Interstellar Ark

Gilgamesh, 14 February 2007 in Philosophy

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The concept of star travel, from planetary system to planetary system, is at the same time completely familiar and completely uncharted. Familiar, as we have certainly all heard of science fiction stories set on a far galaxy, where planets are nations or provinces of an empire. The characters usually move from one planet to another during intervals of time consistent with the story. The actual travel appears just like a formality, which the future advancements of a Triumphant Physics will put within reach.

This is what I’ll call the “strategy zero” (S0) : here travel is “instantaneous” or at the very least quicker than one year, eg. comparable to the durations of terrestrial travels or manned missions to the moon or other solar system’s bodies.

The way toward stars becomes however quite unfamiliar if we consider that such Triumph of Physics could possibly not happen, and that the famous constant of Einstein c, the speed of light (3E8 m/s), represents an horizon speed which is impossible to exceed and which is even extraordinarily difficult to approach, so that we would begin to see outer space like it is seen by astronomers: a vastness compared to which that of terrestrial oceans is nothing.

It is not without reserve that our mind adapts to the true dimensions of interstellar space. The insanity of these distances is not the only reason: in a sense, one could say that the “strategy zero” is enracined in a child’s desire of space. Not of a space-distance, of a horridly naked space, speechless and fearless, but of a space-treasure, and of the worlds which roll within its vastness. All these worlds whose reach should not suffer any delay and whose discovery turns on our imagination.
Realism helping, we leave with some regret the green paradise of “strategy zero”, but we can still consider a little more “teenager” strategy, within the framework of Special Relativity, which we will name “short strategy” or SI, which promises a travel duration within a man’s lifetime.

Short strategy : the relativistic rocket

In SI, which is specifically relativistic, one benefits from the contraction of traveller’s proper time (\tau) when its velocity approaches c. If t is the time for a stationary observer,

where \gamma is the Lorentz factor,

When v/c approches one, \gamma tend toward infinite and \tau tend toward zero. In other words, by approaching the speed of light the traveller’s time runs slower and slower so as one light-year may be traversed in less than one year of traveller’s proper time. This is the principle of the relativistic rocket. The only theoretical limit is the acceleration, which should be kept within physiologically acceptable limits for a human, that is to 1 g or 9,81 m/s².1

The table below gives an idea of the travel durations and the reachable distances depending on the v/c ratio, by constraining acceleration to a constant value of 1 g (this constraint is extremely demanding as we will see):

ly = light-years (1 ly ~ 10 000 billions km)
1 g = acceleration of 9,8 m/s per second, measured in the traveller’s reference frame
\tau and t in years, d in light-years, v/c and \gamma dimensionless

Thus, in just 12 years of proper time (but 113 243 years for the stationary observer), which is a long time but still bearable in a comfortable spaceship, one could traverse the whole Galaxy, whose diameter is 100 000 light-years. But this still requires to approach very closely the speed of light. It should also be considered that, if one wants to arrive at destination at zero speed, it is necessary to reverse the direction of thrust at midway in order to slow down; so that the travel duration is appreciably twice longer, which still remains reasonable. The total travel time (proper time of the traveller) to arrive at zero speed on a target located at D years light, while accelerating and decelerating at the constant rate of 1g in its reference frame is:

\tau = 1,94 arccosh (d/1,94 + 1) years

For d = 100 000 ly (Galaxy extent), \tau = 22.4 years. Thus, if we consider travel duration, SI allows to reach targets as remote as wanted within durations not exceeding human lifespan. It is on this basis that we define this strategy : duration of terrestrial travels (1 year) < \tau < human lifespan (less than 100 years).

Energy considerations

The difficulty of SI is about energy. All happens as if one must pay on the side of energy what one did not spend on the side of travel duration. Let us consider the most favorable case. The propulsion is more effective when one ejects behind oneself the lightest possible projectile at the highest possible speed. The absolute optimum is thus reached when all the fuel is converted into photons (zero mass) collimated behind the spaceship. The only reaction allowing 100% conversion of fuel into photons is the reaction matter-antimatter. It would be then needed to collimate the photons in a gamma laser beam (“graser”) in the ideal case. Neither the antimatter fuel, neither its combustion, nor the gamma laser collimation are within reach of our current technology, but this sets the theoretical maximum. The ratio of the total fuel mass (matter + antimatter) M0 on the spaceship mass M is in this case :

with a = 9,8 m.s-2 = 1,02 ly.year-2
c = 3e8 m.s-1 = 1 ly.year-1
\tau in years

To reach the other end of the Galaxy (\tau = 22.4 years) it would be needed to take 10 million tons of fuel for each kilogram of spaceship structure. This is the absolute theoretical minimum, based on a propulsion efficiency of 1 (which cannot never be reached, as the reaction would produce neutrinos carrying part of the impulse in all directions) and which does not take into account the energy costs of the antimatter production. Antimatter must be produced in particle beams accelerated by classical means. For fundamental reasons (conservation of the baryon number), the maximum theoretical conversion rate is 1/2. In practice in current accelerators it is much lower, about 4E-8 (that is, one antiproton produced every 400 million collisions). One can reasonably hope to gain 3 to 4 orders of magnitude but nothing more can be foreseen beyond such technological horizon.

The SI strategy, realistic on the level of travel durations, quickly ceases to be realistic on the level of energy requirements. Of course, when considering the alternate “long” strategy (SII), one should keep in mind that SI-SII form a continuum, and that what is required is to find the intermediate optimum. In particular, the above scenario is a limiting one which would be not considered, as requiring a constant acceleration of 1g throughout the way is an extremely expensive condition in term of fuel consumption.

In a more general case, where a period of non-accelerated flight is allowed before decelerating, and without reaching the limiting case of an ejection of photons, the ratio of the departure mass M0 (structure + fuel) on the arrival mass M (structure alone) is calculated as follows:

\frac{M_0}{M}= \left[\frac{1+\frac{v}{c}}{1-\frac{v}{c}}\right]^{\Large{{\frac{c}{v_e}}}

with c the speed of light
v the coasting flight velocity (which is also the maximum velocity)
ve the fuel ejection speed (ve < c)
The above mentioned optimum should mean determining the energy minimum allowing to reach a potential stellar target. Such minimum is determined according to the technological and political level of humanity, and there is of course a possible interaction between the goal and the actors. One can suppose reasonable that the interest expressed by mankind for its galactic environment will imply a "passage to the act" as soon as the energetic barrier problem is considered solved at some point, within the first target falling in range within this barrier. And this, even if the duration of travel is very large. This is because, unlike for the energy barrier which has no maximum, the time barrier has a kind of plateau at some canonical duration which one may set equal to about a century. If a single man is able to consider without regrets to live his whole life in a spaceship on the way towards stars, then time is no longer a barrier and the only constraint is the energy needed to build, drive and maintain the spaceship structure. It is this temporal plateau which makes it possible to lower the energy barrier, and the long strategy is based on a multi-centuries duration.

Scarcity of planetary systems

There is another aspect than energy to take into account, which pertain not to astronautics (rockets, engines…) but to astrophysics and exobiology. It is the planet-target or rather the entire stellar system target, including its small gray bodies (asteroids, comets).

One can only accelerate small structures at relativistic speeds, given the magnitude of the M0/M ratio allowing to reach such speeds. Small structures which should nevertheless accommodate as more human beings as possible, so as to ensure sufficient genetic diversity, that is at least 1,000 people.

It would be possible to lower the structure requirements in the relativistic case by hibernating an appreciable fraction of the occupants.

But this would imply the strong requirement to find a target planet immediately habitable without terraforming.

On at least a few hundreds contiguous km², one should ensure of the following minimal environmental conditions:
- Gravity: 0,5 - 2 g
- Annual amount of radiation: < 100 milliSievert
- Atmospheric pressure: 0,5 and 5 atm
- O2 partial pressure: 0,1 - 0,5 atm
- Temperature : -50 and +50°C
- Presence of water at surface or sub-surface
- Absence of pollutants

To reach Earth’s standards, we should add:
- diversified climatic systems based on the water cycle
- liquid water oceans
- stellar spectrum at ~6000°K
- lenient ecosystem

In addition, planning a short strategy to reach a very distant body implies to reach a target which one knows only remotely. Because if it is question of sending automatic probes to explore the system beforehand, then they should also move at relativistic speeds while the future travellers would be at rest, waiting for information to return by radio transmission. But since the target is located beyond a few light-centuries, the latent period (a millennium for a target located at 500 ly) exceeds the travel duration to a closer system in the long strategy, even if the target is less viable, we will see why.

Concerning the frequency of livable planets in the stellar systems, next decades should bring much information which we await with much impatience. But it does not seem presumptuous to me to predict that this frequency will be found low, when considering the requirements enumerated above. Consequently there could be very little chances to find some in the vicinity, say to less than 20 light-years. The number N of stars located at a distance R from the Sun is:

N = bR3
b ~ 0,017 for R<250 ly
R in light-years

The star spectral type (i.e. its surface temperature) should be similar to that of the Sun (G2), which restricts the potential targets to types F, G or K, accounting for approximately 20% of nearby stars. We may add that about half stars belong to a multiple system, which is less likely to host stable planetary orbits.

Let's set to 10% the ratio of FGK systems tolerating the presence of a telluric planet in a stable orbit at the right distance from the star.

Nhab = 0,1 N

That gives a series of sample values for N and Nhab for increasing distances in light-years from the Sun:


We can see for example that if less than 1% of stellar systems include a planet of terrestrial morphology, then there is little chance of finding one at less than 40 light-yearsl.

Even by assuming exponential advances in observational astrophysics, when one can expect that a purely radioelectric observation of a remote system, located at hundreds, thousands or tens of thousands light-years could deliver completely satisfactory information allowing to predict the possibility to colonize it “right from landing”? And this, particularly with regard to its ecosystem. There is undoubtedly not much to fear from the very large (wild animals…) or nano (viruses, requiring compatibility of genetic systems). But mico-organisms of bacterial or fungic type require a simple organic substrate to develop. Such risk remains reasonable, but it counts in the total incurred risk. Everything can happen, and everything should be considered by the travelers, in a much more dramatic way than this reflection may do. A colony reduced to the minimum in a vessel itself minimal, is delivered feet and fists tied to the slightest unforeseen, without hope of any terrestrial help, not even moral. How can one imagine more unforeseen than such a first way out of the solar system? What would happen if the 20 planned years would result in 200 years of confined life?

From this, one may conclude that humankind cannot reasonably venture in the surrounding immense spaces without being rigorously autonomous and detached of any calendar, with the only exception of the energy requirements.

The long strategy aims this autonomy. The energy needed for propulsion at “low” speed (0.015 c) and for the maintenance of a large structure, comparable to a micro-planetary body of gigaton mass, for a duration close to a millennium is comparable with that necessary to the propulsion of a thousand times less massive body at relativistic speed (0.9 c), which supposes ejection of great amounts of fuel at a speed very close to c (say 0.99 c), which is at the borders of our technological horizon. The long strategy constitutes the more “classical” of the solutions, thus the least technologically demanding. S0 strategy is based on theoretical advances, even more than technological ones, located outside of our horizon, and supposing that such solutions do actually exist. It can neither be evaluated nor discussed. SI is based on a well established relativistic physics, but whose practical application demands energy sources which are not available when aiming a very distant target in a lifetime duration. Technologically, it needs to achieve an ejection velocity at the borders of our technological horizon. It also implies a small structure which cannot stand a long term autonomy. SII is both safer and located within our horizon of possibility, even if it is not one of this century.

Long strategy: the Ark

The most effective propulsion which may be achieved within our technological horizon is nuclear fusion. The principle is to confine light nuclei at very high temperature to produce their fusion in a very hot plasma, with electric power allowing ejection of plasma in a magnetic conduit.

This principle would allow to reach ejection velocities of 20,000 km/s. In order to be used as a source of energy, a nuclear fusion reaction must satisfy several criteria. It must:

  • be exothermic, which limits reagents to the part of the binding energy curve corresponding to light nuclei, with few protons, making helium-4 (more rarely deuterium and tritium) the headlight reaction product because of its strong binding energy,
  • imply nuclei with few protons because of the need to overcome the coulombic repulsion so that nuclei may approach sufficiently to achieve fusion,
  • have no more than two reagents: for densities lower than those of stars, the production of three simultaneous collisions is too improbable. It should be noted that in inertial confinement one both exceeds stellar densities and temperatures, which makes it possible to compensate for the weakness of the third parameter of Lawson’s criterion, the very short duration of confinement,
  • have at least two products of reactions allowing the simultaneous conservation of energy and impulse,
  • conserve at the same time the number of protons and neutrons. Cross sections for the weak interaction being too small, the reaction p + p -> D is unusable, even if it is the one which takes place in the Sun and is constantly flooding us with energy. Proton half-life (the average time needed by a proton to react with another proton to form Deuterium, starting the feedback chain leading to helium-4) under the extreme conditions of temperature and density in the nucleus of a star (density: 150 g/cm3, temperature: 13 millions degrees Kelvin) is about 10 billion years. This as in order to occur, the reaction requires a beta decay, that is the spontaneous conversion of one of the two reactional protons into a neutron (a purely “weak” phenomenon), and this, at the very moment of the interaction p-p.

Nuclei (or isotopes) available for the reactions are:

(01) 1H or p, light hydrogen or proton, the most usual,
(02) 2H or D, heavy hydrogen or deuterium, present in small quantity (0.0015% in terrestrial water that is 15 ppm), and maybe 10 times more in some small bodies of solar system, primarily as heavy water HDO),
(03) 3H or T, tritium, instable, with period 12.3 years, therefore absent in natural materials,
(04) 3He, He3, helium-3 present as a trace in the lunar ground and the atmosphere of giant planets,
(05) 6Li, Li6, lithium-6,
(06) 7Li, Li7, lithium-7,
(07) 11B, B11, boron-11,
the latter 3 elements being present as a trace (estimated 6-7 ppm) in the small bodies of solar system.

The fusion reactions concerning these isotopes are:

n represents neutron (in bold when it can breed deuterium).
1 MeV : 1 million electron-volts (eV). 1 eV = 1,602E-19 Joules


We may distinguish two kinds of reactions:
- those producing neutrons and gamma radiation: (02), (04), (06), (07), (09). The reaction (12) produces a neutron but should be put aside because it is endothermic. However coupling the two ways of Li-7 (11) + (12) is exothermic (balance: +2,2 MeV) and this reactional way could be interesting.
- those which produce only charged nuclei: protons, deutons, alpha particles (4He) : (01), (03), (05), (08), (09), (10) (11), (13), (14), (15).

The reactions of the first kind may seem at first unfavorable, as neutrons and gamma photons are not sensitive to electromagnetic fields, and so cannot be ejected by a conduit: their contribution to propulsion is zero whereas they carry the majority of the impulsion. Moreover they are very aggressive and “activate” the metal structures. On the other hand, the reactive nuclei are relatively abundant: these reactions imply deuterium in (3). The reactions of the second kind are ideal for propulsion but tritium is not naturally available and helium-3, lithium-6-7 and boron-11 are much more rare than deuterium in the small bodies of the solar system. However the required fuel mass is considerable, about 21 Gt (gigaton, 1 Gt = 1 billion tons) on the hypothesis adopted hereafter.

One possibility is to use the neutron produced to breed deuterium in a fertile layer of hydrogen 1H.

The D-D reaction has two equiprobable ways, (2) and (3). In the first, pn+pn produces pnn+p (a tritium nucleus and a proton), in the second ppn + n (a helium-3 nucleus and a neutron). The produced tritium is then likely to react in (5) pnn+pnn -> ppnn + 2n (a helium-4 nucleus and 2 neutrons). The two neutrons produced in this second stage of reaction may in their turn breed deuterium by reacting with the fertile layer of light hydrogen.

The great technical difficulty consists in -amongst other- not “wasting” impulse when passing from the first reactional stage (D-D) to the second (T-T, or He3-He3). The ejection velocity is the key parameter in determining the engine efficiency, and it is primarily allowed by the high temperatures of the reactions products. A proton at 20,000 km/s has a kinetic energy of about 4 MeV, which well represents the magnitude of the reactions described above. The second fusion reaction must thus occur within the jet of plasma. Whatever is the difficulty, a complete breeding (at 1:1 rate), even with a very slight surplus, represents an absolutely crucial factor to judge feasibility. The fusible isotopes are only present at the state of traces in the small bodies. A complete breeding only requires to take on board small initial quantities, since each gram undergoing fusion breeds one gram in the fertile layer. If breeding is only partial, it is needed before departure to distil enormous quantities of hydrogen so as to take on board a fuel already strongly enriched in deuterium or other fusible nuclei. The mass needed to provide fuel vary between 1 to 5000 between the two.

Another concept which may be interesting to consider, given the vast surface of the engine to be manufactured, is that of the ice rocket:2 frozen hydrogen and deuterium are used at the same time as reactor, conduit of fuel and screen against the products of the reaction.

Choice of the target: small bodies first

Paradoxically, the fact of travelling in a vessel-world makes it possible to be much less selective on the choice of the stellar target, and to get more chance to find one at short distance (on an astronomical scale). It is not indeed necessary to find a livable planet but simply a system including a star of a spectral type not very far away from the solar type (K, G or F) and abundance of small bodies. Of course, the presence of a planet offering a “practicable” surface, of Martian type for example would be very appreciable.

Among close systems, Epsilon Eridani (the system gravitating around the star designated epsilon in the constellation of Eridan) can be most interesting. It has been, by the way, the subject of research with the Green Bank radiotelescope in 1960, to seek signs of intelligent life, with negative results of course.

The system is a close neighbor of the Sun, which is its first criteria of choice. It is located at only 10.5 light-years (3.2 parsec). Ironically, Eridan is the name of the river where fell Phaeton after its disastrous race too close to the Sun. Let us wish to fall there while moving away from the Sun! The orange color star is rather close to the solar type (0.82 solar mass, spectral type K2 V).

Spectral type of the star closest to the Sun. The dotted line indicates the solar spectral type (G2). Epsilon Eridani offers a good compromise  distance - spectral quality
Click the image to zoom

The infrared observation satellite IRAS detected much dust around the star, a possible indication of a planetary system in formation. It is thus very probable that the system abounds in small bodies. In august 2000, a planet of the size of Jupiter was detected at a distance of 3.2 UA (480 million km) from the star, in a strongly eccentric orbit (e=0.702) which makes it inside the ecosphere on a little more than 10% of its short orbital period (2502 days).

The Epsilon Eridani system, showing its jovian planet and the disk of dust
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If this planet has giant satellites, like Jupiter or Saturn, they could represent a semi-habitable rest place for the arkonauts.3

Epsilon Eridani, artist drawing (© Fahad Sulehria) Epsilon Eridani and its planet, artist drawing
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Energy and travel time: the Tsiolkovski equation

The way to Epsilon Eridani will serve as a textbook case to evaluate SII strategy. We will consider a mean ejection velocity of 15,000 km/s. The fundamental Tsiolkovski equation, in its non-relativistic version (v/c<<1) gives the ratio of the departure mass to the structural mass needed to reach a velocity of v with an ejection velocity of ve :

\frac{M_0}{M}= e^{\Large{\frac{v}{v_e}}}

M0 : total departure mass.
M : mass of the structure without fuel
M0 = M + Mc, with Mc the fuel mass
ve : fuel ejection velocity
v : final velocity at the end of the acceleration period
After the acceleration phase, there is a period of coasting flight at constant velocity, then deceleration to arrive at destination at zero velocity. This implies more fuel since we need to accelerate in the first phase a mass of fuel which will only be consumed during braking, which translates into the squaring of the exponential:

\frac{M_0}{M}= e^{\Large{\frac{2v}{v_e}}}

The quantity of fuel determines final velocity and so, travel duration.

We note:
Da : acceleration and braking distances (cumulated)
Dl : coasting flight distance

We define k, the ratio fuel mass/total mass:

We set:

We have Ta, the acceleration and braking durations (cumulated):

Tl, the coasting flight duration:

One wishes the acceleration and braking duration the shortest possible, so as that most of the travel is at the maximum speed. But on the other side an intense acceleration implies a strong thrust and so more massive engines and reinforced structure, to resist the thrust without deformation.

A technology being given, defining the fuel ejection velocity (ve = 15,000 km/s) it remains two free parameters to calculate the travel duration (t), the average acceleration (a) and the thrust (f) : the fuel mass Mc and the acceleration distance Da (it is supposed that acceleration and braking are totally symmetric). In the graph below, we represent the variation of the three output variables (t, a, f ) as a function of the two input (Mc and Da).

Travel duration, acceleration and thrust as a function of the fuel mass and acceleration distance
Click the image to zoom

We cannot yet carry out a reasoned choice of the input values. We know simply that we should if possible maximize Mc and minimize Da. Mc consists of fusible substance, a rare material (undoubtely deuterium for the most part). Of all the parameters which condition the feasibility of a gigaton-mass Ark, the mass of the fuel to be extracted from small bodies is undoubtedly the one which poses the most acute problems. The illustration below represents an alternative solution, a photonic sail, making it possible to reduce the structure.4 Propelled by fixed laser of very great power, for example installed on the Moon, the Ark would only take on board the braking fuel. Even with its immense surface, the sail insolation reaches more than 1000 times the solar constant in terrestrial orbit (1400 W/m²) : surface must be perfectly reflective in order not to be evaporated by the receiving power. Also, the Ark would depend on an external source which it does not control.
Arche - Voile

Click the image to zoom

For the following, we will adopt the solution of a completely autonomous acceleration with the illustrative ratio M0/M necessary to reach 1.5% of c, that is 4500 km/s. This requires a total departure mass of 46 Gt (25 for structure and 21 for fuel). For a cumulated acceleration and braking distance of 0.5 ly, one obtains an accelerated flight duration of 67 years (34 years for the acceleration and as much for braking), and a coasting flight duration of 667 years, that is a total travel duration of 734 years, to traverse 10.5 ly separating us from Epsilon Eridani.

The Ark structure, its philosophy in brief

The long strategy is based on the construction of a structure, the Ark, within which a small population, the Space Nation, could live an independent existence. This structure must allow a life at the same time completely free from Earth, organized as a whole nation and with enough diversity on all plans of interaction under which we consider the individual life.

When one wants to represent oneself physically what the Ark would look like, three preliminary constraints prove strong enough to define its general architecture.

a) Artificial gravity
The Ark must allow a normal life, according to terrestrial standards, and the first requirement relates to gravity. The acceleration of gravity results from the enormous mass of the Earth (5.97E24 kg) and it is obviously impossible to recreate it in this manner. The only alternative solution is to circularly accelerate a cylindrical surface inside which the habitants take place. The acceleration g is then:

g = \omega^2R

g the acceleration in m.s-2
\omega the angular velocity in rad.-1
R the cylinder radius in m

The acceleration g is set equal to Earth’s, that is 9.81 m/s². Ark radius, discussed below, is 5 km. So:

\omega = \sqrt{\frac{g}{R}}

\omega = 0.044 rad.-1, that is a revolution period of 2 min 22 s.

The structure being of kilometric dimensions, the concerned mass, both for structure and fuel, will be important. Subjecting a mass to an acceleration produces a force, so as the structure needs to be reinforced so that it resists its own weight. Concerning the habitable structure, there is no choice, but concerning the fuel and the engine, one may find it very beneficial not to make them turn with the structure, so as they remain in weightlessness. The cylinder axis offers naturally a space of weightlessness: acceleration depends linearly on distance from center. For R=0, g=0. So, all the engine part should take seat in the center, or Hub of the Ark cylinder.

b) Thrust surface
However, doing a preliminary calculation of the necessary surface of the engines (as each engine is only able to provide a finite and small thrust) we find that is must be far higher than the section of a 5 km cylinder. It is thus necessary to couple to the Ark cylinder a vast thrust surface which should not rotate in order to avoid a structure overweight, while transmitting its thrust to the whole structure.

A first solution would be to place this corolla in the form of a vast ring and to connect it with cables to the cylinder hub. In this approximate version, the Ark would be drawn like princess Elisa by her eleven brothers transformed into swans in the Andersen’s tale of the “Wild Swans”.

Elisa drawn by her brothers, the Wild Swans in the tale of Andersen © Susan Jeffers

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But if the eleven brothers had all softness for their young sister, it would emanate from the corolla a plasma blowing to ten thousands km/s as well as a flood of gamma photons, all very aggressive and extremely detrimental to any structure located under the flow.

The corolla will thus be placed behind. One should imagine the Ark as a motionless disk, the corolla, connected to a turning cylinder, the livable structure, located in front.

The architectural challenge is to transmit to the cylinder the thrust of the disk, without this coupling implying the rotation of the disk. Without also making the cylinder ‘dance’, which would occuper by gyroscopic effect if the thrust axis does not strictly merge with the revolution axis. The coupling of the two elements can only be done in one point, and this point must be located at the exact geometrical center of the Ark. Application of thrust in a single point of small surface, the central stage (diameter ~ 25 m), makes it possible to limit friction which would insidiously transmit the rotational movement of the cylinder to the corolla.

It should be added to that that the disk is not made of very rigid elements. It is essentially made of not very cohesive hydrogen ice. Such a surface cannot work in shearing. It would be the case if the corolla were to directly push on the cylinder by its center. A structure doubling the corolla, rigid enough to collect the thrust on all surface and to work directly in shearing should be extremely massive. The general rule for structures of very long range it is that a load-carrying element working in compression (as the walls of a house) is much more massive than a load-carrying element in traction (such as a cable), for an equal stress. And this more especially as the range increases. This is why it is difficult to build very high towers on Earth. Here, there are no other choices that to make the corolla work in traction, according to fasteners regularly laid out along its perimeter and its rays, by staying it to a beam which will transmit the thrust to the center of the Ark, via a bearing. This beam becomes the only element to work in compression.

At midway, we remember to reverse the thrust, so as to arrive at destination at null velocity. To turn around a structure of this size is as much less easy if its moment of inertia perpendicular to the axis of rotation is large, which is the case. It is simpler to have a second corolla, symmetrical with the first, on the front side, so as the braking jet may be correctly directed to the target when needed. The tick cushion of hydrogen ice which would form on the behind would secure the Ark from collisions with small interstellar bodies, which is very rare but devastating at a velocity of 4,500 km/s.

Ark - Draft

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c) The mass of the structure
The dry mass of the Ark as discussed here represents some 25 Gt (gigatons) or 2.5E13 kg. It would be necessary, of course, to discuss the basis of this estimation and the parameters which can intervene to reconsider it. But in any case, one has a presentiment that for a kilometric structure under tension it must be question of “gigatonnic” mass. A take-off of such structure from the Earth would be whimsical. The gravitational well is too deep and the body should be unreasonably reinforced to resist to the departure thrust. Construction will have thus to be done entirely in space. But even then, the contribution of materials neessary for its construction could not realistically come from Earth, because of energy requirements. The extraction would be done preferentially from small bodies of the solar system (asteroids and comets), whose gravity well is tiny, then conveyed in terrestrial orbit. The structure of the Ark is much larger and massive than any human artifact ever considered, and it is also that which should remain intact over the longest duration, with absolute requirements of resistance and sealing. To satisfy only one of these two requirements would require a new reflection. It is a fortiori the case when both are joined. With some exceptions, of which the structure of the first aircrafts in wood and textiles, all aereonautical structures are metalilc. There exist in the solar sytem a rather large abundance of small metallic bodies, the asteroids of type S which account for 17% of all known asteroids : even concentrating on the only light metal elements (Al, Mg, Ti…) abundance is not a problem. But to conceive a structure entirely made of metal by conventional means is hardly possible, for several reasons. Metals at native state are in the form of oxides (combined with oxygen: XnOm) and their reduction (to make them pure metals) requires high temperatures or intense amperages, which implies mass production of electrical energy. Their shaping and assembly are also energy-expensive and claim much care. They are dense bodies with a rather modest ratio of resistance in tension over specific mass. They are subjected to a phenomenon of tiredness (dislocations in the crystal lattice) which rigidify them and lead to the formation of cracks. They are oxydable in various ways, while at the same time the Ark’s interior is erosive (seasonal atmospheric cycle, humid air, oceanic layer..).

Considering by contrast that the chemical elements which mainly make the small bodies of the solar systems are lighter than metals, and that one seeks a light structure, and considering in addition that even if we are not the authors, thanks to Evolution we have in Plants an immense natural know-how in the construction of fibrous structures both resistant and self-sustained on the basis of light atoms tied to solar energy available in abundance, we come to imagine that the structure of the Ark could be built by natural growth, rather than by construction, with vegetable fiber walls. The components are as said, lighter and more abundant (CHON), they offer an excellent ratio resistance/weight and are of very safe design (they “prevent” before yielding). Especially: they are regenerated, that of which no traditional structure is able. It may seem iconoclast enough to put a ‘vegetable’ in the spatial vacuum. However, the only thing to ensure is to isolate the living cells from this vacuum, and here too biological operation can take care of it by production of a cohesive skin of dead cells in a rubber-like matrix, over some decimeters. The other advantage relates to the construction: engineering simply resumes to nourish an alive structure with simple elements taken from small bodies: H2O, CO2, nitrogen, phosphorus… The structure grows on interior orbit, by using solar energy, from an embryonic stage to kilometric adult dimensions in two or three centuries. During this lapse of time, it is habitable by its host builders. On the interstellar way, it is then necessary to provide the energy of maintenance in luminous form.

The Ark’s biosphere is made of an oceanic film, of a depth of approximately 25m, on which float ballast supporting a thin soil.

Ark - Cut Ark - Hub Ark - Ground
Click the image to zoom

An entire life in the Ark ?

Can one seriously consider a normal life, accomplished on all the plans, within an artificial structure far away from the Earth? Could we do it by ourselves and could we imagine without quivering a line of generations living there, which we would be the ancestor? This prospect undoubtedly constitutes the most immediate psychological brake, but not inevitably the deepest, that every normally made human being will oppose first of all to the idea of a life in the Ark.

To approach this central point, we will use of a concept that one could call the individual horizon, which parameter is a radius defined as the depth of action of the individual, on the level considered essential to an existence deserving to be lived. On all these plans, one seeks the minimum radius for which these requirements are satisfied, if it is not fully at least by combining them when it is possible.

Horizon of visual space: Dimension in which the radius is expressed: extent of the land which can be seen at a glance. It is the first signification apperception of the space, and it dimensions the whole project. On Earth, which is the radius of our visual horizon? From 1 to approximately 10 km depending on landscape. This defines the typical dimensions of the Ark. Concretely, the glance goes sufficiently far without giving the impression to feel at narrow. On the ground, a thickness of 1 to 10m of topsoil and rocks is enough.

Physical horizon of circulation: Dimension of the radius: the surface or volume which can be explored by an individual. On Earth the glance goes from 1 to 10 km, but we can move within a far bigger space, like thousands or even millions of km². From this point of view, it appears frankly impossible to reproduce what potentially offers us the whole Earth. However few individuals on Earth realize such potential in their life. A majority of people, considered in the space-time of their existence, their whole horizon of circulation does not exceed a few hundred km².
One can add to this that the radius of this horizon may be appreciably increased if there is great diversity. A thousand km² of desert do not offer the same radius of action, on that level, than a hundred km² in which one would find a city, a forest, fields, a river and any other elements of landscape diversity. One would want to maximize from this point of view the diversity offered by the natural environments of the Ark by reproducing the main part of the great terrestrial ecosystems.

Moreover, an artificial structure as the Ark offers by its design several levels, going from the center towards the periphery, while terrestrial surface is presented as purement bi-dimensional, without thickness to be explored with the exception of sea-beds or cavities. These various levels offer a completely new diversity unknown on Earth: micro-gravity spaces, spacewalk on the central beam or in galleries within the fuel hydrogen ice, oceanic background and other surfaces and volumes which will be detailed when discussing the structure. All places in the Ark, in surface as in volume, can be designed to be accessible at walk. Some will be very diversified, others rather monotonous. Together, they offer a very large radius of circulation. The maximum visual horizon being about 10 Km, one can start to consider on the same basis the habitation area. It would be a cylindric surface of length L=10 Km on a diameter of 10 Km (or a radius of 5 Km). The habitable surface A0 is:

A0 = 2πRL

That is A0 = 314 km², representing the primary radius of the circulation horizon, something like ‘dry land’, offering conditions of existence in all points comparable to terrestrial standards. It is difficult to rigorously quantify what the spaces developed on the basis of this primary radius represent, since surfaces and volumes are concerned at the same time. Without claiming to give anything other than an estimation, one can however ensure that this radius will be multiplied by ten. In magnitude, an Ark whose visual space horizon is set to 10 km should offer a space of circulation comparable to a space on Earth that a horse can traverse during one day.

Of course, even if one does not feel at narrow in such radius of circulation, it could look very thin when one imagines – and how not to do it - about the terrestrial immensities. But two plans are then confused. Terrestrial space, let us repeat it, is only potentially available to us. Only a negligible minority of people happen to travel from one pole to another or through all timezones. And even the great travelers never explore more than what is within reach of their steps. When we go to China, which we explore best is the seat of the airplane taking us there. And it would be specious to affirm to have been “crossing Japan” because one flew over it at 10,000 meters of altitude. And when one visited China, the actual radius of circulation was by no means comparable to the size of this nation. It was summarized to some visited places, some natural curiosities, a city or two, and within these cities a few remarkable places, with our hotel room probably the best explored place of all the tour, said without any irony. The real circulation space is not indexed to terrestrial vastness but to time. That is the actual criterion. The fact of potentially having an immense world is not at all negligible. But that is a moral aspect which should be considered separately. It may seem important that a vast world is available where to carry our steps, but if one adds up over our whole lifetime the spaces where one will have been able indeed to note our presence, it is probable that they will not exceed the radius of circulation under consideration for the Ark.

Horizon of social interaction: Radius dimension: number and diversity of the arkonauts population. The term arkonaut designates the inhabitants of the Ark. To reach the minimal threshold of diversity, it is needed in other words that one can during his whole life meet people who he never met before. It is also what one could call the threshold of anonymity: while walking in the crowd, one meet unknown people in proportion as large as known people. That corresponds to what happens in a small city, that is a population ranging between 10,000 and 100,000 inhabitants, with an average set for the convenience of this article to 50,000 inhabitants.

Comparing to Earth, Ark should carry a maximized human diversity. However, it is undoubtedly necessary to imagine that the settlement is done primarily by natural growth. The initial settlement, carrying out the way Ground-Ark could include as few as 2,000 families or 5,000 people, adults and their offsprings, knowledgeable in fields useful to construction and maintenance of the structure and the engines. And for the successor, a “meritocratic” ascent formed of volunteers selected for their social utility within the framework of the project. The time of acclimatization and demographic balancing, including the possibility of a return to Earth, will undoubtedly be higher than a century.

Horizon of social activities: Radius dimension: diversity and intensity of the activities constituting the reason to act at the collective level of individuals. An Ark carrying a branch of humanity towards a nearby stellar system, on secular durations, would structure around two primary functions: ensure propulsion of the engine, and maintain the life.

Propulsion: it consists of two symmetrical phases, acceleration and braking. Between the two, the Ark is in coasting flight at maximum speed. As one wishes to carry out the maximum of the way at this maximum speed, in ordern to shorten it, one will seek to reduce as much as possible the duration of the phases of acceleration and braking, and the coasting flight will have to represent the major part of it. Also the strictly propelling function should only concern a short segment of a few decades before departure and preceding arrival. However, the competences concerned with the propelling function preserve an essential preroagative during coasting flight: provide the energy needed to maintain the life in the Ark, that is for the main part the solar energy (when the term does not carry confusion, we will use it to indicate luminous energy) diffused within the structure and allowing life of the ecosystems, as well as the maintenance of the “thermodynamic machine” regulating Ark climate. The energy used by the anthropic activities (industry, transportation, domestic activity) is included, representing a negligible sub-total. Physically, this function is achieved in the center of the Ark, in what one will indicate as the Hub, in a microgravity area, as well as in the Beam and within the corolla, in weightlessness (or almost, since the mass of the structure will create a natural gravity of a millionth gram). One will include in the activity of the Hub all the industrial activities which it may be interesting to perform in low weight.

Life: it is at the same time about the interior ecosystem of the Ark (content) and its walls (container). The Ark is a closed system with a short cycle if compared to those known on Earth. Given the immediate cause/effect relationship between the global functioning of the Ark and the existence of its inhabitants, one can predict that those will develop particularly sharpened skills. The Ark being globally, and totally, an alive being, it represents a source of constant interactions for its inhabitants, either if it question of taking care of it or to be looked after from it. The nature of the “propulsion-life” bond represents something philosophically stimulating, because of the “mise en abyme” of the action which it founds. The Ark shelters man, and man injects energy making it possible to the Ark to live. That resembles the endosymbiose joining together the eucaryotes cells and the mitochondries. It is about a relationship of total mutual dependance, in a conscient form at one part (asymmetry founding a total responsibility) and which seals a unity of destiny all the more intensely felt than an alive being is totally master and totally slave of an another. Interesting dialectical. On the level of individual moral construction founding political systems, the small number of the population is an asset to develop an “Athenian” democracy without political representative body, which increases the richness of social interaction of individuals, insofar as a direct and not mediated power is exerted.

Genesic horizon: Radius dimension: capacity to generate. A very delicate question would be that of demographical control. The Ark being a closed world largely optimized, there is no question of leaving the simple natural growth to control demography. One does not need either to set excessive constraints, population being able without any doubt to grow of up to a factor of 3 without consequent damage. However, the question would be obligatorily set for a so long duration that the balance between personal freedom and collective interest should be solved theoretically. Within a democratic framework one can imagine the following social pact. The first child would constitute an indisputable right, that each woman would concretize when she would like it, with the simple duty of declaring it to allow for demographical planning. The extension of the family to two children would then be decided at random depending on the balance requirements formulated by demographic projections. Concretely, each woman would declare each year their “child project” for that year, in yes/no form, and for the whole of their fertile period (I would desire N children in all). That would be a simple declaration, revisable without notice and nonconstraining. One would draw a demographic projection from it, from which an opinion would result, in the form of a random choice. Each troop of women (a troop is defined by individuals of the same age) would be entitled to the random choice. The randomized names would depend on the wish expressed for the year (if a woman desires a child on that year, his name would be in the list to be randomized) and each name would be weighted in proportion of their parental project already carried out (a woman wishing 3 childrens and not having any would have more rights than a woman wishing 4 childrens and already having 3). The remaining question, subjected to the political appreciation of the arkonauts, would be to determine the applicable constraint (if any) when a woman falls pregnant when the random choice had not granted child to her. If the wishes not carried out by other woman in the troop balances it then all is well. The question takes a more delicate turn if the total indiscipline of a troop would burden the procreative right of the youngers. At worst one can imagine a penal sanction, but it is foreseeable that, like all social constraints originating with a very comprehensibile need, education would suffice so that things occur orderly or almost.

Spiritual horizon: Radius dimension: intensity and diversity of spiritual life, intended as the whole of the activities mobilizing cognition. A way of measuring the radius of this horizon could be to evaluate the depth and the richness of the materials available to mind to think about reality and itself.

Compared to what the Past can deliver, the Ark would embark the whole of terrestrial memories which should represent some 1E20 bytes in magnitude, that is the whole of what is currently registered on paper, magnetic or optical media, with or without repetition, everywhere and in all languages. The Ark will also have access to a “differed present” thanks to the radioelectric link with the Earth, all the more differed since the Ark is moving away. A laser link could be imagined, so as to minimize dispersion, in the infrared band which would get less noise from the plasma emissions. For a laser power of 1 MW at lambda = 1 micron, the bandwidth would be of magnitude of 10 MB/s at 1 ly and 100 KB/s at 10 ly, similar to an average internet link.

Compared to the Present, the spiritual horizon is identified to the horizon of social activity previously discussed. Arkonauts live a morally new situation, in lived and in long-term aims. They should develop an original mentality. The need of getting along would come first. The word trajectory reminds of “transient”. But such transient is a whole life and a whole civilization, with the result that the stellar goal will become almost secondary. The majority of individuals populating this travel will belong to a generation “not leaving, not arriving”. For this majority, the end of the way would just appear like a distant future. Certainly, in background the reach of targets will structure the community, but the stake making the happiness of the travellers will remain for the humankind its present.

Compared to the Future, the colonial objective will require a thorough reflection of what will have to be done once arrived at destination.

Either the target planets present conditions of existence allowing a settlement “naked head”, either they require terraformation. With the first case is associated a weak probability, as previously said. It is thus rather necessary to imagine a life of orbital two-ways travels between an Ark forming a comfortable base camp and life on surface in protected conditions. A terraformation represents a long-term task, exceeding individual lifespan, and arkonauts would live again what lived their ancestors who built the Ark, with this only difference that for the ancestors the base was a planet and their future horizon the Ark, while for the newcomers the base will be the Ark and their future horizon, the planet which becomes gradually livable.

To the multi-century duration of the task, there is a real ethical problem, which arises in all cases where the target planet is not deprived of life: what to do with indigenous life ?Of course, one does not imagine to colonize a planet populated by beings morally equivalent to the human being. But if the planet is ready to receive a form of life based on the chemistry of carbon, then it is possible, to differing degrees, that this one already developed at its surface or in sub-surface an original form of life without continuity with the forms of terrestrial lives which contain the Ark. The idea to sterilize a biosphere appears at the very least monstrous. On this assumption, it is necessary to imagine the existence within a mixed ecosystem.

As a conclusion

The travel duration, more than seven centuries on the adopted hypothesis, constitutes the most striking characteristic of an interstellar travel led by classical means. The most immediate objection which comes to the mind is that it would be perhaps wiser to wait that Physics makes sufficient progress to authorize interstellar travel in “decent” durations. And partly, history of physics seems to encourage us there. Few fields of knowledge indeed progressed at a faster rate than physics over the last four centuries. But on another side, in fact precisely the already achieved progress, crystallized in a solid building, shows well and to the point that crossing of immense spaces is expensive, either in time or energy. Perhaps is physics mature enough so that we should take it seriously as of today? To abolish time while spending immense quantities of energy does not cost anything to imagination, but nevertheless, energy keeps a rare good. Would not be time to drop sterile dreams to embrace fertile dreams? It is on this bet that this reflection was carried out. Energy is for humanity an exogenic good which must be extracted to the Universe, it is a conquest. As the fable says, “One fights for everything he swallows”. Humanity however has its time. It renews itself with its generations. It does not require, to last without effort, of nothing else than a reconstituted environment and a modest amount of energy to maintain it. And yet, even in the long strategy, one need an insane amount of energy. 99% of fuel disperse in the Great Outside and propels the Ark, 1% only feeds the sun shining on such a world.

At the scale of the beginning of XXI century, the Ark is a project at the border of fantasy. Everything appears disproportionate, whether it is the amount of fuel or the dimensions of the structure. But to set the real height which is necessary to cross, to set masses, lengths and energies, to determine the level of technological advance of the travellers, draw the broad outline of a society to come, all of this, even if future would not retain nothing of it, may help the birth of a future.

  1. For more details check []
  2. J. Post, “Hydrogen ice spacecraft“, AIAA, Space Programs and Technologies Conference, Huntsville, AL; (1990) []
  3. A simulation of the system can be seen here: []
  4. G. A. Landis, “Small Laser-propelled Interstellar Probe“, Presented at the 46th International Astronautical Congress, Oslo, Norway (1995) []

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135 Comments to “Interstellar Ark”

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  1. 101

    1. reflash wrote:The original YOU that the copy was made from (even assuming a perfect copy) would not be able to access the experiences of the copy unless those experiences were somehow united with the original conciousness.

    The experiences do not need to be transmitted back and united with the original. Yes, the original YOU that the copy was made of is not able to access experiences of the copy. And at the time the copy is made, the original is also not able to access experiences that the original will gain afterwards on earth. At this point it is crucial how you define “you”.
    For the original before the copy, both the original after the copy and the uploaded being will be the YOU (or neither of them depending on definition).

  2. 102

    100. Keko wrote:

    Chris Wren claimed that there is no fun for you by exploring a distant place with an uploaded copy since the copy is not you. I think that is wrong. The copy is you - given that consciousness arises from information processing, which is an essential precondition for uploading.

    Yes, that is correct. I was merely restating for clarity of the discussion that, despite Chris agreement on previous comments, his argument was in fact forking away from them.

    100. Keko wrote:

    Therefore, uploading would be a way for a human consciousness to explore a distant solar system.

    This is certainly an option, but uploaded humans will need uploaded worlds, in order to avoid living in a hell just for the sake of the experiment (cfr comment 93). This whole dream is about creating worlds and expanding the horizons of our consciousness, not reducing them. Be these worlds Arks engineered in vegetal structures, or computational Arks taking advantage of different physics, one will still need some Gilgamesh to dream and design them.

    100. Keko wrote:

    As far as we know no miracles in physics have to be discouvered to make uploading possible, although our knowledge about the mind and consciousness is quite limited. Thus uploading does not belong to S0. However, I also believe that building an ark is easier than uploading.

    No miracles in physics are needed, but miracles in understanding our mind are needed. Just like one cannot forget the wall of special relativity, one cannot forget the wall of complexity. Blindly reproducing brain maps or evolving a simulated DNA would still not imply mastering the engineering of a civilization uploading (and not to speak about the required computational power and its reliability). To locate and arrange the correct degrees of freedom, understanding is needed.

    100. Keko wrote:

    For the second point, as was mentioned in comment 90, it could be possible to stop the information processing until arrival. After that there should be enough to experience and to do. A simulated world is not really necessary.

    Cfr also comment 53, one goal of the project is also to live autonomously in outer space, not only to get to the target planet and colonize it. The planet could be a new Earth for some, and a fuel station for others which would be leaving again. While a simulated world is not a technical requirement if the persons are dead during travel and resurrecting at arrival, the point is that we build or simulate a world not because it is a technical requirement, but because we desire it.

  3. 103
    Steve Bowers

    I would say that eventually, all feasible different strategies for interstellar travel will be considered seriously, and they may all be used. Future humanity would be ill advised to limit itself to simply considering one method.
    Here is a page I put together describing four major concepts;

    The methods described are
    Methuselah ships; ships containing a few long-lived individuals if such life-extension is in fact possible
    Hibernation Ships ships with frozen or otherwise preserved but inactive individuals if possible (here I refer to K Eric Drexler’s idea of ‘biostasis, nanotech devices prolonging the life of non-frozen but inactive humans)
    Generation Ships As described in Gilgamesh’s excellent essay, but perhaps with a smaller breeding population backed up with preserved genetic material
    Data-only Ships ships with uploads or other forms of AI on board (if possible).

    I think it is important to realise that the next few centuries might bring advances in many fields, particularly biology, while spaceship technology itself might quickly reach fundamental limits due to well-understood physics.

  4. 104

    Concerning uploading,

    100. Keko wrote:

    As far as we know no miracles in physics have to be discouvered to make uploading possible, although our knowledge about the mind and consciousness is quite limited. Thus uploading does not belong to S0.

    Admittedly, we conceive that our mind results from known laws of physics. But what S0 indicates, are in fact solutions which simply cannot be discussed, apart in saying: “when it will be possible, then…”.

    It is sort of like a patent. A patent is a solution which can be explained to a qualified person, so that there is something to discuss concerning the operation of the object. I cannot register a patent allowing to paint one red blood cell out of two in blue, so as we can look purple, as I don’t have any solutions to propose. And I cannot get it registered by saying that I will do it with a “nanorobot painter”.

    Science fiction is not science, and the great SF authors who also introduced new ideas in science were scientists. It is not astonishing. They wrote under constraint, like as for poetry. Poets like words, their forms, the rigour of their assembly, and it is when they master this grammar that they can imagine new forms, simple, which seems from now on obvious, because carried by the necessity of language.

    To take an example in physics, special relativity is much more “simple and obvious” that Jules Verne’s Space Cannon towards the Moon.

    What do we know about human mind? At the same time very little, and much more than simply 20 years ago. We start to see articulation of cerebral zones. We start to partly understand their functions and what they communicate. But we do not know, really not know, how one goes from an exchange of electrochemical pulses to something like “a feeling”. We are in front of an opaque Wall. Quite opaque for the moment. Admittedly, we can today bet that there is nothing else than alive cells which communicate. And since living beings are well established within the limits of Physics, reproducing this on another support appears “theoretically possible”. And I think that it will be done. But is it meaningful to seek to create now a “world” of which we know well that we ignore everything, starting from the essential of its nature and its laws? A chemist in a world without water, who would synthesize one milligram of H2O in its test-tube could deduce its molecular weight, its bond angles, interesting considerations about hydrogen bond. But would he simply imagine the color of the ocean under the sun? Waves and foam on the shore? The power of a cyclone and its elegant geometry? A tempest of hail ? A sparkling snow mantle? A glacier flattening the side of a mountain? An icefield tormented by the currents? Or simply the dawn? In short, a water world? No. He would miss, to resume, “complexity”.

    For the mind, we are able to synthesize the microgram (the electrochemical pulse) and we know what “the ocean” (the thought) resembles. Compared to a water world, we start to say that water is soft or salted, solid or liquid, perhaps gaseous, and to detail the large oceanic basins and the principal rivers. But recreate a water world! No, we are well, well far from it.

  5. 105

    xantox, Gilgamesh: Yes, I agree with you in all respects. My different opinion was a result of the anticipated purpose of the Ark. If the purpose is to build a comfortable world outside of our solar system, be it earth-like or simulated, be it stationary in another solar system or between the stars, then all of your arguments are completely sound. I had more of an exploration and accomplishment idea in mind.

    104. Gilgamesh wrote:Admittedly, we conceive that our mind results from known laws of physics. But what S0 indicates, are in fact solutions which simply cannot be discussed, apart in saying: “when it will be possible, then…”.

    I see. So I got the meaning of S0 wrong. I did not mean to say that we are anywhere near the capability to upload. Instead, I just wanted to place uploading between the ark and some triumphant Physics. I think that uploading is actually much harder (maybe impossible) compared to building a working ark, but far more likely than instantaneous travel by miracle Physics, which is in S0.

  6. 106

    105. Keko wrote:

    xantox, Gilgamesh: Yes, I agree with you in all respects. My different opinion was a result of the anticipated purpose of the Ark. If the purpose is to build a comfortable world outside of our solar system, be it earth-like or simulated, be it stationary in another solar system or between the stars, then all of your arguments are completely sound. I had more of an exploration and accomplishment idea in mind.

    I think that we are well in the attitude of considering all the possible different ways. I am trying to create a “Wiki” website whose topic would not be centrally the Ark, but “autonomous life in outer space”. Capacity to live without a mandatory bond with Earth, the “cradle”. That could be entitled “Constantin’s (Tsiolkovski) Dream”, which would also make a nice echo to “Centauri Dreams” :) . The inhabited interstellar travel constitutes for me the result of such a process, as the seed disseminated by a matured fruit.

    105. Keko wrote:

    So I got the meaning of S0 wrong.

    It is not your understanding of S0 which is wrong, it is probably the definition of S0 which should be refined. In principle, which arises from S0 are in fact all the the ideas which, within the discussion of targeting distant objectives or autonomous life in space, bring to a solution by the means of sort of Seven-League Boots, by means of ad hoc assumptions. They imply inevitably a part of subjectivity and personal judgement. That depends on what each one believes within the field of possibility in - say - a millenium. I propose already this practical threshold as a definition: 1000 years. All that is not realizable within this delay is automatically “S0” :) . The idea is that the man who will take the first possible way and what is discussed is the way of carrying out the first way towards stars, or to conclude the first project of autonomous live in space. If not, as the discussion concerning uploading shows it well, that does not only relate to theoretical physics but to the whole field of sciences: biology, neuroscience, nanotechnology…

    So is uploading part of S0? In my appreciation yes, but it is only my appreciation of possible progress of science in the millenium to come. Nobody can claim authority on such appreciation, it seems to me.

    That brings us to an answer to Steve, i.e. the link with science fiction:

    103. Steve Bowers wrote: The methods described are
    Methuselah ships; ships containing a few long-lived individuals if such life-extension is in fact possible
    Hibernation Ships ships with frozen or otherwise preserved but inactive individuals if possible (here I refer to K Eric Drexler’s idea of ‘biostasis, nanotech devices prolonging the life of non-frozen but inactive humans)
    Generation Ships As described in Gilgamesh’s excellent essay, but perhaps with a smaller breeding population backed up with preserved genetic material
    Data-only Ships ships with uploads or other forms of AI on board (if possible).

    I quote the homepage:

    * hard science
    * plausible technology
    * realistic cultural development
    * vast setting
    * 10,000+ year timeline
    * no humanoid aliens
    Thousands of years in the future, civilization spans the stars, and humanity has branched into myriad directions. Godlike ascended intelligences rule vast interstellar empires, while lesser factions seek to carve out their own dominions through intrigue and conquest. And beyond the safety of the human-friendly worlds, adventure awaits those prepared to risk all.

    Let’s say that even in this “hard science” point of view, the program is still too vast for me. I am interested only in the first step around the cradle. If the Galaxy would be 100 m in diameter and 5 m thick, the playground of Orion’s arm would represent say a zone of the dimension of the Galaxy thickness (5000 x 5000 x 5000 ly) that is 100 m3 on that scale.

    I am interested in just one cm3 of stars around the Sun :)

    And the same for the technical solutions. It is less imposing, I know, but there are nevertheless lot of things to discuss. Also, I do not have at all the same vision of catastrophic events to come which would plunge humanity in the type of crises exposed in the timeline of the site. I do not believe that one can predict the future, but I think that when one risks a prediction, the only data one has to analyze, is our past. For me, we are still essentially men of the Neolithic era, sedentary beings which have lived of agriculture, with a division of labour allowing a technical progress. Our political practices are derived from what was invented in the Mediterranean basin 2500 years ago, and which were disseminated worldwide for the last 500 years. One does not change his own nature like a snap and I have a rather conservative vision of our own future.

  7. 107

    I agree with all of you that it would be cool if, centuries from now, unmodified bio-humans launched themselves to the stars. I would be proud of us. I just don’t think it is a likely scenario.

    From one angle I can see how building a multi-gigaton interstellar rocket would seem to be easier than creating a posthuman AI or uploading human minds into cybernetic substrates. We already know how to build rockets. Except for the fusion power and the closed-cycle environment system (artificial world), we could start on the Ark today, if we were willing to commit a sizeable share of the global economy to the task. We don’t know how to upload human consciousness or build sapient AI’s.

    However, there are two factors I think will prove more important than our current state of knowledge: technological advancement, and economics.

    One of the major premises of the article is that we know enough about physics to say with considerable confidence that space travel 500 years from now will be done the same way Newton and Goddard taught us: by throwing stuff out the back of a rocket at high speed.

    We cannot say with similar confidence what the state of computer technology, nanotechnology, genetics, medicine, and robotics will be then, except that (barring the adoption of some catastrophic failure-mode that destroys our civilization) they will be far, far more advanced than what we have today.

    Except for a few new tricks like better construction materials and fusion power, the science of rocketry is complete. It will always be expensive and potentially dangerous to accelerate a pound of material to extreme velocities. As long as we’re using rockets, a low-mass payload will always have an enormous advantage over a high-mass payload of similar mission utility when it comes to actually getting financed and launched.

    The technologies related to consciousness-uploading and/or sapient AI are all advancing rapidly, with no comparable “final end” in sight. The average game console contains more computing power than was available to NASA for the Apollo program. Not only is the hardware better by orders of magnitude, so is the software in relevant areas such as simulating a world. Look at the high realism of PS3 and XBOX 360 games.

    In terms of rocketry and manned space, we have less capability now than we did in the Apollo era. Others here have pointed out that neurology has advanced quite a bit in the last 20 years. We’re still flying the exact same spaceships to Low Earth Orbit that we were 20 years ago, with no real prospect of an improvement. Burt Rutan and Virgin Galactic offer some hope for the future, but in a race between galloping “GNR” (Genetics, Nanotechnolgy, Robotics) technologies and snail’s-pace manned rocketry, I expect GNR to win hands down.

    Back when Arthur C. Clarke and Stanley Kurbrick gave us 2001: A Space Odyssey, they provided the “obvious” answer to the question, “How will we explore the outer planets?” We’ll build a huge rocketship carrying men and an intelligent computer!

    It turns out we didn’t even come close to sending men, and we didn’t even send HAL. We sent his primitive ancestors mounted on probes as small as we could make them. This may have been less “fun” than manned missions to Jupiter and Saturn, but it was also economically possible. Our probes, rovers, and other machines continue to become smaller, lighter, and more capable. Astronauts don’t.

    Even though Clarke’s estimate for the development of sapient AI turned out to be wildly over-optimistic, we’ve made alot more progress in the direction of HAL than we have toward giant ring-shaped space-cities and huge manned space-cruisers.

    Manned space exploration has always been mostly ceremonial. We sent men to the Moon to beat the Russians and show off American technical prowess. Once the Russians conceded victory in the “Space Race,” we left the Moon, maybe forever. We continue to piddle around a little in LEO, but again, I see this as ceremonial.

    If NASA were to fire all the astronauts and do everything with probes and satellites, it would “look bad,” as giving up on a manned future in space. As long as they send astronauts up to the ISS once in awhile, the government can pretend that 30 years from now we’ll have a real space-faring civilization–like they’ve been doing for the last 30 years.

    All real space exploration is done by machines, and I don’t see this changing in the foreseeable future. While we hope that 30, 50, or 100 years from now people will go beyond LEO again, machines continue to become more capable at an accelerating rate, neurology comes closer and closer to giving us the ability to interface the brain with machines and reverse-engineer the human brain.

    The technology of implanting cybernetic interfaces in human brains and nanotechnology are both beginning to produce real, if rudimentary progress. 100 years from now, or 500, those technologies will have reached a much more advanced state.

    I think a “gradual replacement” scenario (in which cybernetic implants and/or nanobots are introduced into the human brain, allowing the person’s consciousness to gradually migrate to the new substrate with vastly-expanded intelligence and calculating power so that the bio-brain may be transcended entirely) will probably be the path taken rather than making copies of human consciousnesses that exist simultaneously with the unmodified bio-human.

    No matter how it takes place, it will always be more economically feasible to send machines into space than humans. As machines become more advanced and more “humanlike” in their intelligence (then becoming genuinely “human,” then trans-human) the economics will continue to weigh more and more heavily in their favor.

    500 years from now (or whenever we or our descendants get around to building an Ark) it will be far easier and cheaper to launch a crew of cyber-intelligences and their simulated world in a package the size and weight of a PS3, than to send gigatons of soil, water, plants, animals, and people.

    This may be less “fun” or less “inspiring” to us bio-humans than the thought of people just like us going. However, I do not think our descendants will be any more willing to pay several orders of magnitude more for a mission so it can be more fun and inspiring (i.e. meet ceremonial/emotional needs) than we are now when it comes to our missions to Mars, Jupiter, and Saturn.

    This may sound pessimistic; however, if Kurzweil is right and we get something like radical life-extension, uploading or “gradual transition” from bio-mind to cybernetic mind within our lifetimes, then it could be us (what we grow to be in succeeding centuries) rather than our distant biological descendants who get to go.

  8. 108

    Your vision of the future is perfectly rational and argued and I agree that if the traced tendency is continued (success of the probes, cost and relative uselessness of the inhabited missions), things go rather in the direction of a robot-like exploration of space. Today, I agree completely that it is better to send 100 probes than an inhabited mission!

    What could reverse the tendency in a near future would be democratization of space and development of leisure space stations in orbit. These could initially concern a small number of persons, but if it becomes lucrative to send people in space (while remaining close to Earth of course), then tendency could reverse and with it the search for new solutions allowing to satisfy an increasingly large number of persons.

  9. 109
    Blogcoven » Blog Archive » Analysing our future in Space

    […] Analysing our future in Space The vision of humanity departing this planet for one in a far-flung planetary system is deeply ingrained in any of us who have read or watched Science Fiction. In presenting his* analysis, Gilgamesh has demonstrated quite effectively the almost-maddening scale required to conduct any sort of viable interstellar travel. He puts the mass of the “Ark” in the order of Gigatons, which is pretty big, but still not on the serious planetary body scale. […]

  10. 110
    Dan Dx

    I suppose that everyone here knows Asimov’s “Foundation”. Otherwise I summarize: The Galactic Confederation is in crisis, on the way to burst and sink in a new Middle Age. A group of scientists, under the leadership of Hari Seldon, undertakes to gather in various secret places (the Foundation) the whole of human knowledge and to try, thanks to the “psycho-history”, to foresee the evolution of the society, the crises which it’ll pass through, the means to be implemented to solve them.
    Throughout the novel (3 volumes initially, then 2 sequels), whose action proceeds over several centuries, one follows the birth and evolution of various crises, the intervention of the Foundation to solve them.
    The argument of this novel relates to a whole humanity spread in the whole Galaxy. The techniques of history forecasting worked out by the Foundation have to treat a complex object with galactic dimension. In comparison, our algorithms of weather forecasts would look like children’s games.
    But, let’s imagine a project “à-la Hari Seldon”, applied to an object such as the Interstellar Ark and its population. The ultra-determinism of Asimov’s “psycho-history” is no more that absurd: the number of variables (technical events, breakdowns, diseases, social crisis, etc. as well as the various possible solutions and parries) is reduced to a reasonable size and manageable while putting many people at work on it, why not in a co-operative project “à-la Wikipédia”. The computing power would be provided by an alternative of seti@home! (#74 AsCii)
    Ready? Hari Seldon, here we are!

    And then?… Because may be one day, they’ll be real descendants of ourself, locked up in a real ark, subjected to a real form of social organization which we’ll have imagined from now! Will it be a social organization of the Ark looking like a remake of the Platonic Republic, with philosophical tyrants ruling the mass of the travelers and every aspect of their life, under the requirement of the single Article of the Fundamental Law “Thou shalt listen to the Ancients, if not, be careful with your ass!”? Will the so-called “Ancients” have alone a total access to the “World of Ideas”: the database of the “Interstellar Ark Foundation”, established before the departure, including all Wikis and HowTo possible and imaginable: “Culture of Cress For The Lame”, “Protected Use of N2O Gas in Case of Riot”, “Prevention of A Coup d’Etat Without Sorrow”… and will the so-called “Ancients” hold the “root” password, distribute “user” restricted access towards pieces of selected fragments of knowledge?… Nausea!
    Outer Space is hostile; the fragile Ark will be populated within its limit. In this context, the invisible hand of free market and its minimal and weak government (#73 Jc and #83 Michael Dunkel), or anarchy, or libertarianism are unconceivable as adequate forms of political systems. Then, democracy, scientocracy, technocracy, theocracy, oligarchy?…
    I’m afraid, the Ark (like in the long-term Earth, I’m afraid of it too…), because of its brittleness, is condemned for its own survival to be ruled by a strong government, total, perhaps totalitarian, heavily concerned with ecology, hygiene, safety, prevention, a society in which each one will be tyrant of each other and the people’s life will be like the life inside a SSN or an oil platform. And no place for the rebel, unless he’ll be a remote-controlled one, like a sort of backfire or a safety valve, planed and ruled by the “psycho-history”, of course…
    As far as I’m concerned, I’m not that enthusiastic to get on board, I’d prefer to await a possible progress in quantum teleportation.
    But I’m fully aware that I won’t have to be too in hurry, because the future “Stargate’s Galactic Network”, if it works some day without too much entropy (I’ve the weakness to care about the integrity of all of my body’s devices!), in fact, the Arks, running at a miserable sub-luminic speed, will have to carry it trough the space for centuries before to install it on other new worlds!!!

  11. 111

    more likely that the arc will exist, but only after we will be able to expand a human lifespan long enough so that you will be the one to arrive at your destination, and not your descendants.

  12. 112
    Dan Dx

    For sure, Justin, it would be a great solution.
    Let’s see: Today, mean expectancy of life for human been: 65 y.
    Ok, they are geographical disparities:
    The winners are:
    1 Andorra 83.52
    2 Macau 82.27
    3 Japan 82.02
    4 San Marino 81.8
    5 Singapore 81.8
    6 Hong Kong 81.68
    7 Sweden 80.63
    8 Switzerland 80.62
    9 Australia 80.62
    10 France 80.59
    11 Guernsey 80.53
    12 Iceland 80.43
    13 Canada 80.34
    14 Cayman Islands 80.2
    15 Italy 79.94
    16 Gibraltar 79.93
    17 Monaco 79.82
    18 Liechtenstein 79.81
    19 Spain 79.78
    20 Norway 79.67
    21 Israel 79.59
    22 Jersey 79.51
    23 Faroe Islands 79.49
    24 Greece 79.38
    25 Austria 79.21
    26 Virgin Islands 79.2
    27 Malta 79.15
    28 Netherlands 79.11
    29 Luxembourg 79.03
    30 Montserrat 79
    31 New Zealand 78.96
    32 Germany 78.95
    33 Belgium 78.92
    34 Guam 78.76
    35 Saint Pierre and Miquelon 78.76
    36 United Kingdom 78.7
    37 Finland 78.66
    38 Man, Isle of 78.64
    39 Puerto Rico 78.54
    40 Bosnia and Herzegovina 78.17
    41 Bermuda 78.13
    42 Saint Helena 78.09
    43 United States 78
    44 Denmark 77.96
    45 Ireland 77.9
    46 Portugal 77.87
    47 Albania 77.6

    We are a long way out!
    But women have better chances to live longer than men.
    Let’s go for an Amazon’s Ark (and deep frozen sperm’s lighter to carry than men!)
    Back to the charts, the world’s champions are:
    1/ Andorran women 86,61 y (Andorra is a tiny principality on the border between France and Spain. Population: 71 201)
    2/ Japanese women 84.6 y
    3/ French women 82.8 y

    We are a long way out again! And, btw, I ain’t sure that an US administration will finance any part of such a project if no american citizen is able to be part of it, due to a low expectancy of life… Imagine! The main part of the crew being japanese and french women (”Konichi wa?” - “Pas mal! Et toi, chérie, ça va?”… I would love it, I’m french! And Bill O’Reilly’s getting mad!… How yummy!)

    You’re right, if we want to exclude the option “Generation Ark” the solution is to try to extend lifespan. There is a way through genetic engineering; we’ll extend cell’s DNA telomeres Then, the body’s cells will be able to divide itself much more times and the DNA will keep longer in a better condition. (Would be long to explain the function of telomeres in the duplication of human cells, so if you need, have a look at )

    Ok, let’s say that we have extended the lifespan of the Ark’s crew. Now, at the arrival, we have on board an army of ardent multi-centenarian pioneers and, as far as pioneer women are concerned… post-menopausal for long and unable to give birth to a single child!
    The amount of available ova in the female body is physically limited, you know…

    Ok! It’s possible to increase it a bit with gonadotropin-releasing hormone!… But in the long term, it increases dramatically the risk of cancer, and during an extended lifespan, that risk will quickly reach 100%.. Need to have a strong medicine against cancer too.

    Ok, don’t worry! We’ll manage to solve that issue trough a slow-down, with drugs, of the women’s ovulations frequency (at least by a factor of ten because the travel will be, let’s say, 400 years long!)… with the collateral consequence to decrease their fertility at the same rate.

    Ok, ok! It’s also possible for a post-menopausal woman to carry an implanted embryo! We have already today a lot of unused frozen embryos, many more as time will pass, a good way to use them!…
    With a 55 years old woman the rate of failures is already crippling, I won’t bet on the chances of a 400 years old mummy wannabe (not the egyptian one! :-) ).

    In everything, you solve a problem, many more arise! “C’est la vie!”
    In spite of those issues, I keep my faith in the progress of both medicine and technology, even if, for a long while, we’ll need many magic wands, :-)

    Justin, back to you!

  13. 113
    Dan Dx

    #2 Alan Crowe
    #111 Justin
    Me again, sorry. I read over your posts and mine and you know what? I doubt… A philosopher’s bad habit (and what is more, a french one. You know, we are breast-fed by Descartes, it gives bad manners!) Anyway, I doubt.
    I tried last night to imagine the mug of the first genetically engineered human been, being informed that: “My son! Your mother, myself and all your uncles, the doctors, are proud to tell you that you’ll live for about 2000 years!”…

    We are the sole animal on Earth who knows that some day he’ll die. We are born, we live, then we die. For ages, the expectancy of life for an adult was, let’s say, 40 years for a male, much less for a woman. The way we live, the way we behave, the way we think has been, through centuries, determined by that certainty: We don’t live forever and life is short!
    The risks we take and the expected winnings are, consciously or unconsciously, hung in the balance and calculated, and the expected duration of life is among the terms of the equation. When I was younger I used to dive, risky activity. The balance between the winnings (in this case, the pleasure of discovery) and the risks (death or severe injury) is tipped in action’s favor. We are conditioned since ages to weigh up that way.

    Today, the expectancy of life (at least in developed countries with a developed health care system) is growing by and by, some few months added every year. Some scientists think it’ll reach 120 years, maybe a limit. That change is slow, giving enough time to the society, habits, mentality to adapt gradually, we carry on taking risks the way our fathers and ancestors did.

    Now let’s imagine! Suddenly, a new generation of genetically engineered very long livers will born, surrounded by short living parents, teachers, physicians, scientists
    Consciously or unconsciously, they’ll hand on to those strange wonder kids a different message than the one we received. Instead of “Live a fulfilled life!”, those “neo-mathusalems” will grow with their parents’obsessional fear in the background of their mind: “Be careful, my son, save your precious long life!” Because their life will be long indeed, but as fragile than ours!
    Will they agree to embark on the Ark’s adventure? Maybe some of them, driven by despair or boredom, wrong motivations indeed… Will they leave the safe Earth enthusiastically? I’m afraid, no. It will takes generations before this new man would internalize Nietzsche’s “Was mich nicht umbringt, macht mich stärker!” (What doesn’t kill me makes me stronger)
    Zi.Ud.Sud.du, the Sumerian Noah, to whom Gods gave “a life of very long days” (his name) after the Deluge, never went to the sea again.

    Our live is short indeed, but that short life is a great adventure and to take risks is a part of the adventure: Life is wider than our dreams! :-D

  14. 114

    There’s noway to tell what will happen to the psyche of man when we achieve clinical immortality. Would we be forever physically 25? 35? Our age effects our emotional states and our capacity to learn. If I could be returned to my 20 year old body, would I have the focus that I have now? Would I still be able to apply the wisdom I have? Or is my wisdom solely a chemical reaction of my age?

    Anyway, when you talk about a generational ship, you are ‘dooming’ the next generations to spending their whole lives on the arc. They may not wish to live there and long to return to Earth. I think the decision for a generational ship would immoral on that basis unless it was the only hope for mankind’s survival.

    The only way for the trip to be morally correct for the individual would be for immortal interstellar cosmonauts to be the ones making the entire trip.

  15. 115

    Justin, the same argument could be made about any colonist. The first immigrants to the Americas “doomed” their children to living on the continent. Some colonies failed, and entire populations died. At the lost colony at Roanoke, “some ninety men, seventeen women, and eleven children” disappeared. I expect that a generation ship would not be built and launched “from scratch”, but would rather evolve from similar space colonies that would themselves have already evolved stable societies. The initial crew would have been born and raised on the ship, possibly exploring the gas giants of our own solar system, to work any bugs out of the system well before leaving for another start system.

  16. 116
    Chris Wren

    The same ethical argument could be applied to parents who decide to give birth here on earth, “condemning” their offspring to illness, the general travails of an uncertain life, and eventual death. I would think that the decision to sign aboard an interstellar mission, even one with an uncertain guarantee of success would be an ethical decision for the colonists alone, and none of anyone else’s business.

  17. 117

    Sorry, I’m a bit late :)

    Justin, the crucial question that you’re asking about the morality of the Ark project is one of the very first that I asked myself.

    The philosophico-technical question is: TO WHICH CONDITIONS the fact of embarking the human species far from Earth (let’s say far enough so as it is impossible to get back) can be considered moral? Which conditions guarantee that we stay within a humanistic framework, that is, a point of view conceiving the human being, individually and collectively, like an end and not like a mean?

    To this question, can one answer, right from the start and without reflecting: “to NO conditions”? It seems to me that the question deserves at least some reflexion. It can be indeed that the answer will be at the end “none”; but it can also be that there is a solution. Why not? And if it would be the case that there is a solution, let’s call this solution “the Ark”. At the beginning it may be a concept indicating simply that - and which I would reformulate as follows: the Ark indicates the whole of the necessary conditions, most easily accessible in the nearest future, allowing without moral concerns to embark a branch of humanity far from Earth, and without a project to come back.

    One can then dispute such or such point amongst the solutions that I propose. But why affirm that it is anyway not possible? Man does not need the Earth, he needs a terrestrial environment. It is very different. The Ark must take the Earth as a model, but it will not be the Earth. The Ark is “all which Man needs”, needs which were developed in him by the Earth. So that this project is centered on Man, not on Planet.

  18. 118

    Justin, Gilgamesh (Re:117):

    When considering the morality of the ark, one must also take into account the morality of NOT building one. Assuming the ark is the only way the human species is going to spread beyond this little rock, not building one is equivalent to dooming the entire human race to a catastrophe that’s bound to happen at one point or another.

    What is worse - condemning several generations to cramped and lonely living conditions, or endangering the existance of the entire race?

    Another point to consider, while on the subject, is the conditions on Earth at the time of departure. A good reason to spread to other plantets, and a great motivation for colonization is if conditions on Earth become intolerable (due to climate change/pollution/war/oppressive government/whatever comes in the future), and in that case the question of morality is void, since forcibly keeping people on an inhospitable planet seems to be by far the more deplorable option.

  19. 119
    Steve Bowers

    You might be interested in this fictional blog, which I started back in 2005 but have only recently had time to bring up to date.
    it is set in the Orion’s Arm scenario, but is mostly separate from the main events of that timeline. The interstellar ship uses two speculative technologies- antimatter catalysed fusion, and hibernation by vitrification; however the vitrification process is far from perfect.

    If hibernation cannot be perfected, then journeys between the stars will be long and difficult; but generation ships are not the only alternative (as I mentioned in my earlier post).

  20. 120


    You said:
    “What is worse - condemning several generations to cramped and lonely living conditions, or endangering the existance of the entire race?”

    The first alternative is maybe “preferable”, but I do not think that one can send people far from the Earth by authority: I think that it is possible for people to found an autonomous community wanting to leave, which is not at all the same thing. An authoritative project, where one would place people in ships and send them in remote regions (as for the colonization of Australia, for example) is possible only if the travel lasts for a few months. The sailors are voluntary, because they know that they are able to return! But if the travel lasts for tens of years, then it should be thought that the people who are leaving are collectively the crew of this vessel. If the motivation does not come from the crew, but that it is imposed from an external agency, I do not give a long time to the project to fail lamentably by mutiny, scuttling, and so on… It is difficult to be morally bounded (to ensure the immortality of humanity is a high goal) to a terrestrial community which despises your existence and condemns you to a containment worse than a prison. Especially that at the end of the travel there is not a viable place (like it was Australia, with a gravity of 1g, a respirable air, water, cultivable grounds, wild mammals, etc) but a planet undoubtedly inhospitable, which obliges to a confined life during whole generations.
    To ensure the immortality of humanity is an immense goal, one of the most beautiful conceivable. It may constitute a very strong motivation, a moral ‘raison d’être’ to try the project voluntarily. One does not need to imagine that it must be imposed by someone. It is necessary that it is a free community, free of its destiny, and consequently the question of the following generations is very different. A community which choose its destiny rises his/her children in the love of this goal, and shares this destiny with them. That completely changes the appreciation which the future generations will have of their destiny.

    Concerning the second point, I think that over geological durations (1 million years or more) the survival of humanity is in question. But that on a few thousand years, it is difficult to imagine that the Earth becomes less viable than close planets, even after 1000 Tchernobyl or Seveso, even if temperature increases by 10°C on average or that we return to Ice Age. Earth would still remain an extremely hospitable place, compared to Mars for example. If the problem is political, eg that the Earth becomes intolerable for lack of freedom, then we return to the above and moreover, it is difficult to see WHO would undertake such an adventure in such case, who would have the means for it?

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