For fictional structures for living in space, see Space stations and habitats in popular culture.

## The O'Neill cylinder

Island Three The O'Neill cylinder

The O’Neill “Island Three” habitat is a gargantuan cylinder with hemispherical end caps, 32 kilometers (20 miles) long and 6.4 kilometers (four miles) in diameter, with a habitable surface area of 325 square kilometers (125½ square miles) or 32,500 hectares (80,310 acres) supporting a population in the tens of millions.

(In the Gundam canon, the population is generally given as three to ten million.) The cylinder is rotated on its long axis at ½ RPM (one revolution every two minutes) to simulate Terrestrial gravity for the people living inside. (½ RPM is not very impressive visually, so the apparent rate of rotation is exaggerated to about two RPM in the animation.)Orbiting with one end facing the sun, it’s divided lengthwise into six alternating “ground” and “sky” panels, so only half of the inner surface is actually available for habitation.

Three mirrors project outward at a 45° angle from the end facing away from the Sun and reflect sunlight through the translucent “sky” panels to the landscaped “ground” panels opposite them.

Because the end caps of the cylinders are domed, each of the “ground” panels has what, from an inhabitant’s point of view, appears to be a 3.2-kilometer (two-mile) high “mountain” at either end.The simulated “gravity” resulting from the rotation varies from one “G” at the base of the mountain to zero-G at the apex. The drop-off is linear—at the 1.6-kilometer (one-mile) level, midway (45°) up the mountainside, the pseudo-gravity is 50% (½ G). You can calculate the acceleration that produces this pseudo-gravity using the formula F=rω²/g, where F is the resulting acceleration, r is the distance from the central axis, ω is the angular velocity (a constant equal to times the number of rotations per second) and g is the acceleration due to gravity experienced on Earth (9.80665 m/s² or 32.174 ft/s²).

This is equivalent to the more familiar F=mV²/r formula, only substituting V=rω.

(On 7 November 2002, Ian Woollard wrote me to correct my math regarding the drop-off rate.)

The mountains and the “valleys” between them are landscaped to an idyllic green splendor, supporting six densely populated urban and suburban civic and residential centers. The underlying cylinder hull is a meter (3 feet, 3 inches) of titanium-reinforced “mooncrete” or lunar concrete, a mineral aggregate of anorthosite, ilmenite, and “KREEP,” an acronym for potassium (K), rare earth elements (REE) and phosphorus (P).The three “ground” panels are covered with an average 5-meter (16.4-foot) layering of landscaped topsoil.

The three “sky” panels are composed of quartz glass, vitreous silica prepared from pure quartz and noted for its transparency to ultraviolet radiation. Each “sky” panel is 3.2 kilometers (two miles) wide and 25.6 kilometers (16 miles) long, divided into eight square “windows” 3.2 kilometers on a side. Bridges connecting the “ground” panels span the “sky” panel at the junctions of these windows, seven bridges across each of the three “sky” panels, for a total of 21 “sky” bridges in all.

The basic element or building block of the “sky” panels is a cubical quartz glass prism 3.2 meters (10.4 feet) on a side, massing about 80 tonnes (90 tons). The prisms are mounted in a five-by-five titanium grid to form a square “frame” 16 meters (52 feet) on a side and three meters deep, with 25 prisms per frame.These frames are mounted, four ply, in a five-by-five array “pane” 80 meters (260 feet) on a side and 12.8 meters (41.6 feet) deep, with 100 frames (2,500 prisms) per pane.

The panes are mounted in a five-by-five “sash” 400 meters (1,312 feet) on a side, with 25 panes (2,500 frames or 62,500 prisms) per sash. Each of the eight windows is thus an eight-by-eight array of 64 sashes, containing 1,600 panes (160,000 frames or four million prisms), so each “sky” panel contains 512 sashes (12,800 panes or 1,280,000 frames or 32 million prisms).

Since there are three such panels, each colony has 24 windows (1,536 sashes or 38,400 panes or 3,840,000 frames or 96 million prisms) containing a combined mass of about 7,680 megatonnes (8,640 megatons) of quartz glass.

Docking ports called “bay blocks” at either end of the colony’s central axis rotate in the opposite direction, maintaining a “stationary” position around which the colony proper appears to rotate. Laser beacons line a five-kilometer approach path for incoming spacecraft. A solar power station (SPS) generating a gigawatt per hour is built into the port docking port.

Each docking port contains six docking bays, arranged around the axis like the chambers of a revolver. Each docking bay has six docks, arranged in a similar fashion around the centerline of the bay. Each dock can accommodate three 300-meter ships, for a total capacity of 108 ships. Zero-G industrial blocks are strung out along the axis between the docking ports and the end caps, standard-G industrial blocks are mounted on the exterior of the colony cylinder. All of the agriculture and industry is external to the colony proper, so all of the space within the colony cylinder is actual living space for the colonists, pure and unpolluted.(The O’Neill design specified solar power to supply the colony’s needs, but there’s another simple, effective and continuous sources of energy readily available, which is to run thermally conductive material from the interior to the exterior and from the north end cap to the south end cap and use the temperature differential—an extraterrestrial equivalent of “geothermal” power.)

Since the spacecraft bay blocks are necessarily at the center of the end caps, in line with the axis of rotation, the “mountainsides” on the interiors of these end caps are heavily urbanized. Six major cities are built at the bases of these mountains, three at either end, thinning out as they spread down the “foothills” and into the “valleys” toward the equator. (In a reversal of the mundane trend, it is the “hillside” which is the less desirable, “poor” side of town) The central zone at the equator is kept in a state of artificial “wilderness” dotted with a few small rural villages and highly prized resorts. Each colony thus contains six separate urban civic centers, six suburban residential zones and three rural recreational areas, each with its own distinct identity, as a safeguard against inbreeding and cultural stagnation.

Each of the three valleys within the colony is an elongated rectangle 32 kilometers (20 miles) long and 3.2 kilometers (two miles) wide, yielding a total area of 105 square kilometers (40 square miles). The six cities and their associated suburbs cover an area of 41.4 square kilometers (16 square miles) each. The three rural areas cover an area of 20.7 square kilometers (eight square miles) each, which must be shared evenly between the two urban/suburban centers at either end.

Travel from the docking bay and industrial blocks at the axis “down” to the residential areas in the valleys or “up” to the agricultural block ring is via elevator, usually depicted as a set of three vertical tubes spaced 120° apart. If so, riding them would be murder, due to the same Coriolis effect that produces the artificial “gravity” at the hull. As the elevator “rises” from the hull to the axis, the passengers are going to be pushed downspin at the same rate as they are inward, with the result that the “floor” is going to feel as if it’s been upended at a 45° angle.

The same applies going “down” from the axis to the hull, except that the push is going to be upspin. A body dropped from the axis to the hull would fall in a Nautilus-shell helical spiral, appearing to travel in an upspin arc around the axis until it finally impacted, not on the ground panel immediately below, but the ground panel upspin from there. The fall would take about five minutes 20 seconds and make one and one-third revolutions, with a terminal impact of 644 KPH (400 MPH).

Presuming that the elevator accelerates and decelerates at the same rate, minus the sudden sharp stop going from axis to hull, travel time would be the same as it is for a free fall, with the Coriolis effect converted into lateral forces on the vertically restricted passengers. That being the case, the best design for the elevator would be an upspin spiral for the cars going from axis to hull and a downspin spiral for the cars going from hull to axis. The cars would not run “vertically” (i.e., perpendicular to the “ground”), but drive “parallel” to the hull the entire trip.

Population of a colony is, as noted above, somewhat problematic. O’Neill was very detailed in his descriptions of the Island One and Island Two configurations, which he was trying to persuade the U.S. Congress to try and build, but much less so for Island Three, which he held out as the pot of gold at the end of the rainbow. In most instances, he merely referred to “populations in the millions” but on at least one occasion he stated: “Island Three … could support quite easily a population of ten million people.”Most of the Gundam references cite populations of three to ten million per colony, but the question is confused by the fact that there are two types of colonies: the “open type” colonies using the O’Neill design and the more efficient “closed type” colonies with twice the habitable area. It would not be unreasonable to assume that doubling the habitable area would also double the population capacity. (In reality, it’s not that easy, as doubling the population quadruples the strain on the environment.) In any case, a closed type colony should support at least half again as many people as an open type.

Population figures are few and far between throughout the Gundam Saga. Six and a half million people had to be evacuated from Mahal, a closed type colony in Side 3, so that the colony could be converted into the Solar Ray System in UC 0079. Three million colonists were killed in Bunch 30, an open type colony in Side 1, when it was nerve-gassed by the Titans in UC 0085. Eight million people were killed in Bunch 21, an open type colony in Side 2, when it was blown apart by the Colony Laser in UC 0087.

Five million people lived in Londinium, an open type colony in Side 1, when it served as the Londo Bell’s homeport in UC 0093. Ten million people lived in Frontier IV, a “60% to 70% completed” open type colony in Side 4, when it was invaded by the Crossbone Vanguard in UC 0123.

The only populations figure that is consistent throughout the Gundam Saga is that, at the start of the One Year War, there were a total of eleven billion people in the Earth Sphere, nine billion of whom lived in space. Of these, it is estimated that a billion lived in subterranean colonies on the Moon. Another billion were scattered among the various asteroid settlements and geosynchronous satellite stations.

The remaining seven billion lived in the six “Sides” orbiting the Lagrange points, one billion per Side except for Side 3, which alone used the newer closed type colonies to support a population of two billion.

If each Side contained a hundred open type colonies, a population density of ten million per colony yields the requisite billion per Side. A hundred closed type colonies with twenty million people each would yield the requisite two billion for Side 3. The highest number of colonies ever given for a Side is eighty-five (Side 2 in UC 0087), but that just tells us that the top end is at least eighty-five.

If eighty-five is actually in the mid-range, the top end could easily be up to 150 colonies per Side, with populations of 6.67 million apiece. Population estimates of three to ten million per open type colony and six to twenty million per closed type colony are therefore most probably correct.The issue is further confused by the fact that O’Neill envisioned his colonies being built not as single units but as ballistically coupled pairs, 80 kilometers (50 miles) apart. Was the “population of ten million people” that O’Neill cited the population of both cylinders, yielding five million people per unit, or the population of each cylinder, yielding twenty million per pair?

The former puts 1.67 million people in each valley, with as many as 835,000 in each of the six urban centers, at an urban-to-rural ratio of four to one (80% to 20%). The latter puts 3.34 million people in each valley, with up to 1.67 million in each of the six urban centers, with the same 4:1 urban/rural ratio.

This is not so dense a population as it sounds. By way of comparison, the Manhattan Borough of New York City is an elongated irregular strip twenty kilometers (12½ miles) long and four kilometers (2½ miles) wide. It has a total area of 32½ square kilometers (12½ square miles)—roughly equivalent to one of the three valleys. Central Park is an elongated rectangle four kilometers (2½ miles) long and 800 meters (½ mile) wide, giving it a total area of 3.2 square kilometers (1¼ square miles). As of 2000 AD, Manhattan supported a population of 1,537,195 at an urban-to-rural ratio of 10:1—just under half as dense as the ten-million-per-cylinder scenario described above, but with 2½ times as much urban sprawl.

(On 12 September 2002, Julian H. Fong wrote me to note that, while the Gundam animation and artwork ignore it, there’s an important reason why O’Neill colonies must be ballistically coupled pairs. A single cylinder, rotating independently, is gyroscopically stable but it’ll always point toward the same point in space and thus only faces the Sun once a year. In the O’Neill design, the paired cylinders rotate in opposite directions, so the net angular momentum of the system is zero and the linked cylinders can be made to precess with a one-year period, keeping them aligned with the Sun. Without this precession, and the zero angular momentum necessary to achieve it, the “sunflower” illumination scheme simply won’t work!)All of the available space within the colony is given over to habitation. Agriculture, as noted above, is external to the colony proper, outside the residential cylinder. Seventy-two “hatbox” cylinders, each 645 meters (2,110 feet) across and 645 meters (2,110 feet) deep, enclosed by a 1.3-kilometer (4,265-foot) parabolic solar energy concentrator, orbit the industrial block at the north end of the colony. Linked into a giant ring by an annular access tube and connected to the end cap by three 32-kilometer (20-mile) radial spokes, they’re called agricultural blocks or farming satellites (“farmsats”). Each contains 1.3 square kilometers (½ square mile) or 129.4 hectares (320 acres) of hydroponics greenhouses, warmed and illuminated by the concentrated sunlight.

Altogether, each colony has 93 square kilometers (36 square miles) or 9,325 hectares (23,040 acres)—the equivalent of an American rural county “township”—of dedicated, arable farmland.

(In the Gundam animation and artwork, the agricultural ring is often shown at the far end of the colony, but this is technically incorrect, as it would result in the farm modules being eclipsed by the mirrors. And, just as the rotation of the colony is either ignored or exaggerated in the animation, the apparent size of the farm satellites is also exaggerated to make them visible next to the colony proper, with the result that their number is reduced to fifty or sixty. More often, a complete ring isn’t even shown. The sketches of the O’Neill colonies often included only a few representative farm satellites instead of a complete set and the Gundam artists slavishly copied these incomplete drawings in their animations and production art.)

It should be noted that the ring does not rotate along with the colony proper. If it did, the farms would be subjected to pseudo-gravity close to five times that of the Earth. Instead, the ring remains fixed and each of the seventy-two agricultural blocks rotates at two RPM to produce Terrestrial gravity at its inner hull.

The colonies run on a 24-hour clock set to the Universal Time Coordinate (Greenwich Mean Time adjusted to the Terrestrial equator), with “sunrise” at 06:00 UTC and “sunset” at 18:00 UTC. Varying the angle and pitch of the external mirrors can simulate day and night cycles and even seasonal changes. Any Terrestrial climate can be simulated, but generally the air temperature is held between 5° to 25° C (40° to 80° F) and averages 15° C (60° F), with a relative humidity of 40% to 60%—the temperate climate that southern California promises but seldom delivers. The ground temperature ranges from 5° to 50° C (40° to 120° F), with ground water temperature falling midway in between at 10° to 40° C (50° to 105° F), averaging 25° C (80° F) for both. The air pressure is equivalent to that at a Terrestrial elevation of 1.6 kilometers (one mile) above sea level, about the same as Denver, Colorado.

Transportation within the colony is by the ubiquitous “elecar” or electric-powered car, which range in size and power from a two-seat “go-cart” to a containerized cargo “mono-wing” truck. Powered by a fuel cell that burns hydrogen and oxygen to produce electricity and water vapor, which can be broken down and recycled almost endlessly, they are clean, quiet and economical.

Airtight “linear cars” traverse the outer hull in a manner analogous to the metro subway, riding on superconducting magnetic-levitation (“maglev”) rails at the colony’s rotational speed of 644 kilometers per hour (400 miles per hour) and admitting a spectacular view. “Linear trams” resembling the cable cars of San Francisco run up and down the end cap mountainsides, connecting the urban centers to the zero-G industrial blocks and bay blocks. They also allow for easy transport between ballistic coupled pairs of colonies, 80 kilometers (50 miles) apart, with a transit time of seven minutes and twenty-seven seconds (00:07:27) each way.

The O'Neill cylinder, also called an Island Three habitat, is a space habitat design proposed by physicist Gerard K. O'Neill in his book The High Frontier.<ref name="ONeill-HighFrontier">{{cite book
| last = O'Neill
| first = Gerard K.
| authorlink = Gerard K. O'Neill
| year = 1977
| title = The High Frontier: Human Colonies in Space
| location = New York
| publisher = William Morrow & Company
| isbn = 0-688-03133-1
}}</ref>
In the book O'Neill proposes the colonization of space for the 21st century, using materials from the Moon.

An O'Neill cylinder consists of two very large, counter-rotating cylinders, each 5 miles (8 km) in diameter and 20 miles (32 km) long, that are connected at each end by a rod via a bearing system.  They rotate so as to provide artificial gravity via Centripetal force on their inner surfaces.<ref>ibid. High Frontier, chapter V</ref>

## == Background ==

While teaching undergraduate physics at Princeton University, O'Neill had students design large structures in space, with the intent to show that living in space could be desirable.  Several of the architectures were able to provide areas large enough to be suitable for human habitation.  This cooperative result inspired the idea of the cylinder and was first published by O'Neill in a September 1974 article of Physics Today.<ref name="ONeill-Colony">{{cite journal
| last = O'Neill
| first = Gerard K.
| year = 1974
| month = September
| title = The Colonization of Space
| journal = Physics Today
| volume = 27
| issue = 9
| pages = 32–40
| issn = 0031-9228
| e-issn = 1945-0699
| format = PDF (subscriber only)
| accessdate = 2009-04-19
}}</ref>

## == Islands One, Two and Three ==

O'Neill has created three reference designs:

; Island One
: A sphere measuring one mile in circumference (1,681 feet or 512.27 meters in diameter) which rotated, and people lived on the equatorial region. (See Bernal sphere.)
: A later NASA/Ames study at Stanford University developed an alternate version of Island One: the Stanford torus geometry, a toroidal shape 1,600 meters (just under a mile) in diameter.<ref>Space Settlements, A Design Study, 1977, NASA SP-413, accessed June 4, 2009</ref>
; Island Two
: Also a sphere, also 1,600 meters in diameter.
; Island Three
: Two counter-rotating cylinders each five miles (8 km) in diameter, and capable of scaling up to twenty miles (32 km) long.<ref name="NSS-ONeill">{{cite web
| url = http://www.nss.org/settlement/space/oneillcylinder.htm
| title = O'Neill Cylinder
| work = Orbital Space Settlements
| publisher = National Space Society
| accessdate = 2009-04-19
}}</ref>  Each cylinder has six equal-area stripes that run the length of the cylinder; three are windows, three are "land".  Furthermore, an outer agriculture ring, as seen in the picture on the right, 10 miles (16 km) in radius, rotates at a different speed for farming.  The manufacturing block is located at the middle (behind the satellite dish assembly) to allow for minimized gravity for some manufacturing processes.

To save the huge cost of rocketing the materials from Earth, these habitats were to be built with materials launched into space from the moon with a magnetic catapult called a mass driver.<ref>ibid, O'Neil, High Frontier, p149</ref>

## == Artificial gravity ==

The cylinders rotate to provide artificial gravity on their inner surface. Due to their very large radii, the habitats would rotate about forty times an hour, simulating a standard Earth gravity. Research on human factors in rotating reference frames
<ref>Beauchamp, G.T.:Adverse Effects Due to Space Vehicle Rotation, Astronautical Sciences Review, vol. 3 no. 4 Oct-Dec. 1961, pp.9-11</ref>
<ref>Proceedings of the Symposium on the Role of the Vestibular Organs in Manned Spaceflight, NASA SP-77, 1965; Especially helpful: Thompson, Allen B.:Physiological Design Criteria for Artificial Gravity Environments in Manned Space Systems</ref>
<ref>Newsom, B.P.:Habitability Factors in a Rotating Space Station, Space Life Sciences, vol. 3, June 1972, pp192-197</ref>
<ref>Proceedings of the Fifth Symposium on the Role of Vestibular Organs in Space Exploration, Pensacola, Florida, August 19-21, 1970, NASA SP-314, 1973</ref>
<ref>Altman, F.:Some Aversive Effects of Centrifugally Generated Gravity, Aerospace Medicine, vol. 44, 1973, pp.418-421</ref>
indicate that almost no-one (at such low rotation speeds) would experience motion sickness due to coriolis forces acting on the inner ear. People would be able to detect spinward and antispinward directions by turning their heads, however, any dropped items would appear to be deflected by a few centimetres.
<ref>ibid. NASA Study SP-413, pp22</ref>

The central axis of the habitat would be a zero gravity region, and it was envisaged that it would be possible to have recreational facilities located there.

## == Atmosphere and radiation ==

The habitat was planned to have oxygen at partial pressures roughly like the Earth's air, 20% of the Earth's sea-level air pressure. Nitrogen would also be included to add a further 30% of the Earth's pressure.  This half-pressure atmosphere would save gas and reduce the needed strength and thickness of the habitat walls.
<ref>ibid. High Frontier, p117</ref>
<ref>ibid. NASA Study SP-413, p22-3</ref>

At this scale, the air within the cylinder and the shell of the cylinder provide adequate shielding against cosmic rays.
<ref>ibid. High Frontier, p113-116</ref>

## == Sunlight ==

Large mirrors are hinged at the back of each stripe of window. The unhinged edge of the windows points toward the Sun. The purpose of the mirrors is to reflect sunlight into the cylinders through the windows. Night is simulated by opening the mirrors, letting the window view empty space; this also permits heat to radiate to space. During the day, the reflected Sun appears to move as the mirrors move, creating a natural progression of Sun angles. Although not visible to the naked eye, the Sun's image might be observed to rotate due to the cylinder's rotation. As an aside, the light reflected from the mirrors is polarized, which might confuse bees.
<ref>ibid. High Frontier, p63..64</ref>

To permit light to enter the habitat, large windows run the length of the cylinder.
<ref>ibid. High Frontier, p63</ref>
These were not to be single panes, but would be made up of many small sections, to prevent catastrophic damage, and so the aluminum or steel window frames can take most of the stresses of the air pressure of the habitat.
<ref>ibid. High Frontier, p112</ref>

Occasionally a meteorite might break one of these panes.  This would cause some loss of the atmosphere, but calculations showed that this would not be an emergency, due to the very large volume of the habitat.
<ref>ibid. High Frontier, p112</ref>

## == Attitude control ==

The habitat and its mirrors must be aimed at the sun.  O'Neill and his students carefully worked out a method of continuously turning the colony 360 degrees per orbit without using rockets that discard reaction mass.
<ref>ibid. High Frontier, p100</ref>
First, the pair of habitats can be rolled by operating the cylinders as momentum wheels. If one habitat's rotation is slightly retarded, the two cylinders will rotate about each other. Once the plane formed by the two axes of rotation is perpendicular (in the roll axis) to the orbit, then the pair of cylinders can be yawed to aim at the sun by exerting a force between the two sunward bearings: away from each other will cause both cylinders to gyroscopically precess, and the system will yaw in one direction, towards each other will cause yaw in the other direction. The counter-rotating habitats have no net gyroscopic effect, and so this slight precession can continue for the habitat's orbit, keeping it aimed at the sun.

* Dyson sphere
* Bernal sphere
* Centrifuge Accommodations Module
* Globus Cassus
* Stanford torus
* Space stations and habitats in popular culture

## == References ==

* {{cite book
| last = T. A.
| first = Heppenheimer
| title = Colonies in Space
| origyear = 1977
| url = http://www.nss.org/settlement/ColoniesInSpace/index.html
| format = HTML
| access-date = 2009-04-19
| edition = online book
| year = 2007
| publisher = National Space Society
}}

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