So you give someone an inch and they want a yard. Given them a rocket ship and suddenly they want a star ship. SF writers want to use exotic settings on alien planets, but the real estate in our solar system mostly looks like a bunch of rocks. "That's OK," the writer thinks, "There are a million-jillion other solar systems in the galaxy, surely they are not all a bunch of rocks (I know they are there, I've got a map). I know that those spoil-sports at NASA have ruined our solar system for SF writers since their nosy space probes failed to find dinosaur-infested jungles of Venus and scantily-clad Martian princesses. But they haven't sent probes to other stars yet! Why not turn my rocket ship into a star ship?"
Unfortunately it isn't that easy. The basic problem is that interstellar distances are freaking huge.
The introduction begins like this: "Space," it says, "is big. Really big. You just won't believe how vastly hugely mindboggingly big it is. I mean you may think it's a long way down the road to the chemist, but that's just peanuts to space. Listen ..." and so on.
From THE HITCHHIKER'S GUIDE TO THE GALAXY by Douglas Adams (1979)
Consider: a single light-year is an inconceivable abyss. Denumerable but inconceivable. At an ordinary speed — say, a reasonable pace for a car in a megalopolitan traffic, two kilometers per minute — you would consume almost nine million years in crossing it. And in Sol's neighborhood, the stars averaged some nine light-years apart. Beta Virginis was thirty-two distant.
From TAU ZERO by Poul Anderson (1970)
Let's make a mental model. Say the scale is such that one astronomical unit is equal to one millimeter (1/25th inch). There is a glowing dot for the Sun, and one millimeter away is a microscopic speck representing the Earth. The edge of the solar system is about at Pluto's orbit, which varies from 30 mm to 50 mm from the Sun (about 1 and 3/16 inch to almost 2 inches). Imagine this ten-centimeter model floating above your palm.
This would put Proxima Centauri, the closest star to the Sun, at about 272 meters away. That's 892 feet, the length of about two and a half football fields or four and a half New York city blocks! Glance at the ten-centimeter solar system in your hand, then contemplate the nearest solar system four and a half city blocks away.
And the center of the galaxy would be about 1600 kilometers away (about 990 miles), which is a bit more than the distance from Chicago, Illinois to Houston, Texas.
"All right, all right!" the SF author grumbles, "So the distance is outrageous. What of it?"
This of it. How long do you think it is going to take to travel such distances? As an example, the Voyager 1 space probe is currently the fastest human made object with a rest mass, zipping along at a blazing 17.46 km/s. This means that in the space of an eyeblink the little speed demon travels a whopping eleven miles! That's smokin'. What if it was aimed at Proxima Centauri (it isn't), how long would it take to reach it?
About 74,000 years! Which means that if Neanderthal men had launched something as fast as Voyager 1 to Proxima, it would just barely be arriving right now. And the joke's on them. Neanderthals are extinct so not even their descendants would reap the benefit of any scientific broadcasts from the Proxima probe. A similar argument could be used against any interstellar probes we could launch.
This leaves us with two alternatives: deal with the fact that average human lifespan is 74 years, not 74,000; or make the starship go faster.
Well, three, if you count "faster than light", but that will be covered later.
For arbitrary reasons I am defining an Apocee starship as one which cruises at a speed below 14% of the speed of light (0.14c). This is because that is the speed where the relativistic gamma factor reaches 101% (γ = 1.01). I warned you it was arbitrary.
Overview
PINECONEZ' FIRST LAW
Pineconez' first law:
A society capable of building a successful interstellar generation ship will also be capable of building an interstellar relativistic ship simply by virtue of its tech level.
First Corollary:
Building a perfect, failsafe biosphere (as required for
a generation ship) is not necessarily simpler than building an antimatter-fuelled
torch drive, and (unlike the latter) can't be solved by throwing more
power at it. And this is not even discussing cryogenic sleep.
Second Corollary:
It is not necessarily simpler to build a successful,
interstellar generation ship than it is to build a successful,
interstellar relativistic ship, and the latter is preferable for
almost any use case. (The one major advantage of generation ships is probably
payload.)
From Tobias Pfennings (2015)
CHALLENGES OF INTERSTELLAR TRAVEL
artwork by me
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 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.
8 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
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.
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.
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.
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.
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.
19 Cairns-Smith, A.G.: Seven Clues to the Origin of Life, Cambridge University Press, Cambridge, UK,
1991; Dessy, R.: Could silicon be the basis for alien life forms, just as carbon is on Earth? Scien.Am. 2/23/98,
https://www.scientificamerican.com/article/could-silicon-be-the-basi/.
One must carefully understand the ground rules when speculating about
interstellar travel. Compared to most discussions of vehicles, systems or
capabilities, the ground rules are totally different. In the latter half of this
talk, I'm going to pay a great deal of attention to confining myself to such
things as radiator temperatures which are reasonable, and various other
practicalities. When one sits back and discusses interstellar travel, however,
one talks of not just now or the next century, but of cosmic time
scales. Vast advances in technology throughout the centuries are assumed,
and all engineering problems are assumed solvable. One worries only about
violating physical fundamentals. The more intelligent people worry about
whether we even know what fundamentals to violate, but that makes the story
even more complicated. In general, one talks about grand things. Are there
other civilizations out there? If there are, are the fundamental barriers
due to Einstein's limitations on velocity of travel so great that no. civilization
imaginable could ever hope to travel such distances? Should we listen,
as the radio astronomers say, and hope to learn something from these supercivilizations?
The discussion is always in the context of an overall deep
philosophical sort of thing. That's the context of the first part of this talk.
I will tell you when I shift gears and get rational. Unfortunately, you may
have to be told this.
Figure 1
We can delineate these two regions by means of Figure 1, which is a
plot of specific impulse versus dilution ratio for perfect containment. Fission
rockets are on the lower curve and fusion rockets on the higher. A perfect
mass annihilation system is shown at the top. I've defined several regions
on Figure 1. If we were to operate a rocket with nothing but nuclear fuel
(very low dilution ratio), a very high specific impulse, over a million seconds,
would result. The temperatures are just tremendous, however, and no
one knows how. to begin to handle them. A lot of hydrogen, or some other
propellant, can be put through the reactor to decrease the temperature. The
solid core region down at the bottom, which we're all familiar with, is
limited to a low value of specific impulse because of the temperature limitations
on the solid-core materials. We can get higher performance by going
to gaseous-core rockets or Orion which at least do not run headlong into the
materials temperature barrier. This higher region I have labeled the Solar
System Transport Region, and is the region I'm going to cover in the second
half of the talk. The top region, labeled Early Interstellar Travel, is the
region of the first part of my talk. I'm going to cover only undiluted fusion
rockets. I will not bother with mass annihilation rockets, although people
who discuss interstellar travel are not at all adverse to describing rocket
ships operating with 100 percent efficiency on the complete annihilation of
matter. It's bad enough to talk about undiluted fusion rockets, which I'm
sure you'll recognize we do not know how to build.
Figure 2
Under interstellar ground rules, some very interesting things materialize.
I find that I disagree with a number of basic points which some people seem
to think are great. Figure 2 contains most of my complaints all in one
place. It shows a, curve of initial weight of rocket over final weight as a
function of rocket maximum velocity divided by the velocity of light. This
curve is for what I call a perfect fusion rocket. This means that not only
is the fusion reaction running like mad with perfect efficiency while throwing
only fusion fuel out the back, but in addition the rocket has a reasonable
thrust/weight ratio like one or two. I haven't the remotest idea of how to
build anything like that. still, at least it is something that I am using a
fairly legitimate fusion reaction rather than talking matter annihilation.
The weight variation of Figure 2 was calculated including relativistic
effects. The interesting thing, as we all know, is due to a curve ball thrown
by Einstein. In the region of one-third the speed of light, the rocket initial
weight is about 100 times the final weight. In actuality, we build probes today
with weight ratios in the thousands, so that fusion rockets of up to 0.4
the speed of light are imaginable. From there on, however, they start getting
very, very large. To get very close to the speed of light, the weight
of the rocket becomes ridiculous. Now almost everyone who studies interstellar
travel assumes that it does not make sense until 99 percent of the
speed of light has been attained. You can guess the kind of rocket required
at that speed. I didn't even bother to plot it, and I am almost fearless as
far as plotting rocket weights is concerned.
I, myself, do not understand why people seem to have this compulsion
to examine casually low velocity rockets, then immediately jump to 99 percent
of the speed of light. As a so-called engineer, I've made many mistakes
in my life by taking only one point at each end of a curve and thinking
I understood what went on in between. At one-third the speed of light, the
travel duration in earth time is only three times that at the speed of light.
Of course, we might decide to approach the speed of light in order to reduce
ship time by means of the time dilation effect. This relativistic time dilation
is also shown on Figure 2.
Approaching the speed of light closely is the only way open to physicists
for dilating time. Presumably, there are no narrow-minded physicists
here, however, and we all recognize that there are other disciplines in the
world. One of them is biology. Although I am never quite sure of what is
going on in the field of biology, some pretty weird things have been happening
in the last few years. I get the impression that we are getting closer
and closer, by deep freeze and other techniques, to learning about hibernation.
Hibernation is biological time dilation. With biological time dilation,
it is conceivable not only that one could come clear down to zero time, but
also that this could be both for ship and for some earth time. If your wife
loves you enough, she, too, can step into a deep freeze until you get back.
This brings up a small question as to who has the key to the deep freeze.
Regardless of such practical problems, the point is you can't dilate earth
time by ship velocity, no matter how fast you drive the ship.
The question of whether one is at all interested in ships which travel
at one-third the speed of light, or feel that almost the speed of light is required,
therefore, has a great deal to do with a totally different discipline
from physics. If the biologists do something about hibernation, they will
exert a much greater leverage, both on earth and in the ability to build
reasonable starships, than any possible attempt to drive ships out to the
speed of light. So far as I am concerned, the people that make analyses
with speeds only 1 percent lower than the speed of light, then conclude,
"This is preposterous; we could never go there," are really performing a
pretty naive systems analysis of interstellar travel.
Even at only one-third the speed of light, these are pretty cute ships.
Other than bombs, I'm not sure that this Laboratory has done a very good
job of controlling fusion reactions yet; and this rocket must be light weight,
have perfect efficiency, and be safe. Furthermore, this ship, compared to
one utilizing a gaseous fission engine, must control about three orders of
magnitude higher thermal fluxes in order to keep from vaporizing. In addition,
there is another factor of about four orders of magnitude on total power
generated to obtain these speeds. Because the resulting shielding penalties
are pretty horrendous, the actual payload carried will be a small fraction of
the final weight.
Figure 3
Figure 3 is a plot of the initial power of a perfect rocket with final
weight of 10,000 pounds as a function of maximum design velocity. The
right-hand scale gives the power which would have to be rejected by a radiator
system, assuming 10 percent of the energy soaked into the structure.
Also shown is a typical number for a gaseous fission engine of about 2,500
seconds specific impulse and one million pounds of thrust, the sort of engines
we'll talk about later. For a ship to generate 0.3 the velocity of light, it
must improve three orders. of magnitude or so in its energy handling capability
for the same thrust level. If you did get these reactions running, if
you could understand how to do this at a reasonable weight, we still have
three orders of magnitude of energy which somehow has to be taken in and
out of the structure, or we're going to vaporize the ship right on the spot.
So, even if you could turn around tomorrow and say, "Here' s the engine,"
it's not clear at all that we could use it on these missions.
On the other hand, this is only 3 orders of magnitude, not 30 orders
of magnitude. In any given year, 3 orders of magnitude sounds pretty grim
to us, but that kind of number has been known to be run over in development
programs in a relatively few decades. There are ways in which it
might be possible to cut this number down. Ten percent of energy soaked
into the structure is typical of a gaseous fission engine. An Orion system
does not put as high a percentage of its energy into the structure. Any
case where a fusion reaction would be different from a fission, reaction and
put less energy into the structure lowers the number. When the reaction is
not moderated, then we might have the reaction running in a relatively transparent
engine shell, so that a lot of the energy would go straight through.
If the opaqueness were only 1 percent, that would be an order of magnitude.
I'm not saying that I know even remotely how to begin this. I'm simply
throwing out some suggestions to indicate that from here to there just may
not be centuries, it may be something like decades. Many people throw up
their hands and say, "Forevermore, there will never be any interstellar
travel. It doesn't make any sense." They are saying that forevermore
we're not going to improve our energy control by three orders of magnitude.
I' m not sure that is a suitably cosmic viewpoint.
I couldn't resist spotting the power of the sun on Figure 3. In the
region beyond 96 percent of the velocity of light, the rocket is putting out
more power than 'the whole sun. Once again, it's easy to decide that it's
a pretty preposterous idea — and it is. Although, I don't know; I don't trust
you people. I think maybe a design that would do that might be appealing
to some here.
Figure 4
Now that we have settled the fact that we can have such ships, it
seemed appropriate to present a picture of the whole galaxy as seen by a
star ship designer. Figure 4 shows the number of stars in our galaxy versus
the distance in light years away from the star we're located near now. I
would prefer not to put much of my reputation behind the accuracy of these
curves. The top curve shows the total number of stars. Presumably, a
good astrophysicist, at least for a while, would be interested in a close look
at most any of them. Furthermore, we have reason to suspect that F, G,
and K type stars, considering their rates of rotation, have planetary systems.
They constitute about 5 percent of the stars. They are likely to be
of more lasting interest than stars without planets. This was the basis for
drawing the curve labeled planetary astrophysical interest.
This still leaves the question of contact with an alien race. Since the
radio-astronomers say we should do nothing but listen for the rest of our
lives, the question of the probability of an alien transmission arises. It is,
to say the least, a difficult estimate to make. We have a pretty good reason
to believe that there are an awful lot of stellar systems with planets. We
also have a lot of reason to believe, due to the researches on chemical
evolution, that life would arise spontaneously on most of these. There still
remains the question of the rise of intelligence and the rise of culture.
Furthermore, if a culture reaches the point where it wants to communicate,
how long will it have the urge? Our culture has not been communicating
very long. Over any distance, it's only a few decades and in terms of
written records, only a few millenia. It could be that after another 5,000
years, the human race won't have a scientific culture. We may be living
at the height of the scientific society. Maybe in another hundred years, it'll
all be philosophical and no one will develop anything — a hundred years, that
is! Perhaps our descendants will not care about communicating with anyone.
Even today, there are a lot of people on this planet that I couldn't care less
about communicating with. I might add that this is healthily returned with
respect to me by a lot of people on the same planet.
The bottom curve labeled social interest assumed that life would develop
at each F, G, and K type star, that after 5 billion years it would produce
a society, and that the average society would only be actively interested
in communicating with other civilizations for about 50,000 years. The 5 billion
years is based on precisely one data point; namely, the time required
by our star to produce a society. I've often wondered what will happen if
we get two data points on that subject. The 50,000 years is based on even
less data. If those assumptions are correct, however, the bottom curve results.
It is not surprising that there is a tendency for the radio-astronomers
to say that we should never try to go to the stars. The galaxy is a big
place and there should be plenty of communicating societies, but the nearest
one is a very long ways off. If only currently communicating societies interest
us, perhaps all we should do is listen from here, and hope to learn
something.
I think the astronomers are missing a point, not even counting the fact
that I don't think they know very much about rockets. There is another
class of stellar system which should interest us. This interest is created
because a ship that goes there is, in a way, a time machine. We only possess
deliberate communication records of a society on this planet for a few
thousand years. We have looked hundreds of millions of years into the past,
however, learning things of biological interest such as the patterns of the
development of life. Therefore, if one goes to a place and explores, one
can look both back and ahead in time as compared with the limited real
time contact with any currently communicating society. I don't think anyone
in this room has ever talked to a dinosaur, but we've learned quite a
bit about the age of the dinosaurs over a hundred million years ago. You
may not know whether to bring micro-biolbgists or archeologists, but you
are able to look both back and forward in time. If you assume 500 million
years as the time during which a planet has biological interest based on our
own use of data from a comparable time on this planet, then the remaining
curve on Figure 4 results.
Figure 5
The probable time of data return from the stars is shown on Figure 5.
For travel, it was assumed that the ships would travel at one-third the speed
of light, then transmit data back at the speed of light after arrival. For
communicating, the assumption was that a signal was received from the most
probable distance tomorrow which we immediately returned to this advanced
civilization which then, in turn, sent it back to earth. The travel curves
show data return if you start sending ships tomorrow and the communication
curve is the time for data return if you receive a signal tomorrow.
The receipt of any signal tomorrow from an alien race would be extremely
stimulating, and it is obviously well worth listening. It would seem
that if you stick only to listening, however, it would take 1,000 years for a
reply if we heard tomorrow from the most probable distance. If one travels
for purely stellar physics interests, one can get results much earlier. Even
for planetary interests as well as stellar, the results are earlier. In fact,
within 100 years, information should have been picked up from 15 or so
stars with planets, one or two of which should have data of biological interest.
If one sticks to only listening, another 900 years must pass before
anything happens.
It is apparently fashionable today to say, "Only communicating is the
thing to do. Travel is nonsense, and belongs back on the cereal boxes."
But only the bottom curve of Figure 5 is available to the listeners and
thinkers, while the other curves are available to the 'goers and doers.' I
wish to make a historical point which is true, regardless of what you may
think today in our current intellectual framework. All of the history of this
race is squarely on the side of the 'goers and doers.'