Traveling to a distant star presents a number of challenges. First and foremost is the immense distance involved. For example, the nearest stars to us are in the Alpha Centauri system. The closest of these, Proxima Centauri, is 4.22 light-years away, which translates into nearly forty trillion kilometers (or 24 trillion miles). This is around 271,000 times the distance between the Earth and the Sun. Other stars are much farther away. These tremendous distances raise a number of issues related to methods of getting there, the long-term effects of time and space on the physiology and psychology of space travelers, and the chances of finding planets with life around a selected star. It is likely that the first manned interstellar missions will be decades to centuries long, requiring a multigenerational approach where crewmembers will live, give birth, and die during the course of the mission. But putting some or all of the crew in suspended animation is also a possibility. Both of these scenarios will be discussed.

Traveling to the Stars: Distance, Propulsion, Radiation

In considering where to go, the stars closest to us are the likeliest candidates for the first multigenerational starship mission. In our Sun’s neighborhood, the closest stars and their distances in light-years (in parentheses) are: Proxima Centauri (4.2), Alpha Centauri A and B (4.4), Barnard’s Star (5.9), Wolf 359 (7.8), Lalande 21185 (8.3), Sirius A and B (8.6), UV Ceti A and B (8.7), Ross 154 (9.7), Ross 248 (10.3), and Epsilon Eridani (10.5). All of these stars are a long way away: trillions of miles. Using current technology, interstellar travel is highly unlikely. For example, a starship traveling at the same speed as Voyager 2 would take around 497,000 years to reach the Sirius star system . In contrast, a ship traveling at 5% the speed of light (.05c) would take 88 years to reach Alpha Centauri. Although an improvement, this still would be longer than the expected lifetime of most of the crewmembers and would necessitate a multigenerational approach or the use of suspended animation.

Since faster than light speeds, traveling through wormholes, or using a “warp drive” to distort space-time are not scientifically credible options at present, new propulsion systems that can reach a significant fraction of the speed of light will be necessary. In a typical mission, the vehicle must first accelerate up to this speed, then coast along through much of the mission at this velocity, and finally decelerate to orbital or landing speed as it approaches its destination. By accelerating such a starship at the force of one g (producing an Earth-like gravity situation for the crewmembers), it would take about a year to reach a cruising speed close to that of light. The acceleration time would be less for a ship reaching a more manageable cruising speed, say around 10% the speed of light (.10c). Relativistic time effects are important to consider when traveling close to light speed, but they are relatively negligible at speeds in the range of .10c. Three kinds of propulsion system have been identified for interstellar missions: those that carry their own fuel, those that rely on some sort of external energy source to move them along, and hybrids of these two2.

Interstellar vehicles using internal energy sources: Traditional rocket-based propulsion systems are self-contained: they carry along their reaction mass, energy source, and engine, all of which greatly increase their total mass and cost. One type is the nuclear fission rocket, which uses a nuclear reactor to thermally accelerate hydrogen atoms to provide thrust; a variant adds a thermal-to-electric generator to expel charged atoms at high velocity (the nuclear electric rocket). An example of the former was a program called NERVA (Nuclear Energy for Rocket Vehicle Application), which developed some prototype engines in the late 1950s and 1960s but was terminated in the early 1970s. An example of the latter, proposed by the Jet Propulsion Laboratory in California in the mid-1970s, was the TAU (Thousand Astronomical Unit) mission. Although useful for outer Solar System travel and transport, such fission rockets do not produce enough thrust to reach a star in a reasonable amount of time.

A second and more powerful system that contains its own energy source is the nuclear pulse rocket, which is propelled by small nuclear bombs ejected and exploded every few seconds or so against a heavy-duty pusher plate at the back. The pusher plate absorbs each impulse from the hot plasma and transfers it to the vehicle through large shock absorbers. The prototype system for this method of propulsion was Project Orion, proposed by the Los Alamos National Laboratory in New Mexico in the late 1950s and early 1960s to use small nuclear fission bombs and in the late 1960s by Freeman Dyson to use fusion devices. It was estimated that some three hundred thousand bombs would be needed to propel the massive space ship, which would weigh four hundred thousand tons and accommodate a crew of several hundred3.

A third internal energy system was explored by members of the British Interplanetary Society in the 1970s and was termed Project Daedalus. This was a fusion-powered interstellar rocket where pellets of helium-3 and deuterium were compressed and heated in a combustion chamber inside the ship by high-energy electron beams or lasers. The resulting fusion reaction provided energy to power the vessel. Although some have proposed using tritium instead of helium-3 since the reaction is easier to initiate, helium-3 results in charged particles that can be confined and directed by a magnetic nozzle (rather than the leaky neutral neutrons that are produced by the tritium reaction). Since helium-3 is rare on Earth, it would have to be mined elsewhere, such as the atmosphere of Jupiter or Saturn, possibly using robotic helium mines suspended by balloons. Deuterium could be obtained from cometary nuclei in the Oort cloud. A followup version to Daedalus, Project Icarus, was examined in 2009 to explore similar concepts using newer twenty-first-century notions and to develop some sort of deceleration mechanism when the target star was reached. Other variants included the novel use of autonomous robotics and artificial intelligence for onboard planning, maintenance, and self-repair, and new propulsion concepts, such as plasma jet driven magneto-inertial fusion. Another Daedalus follow-up examined the use of the entire spacecraft as a magnetically-insulated capacitor which would ignite the deuterium- tritium reaction using an intense ion beam.

A fourth internal energy system depends upon the reaction of matter and antimatter to provide energy to move the vehicle. Although the concept has been discussed since the 1950s, it was more fully developed in the early 1980s by Robert Forward. The notion was that the reaction of protons and antiprotons would produce electrically charged elementary particles that could be focused by a magnetic nozzle and expelled out the back of the rocket ship as exhaust. Although more powerful than fission or fusion, this system presents technical issues related to storing antimatter in a manner that would prevent it from touching and reacting with the walls of the ship, such as in a magnetic or electric field. In addition, antimatter is very rare, and it would be a challenge to obtain enough of it to propel a giant starship.

Hybrid interstellar propulsion systems: External energy propulsion systems solve the major problem that decreases the efficiency of systems using internal energy: the need to take along large amounts of heavy fuel. Hybrid systems likewise rely on external energy sources to decrease mass, but they also use small amounts of internal energy. One such system was the Bussard interstellar ramjet, which was proposed in the early 1960s by Robert Bussard. This vehicle consisted of the payload, a fusion reactor, and a large electrical or magnetic scoop to collect onrushing charged particles along the flight path. Interstellar hydrogen was the main fuel source. However, some supplemental intrinsic fuel was necessary for travel through low hydrogen areas, such as in our Sun’s vicinity. Although this model employed a heavy rocket engine whereby the energized helium exhaust resulting from hydrogen fusion was expelled from the rear of the spacecraft to accelerate it forward, such a starship would not need to carry a lot of fuel during the trip, thus cutting down on mass and cost. Since the amount of hydrogen collected by the ramscoop increases with speed, this system could reach high velocities and would be suitable for interstellar travel, assuming it was designed well enough to minimize drag. The scoop would need to be large and structured using lightweight material, or it could consist of a magnetic or electrostatic f ield that would collect hydrogen that has been ionized by a forward pointing laser.

One variant of the Bussard approach is the Ram-augmented Interstellar Rocket. This system incorporates a separate fusion reaction that uses a small amount of intrinsic fuel such as helium-3 and deuterium (see above). But in this case, the reaction serves to energize the hydrogen that is collected from space by a ramscoop, which it does in a very efficient manner. Note that the hydrogen is not used as fuel but as reaction mass to produce thrust for the starship.

Interstellar vehicles using external energy sources: A purely external energy system discussed as far back as the 1920s employed beamed power. The type usually mentioned uses the momentum of massless light photons from the Sun to “push” against a solar sail, thus moving the vehicle in the direction of the beam. In contrast to the solar-electric drive that uses sunlight falling on solar cells to convert fuel to ion propulsion, a beamed system would only need a payload and the structure of the vehicle; there would be no need for heavy intrinsic fuel or any kind of engine. The solar sail concept has been tested on Earth and in space with some success by several space agencies. Being located within our Solar System, the beaming system could be monitored and maintained relatively close to home. However, the space vehicle would be a relatively slowly accelerating system, and the larger the payload, the greater the need for a very large sail. This system would likely be better for unmanned interstellar missions carrying small payloads.

A number of other beam/sail systems have been suggested, such as using small charged pellets accelerated by an electromagnetic mass driver that strike a magnetic field sail; microwave photons pushing against a wire mesh sail containing microcircuits at the wire intersections; or lasers aimed by a Fresnel lens reflecting against a large light sail. Much like a tacking sailboat, some of these systems allow the craft to turn or even decelerate upon reaching a stellar destination. Methods for using the solar wind have also been considered for travel in the Solar System. More novel approaches for beamed propulsion have been proposed as well, such as using gravitational waves and antimatter to generate thrust.

Several of the above propulsion systems are capable of achieving very high speeds that would cut down on travel time. A round trip to Proxima Centauri could be made in 11 years, assuming a one-year acceleration to near-light speed, then a 3½-year coast in deep space, a one-year deceleration to the star, then a similar flight plan on the return. But traveling at near-light speed presents difficult technological problems. In addition, the rapidly oncoming flow of interstellar gas and dust particles and cosmic rays on the starship and its inhabitants could present unique particulate and radiation hazards4. Some kind of deflector shield and laser combination in the front will be necessary to block oncoming dust particles and vaporize larger bodies, although it has been pointed out that a massive deflector system might interfere with the maneuverability of a starship traveling at relativistic speeds. In a ramjet type of vehicle, micron-sized bits of dust likely will be vaporized by protons in the electromagnetic field of the scoop. To protect against oncoming cosmic rays, a passive rock or metal shield or an active magnetic or electric field deflector could be used.

Economic Considerations

The technology to propel and protect a starship would be enormously complicated and expensive, especially when one considers the massive size of the ship itself. Consider the scenario of a huge, self-contained multigenerational starship full of colonists needing to be kept alive for decades while traveling to a distant star. Strong has envisioned giant one-hundred-megaton starships containing 100-150 people that would be equipped for a centurylong journey to the stars5. Woodcock imagines even larger one million metric ton starships the length of 11 football fields that would carry ten thousand people6. Accelerating to a maximum velocity of 15% the speed of light (.15c), then decelerating to reach a star some ten light-years away, such a behemoth would complete its journey in about 130 years.

Zubrin has taken a look at the economics of a starship with a dry mass of one thousand tons that can cruise at .10c and carry a few score colonists on a trip lasting several decades7. He estimates that if this ship operates at 100% efficiency (an unlikely occurrence), the energy costs alone would amount to 12.5 trillion dollars. The addition of other costs, such as technology development and hardware manufacture, raises the price tag to 125 trillion dollars! This is roughly one thousand times the cost of the Apollo program in today’s dollars. He estimates that to keep the cost of this interstellar mission at Apollo levels in proportion to the total wealth of human society (about 1% of GDP), a future spacefaring civilization will need a GDP two hundred times greater than today and a total human population of some forty billion. He foresees fusion reactions using helium-3 and deuterium for fuel as the power source to most cheaply meet the high-power needs of this civilization. The fuel could be mined from the atmospheres of the outer gas giant planets in our Solar System. He believes that the helium-3/deuterium fusion reaction would be the power source for an interstellar vehicle as well, with the super-hot, plasma-charged particles being confined and reacting in a vacuum chamber using magnetic fields, and the exhaust mixture being directed away by a magnetic nozzle to provide the thrust. A number of technological issues need to be addressed before such a system is possible (e.g., containing the super-hot plasma, using catalytic methods to enhance fusion at a lower temperature), but Zubrin presents a good case. Of course, political, scientific, and economic stakeholder considerations (e.g., national policy priorities, scientific benef its, profit generation) will also influence the likelihood of such a mission.

Assuming that there is a will to undertake such an interstellar mission, that appropriate resources are devoted to it in a sustained manner, and that technological breakthroughs occur in a timely sequence, it is reasonable to assume that small, unmanned, beam-powered interstellar probes could be launched to a nearby star like Alpha Centauri by the twenty-third century. Such probes might even use nanotechnology. After they report back their findings, massive, manned, fusion-powered colony ships could be built and launched by the twenty- fourth and twenty-fifth centuries. Due to the scale and economics of the situation, a fusion propulsion system may not be used for the colony ship. Instead, beamed propulsion might be adequate, especially if several probes are launched sequentially that can use the same beaming source. However, travel by this method would be slow and require much more time to reach the destination stars.

Psychological and Sociological Issues

In past Analog Science Fiction and Fact articles, I have discussed a number of psychological and sociological issues that affect crewmembers during long-duration space missions.These are reviewed in Table 1 with particular reference to an interstellar mission and won’t be discussed further here.

Table 1. Psychological and Sociological Issues during an Interstellar Mission

Selection issues: Who would want to go? Who would be excluded? What kind of diversity would there be in the crew? Feelings of isolation and loneliness in deep space Earth as an insignificant dot in the heavens—Earth-out-of-view phenomenon Lack of novelty and social contacts in deep space Dealing with monotony and leisure time through meaningful activities and habitability design Autonomy from Earth and over-dependence on on-board resources: computers, machinery Dealing with mentally or medically ill people in a confined space Unknown physical and psychological effects of radiation due to traveling at near-relativistic speeds Starship environment: sustainable resources, artificial gravity, population control Intolerance of diversity: cultural factors, religion, language differences Feelings of homesickness, especially people in the first generation who directly remember the Earth Dealing with myths and folklore regarding the Earth in later generations Keeping the original colonizing goals: rebellion by later generations who want to go back or keep traveling in space, flexible governance Dealing with criminals and sociopaths in a relatively small social network Psychological and ethical effects of social engineering: regulating coupling, birth rate Psychological and medical issues related to suspended animation

Putting crewmembers in suspended animation has been a well-utilized novum in science fiction as a way of conserving resources and dealing with the long durations inherent in interstellar missions. It has been employed in written stories (e.g., Don Wilcox’s 1940 “The Voyage that Lasted 600 Years,” A.E. van Vogt’s 1944 “Far Centaurus” ) and popular movies (e.g., 2001, Alien). In this scenario, after the critical activities involving the launch and the setting of the course for a distant star have been accomplished, the crew would be put in a state where their physiological functions are slowed down until such time as they are near their destination, when they would be “awakened” to perform their landing and exploration duties. This notion proposes the effective cessation of metabolism in the crewmembers due to drugs and/or extreme cold (i.e., cryosleep). Certain key crewmembers could be revived periodically to perform mission critical activities, then go back into suspended animation when these are completed. The starship would be on autopilot during the bulk of the mission, and computers would handle life support and navigation, as well as the revival process.

The problem is that the technology to put an entire human being in suspended animation has yet to be developed, and the process is fraught with difficulties9. Although freezing is used to preserve red blood cells and corneas for transplantation, the ability to freeze and later thaw complete organ systems and whole bodies composed of differentiated cells with different freeze-thaw rate profiles is beyond our abilities in the foreseeable future. Ice crystals can form, which can be lethal to cells, and areas of the body can be deprived of oxygen from blood clotting or premature freezing before metabolism is slowed down. Even the use of cryoprotectants such as glycerol, sucrose, or ethylene glycol presents technological challenges. The thawing of previously frozen cells and tissues presents risks of ice crystal formation and damage as well.

A related idea is to cryopreserve sperm, ova, or actual embryos in liquid nitrogen or via other techniques for later implantation in female crewmembers or in an artificial womb. This would present a possible backup system for fertility problems that might develop in transit to a distant star, or it could be used to increase the colony population after landing on a suitable exoplanet. Such preservation for up to two decades has resulted in successful implantation and birth.

One notion of preserving cells in the human body is through the process of vitrification. In this process, the water in the body and its cells is cooled in such a way that it does not actually freeze. Instead, it is supercooled to a kind of glass-like state where cellular molecular motion and metabolism cease and cell components are preserved in place due to the arrested state of motion. In theory, the dangers of freezing should not be present; however, ice crystal formation and cell damage could still occur during the thawing process.

Even if suspended animation becomes technically possible, problems could still occur. Perhaps there are unknown physical and physiological effects of long-term suspended animation lasting up to a century or more that might result in permanent organ damage or impaired brain function. This risk could be enhanced by power surges or breakdowns of the equipment during this long period of time. In addition, psychological problems could result prior to freezing in people fearful of being incapacitated for years at a time or worrying that some catastrophe could occur, such as a collision or equipment failure. For example, what if a meteoroid hit the ship and negatively impacted life support equipment before crewmembers could be aroused? Computers and other machines are not perfect; the notion of being helplessly dependent on them to maintain your body and revive you later is not a comfortable thought and could create anxiety. Many people would prefer the awake multigenerational option for the f irst space colony mission, since they would be in more control over their destiny.

Exoplanets and Colonization

Planets revolving around distant stars can be detected using several techniques, such as astrometry, which measures a star’s wobble due to the gravitational influences of an orbiting planet; Doppler changes in stellar spectrum due to this wobble; pulsar timing variations resulting from planet-caused gravitational perturbations as the pulsar rotates; changes in a star’s luminosity resulting from a transiting planet; and gravitational microlensing, where the light from a background star is bent by the gravitational effects of a closer inline star with planets10. Often, the mass and distance of the exoplanet from its star can be determined. These detection methods bias the search in favor of finding larger planets, but as the techniques become more refined, more and more exoplanets approaching the size of Earth are being discovered. Thanks to the sensitivity of the Kepler Space Telescope, the NASA Exoplanet Archive on December 3, 2014, listed 1,780 confirmed exoplanets (and 459 multi-planet systems), and more continue to be listed every week as the Kepler data are processed11. Some of these planets are in the star’s so-called habitable (or “Goldilocks”) zone: not too hot or too cold, but at the right distance to have surface temperatures in the range supporting the presence of liquid water, thus making them possible candidates for life. In fact, a recent study found ten Earth-size exoplanets orbiting in their respective star’s habitable zone12. The study results supported the conclusion that 22% of Sun-like stars in our galaxy may in fact harbor Earth-size planets that orbit in their habitable zones, and that the nearest such planet may well be within 12 light-years from us. Nineteen single or double star systems lie within this distance.

Three of these systems are thought to have at least one planet orbiting a star13. In November 2012, an Earth-like star was thought to have been detected around Alpha Centauri B, located 4.4 light-years from Earth. If confirmed, the planet would likely be very close to its star and therefore too hot to be habitable. Work published in December 2012 has suggested that the Sun-like star Tau Ceti, located 11.9 light-years away, may host a system of up to five planets ranging in size from two to seven Earth masses, and that two of these are close to the habitable zone.

A bit nearer to us at 10.5 light-years away, and better studied than the other two star systems, is the interesting system around Epsilon Eridani. With an apparent magnitude of 3.7, this young star is probably less than a billion years old and has a mass of about 80% that of our Sun. It is of spectral class K2 and has an orange hue. A number of components are thought to surround the star. These include: an inner asteroid belt some three astronomical units away (1 AU = the Earth-Sun distance, or 149,597,871 kilometers); a large planet discovered in the year 2000 that is likely 1.5 times the mass of Jupiter and is around 3.4 AU away from its star, with an orbital period of about seven years; an outer asteroid belt some 20 AU away; a more Earth-sized planet about 10% the mass of Jupiter and around 40 AU away, with an orbital period of some 280 years; and a Kuiper belt-like dust disk 35–90 AU away that is relatively devoid of cometary nuclei. There is speculation that other planets exist in the system, especially bordering and helping to form the belts and disk.

Young K2 stars like Epsilon Eridani are seen as good possibilities to harbor planets that support life. This is because they are numerous, are stable for long periods of time, and potential planets orbiting them are less likely to be trapped in a synchronous rotation due to tidal damping than planets around older stars. Although determining the location of a star’s habitable zone is dependent upon many factors, such as the star’s age, luminosity, and f lare activity, as well as assumptions about a planet’s magnetic field, climatic conditions, and cloud formation, a reasonable estimate of the distance of the habitable zone of Epsilon Eridani is around .5 to 1 AU. Furthermore, with a distance of around .5 to .6 AU from this star matching the solar constant and UV flux experienced on Earth, this distance looks promising for any planet found in this location to harbor life. Recently, the Kepler telescope discovered two Earth-size planets orbiting another K2 star (Kepler-62) that is two-thirds the size of our Sun and is located 1,200 light-years away from us in the constellation of Lyra.15 No Earth-size planets have been found yet in the habitable zone of Epsilon Eridani, but should they exist, this would be a good place to look for extra-solar life.

In time, it is likely that exoplanets will be found relatively close to us that are good candidates for colonization. If so, what would such a colony be like? Based on his analyses of thirteen post-migration communities on Earth, Schwartz has conceptualized three typical stages of organization following a migration.16 The first is the pioneering phase, lasting two to four years, where the new settlement may experience tension and factionalism over issues related to physical survival. After food has been provided in a reliable manner, and after permanent shelters have been established, this sense of impermanence disappears. The community now enters into the consolidation phase, where it crystallizes and formalizes its social institutions and associations, and a sense of group solidarity begins to develop. In some colonies, there is pressure to retain the old ways of doing things despite changing conditions, but in others new norms are established and cultural changes occur. As the potential factionalism of the first two stages are dealt with, and ways of resolving disagreements are established, the community enters into the third phase—stabilization—where it continues to develop in ways not directly related to the resettlement. Although initially the settlers may experience a sense of equality with each other, the social class structure of the original migrating group could be reestablished later on. Alternatively, new social interactions may result from the new conditions. In a similar manner, either weak or strong authority systems could occur, largely as a result of the nature of the structure in the pre-settlement culture. In terms of religion, Schwartz outlines three patterns: a simplification of the religious system in the early years following the migration; a rise in its importance as a factor increasing the unity of the community; or as a vehicle for factionalism after the initial period of settlement. How these factors will apply to a new interstellar community is dependent upon the specific conditions and social conventions of the group. Economically, Hodges has written that a newly settled star system community will experience a period of great scarcity of goods, but after basic survival needs are met, and after the population has grown and becomes selfsufficient, the standard of living will improve as industries are established that produce goods beyond the basic necessities.17

Extraterrestrial Life

Could life evolve on a planet orbiting a distant star, especially one like Epsilon Eridani that is less than a billion years old? On Earth, there is fossil evidence that suggests that primitive microbes had developed in shallow ocean environments by one billion years, and that these organisms evolved in many ways, from obtaining their energy through chemical means (chemoautotrophs) to using photosynthesis (photoautotrophs). There likely was little oxygen in the atmosphere at this time, but later on the increasingly wider use of photosynthesis began to change things, as atmospheric carbon dioxide was consumed and oxygen was produced. Irwin and Schulze-Makuch have provided intriguing arguments that under the right conditions, the life evolutionary process can be speeded up as compared to that which took place on Earth, and that such a process could have happened on Mars.18 Specifically, they believe that a billion years would be long enough for multicellular aquatic plants and colonial filter feeders to develop in water environments, and for unicellular extremophiles and organisms living in rock crevices to develop in subterranean and surface environments. With this amount of activity, it is possible that oxygen would have accumulated relatively early in the atmosphere as a byproduct of ongoing photosynthesis. It is unclear how likely photosynthesis would be in the light of a low-luminosity K2 star like Epsilon Eridani. But it should be kept in mind that 4.4 billion years ago, shortly after the Earth was formed, the Sun’s brightness was 25–30% less than today, and that its relative faintness continued for at least another 1.5 billion years. Even under these conditions, photosynthesis- using plants managed to develop and eventually produce oxygen that forms the basis for our existence.

Irwin and Schulze-Makuch further speculate that life could be present in such exotic environments as a watery subsurface on Europa or in aqueous ammonia or liquid ethane habitats on Titan. Alien life has been depicted in a variety of ways living under a variety of conditions, but an exoplanet that has been carefully selected for human colonization will likely have a number of Earth-like characteristics with respect to gravity, a rocky surface, moderate temperatures, tolerable radiation, an atmosphere with oxygen, liquid water, and plant-producing soil. As a result, any life found will likely be carbon-based and require sunlight and water. But even on Earth there are a number of extremophilic microorganisms that survive under inhospitable conditions of temperature, radiation, acidity/alkalinity, and pressure, and some give off methane as a metabolic byproduct. Organisms with silicon-based structures exist, and there is evidence that silicon may have played a role in the emergence of life on Earth.19 So it is anybody’s guess as to what kinds of alien life future colonists will have to deal with.

One possibility is a life form similar to slime molds on Earth, which are very interesting organisms. Some types live as a syncytium of numerous cell nuclei embedded in a glob of cytoplasm surrounded by a single large membrane. Other types typically exist as singlecelled microorganisms that lead solitary lives when their bacterial, yeast, or fungal food is plentiful. However, when food is scarce, they merge together via chemical communication to form a giant amoeba-like organism that is a very efficient finder of food. In addition, in their merged state they adaptively form stalks that produce fruiting bodies that release countless spores to reproduce themselves during difficult times.

In studies where the merged organism is placed on a grid depicting a city like London or Tokyo with its surrounding suburbs, and where food is placed at these suburban locations, the slime mold will extend its pseudopods to find the most direct routes to the food, essentially replicating the city’s efficient highway or railway system. Similarly, slime molds are able to traverse complex mazes in order to find food and to learn ways of anticipating unpleasant cold and dry conditions in the laboratory. This has given rise to the notion that these primitive organisms possess a kind of rudimentary intelligence20.

Conclusions

There are many issues to consider when talking about interstellar travel. Due to the great distances, more efficient propulsion systems are needed, some of which require technology not yet developed. In addition, there is great expense involved, which necessitates a strong financial commitment. The mission will take many decades, and a multigenerational approach likely will be necessary. This will result in a number of psychological and sociological sequelae. Putting some or all of the crewmembers in suspended animation is theoretically possible but practically very difficult. When a distant Earth-like exoplanet is reached, setting up a colony creates its own problems, and if life is found, it may be quite primitive or exotic. Yet, population and climate change pressures at home may lead us in the direction of interstellar travel, not to mention our curiosity of the unknown and our desire to find life among the stars.

2 For good reviews of these propulsion systems, see: Mallove, E.F., Matloff, G.L.: The Starflight Handbook: A Pioneer’s Guide to Interstellar Travel, John Wiley & Sons, Inc., New York, 1989; Kondo, Y., Bruhweiler, F.C., Moore, J., Sheffield, C. (eds): Interstellar Travel and Multi-Generation Space Ships, Apogee Books, Burlington, Ontario, Canada, 2003; Matloff, G.L.: Deep Space Probes: To the Outer Solar System and Beyond, 2nd ed. Springer Science+Business Media, New York, 2005; Johnson, L., McDevitt, J. (eds.): Going Interstellar, Baen Publishing Enterprises, Riversdale, NY, 2012; Benford, J., Benford, G. (eds.): Starship Century: Toward the Grandest Horizon, Microwave Sciences and Lucky Bat Books, Charleston, SC, 2013.

3 Forward, R.L.: Ad astra! In: Kondo, Y., Bruhweiler, F.C., Moore, J., Sheffield, C. (eds): Interstellar Travel and Multi-Generation Space Ships, Apogee Books, Burlington, Ontario, Canada, 2003, pp. 29–51.

4 Semyonov, O.G.: Radiation hazard of relativistic interstellar flight. Acta Astronaut. 64, 644–653, 2009.

5 Strong, J.: Flight to the Stars. Hart Publishing Company, New York, 1965.

6 Woodcock, G.R.: To the stars! In: Schmidt, S., Zubrin, R. (eds): Islands in the Sky: Bold New Ideas for Colonizing Space, John Wiley & Sons, New York, 1996, pp. 183–197.

7 Zubrin, R.: On the way to starflight: The economics of interstellar breakout. In: Benford, J., Benford, G. (eds.): Starship Century: Toward the Grandest Horizon, Microwave Sciences and Lucky Bat Books, Charleston, SC, 2013, pp. 83–101.

8 Kanas, N.: The psychology of space travel. Analog Science Fiction and Fact, October 2009, pp. 33–41; Kanas, N.: To the outer solar system and beyond: Psychological issues in deep space. Analog Science Fiction and Fact, May 2011, pp. 38–43.

9 For a complete and thoughtful review of suspended animation, see: Stratmann, H.: Chapter 7: Suspended animation: Putting characters on ice, in Using Medicine in Science Fiction: The SF Writer’s Guide to Human Biology, Springer Science+Business Media, New York (in press). An older and briefer discussion of this topic is also found in Mallove, E.F., Matloff, G.L.: The Starflight Handbook:A Pioneer’s Guide to Interstellar Travel, John Wiley & Sons, Inc., New York, 1989, pp. 199–205.

10 For recent discussions on detecting exoplanets, see: Coughlin, J.L.: Extrasolar planets: What can be known before going there. J.Brit. Interplanet. Soc. 66, 47–50, 2013; Kanas, N.: Solar System Maps: From Antiquity to the Space Age. Springer Science+Business Media, New York, 2014, pp. 227–230.

11 http://exoplanetarchive.ipac.caltech.edu/index.html.

12 Petigura, E.A., Howard, A.W., Marcy, G.W.: Prevalence of Earth-size planets orbiting Sun-like stars. PNAS, 110(45), 1–6, 11/4/13. http://www.pnas.org/content/early/2013/10/31/1319909110.

13 Matloff, G.L.: Deep Space Probes: To the Outer Solar System and Beyond, 2nd ed. Springer Science+Business Media, New York, 2005, pp. 141–154; Baxter, S., Crawford, I.: Starship destinations. In: Benford, J., Benford, G. (eds.): Starship Century:Toward the Grandest Horizon, Microwave Sciences and Lucky Bat Books, Charleston, SC, 2013, pp. 225–237.

15 NASA: NASA’s Kepler discovers its smallest ‘habitable zone’ planets to date. April 18, 2013. http://www.nasa.gov/mission_pages/kepler/news/kepler-62-kepler-69.html.

16 Schwartz, D.W.: The colonizing experience: A cross-cultural perspective. In: Finney, B.R., Jones, E.M. (eds.): Interstellar Migration and the Human Experience, University of California Press, Berkeley and Los Angeles, 1985, pp. 234–246.

17 Hodges, W. A.: The division of labor and interstellar migration: A response to “Demographic Contours.” In: Finney, B.R., Jones, E.M. (eds.): Interstellar Migration and the Human Experience, University of California Press, Berkeley and Los Angeles, 1985, pp. 134–151.

18 Irwin, L.N., Schulze-Makuch, D.: Cosmic Biology: How Life Could Evolve on Other Worlds, Springer Science+Business Media, New York, 2011.

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