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Stanford torus
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<h2 id="history-of-the-concept">History of the concept</h2> <p>The Stanford torus was proposed during the 1975 NASA Summer Study,<a class="footnote-ref" id="fnref:4" href="#fn:4"><sup>4</sup></a> conducted at <a href="/facts/Stanford_University/JEQeF9z2">Stanford University</a>, with the purpose of exploring and speculating on designs for future <a href="/facts/Space_colonization/3xUDzls1">space colonies</a>, with the conclusions and the detailed proposal being published in 1977 in <i>Space Settlements: A Design Study</i> book, by Richard D. Johnson and <a href="/facts/Charles_H._Holbrow/VHSsnknL">Charles H. Holbrow</a><a class="footnote-ref" id="fnref:5" href="#fn:5"><sup>5</sup></a> (<a href="/facts/Gerard_K._O%2527Neill/vXsLmqw6">Gerard O'Neill</a> later proposed his Island One or <a href="/facts/Bernal_sphere/IIPszXaB">Bernal sphere</a> as an alternative to the torus<a class="footnote-ref" id="fnref:6" href="#fn:6"><sup>6</sup></a>). "Stanford torus" refers only to this particular version of the design, as the concept of a <a href="/facts/Rotating_wheel_space_station/aVlYRocn">ring-shaped rotating space station</a> was previously proposed by <a href="/facts/Konstantin_Tsiolkovsky/s2n3Nb6A">Konstantin Tsiolkovsky</a> ("Bublik-City", 1903),<a class="footnote-ref" id="fnref:7" href="#fn:7"><sup>7</sup></a> <a href="/facts/Herman_Poto%25C4%258Dnik/RB0rAPq6">Herman Potočnik</a> (1923)<a class="footnote-ref" id="fnref:8" href="#fn:8"><sup>8</sup></a> or <a href="/facts/Wernher_von_Braun/pzhbJTCj">Wernher von Braun</a> (1952),<a class="footnote-ref" id="fnref:9" href="#fn:9"><sup>9</sup></a> among others. </p> <h2 id="design">Design</h2> <p>Stanford torus (the proposed 10,000 people habitat described in the 1975 Summer Study, to be distinguished from other <a href="/facts/Rotating_wheel_space_station/aVlYRocn">rotating wheel space station</a> designs) consists of a <a href="/facts/Torus/J9SkaV1W">torus</a>, or <a href="/facts/Doughnut/zVkVhznp">doughnut</a>-shaped ring, that is 1.8 km in diameter and rotates once per minute to provide between 0.9 <i>g</i> and 1.0 <i>g</i> of <a href="/facts/Artificial_gravity/yyzveD81">artificial gravity</a> on the inside of the outer ring via <a href="/facts/Centrifugal_force/CWgvMw6w">centrifugal force</a>.<a class="footnote-ref" id="fnref:10" href="#fn:10"><sup>10</sup></a> </p><p>Sunlight is provided to the interior of the torus by a system of <a href="/facts/Mirror/wMu4u6BB">mirrors</a>, including a large non-rotating primary solar mirror. </p><p>The ring is connected to a hub via a number of "spokes", which serve as conduits for people and materials travelling to and from the hub. Since the hub is at the rotational axis of the station, it experiences the least artificial gravity and is the easiest location for <a href="/facts/Spacecraft/YQqQaUw0">spacecraft</a> to dock. <a href="/facts/Space_manufacturing/5zoFkz5x">Zero-gravity industry</a> is performed in a non-rotating module attached to the hub's axis.<a class="footnote-ref" id="fnref:11" href="#fn:11"><sup>11</sup></a> </p><p>The interior space of the torus itself is used as living space, and is large enough that a "natural" environment can be simulated; the torus appears similar to a long, narrow, straight <a href="/facts/U-shaped_valley/nN7A4GUB">glacial valley</a> whose ends curve upward and eventually meet overhead to form a complete circle. The population density is similar to a dense suburb, with part of the ring dedicated to agriculture and part to housing.<a class="footnote-ref" id="fnref:12" href="#fn:12"><sup>12</sup></a> </p> <h3>Chosen shape</h3> <p>The 1975 NASA Summer Study evaluated several options for the space habitat design, including spherical and cylindrical shapes, in addition to the toroidal one. The torus was chosen as the best option, among other reasons, because it minimized the amount of mass required to have the same area and radius of rotation.<a class="footnote-ref" id="fnref:13" href="#fn:13"><sup>13</sup></a> </p> <h3>General characteristics</h3> <ul><li>Location: Earth–Moon <a href="/facts/Lagrangian_point/XCAN4o3P">L5 Lagrangian point</a>.</li> <li>Human population: 10,000.</li> <li>Total mass: 10 million tons (including radiation shield (95%), habitat, and atmosphere).</li> <li>Diameter: 1,790 m (1.11 mi).</li> <li>Circumference: 5,623.45 m (3.49 mi).</li> <li>Rotation: 1 <a href="/facts/Revolution_per_minute/fdNmLToF">revolution per minute</a>.</li> <li>Temperature: 23 ± 8 °C.</li> <li>Radiation shield: 1.7 meters (5.6 feet) thick raw <a href="/facts/Lunar_soil/1eXW6nT7">lunar soil</a>.<a class="footnote-ref" id="fnref:14" href="#fn:14"><sup>14</sup></a></li></ul> <h3>Components</h3> <ul><li>Habitation tube (torus proper), with a diameter of 130 m (430 ft). 2/3 of its surface are made of aluminum plates, and the remaining 1/3 is filled with glass windows mounted on aluminum ribs, to allow sunlight to enter inside the torus.</li> <li>Non-rotating main mirror, that directs sunlight towards the central hub.</li> <li>Central hub, with a diameter of 130 m (430 ft). Secondary mirrors around the central hub direct sunlight towards the habitation tube.</li> <li>Fabrication sphere, connected to central hub's <i>South Pole</i>, with a diameter of 100 m (330 ft). It is also connected to a solar furnace and the habitat radiator.</li> <li>Docking module, connected to central hub's <i>North Pole</i>, with a diameter of 15 m (49 ft) and a length of 60 m (200 ft).</li> <li>Spokes: 6 spokes of 15 m (49 ft) diameter, connecting the central hub with the habitation tube. They have elevators, power cables, and heat exchange pipes between the torus and the hub.<a class="footnote-ref" id="fnref:15" href="#fn:15"><sup>15</sup></a></li></ul> <h3>Area and volume allocation</h3> <p>The circumference of the torus proper (about 5,600 m in all) would be divided into 6 sections of equal length. 3 of the sections would be used for agriculture, and the remaining 3 for residential uses. Agricultural and residential sections would alternate. A central plain would run through the full length of the torus. To gain space, structures would be terraced over the curved walls of the torus, while many commercial facilities (such as large shops, light industry or mechanical facilities) would be below the level of the central plain. According to the figures included in the study, the plain's floor would be about 1/4 of tube's diameter over the torus bottom, and each spoke would connect at the center of one of the 6 sections.<a class="footnote-ref" id="fnref:16" href="#fn:16"><sup>16</sup></a> </p> <h4>Non-agricultural uses</h4> <table><tbody><tr><th>Use<a class="footnote-ref" id="fnref:17" href="#fn:17"><sup>17</sup></a></th><th>Used land area (m2)</th><th>Number of levels</th><th>Total usable area (m2)<a class="footnote-ref" id="fnref:18" href="#fn:18"><sup>18</sup></a></th><th>Height per level (m)</th><th>Volume (m3)</th><th>Notes</th></tr><tr><td>Residential</td><td>120,000</td><td>4</td><td>490,000</td><td>3</td><td>1,470,000</td><td>Including dwelling units, private exterior space and pedestrian access space. Modular housing, allowing for one-or two-level clustered homes, as well as grouped apartment buildings with 4 or 5 stories, and terraced homes taking advantage of the edges of the central plain that runs through the torus</td></tr><tr><td>Shops</td><td>10,000</td><td>2</td><td>23,000</td><td>4</td><td>92,000</td><td>The authors of the study determined the space use from recommendations that call for 10 shops per 1000 people</td></tr><tr><td>Offices</td><td>3,300</td><td>3</td><td>10,000</td><td>4</td><td>40,000</td><td></td></tr><tr><td>Schools</td><td>3,000</td><td>3</td><td>10,000</td><td>3.8</td><td>38,000</td><td>With community multimedia center. The authors of the study calculated the space use for a student population of 10% of total population</td></tr><tr><td>Hospital</td><td>3,000</td><td>1</td><td>3,000</td><td>5</td><td>15,000</td><td>50-bed hospital with all the different needed facilities</td></tr><tr><td>Assembly (churches, community halls, theaters)</td><td>15,000</td><td>1</td><td>15,000</td><td>10</td><td>150,000</td><td></td></tr><tr><td>Recreation and entertainment</td><td>10,000</td><td>1</td><td>10,000</td><td>3</td><td>30,000</td><td>All commercial entertainment, including indoor activities and restaurants</td></tr><tr><td>Public open space</td><td>100,000</td><td>1</td><td>100,000</td><td>50</td><td>5,000,000</td><td>Parks, zoo, outdoor recreation (swimming, golf, playgrounds)</td></tr><tr><td>Service industry</td><td>20,000</td><td>2</td><td>40,000</td><td>6</td><td>240,000</td><td>Light service industry of personal goods, furniture, handicrafts, etc.</td></tr><tr><td>Storage</td><td>10,000</td><td>4</td><td>50,000</td><td>3.2</td><td>160,000</td><td>Wholesaling and storage</td></tr><tr><td>Transportation</td><td>120,000</td><td>1</td><td>120,000</td><td>6</td><td>720,000</td><td>15 m width for typical streets. Ring road around the torus, at the edge of the central plain. Mass transport system consisting of a moving sidewalk, monorail, and minibus</td></tr><tr><td>Communication switching equipment (for 2800 families)</td><td>500</td><td>1</td><td>500</td><td>4</td><td>2,000</td><td>Communication and telephone distribution</td></tr><tr><td>Waste and water treatment and recycling</td><td>40,000</td><td>1</td><td>40,000</td><td>4</td><td>160,000</td><td>Including water supply, return and recycling, and sewage treatment</td></tr><tr><td>Electrical supply and distribution</td><td>1,000</td><td>1</td><td>1,000</td><td>4</td><td>4,000</td><td>Including transformer substations</td></tr><tr><td>Miscellaneous</td><td>10,000</td><td>2</td><td>29,000</td><td>3.8</td><td>112,000</td><td></td></tr><tr><td>Total</td><td>466,000</td><td>-</td><td>942,000</td><td>-</td><td>8,233,000</td><td></td></tr></tbody></table> <h4>Agricultural uses</h4> <table><tbody><tr><th>Use<a class="footnote-ref" id="fnref:19" href="#fn:19"><sup>19</sup></a></th><th>Used land area (m2)</th><th>Number of levels</th><th>Total usable area (m2)<a class="footnote-ref" id="fnref:20" href="#fn:20"><sup>20</sup></a></th><th>Height per level (m)</th><th>Volume (m3)</th><th>Notes</th></tr><tr><td>Plant growing areas</td><td>147,000</td><td>3</td><td>440,000</td><td>15</td><td>6,600,000</td><td>38,000 m2 for sorghum (yield of 83 g/m2/day), 235,000 m2 for soybeans (yield of 20 g/m2/day), 72,000 m2 for wheat (yield of 31 g/m2/day), 36,000 m2 for rice (yield of 35 g/m2/day), 9,000 m2 for corn (yield of 58 g/m2/day), 52,000 m2 for vegetables (yield of 132 g/m2/day). Part of the plant production is used to feed livestock. Sorghum is used to obtain sugar. Fruit trees are grown in parks and residential areas, providing 250 g of fruit per person each day, and serving at the same time for ornamental purposes.</td></tr><tr><td>Animal areas</td><td>17,000</td><td>3</td><td>50,000</td><td>15</td><td>750,000</td><td>Stable herd of animals: 260,000 fish (0.1 m2 for each one), 62,000 chicken (0.13 m2 for each one), 28,000 rabbits (0.4 m2 for each one), 1,500 cattle (4 m2 for each one). Flexibility is allowed for other animals to replace parts of these numbers (for example, pigs would have area requirements between those of rabbits and cattle).</td></tr><tr><td>Food processing, collection, storage, etc.</td><td>13,000</td><td>3</td><td>40,000</td><td>15</td><td>600,000</td><td></td></tr><tr><td>Agriculture drying area</td><td>27,000</td><td>3</td><td>80,000</td><td>15</td><td>1,200,000</td><td></td></tr></tbody></table> <h4>Totals</h4> <table><tbody><tr><th>Used land area (m2)</th><th>Total usable area (m2)</th><th>Volume (m3)</th><th>Notes<a class="footnote-ref" id="fnref:21" href="#fn:21"><sup>21</sup></a></th></tr><tr><td>670,000</td><td>1,552,000</td><td>17,383,000</td><td>Only part of the 678,000 m2 of land area and 69,000,000 m3 of volume available in the torus are used</td></tr></tbody></table> <h2 id="construction">Construction</h2> <p>The torus would require nearly 10 million tons of mass. Construction would use materials extracted from the <a href="/facts/Moon/zKHqEMPb">Moon</a> and sent to space using a <a href="/facts/Mass_accelerator/SeKkUwED">mass accelerator</a>. A <a href="/facts/Mass_catcher/B8ShU5Up">mass catcher</a> at <a href="/facts/Lagrangian_point/XCAN4o3P">L2</a> would collect the materials, transporting them to <a href="/facts/Lagrangian_point/XCAN4o3P">L5</a> where they could be processed in an industrial facility to construct the torus. Only materials that could not be obtained from the Moon would have to be imported from Earth. <a href="/facts/Asteroid_mining/jT1XUpBn">Asteroid mining</a> is an alternative source of materials.<a class="footnote-ref" id="fnref:22" href="#fn:22"><sup>22</sup></a> </p> <h2 id="world-ship-proposal">World ship proposal</h2> <p>In 2012 paper <i>World Ships - Architectures & Feasibility Revisited</i>, a <a href="/facts/Generation_ship/9cqVJ75r">generation ship</a> (also called world ship) based on Stanford torus was proposed. Stanford torus was chosen over O'Neill colony designs because of its detailed design, that covers in depth aspects such as life support systems and wall thickness. </p><p>Four Stanford torus colonies would be stacked together, each with a population of 25,000 (bigger than the population of 10,000 for the original Stanford torus, while keeping the original general design and dimensions, and almost the same mass, that is increased by only 10% to 11 million tones), for a total population of 100,000, that is the minimum population size that the paper considers for a world ship. </p><p>For propulsion system, the one designed in <a href="/facts/Project_Daedalus/cWTC01HS">Project Daedalus</a> was chosen, to be used in combination with the Stanford torus, being attached to the center of the torus. Daedalus would provide other additional features, such as power generation and a dust shield to protect the Stanford Torus from interstellar dust impacts.<a class="footnote-ref" id="fnref:23" href="#fn:23"><sup>23</sup></a> </p> <h2 id="gallery">Gallery</h2> <h2 id="see-also">See also</h2> <ul> <li>Spaceflight portal</li><li>Engineering portal</li></ul> <ul><li><a href="/facts/Asteroid_mining/jT1XUpBn">Asteroid mining</a></li> <li><a href="/facts/Bernal_sphere/IIPszXaB">Bernal sphere</a></li> <li><a href="/facts/Colonization_of_the_Moon/Sc13PEu3">Colonization of the Moon</a></li> <li><a href="/facts/Rotating_wheel_space_station/aVlYRocn">Rotating wheel space station</a></li> <li><a href="/facts/O%2527Neill_cylinder/ILWtI7v9">O'Neill cylinder</a></li> <li><a href="/facts/Bishop_Ring_(habitat)/wsJP9EGG">Bishop ring</a></li> <li><a href="/facts/Space_colonization/3xUDzls1">Space colonization</a></li> <li>In fiction <ul><li><a href="/facts/Aurora_(novel)/XBWMKudk"><i>Aurora</i> (novel)</a></li> <li><a href="/facts/Gaea_trilogy/VHrTxIS7">Gaea trilogy</a></li> <li><i><a href="/facts/Ringworld/Hpeugaxm">Ringworld</a></i></li> <li><a href="/facts/Interstellar_(film)/7IwvIUpP"><i>Interstellar</i> (film)</a></li> <li><a href="/facts/Elysium_(film)/xKLJMn1m"><i>Elysium</i> (film)</a></li> <li><i><a href="/facts/Mobile_Suit_Gundam_Wing/qmkjhPtH">Mobile Suit Gundam Wing</a></i></li> <li><a href="/facts/Citizen_Sleeper/zysqLSsM"><i>Citizen Sleeper</i> (videogame)</a></li></ul></li></ul> <h2 id="notes">Notes</h2> <h2 id="external-links">External links</h2> Wikimedia Commons has media related to Stanford Torus. <ul><li><a href="https://upload.wikimedia.org/wikipedia/commons/f/ff/Space_Settlements_-_A_Design_Study_%281977%29.pdf"><i>Space Settlements: A Design Study</i></a>, 1977 book, where Stanford torus was described</li> <li><a href="https://www.youtube.com/watch?v=UcO_BjXfhhc">Space Colonization: Report on Stanford Torus Stations – NASA Ames circa 1975</a> on <a href="/facts/YouTube_video_(identifier)/db276jst">YouTube</a></li> <li><a href="http://www.spacehabs.com/357392/station-construction/">Visualisation of stanford torus construction from an asteroid mining facility in 2010</a> <a href="https://web.archive.org/web/20130125012327/http://www.spacehabs.com/357392/station-construction/">Archived</a> 2013-01-25 at the <a href="/facts/Wayback_Machine/nmQ3a6JC">Wayback Machine</a></li></ul> <h2 id="references">References</h2> <ol> <li id="fn:1"><p>"Stanford Torus Space Settlement - NSS". 2017-08-03. Retrieved 2025-04-08. <a href="https://nss.org/stanford-torus-space-settlement/" target="_blank">https://nss.org/stanford-torus-space-settlement/</a> <a href="#fnref:1" class="footnote-back-ref">↩</a></p></li> <li id="fn:2"><p>Johnson & Holbrow 1977, p. 1, "The Overall System", p. 60, Summary - Johnson, Richard D.; Holbrow, Charles (1977). "Space Settlements: A Design Study". National Aeronautics and Space Administration. Archived from the original on 2009-12-14. <a href="https://web.archive.org/web/20091214010802/http://settlement.arc.nasa.gov/75SummerStudy/Table_of_Contents1.html" target="_blank">https://web.archive.org/web/20091214010802/http://settlement.arc.nasa.gov/75SummerStudy/Table_of_Contents1.html</a> <a href="#fnref:2" class="footnote-back-ref">↩</a></p></li> <li id="fn:3"><p>Johnson, Richard D.; Holbrow, Charles (1977). "Space Settlements: A Design Study". National Aeronautics and Space Administration. Archived from the original on 2009-12-14. <a href="https://web.archive.org/web/20091214010802/http://settlement.arc.nasa.gov/75SummerStudy/Table_of_Contents1.html" target="_blank">https://web.archive.org/web/20091214010802/http://settlement.arc.nasa.gov/75SummerStudy/Table_of_Contents1.html</a> <a href="#fnref:3" class="footnote-back-ref">↩</a></p></li> <li id="fn:4"><p>"Stanford Torus Space Settlement - NSS". 2017-08-03. Retrieved 2025-04-08. <a href="https://nss.org/stanford-torus-space-settlement/" target="_blank">https://nss.org/stanford-torus-space-settlement/</a> <a href="#fnref:4" class="footnote-back-ref">↩</a></p></li> <li id="fn:5"><p>Johnson & Holbrow 1977, pg VII, "Preface" - Johnson, Richard D.; Holbrow, Charles (1977). "Space Settlements: A Design Study". National Aeronautics and Space Administration. Archived from the original on 2009-12-14. <a href="https://web.archive.org/web/20091214010802/http://settlement.arc.nasa.gov/75SummerStudy/Table_of_Contents1.html" target="_blank">https://web.archive.org/web/20091214010802/http://settlement.arc.nasa.gov/75SummerStudy/Table_of_Contents1.html</a> <a href="#fnref:5" class="footnote-back-ref">↩</a></p></li> <li id="fn:6"><p>O'Neill, Gerard K. (1977). The High Frontier: Human Colonies in Space. Bantam Books. p. 149. <a href="/wiki/Gerard_K._O%27Neill" target="_blank">/wiki/Gerard_K._O%27Neill</a> <a href="#fnref:6" class="footnote-back-ref">↩</a></p></li> <li id="fn:7"><p>Bekey, Ivan; Herman, Daniel (January 1, 1985). "Space Station and Space Platform Concepts: A Historical Review". Space Stations and Space Platforms-Concepts, Design, Infrastructure, and Uses. American Institute of Aeronautics and Astronautics. pp. 203–263. doi:10.2514/5.9781600865749.0203.0263. ISBN 978-0-930403-01-0. <a href="978-0-930403-01-0" target="_blank">978-0-930403-01-0</a> <a href="#fnref:7" class="footnote-back-ref">↩</a></p></li> <li id="fn:8"><p>Noordung (pseudonym), Hermann (1993) [1929]. Das Problem der Befahrung des Weltraums: der Raketen-Motor (PDF) (in German). Berlin: Richard Carl Schmidt & Co. pp. 136–144. ISBN 3851320603. <a href="3851320603" target="_blank">3851320603</a> <a href="#fnref:8" class="footnote-back-ref">↩</a></p></li> <li id="fn:9"><p>von Braun, W. (March 22, 1952). Crossing the Final Frontier. Colliers. <a href="/wiki/Wernher_von_Braun" target="_blank">/wiki/Wernher_von_Braun</a> <a href="#fnref:9" class="footnote-back-ref">↩</a></p></li> <li id="fn:10"><p>Johnson & Holbrow 1977, p. 46 - Johnson, Richard D.; Holbrow, Charles (1977). "Space Settlements: A Design Study". National Aeronautics and Space Administration. Archived from the original on 2009-12-14. <a href="https://web.archive.org/web/20091214010802/http://settlement.arc.nasa.gov/75SummerStudy/Table_of_Contents1.html" target="_blank">https://web.archive.org/web/20091214010802/http://settlement.arc.nasa.gov/75SummerStudy/Table_of_Contents1.html</a> <a href="#fnref:10" class="footnote-back-ref">↩</a></p></li> <li id="fn:11"><p>Johnson & Holbrow 1977, Chap. 5 - Johnson, Richard D.; Holbrow, Charles (1977). "Space Settlements: A Design Study". National Aeronautics and Space Administration. Archived from the original on 2009-12-14. <a href="https://web.archive.org/web/20091214010802/http://settlement.arc.nasa.gov/75SummerStudy/Table_of_Contents1.html" target="_blank">https://web.archive.org/web/20091214010802/http://settlement.arc.nasa.gov/75SummerStudy/Table_of_Contents1.html</a> <a href="#fnref:11" class="footnote-back-ref">↩</a></p></li> <li id="fn:12"><p>Johnson & Holbrow 1977, Chap. 5 - Johnson, Richard D.; Holbrow, Charles (1977). "Space Settlements: A Design Study". National Aeronautics and Space Administration. Archived from the original on 2009-12-14. <a href="https://web.archive.org/web/20091214010802/http://settlement.arc.nasa.gov/75SummerStudy/Table_of_Contents1.html" target="_blank">https://web.archive.org/web/20091214010802/http://settlement.arc.nasa.gov/75SummerStudy/Table_of_Contents1.html</a> <a href="#fnref:12" class="footnote-back-ref">↩</a></p></li> <li id="fn:13"><p>Johnson, Richard D.; Holbrow, Charles (1977). "Space Settlements: A Design Study". National Aeronautics and Space Administration. Archived from the original on 2009-12-14. <a href="https://web.archive.org/web/20091214010802/http://settlement.arc.nasa.gov/75SummerStudy/Table_of_Contents1.html" target="_blank">https://web.archive.org/web/20091214010802/http://settlement.arc.nasa.gov/75SummerStudy/Table_of_Contents1.html</a> <a href="#fnref:13" class="footnote-back-ref">↩</a></p></li> <li id="fn:14"><p>Johnson, Richard D.; Holbrow, Charles (1977). "Space Settlements: A Design Study". National Aeronautics and Space Administration. Archived from the original on 2009-12-14. <a href="https://web.archive.org/web/20091214010802/http://settlement.arc.nasa.gov/75SummerStudy/Table_of_Contents1.html" target="_blank">https://web.archive.org/web/20091214010802/http://settlement.arc.nasa.gov/75SummerStudy/Table_of_Contents1.html</a> <a href="#fnref:14" class="footnote-back-ref">↩</a></p></li> <li id="fn:15"><p>Johnson, Richard D.; Holbrow, Charles (1977). "Space Settlements: A Design Study". National Aeronautics and Space Administration. Archived from the original on 2009-12-14. <a href="https://web.archive.org/web/20091214010802/http://settlement.arc.nasa.gov/75SummerStudy/Table_of_Contents1.html" target="_blank">https://web.archive.org/web/20091214010802/http://settlement.arc.nasa.gov/75SummerStudy/Table_of_Contents1.html</a> <a href="#fnref:15" class="footnote-back-ref">↩</a></p></li> <li id="fn:16"><p>Johnson, Richard D.; Holbrow, Charles (1977). "Space Settlements: A Design Study". National Aeronautics and Space Administration. Archived from the original on 2009-12-14. <a href="https://web.archive.org/web/20091214010802/http://settlement.arc.nasa.gov/75SummerStudy/Table_of_Contents1.html" target="_blank">https://web.archive.org/web/20091214010802/http://settlement.arc.nasa.gov/75SummerStudy/Table_of_Contents1.html</a> <a href="#fnref:16" class="footnote-back-ref">↩</a></p></li> <li id="fn:17"><p>Johnson, Richard D.; Holbrow, Charles (1977). "Space Settlements: A Design Study". National Aeronautics and Space Administration. Archived from the original on 2009-12-14. <a href="https://web.archive.org/web/20091214010802/http://settlement.arc.nasa.gov/75SummerStudy/Table_of_Contents1.html" target="_blank">https://web.archive.org/web/20091214010802/http://settlement.arc.nasa.gov/75SummerStudy/Table_of_Contents1.html</a> <a href="#fnref:17" class="footnote-back-ref">↩</a></p></li> <li id="fn:18"><p>The total usable area does not always exactly match with the product of the used land area multiplied by the number of levels. <a href="#fnref:18" class="footnote-back-ref">↩</a></p></li> <li id="fn:19"><p>Johnson, Richard D.; Holbrow, Charles (1977). "Space Settlements: A Design Study". National Aeronautics and Space Administration. Archived from the original on 2009-12-14. <a href="https://web.archive.org/web/20091214010802/http://settlement.arc.nasa.gov/75SummerStudy/Table_of_Contents1.html" target="_blank">https://web.archive.org/web/20091214010802/http://settlement.arc.nasa.gov/75SummerStudy/Table_of_Contents1.html</a> <a href="#fnref:19" class="footnote-back-ref">↩</a></p></li> <li id="fn:20"><p>The total usable area does not always exactly match with the product of the used land area multiplied by the number of levels. <a href="#fnref:20" class="footnote-back-ref">↩</a></p></li> <li id="fn:21"><p>Johnson, Richard D.; Holbrow, Charles (1977). "Space Settlements: A Design Study". National Aeronautics and Space Administration. Archived from the original on 2009-12-14. <a href="https://web.archive.org/web/20091214010802/http://settlement.arc.nasa.gov/75SummerStudy/Table_of_Contents1.html" target="_blank">https://web.archive.org/web/20091214010802/http://settlement.arc.nasa.gov/75SummerStudy/Table_of_Contents1.html</a> <a href="#fnref:21" class="footnote-back-ref">↩</a></p></li> <li id="fn:22"><p>Johnson & Holbrow 1977, p. 201 - Johnson, Richard D.; Holbrow, Charles (1977). "Space Settlements: A Design Study". National Aeronautics and Space Administration. Archived from the original on 2009-12-14. <a href="https://web.archive.org/web/20091214010802/http://settlement.arc.nasa.gov/75SummerStudy/Table_of_Contents1.html" target="_blank">https://web.archive.org/web/20091214010802/http://settlement.arc.nasa.gov/75SummerStudy/Table_of_Contents1.html</a> <a href="#fnref:22" class="footnote-back-ref">↩</a></p></li> <li id="fn:23"><p>Hein, Andreas M.; Pak, Mikhail; Pütz, Daniel; Bühler, Christian; Reiss, Philipp (2012). "World ships—architectures & feasibility revisited". Journal of the British Interplanetary Society. 65 (4): 119. <a href="https://www.researchgate.net/publication/236177990" target="_blank">https://www.researchgate.net/publication/236177990</a> <a href="#fnref:23" class="footnote-back-ref">↩</a></p></li> </ol>