Cruising the Infinite

Strategies for Human Interstellar Travel

by Paul Lucas

Image courtesy NASA.

Exploring the universe is one of the human race's great dreams. All but a few believe it is not inevitable that our descendants will someday emerge from our planetary cradle and spread out among the stars.

But the spaces between solar systems are unimaginably vast. The fastest object ever launched, the Voyager 2 probe, is traveling at dozens of miles per second, and yet it would not reach even the nearest star, Proxima Centauri, in less than 100,000 years--twenty times longer than the age of the civilization that built it.

In many science fiction sources, alien star systems are traversed by ships with magical hyperdrives that can make the trip in a relative blink of an eye--days or weeks instead of centuries and millennia. But the reality of how humanity will explore our interstellar neighborhood is shaping up to be of a far different character. Voyages will take many decades, and the greatest hurdle we will face will not be screaming starfighters or glowing space anomalies, but endless, endless tracts of utter emptiness.

We are poised to begin this incredible adventure perhaps within the next one hundred years, as space technology begins catching up with our interstellar dreams. Robot probes will inevitably be our scouts, going before us to tell us what's waiting for us in the great dark. But when human beings are ready to go, how will we get there?

This article is designed to answer that question, looking at cutting-edge theories and technology that can give us realistic options for human interstellar exploration.


In most scenarios of interstellar exploration, immense ships of one design or another leave one life-bearing world in the inner system of a star, fly through the void, then brake around another life-bearing world in the inner system of another star. The old assumption was that the outer reaches of a star system, being cold, lifeless, and only sparsely populated with material resources, wasn't worth the bother.

However, discoveries in the past decade has shown that the borderlands of a star system may reach much farther, and contain much more in it, than previously thought. Frozen worlds and objects that can be used as way-stations and resources for journeys out into the void.

Stepping Stone strategy is mentioned in a number of science fiction sources, such as the novels _Heart of the Comet_ by David Brin and Gregory Benford, _The Gripping Hand_ by Larry Niven and Jerry Pournelle, and _Permanence_ by Karl Schroeder, but to the best of my knowledge no one has yet done a serious technical study of it.

For our first stop beyond Pluto, there's the Kuiper Belt, which extends from the orbit of Neptune to at least 50 AUs (about 7 billion kilometers) out from the sun in more or less the same orbital plane as the planets. It is estimated that at least 70,000 cometary objects with diameters larger than 100 kilometers exist in this Belt, all primordial remnants from the accretion disk that originally formed the solar system. Pluto, its moon Charon, the recently-discovered worldlet Quaoar, and a large object near Saturn's orbit called Chiron are all thought to be large Kuiper Belt objects.

There's also the Oort Cloud, an even larger and more widely dispersed collection of cometary objects that forms a rough sphere around the solar system, starting from about where the Kuiper Belt ends. Comprised of an estimated one trillion significantly sized objects, the Oort Cloud is thought to extend to at least 50,000 AUs from the sun, though some estimates put the outer boundary at 2 light years or more. The total mass of all the comets in the cloud are thought to exceed 40 times the mass of Earth, though individual objects may be tens of millions of kilometers apart.

Rogue planets, worlds either ejected from their home star systems or ones that formed alongside stars in interstellar nurseries but were never bound to them, are now thought to be far more numerous than previously thought. As are Brown Dwarves, objects too big to be a planet but too small to ignite into a star, which have masses between 15 and 80 times that of Jupiter. Both types of objects combined are now considered to be more numerous than mainstream stars, and litter the vast interstellar depths.

The Sun itself is thought by some astronomers to have a distant near-interstellar companion that may either be a rogue planet or a brown dwarf. Named Nemesis, it is theorized to have an orbital period measured in millions of years, periodically sweeping through the Oort Cloud to send comets raining into the inner system, causing mass extinctions such as the one that wiped out the dinosaurs.

The Stepping Stone strategy for exploration takes advantage of all these various objects to very slowly build a 'step ladder' out of one solar system and into another. Using both passive astronomical techniques and powerful active radar arrays, most of the major objects in the Kuiper Belt, Oort Cloud, and nearby interstellar space can be found and their orbits plotted. One first builds bases and/or colonies on the outer planets to use for building and refueling outbound ships. Then one moves into the Kuiper Belt, mining the objects there for fuel and hollowing out larger comets to use for bases and colonies. One repeats the process once again for the Oort Cloud, then again for any significant objects that may exist in interstellar space.

Once a human presence is established on an interstellar brown dwarf or rogue planet or lonely comet, the process is reversed in a nearby system, moving inward from the interstellar way station to the new system's Oort Cloud, then to its Kuiper Belt, to its outer planets, and finally to its inner system. If a conveniently-placed interstellar object is absent, a comet or a group thereof can be towed or boosted into place.

Needless to say, this may be an extremely gradual, multi-generational process. In fact, some think it may not even be done intentionally. As humanity moves out into the solar system, the inner worlds may fill up within a millennium, forcing some elements of its population ever outward looking for new resources and living space.


A vision for an anti-matter rocket. Image courtesy NASA

So we have a potential path to the stars. Now how exactly do we use it?

There are four basic drive technologies with the power and fuel efficiency for possible interstellar flight now being studied. These are:

--Nuclear Pulse Drives, which use rapid detonations of nuclear bombs to drive a ship forward.

--Nuclear Rockets, which use either fission or fusion techniques to superheat exhaust plasmas.

--Solar Sails, which use enormous but gossamer-thin sails to harness sunlight and directed laser light for propulsion.

--Antimatter Rockets, which harness the enormous energies released from matter/antimatter annihilations.

All have been extensively detailed in articles for Orbital Vector, the links for which can be found at the bootom of the page. Though primarily envisioned for near-future planetary exploration, each is capable of reaching nearby Stepping Stone objects or even direct flight to nearby stars within only a few decades of travel time.

A fifth major type of drive has also been proposed, meant almost exclusively for interstellar missions: the Bussard ramjet.


Conceptual image courtesy NASA

This drive was first conceived by Robert Bussard in 1960, and has since been used in countless science fiction stories and novels, such as _Tau Zero_ by Poul Anderson, _A Deepness in the Sky_ by Vernor Vinge, and numerous "Known Space" stories by Larry Niven.

The Bussard Ramjet concept relies on the fact that the vacuum of space is not _quite_ as empty as we think. While matter is spread incredibly thin in the depths between the stars, there still exists about one hydrogen atom per cubic centimeter (compared to the 10^18 atoms per cubic centimeter in Earth's atmosphere at sea level.) The Bussard Ramjet uses an immense forward-facing conical magnetic field to scoop up this interstellar medium as it zooms through space, using its tremendous forward velocity to force the funneled hydrogen molecules to a fusion state at the apex of the converging magnetic fields. The super-hot fusion plasma is then expelled for thrust.

Used in this way, a Ramjet-equipped ship would never run out of fuel as long as it maintained a certain minimum velocity for the system to remain functioning. This exact minimum velocity is a subject for debate; some sources say one percent lightspeed or so, while others quote it as high as six percent. Whatever it may be, the ship would need a secondary drive system that would allow it to not only get up to these velocities without using the ramscoop but to maneuver around within a star system where significant fractions of lightspeed may prove undesirable. Typical secondary drive sources include light sails, fusion drives, and antimatter rockets.

Because it has a practically unlimited fuel supply, a Bussard ramjet is a particularly powerful stardrive. It can theoretically accelerate for any arbitrary interval of time, whether it be a few minute or many millennia. Very efficient ramjet drives could come to within a hairbreadth's of the velocity of light, though some source say that a ramjet's more practical limit may be between 50% and 85% lightspeed. Accelerating at a constant 1 g, a Bussard Ramjet could get to within a few percentage points of lightspeed within a year.

A wide-beam laser is shot ahead of the vehicle, imparting enough energy to any hydrogen atom in its path to force its electrons to fly off. The magnetic field projected by the ship attracts the positively charged ions and repels the now free electrons. The tremendous forward velocity of the ship and the tapering cone of the magnetic fields force the protons together with enough force to spark a fusion reaction. Alternately, the compressed hydrogen can be catalyzed by an on-board antimatter supply to produce more efficient fusion reactions.

One of the main difficulties in building a Bussard ramjet (aside from getting it up to the minimal operational velocity of one to six percent lightspeed) is creating magnetic fields large enough to gather enough fuel to be strong enough to handle the stresses of scooping and fusing hydrogen at significant fractions of lightspeed. In order to obtain enough fuel for continual operation, the scoop would have to be thousands of miles wide and relatively narrow to aid in maintaining magnetic field strength. The strength of the field would also be immense, on the order of ten million tesla, making them instantly deadly to any living creature nearby.

Some concern has been expressed about the amount of drag the interstellar medium will induce on a ramjet. Moving at significant fractions of lightspeed, the repeated impacts of the interstellar hydrogen on the immense ramscoop field is thought by some to offset much of the acceleration produced by the fusion engines, greatly reducing the starship's capabilities. If this is so, a ramjet's top speed may be only 15% to 25% lightspeed. However, it has also been pointed out that the impacts would not necessarily produce nothing but waste energy, as the ramscoop uses the impacts as part of its scooping and fusion processes, so how much drag a bussard ramjet would actually experience is up for debate.

With Stepping Stone objects, we have a sparse but traversible road to the stars. With various drive technologies, we have the means to get there. Now all we have to do is figure out how to get very fragile biological crew through the decades of travel to reach destinations light-years distant.


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. Two of the 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." A generation ship that used time dilation to reduce a five million year journey into a mere subjective one thousand 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.

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'Neil 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.

Extremely efficient resource management would be absolutely essential to the success of a generation ship. Recycling systems would have to work at nearly one hundred percent 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 via Stepping Stone objects 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 social structures are ripped apart by unacceptable thoughts and ideas. Or, in the most cliched type of societal breakdown, 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 dire 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's artificial habitat, which will then grow and "settle" the rest of the ship's living space in the ensuing the decades or centuries. 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.

Experts disagree on the exact minimum crew needed at the start of a voyage. 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 population would probably be a good median strategy.

The most common vision for generation ship design is to model it after an O'Neil colony, first proposed by Gerard O'Neil in the 1970s. This is basically a gigantic rotating cylinder or disk 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 curved 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's systems. On an orbiting O'Neil 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'Neil style generation ship could ultimately hold from 10,000 to several hundred thousand inhabitants, depending on its exact size and design.


A hollowed asteroid generation ship basically converts an entire asteroid a kilometer 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.

The hollowed asteroid scheme has two advantages over the O'Neil 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 many 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 fortuitous. Second, the rock shell can provide megatons of 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 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.


A sleeper ship from the Star Trek universe. Image copyright Paramount.

Sleeper ships have been seen or mentioned in many science fiction sources, including the original Star Trek episode "Space Seed," the original Planet of the Apes movie, the early 90's TV series _Earth 2_, and in "Known Space" stories by Larry Niven.

Basically, the crew of the ship spends most of the many decades in transit in suspended animation, where they age very slowly or not at all. Their physical condition is monitored by computer and they will eventually be awakened by automated ship's systems. Many methods for suspended animation have been proposed, including cryonics, chemically induced hibernation, fluid replacement, cryogenic suspension, nanotech restructuring, or combinations thereof. The length of a voyage may not be limited by the technology of the stardrive but by how long the human crew can safely remain in suspended animation without risking permanent medical complications.

Depending on circumstances, it may not be wise to have the entire crew in suspended animation for the entire voyage. Individual crewmembers may have to be awakened for rotating 'watches' while the rest of the crew sleeps so they can monitor systems and do routine maintenance. For example, on a ten year voyage with a crew of twenty, two crewmembers at a time would spend a year awake to tend to the ship. On very long voyages of centuries or more, the ship would have to run on automatic for most of the time, with the crew being only occasionally awakened for brief periods for routine maintenance or for emergencies.


Seed ships are an intriguing idea for colonizing interstellar space that depends heavily on extremely advanced and reliable artificial intelligence and robotics. It occasionally pops up in various science fiction sources. The novel _The Songs of Distant Earth_ by Arthur C. Clarke, the animated movie _Titan A.E._, and the "Scorched Earth" episode of the TV series _Stargate: SG-1_, all used seed ships as major plot points.

No known technique can yet freeze a human being and bring him back to life. However, it is now a tried and true technology to freeze human zygotes (fertilized ova) for long periods of time, then thaw them and bring them to full term. Since it is assumed no human could survive the centuries-long trips between the stars, seed ships are fully automated. When they reach their destination after many decades in transit, the ship lands, and the vessel's mainframe oversees the thawing and bringing to term human zygotes via artificial wombs. When the ship-born children come to term, robots take over rearing them, educating and training them to become the seed population of a new human colony.

Usually this first round of humans, from a handful to several dozen in number, is then used to awaken and raise several hundred more humans from the ship's biological stores. These then form the first human community on planet, which expands in the usual manner in succeeding generations.

Actually, the most difficult part of creating a seed ship mission wouldn't be the stardrive or plotting an interstellar trajectory or anything so mundane, but in creating artificial intelligences and robots that could handle and raise human children without causing any undue psychological or physiological damage to their charges. The mission designers would no doubt try to make the parental computers as human-like as possible, with true androids being used by advanced enough societies.

An alternative to child-rearing robots would be to keep the children in a total virtual reality environment from birth to maturity, as per the movie The Matrix, with tissue growth, bone density, and muscle tone maintained through electrodynamic stimulation or nanotech restructuring. Within the VR environment, they experience a perfectly normal childhood, then are given "transitional" scenarios that allow them to psychologically ease into their waking lives on an alien planet.

If technology such as memory printing is available, it could also be possible to clone and quick-grow individuals as a seed population, then imprint them with the memories and skills of specialists recorded back on the homeworld.

Besides human zygotes, the ship could also carry frozen genetic material of livestock, crops, and other useful lifeforms.


Both _Titan A.E._ and the "Scorched Earth" episode of _Stargate: SG-1_ used this type of craft.

The Colonizing Seed Ship assumes that the ship will be sent to a previously known planet where the conditions to support its colonists already exist. However, the Terraforming Seed Ship is created with the ability to transform a planet (or, in Titan A.E.'s case, a loose planet-sized mass of ice asteroids) to fit the requirements of its colonists. By necessity, it needs to be much bigger, more powerful, and more advanced that its Colonizing cousin.

Besides the zygotes of the colonists, the seed ship would also need to carry the genetic material of all the plants, animals, and microorganisms that will form the ecology of the terraformed planet. It would also have to be smart enough to set up that delicately-balanced ecology while avoiding the many pitfalls that can plague such an undertaking.

Terraforming an entire planet can prove be a long, arduous, and resource-consuming process, one that could take many centuries. A terraforming seed ship, therefore, would have to be built as tough and as long-enduring as the most rugged generation ship, as it may be a very, very long time running on automatic, between its decades-long journey and centuries or more terraforming a planet or planet-sized mass.

Once a habitable environment is established, the ship gestates its human crew and raises them as per a colonizing seed ship, above.


The Interstellar Cycler concept was envisioned by science fiction author Karl Schroeder in his novel _Permanence_.

No matter what technique you use, it takes an enormous amount of energy to accelerate a craft up to near-light speed. So much so, in fact, that it may not be worth an equal amount of energy to simply decelerate it again at its destination. The interstellar Cycler envisions a vessel that never slow down, but uses various techniques to affect turns around a local neighborhood of stellar objects to inscribe a rough circle. A cycler may visit anywhere from a handful to many dozens of star systems and Stepping Stone objects before returning to its point of origin and completing a circle.

The inhabited systems it passes accelerates smaller cargo vessels to catch up to it and decelerate cargoes the cycler drops off. This way, no one world or outpost has to foot the entire cost of building and launching the interstellar vessel; they only have to absorb the cost of accelerating and decelerating much smaller cargo vessels. The cycler also takes some of the cargo to slowly build itself up. In the history of Schroeder's novel, the original cycler started out as an unmanned magnetic sail probe; after many centuries, it grew into a full-scale, fully-inhabited fleet of city-sized structures.

To begin the process, a magnetic sail is accelerated by an array of powerful particle beams out of its home solar system. It then deploys large loops of highly charged superconductive wires. These interact with the interstellar medium and galactic magnetic fields, using the Lorentz Force (the force exerted on charged particles in a fixed magnetic field) to very gradually turn the vessel. This is combined with other techniques, such as long-range assists from the home system's particle beams, gravity slingshots, and on-board reactive engines such as ion drives, to slowly turn the vessel and align it with its next destination. After it reaches one to six percent lightspeed, it could perhaps even use magnetic field scoops to gather hydrogen from the interstellar medium to power a Bussard ramjet system.

Its a matter of some debate whether affecting large, gradual turns at significant fractions of lightspeed is actually more energy conservative that simply slowing the vessel down at its destination. Obviously, the shallower the angle of the turn, the less energy would be needed, and the more significant masses it can pass to use for gravity assists would also be useful. Thus, interstellar cyclers would probably work best when used on routes heavily-populated with star systems and large interstellar objects such as brown dwarves.

As a cycler passes a system, that system's inhabitants use magnetic sail craft to accelerate cargo to meet with the cycler, then use the same system to decelerate cargo left off by the cycler. The particle beams used to accelerate the cargo may also be used to help accelerate and/or turn the cycler as it passes through. The cycler ship is slowly built up as it passes from system to system and cycle to cycle, drawing upon resources sent to it by colonies along its route.

It is very probable that multiple interstellar cycler craft will be used along any one route, staggered anywhere from a few years to a few months apart. In this way, an interstellar trading economy can remain viable even at otherwise prohibitive STL speeds, as new goods from the stars would be arriving at regular intervals. Schroeder proposed that the original cycler on a route can "reproduce" after it accumulates enough resource donations. The crew of the cycler slowly builds up another cycler vehicle en route, and when complete the new vessel slows down and drops behind its "parent" ship before it accelerates again a few months or a few years later. The process is repeated as many times as necessary to create a fleet of cyclers for any one route.


These are just some of the possible strategies that have been proposed thus far in getting humans to the stars. In the century to come, as space technology advances, we will no doubt see an explosion of conjectures dealing with the possibilities of interstellar travel. Some of us may even be lucky to live long enough to see some of these theories become reality, and witness the beginning of the human race's interstellar age.


The Kuiper Belt

The Oort Cloud

Rogue Planets

Brown Dwarves

More Brown Dwarves

On Generation Ships:

David Darling's Page

Astrobiology Page

On Suspended animation, cryogenic suspension, and cryonics science:

Deep Space Propulsion

Light Sails

Nuclear Pulse Drive

Nuclear Rockets

Antimatter Rockets

Interstellar Cyclers

On Bussard Ramjets:

Bussard ramjet Page

Bussard Ramjet Discussion

Article added 31 January 2009