The colonization of space 

Careful engineering and cost analysis shows we can build pleasant, self‐sufficient dwelling places
in space within the next two decades, solving many of Earth's problems.
Gerard K. O'Neill

Physics Today 27 (9), 32–40 (1974);

https://doi.org/10.1063/1.3128863

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The colonization of space
Careful engineering and cost analysis shows we can build
pleasant, self-sufficient dwelling places in space within the
next two decades, solving many of Earth's problems.

Gerard K. O'Neill

New ideas are controversial when they lectures at universities. The positive tic exploration before it, has consisted
challenge orthodoxy, but orthodoxy response (especially from students) en- of short-term scientific expeditions,
changes with time, often surprisingly couraged me to dig harder for the an- wholly dependent for survival on
fast. It is orthodox, for example, to be- swers to questions about . meteoroid supplies brought from home. If, in

25 August 2023 18:23:10

lieve that Earth is the only practical ha- damage, agricultural productivity, ma- contrast, we use the matter and energy
bitat for Man, and that the human race terials sources, economics and other available in space to colonize and build,
is close to its ultimate size limits. But I topics. The results of that study indi- we can achieve great productivity of
believe we have now reached the point cate that food and material goods. Then, in a
where we can, if we so choose, build new • we can colonize space, and do so time short enough to be useful, the ex-
habitats far more comfortable, produc- without robbing or harming anyone and ponential growth of colonies can reach
tive and attractive than is most of without polluting anything. the point at which the colonies can be of
Earth. • if work is begun soon, nearly all our great benefit to the entire human race.
Although thoughts about migration industrial activity could be moved away To show that we are technically able
into space are as old as science fiction, from Earth's fragile biosphere within to begin such a development now, this
the technical basis for serious calcula- less than a century from now. discussion will be limited to the tech-
tion did not exist until the late 1960's. • the technical imperatives of this kind nology of the 1970's, assuming only
In addition, a mental "hangup"—the of migration of people and industry into those structural materials that already
fixed idea of planets as colony sites— space are likely to encourage self-suffi- exist. Within a development that may
appears to have trapped nearly every- ciency, small-scale governmental units, span 100 years, this assumption is un-
one who has considered the problem, cultural diversity and a high degree of realistically conservative. We shall
including, curiously enough, almost all independence. look at the individual space communi-
science-fiction writers. In recent • the ultimate size limit for the human ties—their structure and appearance
months I learned that the space pioneer race on the newly available frontier is at and the activities possible for their in-
Konstantin Tsiolkowsky, in his dreams least 20 000 times its present value. habitants, their relation to the space
of the future, was one of the first to es- How can colonization take place? It around them, sources of food, travel be-
cape that hangup. is possible even with existing technolo- tween communities as well as to Earth,
By chance, and initially almost as a gy, if done in the most efficient ways. the economics of the colonies and plans
joke, I began some calculations on the New methods are needed, but none goes for their growth. As is usual in physics,
problem in 1969, at first as an exercise beyond the range of present-day knowl- it is valuable to consider limiting cases:
for the most ambitious students in an edge. The challenge is to bring the goal for this study, the limits are an eventual
introductory physics course. As some- of space colonization into economic fea- full-size space community on a scale es-
times happens in the hard sciences, sibility now, and the key is to treat the tablished by the strength of materials,
what began as a joke had to be taken region beyond Earth not as a void but and a first model, for which cost esti-
more seriously when the numbers began as a culture medium, rich in matter and mates can reasonably be made. The
to come out right. There followed sev- energy. To live normally, people need goals of the proposal will be clearer if
eral years of frustrating attempts to get energy, air, water, land and gravity. In we first discuss the large community.
these studies published. space, solar energy is dependable and
Friends advised that I take my ideas convenient to use; the Moon and aster- A cylindrical habitat
"to the people" in the form of physics oid belt can supply the needed materi- The geometry of each space commu-
als, and rotational acceleration can sub- nity is fairly closely defined if all of the
for Earth's gravity.
Gerard K. O'Neill is professor of physics at following conditions are required: nor-
Princeton University. Space exploration so far, like Antarc- mal gravity, normal day and night cycle,

32 PHYSICS TODAY/SEPTEMBER 1974

 Natural sunlight from planar mirrors

Illllil

Rotation

natural sunlight, an earthlike appear-

ance, efficient use of solar power and of
materials. The most effective geometry
satisfying all of these conditions ap-

25 August 2023 18:23:10

pears to be a pair of cylinders. The
economics of efficient use of materials 1000 2000 3000
Earthbound structures for scale
tends to limit their size to about four Meters
miles in diameter, and perhaps about 16
miles in length. (See figure 1.) In
these cylinder pairs, the entire land
area is devoted to living space, parkland
and forest, with lakes, rivers, grass,
trees, animals and birds, an environ-
ment like most attractive parts of
Earth; agriculture is carried on else-
where. The circumference is divided
into alternating strips of land area
("valleys") and window area ("solars").
The rotation period is two minutes, and
the cylinder axes are always pointed
toward the Sun.
Because the Moon is a rich source
both of titanium and of aluminum, it is
likely that these metals will be used ex-
tensively in the colonies. For conserva-
tism, though, the calculation of the cyl-
inder structure has been based on the
use of steel cables, to form "longerons"
(longitudinal members carrying the at-
mospheric forces on the end caps) and
circumferential bands (carrying the at-
mospheric force and the spin-induced
weights of the ground, of the longerons
and of themselves. For details of this
calculation and the assumptions it in-
cludes, see the box on page 34.) The
steel cables are bunched to form a Section of a space-community main cylinder (top). The circumference is divided into alternat-
coarse mesh in the window areas. The ing strips of land area (valleys) and window area (solars). Although the space-community val-
bands there subtend a visual angle of leys offer new landscaping opportunities and architectural possibilities, it is reassuring to note
2.3 X 10~4 radians, about equal to the that certain Earth features can be recreated: the side view of a cylinder end cap (bottom) in-
diffraction limit for the sunlight-adapt- cludes a mountain profile taken from an aerial photograph of a section of the Grand Teton range
ed human eye, and so are nearly invisi- in Wyoming. Figure 1

PHYSICS TODAY/SEPTEMBER 1974 33

 Table 1. Possible Stages in the Development ble. The windows themselves are of
of Space Communities glass or plastic, subdivided into small
panels.
There is no sharp upper limit on the
Earliest size of a space-community cylinder;
Length Radius Period estimated with increasing size, though, a larger
Model (km) (m) (sec) Population* date
fraction of the total mass is in the form
1 1 100 21 10 000 1988 of supporting cables. The figure 3200
2 3.2 320 36 100-200 X 103 1996 meters for radius R is somewhat arbi-
3 10 1000 63 0.2-2 X 106 2002 trary. Economoy would favor a smaller
4 32 3200 114 0.2-20 X 106 2008
size; use of high-strength materials, or a
strong desire for an even more earthlike
* Population figures a re for double unit; higher figures are the approximate ecological limits, for
conventional agriculture. environment, would favor a larger. In-
dependent of size, the apparent gravity
is earth-normal, and the air composi-
tion as well as the atmospheric pressure
are those of sea level on Earth. For R
equal to 3200 meters, the atmospheric
Table 2. Masses of Materials Required for depth is that of an Earth location at
Model 1 (Metric tons) 3300 meters above sea level, an altitude
where the sky is blue and the climate
habitable: At any radius r within the
Total mass Mass required
required from Earth cylinder we have
Aluminum (container, structures) 20 000
Glass (solars) 10 000
Water 50 000 l where
Generator plant 10002 1000
Initial structures 1000 1000 a = gpJ2Rp0 =
Special fabricated hardware 1000 1000 X 10-4/meter)
Machines and tools 800 800
1
Soil, rock and construction materials 420 000 The length of a day in each communi-
Liquid hydrogen 5400 5400 ty is controlled by opening and closing
2000 people and equipment 200 200
Dehydrated food 600 600 the main mirrors that rotate with the
Totals >500 000 10 000 cylinders. The length of day then sets
the average temperature and seasonal

25 August 2023 18:23:10

1
Includesreplenishable reserves to be used to initiate construction of Model 2, and so are higher
variation within the cylinder. Each
than the minima required for Model 1. cylinder can be thought of as a heat
2
For 100 MW plant. sink equivalent to 3 X 108 tons of water;
for complete heat exchange, the warm-
up rate in full daylight would be about
0.7 deg C per hour. As on Earth, the
true warmup rate is higher because the
ground more than a few centimeters
Steel structure below the surface does not follow the
For the structure, steel cables are assumed to be formed into longerons (average thick-
diurnal variation.
ness ArL) and circular bands (average thickness lrB). The value of ArL required is Bird and animal species that are en-
Ar,. = Rp,,/2T dangered on Earth by agricultural and
industrial chemical residues may find
where R is the cylinder radius, po the atmospheric pressure and 7"the tension. For land havens for growth in the space colonies,
density pL and depth x,_, and bands of density pF, the total equivalent internal pressure pj where insecticides are unnecessary, ag-
is ricultural areas are physically separate
from living areas, and industry has un-
limited energy for recycling.
To solve for pj we note that
As we can see in figure 1, it is possible
Ar h = pTR/T to recreate certain Earth features: the
so that mountain profile is taken from an aerial
PT = photograph of a section of the Grand
(pt, + gp,,xL + gpFR/T)/a - Teton range in Wyoming. The calcu-
For an average soil depth of 150 cm, with an average density of 1.5 gm per cc, lated cloud base heights as seen in the
figure are typical of summer weather on
Pn = gP[XL = 1.2.3 X 10' newtons/m 2 Earth: For a dry adiabatic lapse rate of
To arrive at a conservative value for T, we note that half a century ago, the working 3.1 deg per 300 meters and a dew-point
stress for suspension-bridge cables was 70 000 to 80 000 pounds per square inch.1 At lapse rate of 0.56 deg per 300 meters,
that time, D. B. Steinman argued1 for the use of stresses over 100 000 psi. If we use 50% relative humidity and a tempera-
1920's steels, hardened to bring the yield point to 90% of the ultimate strength, and work ture range between zero and 32°C, the
at 75% of the yield point, the working stress can be 152 000 psi. If we take f a s
cloud base heights range between 1100
150 000 psi and R as 3200 meters, the averaged surface mass density is 7.5 tons per
square meter.
and 1400 meters.
In the window (solar) areas, the longerons can be 0.8-meter cables in stacks of four at
14-meter intervals. The bands can be in the same arrangement, but with a 1.5-meter di- Environmental control
ameter, and the mesh transparency will then be 84%. Considerably larger values of R The agricultural areas are separate
would result from the extensive use of titanium in the structure, together with a thinner from the living areas, and each one has
layer of earth. the best climate for the particular crop
it is to grow. Gravity, atmosphere and

34 PHYSICS TODAY/SEPTEMBER 1974

insolation are earthlike in most agricul- oid damage should not be a serious dan- of thrusters. The force and torque di-
tural cylinders, but there is no attempt ger. Most meteoroids are of cometary agram for this arrangement is seen in
there to simulate an earthlike appear- rather than asteroidal origin and are figure 3. To accelerate the cylinders up
ance. Selected seeds in a sterile, isolat- dust conglomerates, possibly bound by to the required rotational speed, static
ed environment initiate growth, so that frozen gases;3 a typical meteoroid is torque is transmitted through the com-
no insecticides or pesticides are needed. more like a snowball than like a rock. pression framework that joins the two
(The evolution time for infectious orga- Spacecraft sensors have collected abun- cylinders of a pair. For a spin-up time
nisms is long, and resterilization of a dant and consistent data on meteoroids of three years, a constant 560 000 horse-
contaminated agricultural cylinder by in the range 10~6 to 1 gram, and the power is needed; this is 3% of the gener-
heating would not be difficult.) All Apollo lunar seismic network is believed ator capacity of a cylinder. After spin-
food can be fresh, because it is grown to have 100% detection efficiency for up, the same motors can provide main-
only 20 miles from the point of use. meteoroids4 above 10 kg: Data from tenance power for frictional losses and
The agricultural cylinders can be evenly these sources are consistent with a sin- for attitude control about the spin axis.
distributed in seasonal phase, so that at gle distribution law. Each cylinder's angular momentum is
any given time several of them are at The Prairie Network sky-camera 1.5 X 1018 kg2 rad per sec; the torque
the right month for harvesting any de- data, 5 after substantial correction for needed to precess this angular momen-
sired crop. assumed luminous efficiency, agree with tum once each year is 3 X 1011 newton
Figure 2 shows side and end views of data from the National Aeronautics and meters, corresponding to a constant
a space community as a complete eco- Space Administration for 10-gm mete- force of 1200 tons on a 26-km lever arm.
system. The main mirrors are made of oroids. The spacecraft and seismic The phase difference of seasons be-
aluminum foil and are planar. Moving data indicate a mean interval of about tween the two cylinders permits "sea-
these mirrors varies the angle at which one-million years for a strike by a heavy sonal counterpoint," midsummer in one
sunlight hits the valleys (controlling the (one ton) meteoroid on a space commu- cylinder during midwinter in the other.
diurnal cycle), and the Sun appears mo- nity of cross section 1000 square kilo- Travel between the two requires no
tionless in the sky, as it does on Earth. meters. Even such a strike should pro- power and only nine minutes of time.
The solar power stations, which consist duce only local damage if the structure They are only 90 km apart, and engine-
of paraboloidal mirrors, boiler tubes is well designed. For 100-gram meteor- less vehicles can unlock from the outer
and conventional steam-turbine electric oids, the mean interval for a strike is surface of one cylinder at a preset time,
generators, can provide the community about three years. From the combined move in free flight with the tangential
with sufficient power, easily up to ten viewpoints of frequency and of momen- velocity (180 meters per sec or 400 miles
times the power per person now used tum carried, the size range from one to per hour) and lock on to the other cylin-
(10 kw) in highly industrialized re- ten grams may need the most care in der at zero relative velocity.
gions.2 For such energy-rich conditions window design and repair methods. Travel between communities can also
(120 kw per person) the power needed For total breakage of one window panel, be carried out with simple engineless

25 August 2023 18:23:10

for a cylinder housing 100 000 people is Daniel Villani at Princeton has calcu- vehicles, accelerated in a computed di-
12 000 megawatts: The solar power in- lated a leakdown time of about 300 rection by a stationary cable-pulling
cident on a cylinder end cap is 36 000 years. Meteoroid-damage control is, electric motor and decelerated by an ar-
megawatts, adequate if the thermal effi- then, a matter of sensing and of regular resting cable at the destination. The
ciency is 33%. Extra power plants near minor repair rather than of sudden "cable-car" vehicles for such free flight
the agricultural ring would be needed emergencies. need no fuel, no complex maintenance
for higher population density. Waste nor a highly trained crew, and should be
heat is sent into space by infrared radi- Axial rotation and transport inexpensive. Vehicle speeds permit
ators of low directionality. A key element in the design of the travel among a total population larger
The communities are protected from space colony is the coupling of two cyl- than that of Earth within flight times of
cosmic rays by the depth of the atmo- inders by a tension cable and a com- seven hours. (I have here assumed
sphere and by the land and steel sup- pression tower to form a system that communities spaced at 200-km inter-
porting structure, the bands and longer- has zero axial angular momentum and vals, so that the maximum dimension of
ons being distributed where visual is therefore able to maintain its axis a planar cluster housing 4 billion people
transparency is unnecessary. Meteor- pointed toward the Sun without the use is 29 000 km. For a vehicle with accel-

Agricultural areas
0 5 10 Agricultural areas (37 of 72 shown)

Main mirrors

Sunlight
Distant-travel'
launch track
Space-community Solar-power stations
main cylinder

Midline between members of cylinder-pairs

Space community as a whole is seen in side (left) and end (right) are shown; the ring does not rotate as a whole. Note the lines of
views. For the end view, 37 of the 72 agricultural cylinders in a ring symmetry in both sections of the figure. Figure 2

PHYSICS TODAY/SEPTEMBER 1974 35

eration Ig and the required travel time could be larger. One foreseeable devel- are nonrepetitive and require a sense of
of seven hours, the acceleration length opment is the use of near-frictionless art and beauty.
is 66 km.) With no need for aerody- (for example, magnetic) bearings be-
namic design, the vehicles can be far tween a rotating cylinder and its sup- Our new options
more roomy and comfortable than the porting structure, which need not be It is important to realize the enor-
typical earthbound commercial jet. spun. For eight tons per square meter mous power of the space-colonization
of surface density and a tensile strength technique. If we begin to use it soon
Life in the colonies of 300 000 psi, R would be 16 km, the enough, and if we employ it wisely, at
The key statements so far have been total area would 50 000 km2, and the least five of the most serious problems
based on known facts, on calculations population would be between five mil- now facing the world can be solved
that can be checked and on technology lion (low density) and 700 million (the without recourse to repression: bring-
whose costs can be estimated realistical- ecological limit, the maximum popula- ing every human being up to a living
ly. The discussion, however, would be tion that can be supported). standard now enjoyed only by the most
sterile without some speculations— In Table 1 we see my estimate of the fortunate; protecting the biosphere
speculations that must, of course, be earliest possible schedule for space colo- from damage caused by transportation
consistent with the known facts. nization, beginning with a model com- and industrial pollution; finding high-
With an abundance of food and clean munity in the late 1980's. From about quality living space for a world popula-
electrical energy, controlled climates the year 2014, I assume a doubling time tion that is doubling every 35 years;
and temperate weather, living condi- of six years for the colonies; that is, the finding clean, practical energy sources;
tions in the colonies should be much workforce of a "parent" colony could preventing overload of Earth's heat bal-
more pleasant than in most places on build a "daughter" colony within that ance.
Earth. For the 20-mile distances of the time. In making these estimates I have I hesitate somewhat to claim for
cylinder interiors, bicycles and low- calculated that the first model commu- space-colonization the ability to solve
speed electric vehicles are adequate. nity would require a construction effort one other problem, one of the most ago-
Fuel-burning cars, powered aircraft and of 42 tons per man-year, comparable to nizing of all: the pain and destruction
combustion heating are not needed; the effort for large-scale bridge building caused by territorial wars. Cynics are
therefore, no smog. For external travel, on Earth. Full-size communities at sure that humanity will always choose
the simplicity of engineless, pilotless ve- high population density require 50 tons savagery even when territorial pressures
hicles probably means that individuals per man-year, and up to 5000 tons per are much reduced. Certainly the mani-
and families will be easily able to afford man-year for low population density. acal wars of conquest have not been ba-
private space vehicles for low-cost trav- For comparison, automated mining and sically territorial. Yet I am more hope-
el to far distant communities with di- shipping in Australia now reaches 200 ful; I believe we have begun to learn a
verse cultures and languages. The tons per man-year averaged over a little bit in the past few decades. The
"recreational vehicles" of the colonial town.6 history of the past 30 years suggests
age are therefore likely to be simple

25 August 2023 18:23:10

In the long run, space-colony con- that warfare in the nuclear age is
spacecraft, consisting of well furnished struction is ideally suited to automa- strongly, although not wholly, motivat-
pressure shells with little complexity tion. A colony's structure consists ed by territorial conflicts; battles over
beyond an oxygen supply and with mainly of cables, fittings and window limited, nonextendable pieces of land.
much the same arrangement of kitchen panels of standard modular form in a From the viewpoint of international
facilities and living space as are found pattern repeated thousands of times. arms control, two reasons for hope come
today in our travelling homes. The assembly takes place in a zero- to mind. We already have an interna-
All Earth sports, as well as new ones, gravity environment free of the vagaries tional treaty banning nuclear weapons
are possible in the communities. of weather. By the time that the colo- from space, and the colonies can obtain
Skiing, sailing, mountain climbing (with nies are evolving to low population den- all the energy they could ever need from
the gravity decreasing linearly as the al- sity, therefore, I suspect that very few clean solar power, so the temptations
titude increases) and soaring are exam- people will be involved in their con- presented by nuclear-reactor bypro-
ples. As an enthusiastic glider pilot, I struction. Most of the workforce will ducts need not exist in the space com-
have checked the question of thermal probably be occupied in architecture, munities.
scales: The soaring pilots of the coloni- landscaping, forestry, zoological plan- To illustrate the power of space-colo-
al age should find sufficient atmospher- ning, botany and other activities that nization in a specific, calculable situa-
ic instability to provide them with lift.
At high altitudes, man-powered flight—
a nearly impossible dream on Earth—
becomes easy. A special, slowly rotat-
ing agricultural cylinder with water and Torque (*) _
fish can have gravity 10~2 or 10~3 times Angular momentum of north cylinder
that on Earth for skin diving free of Force t
pressure-equalization problems. Noisy Force
or polluting sports, such as auto racing,
can easily be carried out in one of the Sun
Tension cable Compression tower
cylinders of the external ring.
The self-sufficiency of space com-
munities probably has a strong effect on
government. A community of 200 000 , Force
people, eager to preserve its own culture Force'
Angular momentum of south cylinder
and language, can even choose to re-
main largely isolated. Free, diverse so- Torque
cial experimentation could thrive in
such a protected, self-sufficient envi-
ronment.
If we drop our limitation to present Force and torque diagram for a cylinder. Nondissipative static forces are used to precess the
technology, the size of a community spin angular momenta, so that cylinder axes always point toward the Sun. Figure 3

36 PHYSICS TODAY/SEPTEMBER 1974

tion, we trace the evolution of a worst- cylinders of Model 1 should then be the communities to a stable density of
case example: Suppose the present able to support up to 10 800 people, and 1.43 people per hectare, about one hun-
population-increase rate were to con- the corresponding ecological limit for a dredth of the ecological limit. The
tinue on Earth and in the space colo- full-size community would be 20 million total land area in the colonies would
nies. In that case the total human pop- people. At this limit, all the colonists then be more than three times that of
ulation would increase 20 000-fold in a would have a high standard of living, Earth.
little over 500 years. Space-coloniza- but in apartment-house living condi- We can hope that, in contrast to this
tion would absorb even so huge a tions, looking out over farmland. For a worst-case example, some progress
growth, as we shall see from our calcula- community limit of 13-million people, toward zero population growth10 will be
tions. the main cylinders could be kept free of made in the next 75 years. Any such
The total volume of material needed agriculture. progress will hasten the solution, reduce
in a full-size community is 1.4 X 109 By about 2050, then, figure 4 indi- Earths population peak, and hasten the
cubic meters, and the material available cates that emigration to the colonies day when the population densities on
in the asteroid belt (from which the could reverse the rise in Earth's popula- Earth as well as in the colonies can be
later communities will be built) is esti- tion, and that the acceleration of the so- reduced to an optimum value.
mated to be 4 X 1017 cubic meters, lution could be dramatically fast:
about one twenty-five hundredth the Building the first colony
Within less than 30 years, Earth's popu-
volume of Earth. For a present world lation could be reduced from a peak of A responsible proposal to begin the
population of 3.9 X 109 people and a 16.5 billion people to whatever stable construction of the first colony must be
growth rate7 of 1.98% per year (the value is desired. I have suggested 1.2 based on a demonstration, in some de-
1965-71 average), the asteroidal materi- billion as a possible optimum; it corre- tail, of one workable plan with realistic
al would last 500 years, corresponding sponds to the year 1910 in Earth histo- cost estimates. I emphasize two points
to a 20 000-fold population increase at ry. The reduction in population densi- about any such plan: The details pre-
low population density. ty in the space communities could be sented should be thought of simply as
In figure 4, we see the development of equally rapid, and within another 40 an existence proof of feasibility; and
this worst-case problem. To hasten the years new construction could thin out many variations are possible. The op-
solution of that problem, the initial
space community population density is
taken as the ecological limit; the maxi-
mum number of people that can be sup- Table 3. Guideline Parameters for Transport Linear Accelerator
ported with food grown within the com-
munities, with conventional agriculture.
Richard Bradfield has grown enough to Acceleration 288 meters/sec2
feed 72 people per hectare by the tech- Average accelerating force 900 lbs
Maximum field 10 000 gauss

25 August 2023 18:23:10

niques of double planting and multiple
Bucket dry mass 5 kg
cropping, and with the use of cuttings Payload 9 kg
for livestock feed. These results,8 as Repetition rate 1/sec
published and also as described to me Transport rate 750 tons/day
by Bradfield, were obtained in the Phil- Buckets on accelerator 8
lipines, which has only a nine-month Sector length (accelerator) 50 meters
Inductance per meter 0.6 microHy
growing season and less than ideal Peak stored energy per meter 10.4 KJ
weather conditions. Calculations based Maximum frequency (LSM)* 2500 Hz
on his figures, but assuming an ideal Instantaneous length driven 2 meters
twelve-month season, indicate that the Direct current in bucket coil 75 X 103 amp-turns
Peak current in LSM* 136 X 103 amps
colonies should be able to support 143 Acceleration power 40 MW
people per hectare with a diet of 3000 Ohmic losses (feeders) 15 MW
calories, 52 grams of usable protein and
4.3 pounds of total food per person per ' LSM: linear synchronous motor
day.9 Much of the protein would come
from poultry and pork. The two main

Table 4. Estimated Cost of Building Space Colonies (in 1972 dollars)

Model 1 Model 2
9
Item Unit cost Total (in $10 ) Unit cost Total (in $109)
9 5
Launch vehicles 0.3 X 10 0.9 0.5 X 10 1.5
Transport E —• L5 425/lb 8.5 250/lb 11.0
People E — L5 1000/Ib 2.2 500/lb 8.8
Transport E - • M 1000/1b 6.6 500/1 b 2.2
Equipment for Moon 400/1 b 2.4 400/lb 1.8
Equipment for L5 180/lb 1.2 180/lb 2.0
Machines and tools (L5) 625/lb 1.1 625/lb 2.8
Salaries (Lr,) 50 000/man-year 0.6 (25% on Earth) 2.0
Salaries (Earth) 30 000/man-year 7.2 (30 000/man-year) 2.0
Totals 30.7 34.1*
($5.1 X 109/yr) (4.3 X 10 9 /yr*)

'The cost saving due to the presence of Model 1 can be divided as follows: production, 25 000lbs/man-year; workforce, 4000 people; transport
costs, $250/lb. The saving over the eight years needed to complete the colony is thus a total of $200 X 109.

PHYSICS TODAY/SEPTEMBER 1974 37

timum design and course of action can first colony it is probably best to choose method for transporting raw materials
only be decided on after study and con- a particular point on that sphere, within from the Moon to the construction site.
sultation among experts in a number of easy range of both Earth and Moon, not The discussion of transport methods
fields. so close as to be eclipsed often, and should be taken as an existence proof
The nominal values for the first preferably stable against displacements rather than as a detailed design. There
model colony are taken as: construc- in all three coordinates. The L4 and L5 may very well be better methods than
tion force, 2000 people; population, Lagrange libration points satisfy all those I have considered; however, it is
10 000; total mass, 500 000 tons. When these conditions. They have the fur- enough to show two solutions that ap-
the design and cost analysis are done in ther advantage of forming only a very pear to be workable. Both use the two
detail for the entire enterprise, the need shallow effective-potential well.11 great advantages of the lunar environ-
to fit a budget may force some reduc- Earth, Moon, Sun and the colony ment: an excellent vacuum and a very
tion in size. The initial estimates have form a restricted four-body gravita- low escape velocity, about 1.5 miles per
been aimed at holding the cost equal to tional problem, for which the full solu-. sec, less than one quarter of the escape
that of one project we have already car- tion has only been worked out within velocity from Earth. To bring a kilo-
ried through: Apollo. The choice of the past several years.12 The stable gram to L 5 from the Moon takes less
10 000 as a target population ensures motion is a quasielliptical orbit, of largethan 5% of the energy needed to take a
that, even with some reduction, Model 1 dimensions, about L5. The maximum kilogram from Earth.
will be large enough to obtain econo- excursions in arc and radius are several Both methods assume electric power
mies of scale and to serve as an effective tenths of the Earth-Moon distance. from a conventional steam-electric
industrial base for the construction of On the stable orbit there is room for power plant that uses solar energy, and
Model 2. A much reduced colonization several thousand colonies; a long time both assume that the system runs only
project would be little more than a re- will pass before colonization can fill so during the lunar day, the night being
named space station, perhaps able to big an orbit. used for scheduled maintenance, crew
maintain itself but incapable of build- rest and possibly materials processing.
ing the larger models that are necessary Cost minimization I have also assumed another factor of
if the program is ultimately to support There are several key problems in- two lost to system breakdowns. Overall
itself. It is an essential feature of the volved here, each of which appears to then, each system is assumed to be run-
colonization project that Earth should yield to an efficient solution in princi- ning only one week in four.
no longer have to support it after the ple: reducing freight-shipment cost The first method, called "RPL" for
first two or three stages. from the Earth to L5, the colony site; rotary pellet launcher, is a symmetric,
Ultimately, colonization could take minimizing the mass needed from two-arm propeller-like device, running
place in the entire sphere, 3 X 1017 km2 Earth; designing a device for low-cost at constant speed. (See box on page 38
in area, that surrounds the Sun at the transfer of materials from the Moon to for description). To transfer 500 tons
distance we have evolved to prefer (the L5. in six years, about 26 such RPL's would

25 August 2023 18:23:10

so-called "Dyson sphere"). For the The first problem was considered by be needed, for a total power of 32 MW.
Robert Wilson (NASA), Eric Hannah Precise steering is carried out by a lin-
and George Hazelrigg (Princeton) at a ear electromagnetic deflection-plate
meeting held 9 and 10 May at Princeton system after the launching, to hold
(A Proceedings of this meeting will be down the pellet dispersion and permit
Rotary pellet launcher published). Their conclusion was that easy collection.
The rotary launcher is assumed to be a the best method during the 1980's will The alternative method, called
symmetric two-arm propeller-like device, probably be conventional chemical "TLA" for transport linear accelerator,
running at constant speed, with launching rockets—specifically, the high-quality uses the technology of dynamic magnet-
arms of ten-meter radius.
engines already being developed for the ic levitation and the linear synchronous
Mass 10 tons space shuttle. Among several varia- motor. The TLA is a recirculating sys-
Rotation rate 2300 rpm tions possible, the common feature was tem of small, passive vehicles (buckets),
Tip speed 2400 m/sec reusability, and the cost estimates for each having no moving parts but con-
(escape velocity) shipment varied from $190 to $400 per taining superconducting coils. The
Power 1600 horsepower pound, in 1972 dollars. The cost sum- bucket accelerates a 9-kg payload to es-
mary table (Table 4) therefore assumes cape speed along a magnetic-levitation,
$425 per pound. linear-synchronous track. Deceleration
The transfer rate per launcher is 3250
tons per year for the transfer of 5-gm pel- To reduce the mass needed from then releases the payload, the bucket
lets, assuming a 25% duty cycle. The Earth, most of the repetitive structural slows to a moderate speed, and is recir-
strength-to-mass ratio for the launcher is members (aluminum) and window pan- culated to receive another payload.
within the range attainable by boron-fila- els (glass) must be produced at L 5 from Table 3 shows some guideline parame-
ment technology: An aluminum matrix lunar material. A further, important ters. The mass estimate is 1500 tons, of
containing boron grown on tungsten cores saving is made by getting 89% of the which about 80% is in power-generation
is calculated to have a yield stress of mass of needed water from oxygen in and power-handling equipment. In six
322 000 psi and average density 4.1, so
the plentiful lunar-surface oxides, years, running 25% of the time, the TLA
that
bringing only 11% of the water mass as can transport over 300 times its own
p/T = 135 X 10~6 liquid hydrogen from Earth. Of the weight. (For a short bibliography of
500 000-ton total mass (see Table 2) for early work on the possibilities of elec-
Here p is the density and f t h e tension in
MKS units. For uniform stress, the ratio
the Model 1 colony, 98% can be ob- tromagnetic launching, before the de-
of arm radii at the base and the tip (r, and tained from the Moon. The elements velopment of dynamic magnetic levita-
r2)is most needed are aluminum, titanium, tion, see reference 13.)
silicon and oxygen. Lunar surface soil Both RPL and TLA may have even-
log rjr, = (p/47V is usable for agriculture, with the addi- tual applications as high-throughput,
where v is the escape velocity. For r-[/r2 tion of nitrates and small amounts of energetically efficient reaction motors,
less than 50, pi T must be less than 2.08 trace elements. The remaining 10 000 running on solar power and able to use
X 10" 6 . tons must come from the Earth. any kind of asteroidal debris as reaction
To bring the total cost within practi- mass. They could propel very large
cal limits, we must develop a low-cost payloads, in the million-ton range or

38 PHYSICS TODAY/SEPTEMBER 1974

 forward in time; going back the same
number of years brings us to the era of
the V2 rocket, more than ten years be-
fore the first artificial satellite).
Around the year 2000, a fully reusable
chemical rocket system could transport
payloads to L 5 at a cost of about $100
per pound (again, in 1972 dollars). A
prospective colonist could therefore
save enough money (one or two years'
salary) to emigrate with his family of
three. The near certainty of continued
advances in propulsion systems
suggests that the actual costs will be
lower.
By the middle years of the next cen-
tury, and possibly earlier, production
costs at L5 should be lower than on
Earth. My reasons for this belief are
that:
• the asteroid belt is a rich source of
raw materials, already exposed and dif-
ferentiated.
1975 2150 • transport from the belt to L5 can be
done in a way analogous to ocean
freight on Earth; that is, in very large
Effectiveness of space colonization in solving a hypothetical "worst case " population-growth units, with low fuel costs and very small
problem. The case considered assumes no reduction of population growth rate either on Earth crews. In space, it may be most practi-
or in the space colonies. Here Pe is the population of Earth, Ps that of space, and A^lA^ the cal to eliminate the freighter hulls en-
ratio of land area in space (all usable) to total land area of Earth. Both Pe and Ps/A^ reach sta- tirely. A TLA-type reaction motor can
ble, relatively low values. Changes within wide limits in the assumed input numbers do not af- run on free solar power and transport
fect the reaching of a stable solution, nor do they affect the final stable values of Pe and Psl A$. an entire asteroid to L5, perhaps with
This figure is an example of the power of space-colonization, not a prediction. Figure 4 no crew at all.
• food-raising costs, production costs

25 August 2023 18:23:10

and shipping costs among the com-
higher, between the asteroid belt and 4. For comparison, the Apollo project munities should all be lower than on
the L5 site. cost about 33 billion (1972) dollars. Earth because of ideal growing condi-
The Model 1 colony will be too small We can also see in Table 4 that the tions, proximity of farms to consumers,
to carry out a wide variety of manufac- economic payoff from the construction availability of unlimited solar power
turing processes, but it can perform of the first community will come quick- and the convenience of zero-gravity and
those tasks that are energy intensive, ly, during construction of the second. high-vacuum environments for produc-
not labor intensive, and that will pro- That payoff will be in the form of trans- tion and transportation.
duce a large return in total tonnage. port costs saved because tools and fab- If we are so prodigal as to run
One example is the production of alu- ricated structures will be made from through the entire material of the aster-
minum by the Hall process. An in- lunar material at Community 1 rather oid belt in the next 500 years, we can
stalled capacity of 40 MW is enough to than on Earth. The first colony can ap- even gain another 500 years by using up
produce 20 000 tons of aluminum in two parently pay for itself in one or two the moons of the outer planets. Long
years, for the exostructure of Model 1. years, and, by its presence, can keep the before then, I hope we will have slowed
Another example is the separation of annual cost of building Community the growth of the human population.
oxygen from the lunar oxides to com- 2—with its 100 000 to 200 000 people— And I feel sure that long before then a
bine with hydrogen brought from at about the same level as for Commu- modified version of a space community
Earth. (With 50 000 tons of water, nity 1. After that, construction costs will have travelled to a nearby star.
Model 1 can have lush vegetation as for models 3, 4 and so on, should taper I am left with the desire to communi-
well as substantial streams and small off as space-based industry becomes cate two aspects of this work more com-
lakes). In contrast, small, low-mass stronger, and as the wide range of pletely. On the one hand. I would like
parts are best brought from Earth. chemical elements in the asteroids are to display for review more of the details
The later colonies, perhaps beginning used. of calculations and references than is
with Model 3, will use asteroidal mate- We can speculate that the second or possible here. And on the other hand, I
rial, which is rich in hydrocarbons as third colony may begin to pay back its am acutely aware of the need for discus-
well as in metals. We can speculate construction cost in additional ways, for sion outside our own group of physics-
that, relatively early in the development example by the manufacture of high- oriented people. This work should be
of the colonies, the economics of freight strength single crystals14 in the zero- discussed and debated as widely as pos-
transport will probably dictate that the gravity, high-vacuum environment that sible, by people with a range of techni-
"up" shipments from Earth will consist surrounds it, and by the manufacture of cal and artistic talents, and by people
only of people and labor-intensive, titanium products. who claim no special talent beyond the
miniaturized products such as comput- To follow the economics as far as ability to work hard for a worthwhile
ers and calculators. The "down" ship- Model 3 would be too speculative; its goal. I hope I have conveyed at least a
ping costs may be lower because of the costs to Earth will mainly be those of little of the sense of excitement that I
possibility of atmospheric braking. Be- transporting its one to two million in- have enjoyed over the past few years as
tween colonies, all shipping and travel habitants to L5. Its earlies possible each serious problem has appeared to
costs should be very low. For Model 1, completion date is estimated at just yield to a solution, as well as how much
the project cost is summarized in Table after the turn of the century (28 years more remains to be done and how much

PHYSICS TODAY/SEPTEMBER 1974 39

 need there is for good ideas and hard
work.
* * *
FROM SCHOTT PRODUCTION For private communications leading to ref-
erences, I thank Donald Gault, Barry Royce,
Richard Johnson, George Hazelrigg and
John Breakwell. And it is a special plea-
sure to thank those who encouraged me to
continue this work in the years when it was
little known, particularly George Pimentel,
Freeman Dyson, Brian O'Leary, Roman
Smoluchowski, Richard Feynman and John
Tukey. I am also grateful to Michael Phil-
lips of the Point Foundation, which sup-
ported the first public meeting on this
subject.
References
1. G. A. Hool, W. S. Kinne, Movable and
Long Span Steel Bridges, McGraw-Hill,
New York, (1943), page 328; D. B. Stein-
man, A Practical Treatise on Suspen-
sion Bridges, John Wiley, New York
(1929), page 236.
2. S. F. Singer, Scientific American, Sep-
Finished tember 1970, page 174.
3. "Meteoroid Environment Model—1969
Color Filters (Near Earth to Lunar Surface)," NASA
SP-8013.
4. G. Latham, J. Dorman, F. Duennebier,
M. Ewing, D. Lammlein, Y. Nakamura,
"Moonquakes, Meteorites and the State
MPT DELIVERY AND HIGH QUALITY of the Lunar Interior," and "Lunar
Seismology," in Abstracts of the Fourth
Lunar Science Conference, 1973, Lunar
Science Institute, 3303 NASA Road 1,
Schott's new color filter glass finishing Houston, Texas 77058.

25 August 2023 18:23:10

operation is now on-line . . . adding the serv- 5. R. E. McCrosky, "Distributions of Large
Meteoric Bodies," Smithsonian Astro-
ice of an in-house finishing facility to Schott's physical Observatory Special Report No.
well-earned reputation for precisely-controlled 280.
color glass melts. The newest concepts in 6. K. MacLeish, "Australia's Wild," in Na-
tional Geographic 143, no. 2,168, (1973).
filter finishing, including special equipment 7. "1970 World Population Data Sheet,"
designed expressly for Schott, now insure the Population Reference Bureau Inc, 1755
Massachusetts Ave, N. W., Washington,
best and most competitive product available. D.C. 20036.
8. R. Bradfield, "Multiple Cropping—
No longer must the designer choose be- Hope for Hungry Asia," in Reader's Di-
tween standard, readily available filters and gest, October 1972, page 217.
9. F. M. Lappe, Diet for a Small Planet,
Schott's quality glass. The purchasing agent Ballantine Books, New Vork, (1971).
now has a competitive full service source, 10. "The Limits of Development," Report
by the Systems Dynamics Group, Massa-
from glass to finished filter. And, with design chusetts Institute of Technology (1972),
and purchasing well in hand, the project man- Club of Rome, Geneva.
ager has that much more time to devote to 11. W. H. Michael Jr, "Considerations of the
Motion of a Small Body in the Vicinity
profitability. of the Stable Libration Points of the
Earth-Moon System," NASA TR-160
In short, this new approach offers some- (1963).
thing for everyone. Give us a call and see what 12. R. Kolenkiewicz, L. Carpenter, "Stable
Periodic Orbits About the Sun-Per-
it can do for you. turbed Earth-Moon Triangular Points,"
AIAA Journal 6, no. 7, 1301 (1968); A. A.
Kamel, "Perturbation Theory Based on
" We made the art a science. Lie Transforms and its Application to
the Stability of Motion Near Sun-Per-
Now we've made it a modern manufacturing process." turbed Earth-Moon Triangular Libra-
tion Points," NASA CR-1622, August
1970.
SCHOTT OPTICAL GLASS INC. 13. A. C. Clarke, J. Brit. Interplanetary Soc.
9,261(1950).
DURYEA, PENNSYLVANIA 18642
14. H. C. Gatos, A. F. Witt, "Crystal Growth
(717) 457-7485 TELEX 837423 Studies on Skylab," MIT News Release,
14 May 1974. °
Circle No. 20 on Reader Service Card

40 PHYSICS TODAY/SEPTEMBER 1974