centuries or millennia, a megastructure ship could conceivably hold enough resources to remain en route for far, far longer.

In the novel Wall Around A Star, by Frederick Pohl and Jack Williamson, a Dyson Sphere was used as a generation ship by an alien race to cross not interstellar but intergalactic distances to settle the Milky Way galaxy, a journey that took them over a million years. On the syndicated TV series Andromeda, an alien race called the Magog also used a dyson sphere-like structure to cross intergalactic distances.

In Print:

Ring by Stephen Baxter

“Orphans of the Sky” by Robert Heinlein

Eon by Greg Bear

Wall Around A Star by Frederick Pohl and Jack Williamson

Heart of the Comet by David Brin and Gregory Benford

Rendezvous with Rama by Arthur C. Clarke

Ringworld, The Ringworld Engineers and The Ringworld Throne by Larry Niven

On The Web:

The Unofficial Page dedicated to the TV series, “The Starlost”:

Technical information on generation ships: