Exploring the universe is one of the human race's great dreams. Most people believe it is almost 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 stories, 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 give some answers that question, looking at cutting-edge theories and technology that can give us realistic options for human interstellar exploration.

Stepping Stones

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, aren't worth the bother.

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

The Stepping Stone strategy is mentioned in a number of works of science fiction, 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) 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 that were either ejected from their home star systems or that were formed alongside stars in interstellar nurseries but were never bound to them, are now thought to be far more numerous than previously realized; as are brown dwarfs, 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 theory remains unproven and controversial, however.

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. After plotting most of the major objects in the Kuiper Belt, Oort Cloud, and nearby interstellar space, using both passive astronomical techniques and powerful active radar arrays, we would first build bases and/or colonies on the outer planets to use for building and refueling outbound ships. The next step would be to move into the Kuiper Belt, mining the objects there for fuel and hollowing out larger comets to use for bases and colonies, and then to repeat the process again for the Oort Cloud, and then again for any significant objects that may exist in interstellar space.

Once a human presence has been established on an interstellar brown dwarf or rogue planet or lonely comet, the process would be 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 in search of new resources and living space.

Drive Technologies

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 of these drives have been extensively detailed in past articles for Strange Horizons. Though primarily intended for near-future planetary exploration, each is capable of reaching nearby Stepping Stone objects, or even 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.

Bussard Ramjet

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 is nonetheless about one hydrogen atom per cubic centimeter (compared to the 10¹⁸ 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 material as it zooms through space, using its tremendous forward velocity to force the funneled hydrogen atoms to a fusion state at the apex of the converging magnetic fields. The super-hot fusion plasma is then expelled for thrust.

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.

Used in this way, a ramjet-equipped ship would never run out of fuel as long as it maintained the necessary minimum velocity for the system to remain functioning. This exact minimum velocity is a subject for debate; some sources say 1% of lightspeed would suffice, while others place it as high as 6%. Whatever the figure 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 also 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 minutes or many millennia. Very efficient ramjet drives could come to within a hairbreadth's of the speed of light, though some source say that a ramjet's more practical limit may be between 50% and 85% of lightspeed. Accelerating at a constant 1 g, a Bussard Ramjet could get to within a few percentage points of lightspeed within a year.

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 of 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 would 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% of lightspeed. However, it has also been pointed out that these impacts would not necessarily produce anything other than waste energy, as the ramscoop would use 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 a very fragile biological crew through the decades of travel to reach destinations light-years distant.

Generation Ships

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 through the decades. Two of the most famous examples are Robert Heinlein's Orphans of the Sky, 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'Neill colony (see below), and the crew of the ship live out their natural lifetimes on board, working, playing, building, breeding, etc. Their children and grandchildren and so on are 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 would 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 route with subcraft, or the world where the ship was launched might send 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 (for example, Brian Aldiss' novel Non-Stop) 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 in transit. Sometimes, in these stories, the society which starts the voyage is torn down in a revolution and replaced by something entirely different 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 clichéd type of societal breakdown, the population somehow loses its high-tech knowledge and forgets that it is on a spacecraft. 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 at 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 the inbreeding problems that are sure to arise in succeeding generations. Different sociological and biological assumptions have led to numbers as small as 25 and as large as 10,000 being proposed. However, a compromise range of several hundred to a thousand or so would probably be a good median strategy.

The most common vision for generation ship design is to model it after an O'Neill colony, first proposed by Gerard O'Neill 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'Neill colony, light would be 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'Neill style generation ship could ultimately hold from 10,000 to several hundred thousand inhabitants, depending on its exact size and design.

Hollowed-Asteroid Generation Ship

A hollowed asteroid generation ship essentially 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'Neill-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 shell of rock can provide megatons of additional mineral resources that the crew could 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 would 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.

Sleeper Ships

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

In a sleeper ship, the crew of the ship would spend most of the many decades in transit in suspended animation, where they would age very slowly or not at all. Their physical condition would be monitored by computer and they would eventually be awakened by the ship's automated systems. Many methods for suspended animation have been proposed, including cryonics, chemically induced hibernation, fluid replacement, cryogenic suspension, nanotech restructuring, and 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 might not be wise to have the entire crew in suspended animation for the entire voyage. Individual crewmembers might 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.

Colonizing Seed Ships

Seed ships are an intriguing idea for colonizing interstellar space, although they rely heavily on extremely advanced and reliable artificial intelligence and robotics. The idea 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 technique yet known can freeze and successfully revive a human being. However, we do have a tried and tested technology that allows us to freeze human zygotes (fertilized ova) for long periods of time, then thaw them and bring them to full term. Since it is assumed that no human could survive the centuries-long trips between the stars, seed ships would be fully automated. After reaching their destination after many decades in transit, the ship would land, and the vessel's mainframe oversee the thawing and bringing to term human zygotes via artificial wombs. When the ship-born children have come to term, robots would 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, would then be used to awaken and raise several hundred more humans from the ship's biological stores. These then form the first human community on the 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 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 would experience a perfectly normal childhood, then be given "transitional" scenarios that allow them to psychologically ease into their waking lives on an alien planet.

If technology such as memory printing becomes 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.

Terraforming Seed Ships

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 would 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 might spend a very, very long time running on automatic, given the combination of a decades-long journey and the centuries or more needed to terraform a planet or planet-sized mass.

Once a habitable environment has been established, the ship would gestate its human crew and raise them as per a colonizing seed ship, above.

Conclusion

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.

Paul Lucas grew up on the shores of Lake Erie just a few snow drifts away from Buffalo in the sleepy little town of Dunkirk, NY. He currently resides in Erie, PA, where he freelances as a writer and artist. His previous publications for Strange Horizons can be found in our Archive. To contact Paul, email him at plucas1@hotmail.com.

Further Reading

On Rogue Planets.

On Brown Dwarfs.

On Bussard Ramjets.

On Generation Ships.

On suspended animation, cryogenic suspension, and cryonics science.