Space colonization

Artist rendering of a human habitat on the moon.

Space colonization is the formation of permanent self-sufficient human settlement and the exploitation of natural resources beyond Earth. Space colonization is also referred to as extraterrestrial colonization and space settlement. Reasons for space colonization include ensuring the long-term survival of humanity if something catastrophic happens on Earth and the exploitation of natural resources not found on Earth. Space colonization can shelter humanity from extinction on Earth due to events such as asteroid impacts, nuclear warfare, pandemics, wars, and environmental catastrophes. Arguments against space colonization are primarily regarding the initial economic costs of colonizing space being too high to justify when resources could be used to deal with problems faced on Earth, such as poverty, hunger, and disease.

Often space colonization is associated with the Cold War, which saw the United States place humans on another world in six separate visits. Since then, most exploration has stayed in Earth's orbit and has focused on the orbiting of satellites. These satellites survey the solar system, which is certainly a good first step when planning to explore and potentially colonize near space. The lack of exploration has led to building interest in exploration, with plans to return to the moon in 2024 and further plans to build permanent habitats on other planets.

Certain requirements are necessary to establish a successful space colony: access to space, food, water, construction materials, transportation, energy, communication, life support, protection from radiation, gravity or simulated gravity, and entertainment. Not meeting these requirements and accounting for the change in climate and environment away from Earth could lead to illnesses in human colonizers, including cancer, radiation illnesses, reproductive problems or sterility, muscle degeneration, bone loss, skin burns, cardiovascular disease, depression, boredom, inability to concentrate, high blood pressure, immune disorders, structural changes in the brain, nausea, dizziness, weakness, cognitive decline, and altered gene function.

In addition to the challenges of the physical constraints that life on another planet would place on the habitant, there are also concerns regarding who should be placed in charge of this exploration, especially as concerns and challenges around the habitation of other planets continue to be learned. For example, Pascal Lee, a planetary scientist at the SETI Institute in California and director of the NASA Haughton Mars Project, uses the Mars environment to ask questions concerning how humans might threaten life on other planets.

Not necessarily the intelligent life of science fiction, but rather this would be the microbial life that could live on these planets, and which could in turn be threatened by microbes and similar small lifeforms that humans would naturally transfer to the planet. Which planet is colonized informs the answer to that question, with the answer for Mars being a no, as the harsh climate and high UV radiation would kill the majority of microbes that are brought from Earth.

There are various possible locations or targets for colonization, often focused on the inner solar system, which have been considered for terraforming or colonization. Some main candidates include Mars, the moon, Venus, and Mercury. Whereas other targets that have been suggested for possible colonization include Jupiter's moon Europa, Saturn's sixth-largest moon Enceladus, Saturn's moon Titan, and Ceres located in the asteroid belt between Mars and Jupiter.

Example of a potential Mars colony.

For decades, there has been a relative obsession with the possibility of colonizing Mars. When talking about space colonization, Mars is the most frequently discussed planet for colonization. Studies of Mars have highlighted that the planet probably has water, warmer temperatures, and a thicker atmosphere. Astrophysicists, through these studies, have come to believe Mars experienced some serious climate changes, with rocks, sediments, and soil providing some clues to the planet's past. Understanding this history would help people in the colonization of the planet.

Though Mars is considered the closest planet that is hospitable, the trip would take around six to eight months. Otherwise, despite the smaller size, Mars has about the same total amount of land area as Earth's continents, and the temperatures on the planet are not too extreme even compared with Earth. Further, the gravity is lower on Mars, around 2.7 times less than that of Earth, which makes it reasonable for living and makes Mars an excellent base for missions, with longer-term habitation likely requiring terraforming.

In 1962, John DeNike and Stanley Zahn published a study titled "Lunar Basing," which included their concept for a sub-surface base located at the Sea of Tranquility, which was the landing site of the Apollo 11 mission. Similar to proposals from Arthur C. Clarke, the base would rely on nuclear reactors for power and an algae-based air filtration system, made up of thirty habitat modules and would measure 1300 meters squared in overall size and capable of accommodating twenty-one crew members. During the 1960s, NASA produced various studies that advocated for the creation of habitats inspired by the Apollo Program's mission architecture. These plans often envisioned space station modules being placed on the lunar surface and using existing designs and technology for increased reliability and reduced costs.

Rendering of a proposed Lunar colonization structure.

Since those studies, and with various countries and their respective space agencies working towards returning to the surface of the moon, there has been an increased interest in colonizing the moon. In 2006, Japan announced plans for a moon base by 2030. Russia, in 2007, made a similar proposal to be built between 2027 to 2032. And Jim Burke of the International Space University in France proposed creating a Lunar Noah's Ark to ensure human civilization could survive a cataclysmic event. In 2016, ESA chief Johann-Dietrich Worner proposed the creation of an international village on the moon as a successor to the International Space Station.

Darby Dyar, professor of astronomy and geology at Mount Holyoke College, serves on the Solar System Exploration Research Virtual Institute. She studies the minerals on the moon and other airless bodies and works to figure out how future residents of the moon could get water, with ideas including finding water trapped in the minerals or capturing water from Comets crashing into the moon.

There are various advantages to colonizing Venus, including its proximity to Earth, which is 87 million miles closer to Earth than Mars, meaning it would take 30 to 50 percent less time to travel to Venus compared wtih Mars. This could, in turn, make Venus more economically practical and expose the human body to less of the harmful effects of space travel. Solar panels placed on Venus would provide significantly more power than those on Mars, providing potential inhabitants with increased power generation potential.

Additionally, Venus has an immense buildup of carbon dioxide inside its atmosphere, which in the future may be a resource from which to extract oxygen. The gravitational force of Venus is about 0.9 times that of Earth, meaning humans could not live on the surface of the planet; but with the thick atmosphere, the potential is that Venusian colonists could live in floating habitats.

Potential Venus colonies.

Although there are, arguably, various ways Venus could be colonized, the most popular proposal has considered creating cities in the upper atmosphere, where the pressure is 1,000 hectopascals (hPa), which is close to Earth's 1013 hPa at sea level. Not only would human colonists deal with this well, but the pressure outside the living structure would be similar to outside, meaning a puncture would result in a repairable leak rather than a catastrophe, and people would be able to deal with the temperature ranging from 32 to 122 degrees Fahrenheit.

Example of floating cities in science fiction.

While the development of floating cities sounds like science fiction, NASA scientist and science-fiction author, Geoffrey Landis, has studied the feasibility of human colonies on Venus and has explained that floating a city 31 miles above the planet's surface would be relatively straightforward. Because Venus' atmosphere is mostly carbon dioxide, a mixture of oxygen and nitrogen could generate enough lift to float a one-kilometer diameter sphere of 700,000 tons. A similar two-kilometer diameter sphere would be able to lift six million tons. And, with the amount of space in Venus' atmosphere, these floating cities could have populations of hundreds of thousands of humans and even see billions of humans living in the atmosphere.

A look at Mercury, which has been proposed as a potential location for colonization.

Mercury presents a difficult planet to colonize, based on the nature of its rotation, orbit, composition, and geological history. The planet's slow rotational period means one side of the planet faces the sun for extended periods, reaching temperatures of up to 800 degrees Fahrenheit, while the side facing away reaches extreme cold temperatures around -315 degrees Fahrenheit. In addition, the planet's rapid orbital period of 88 days, combined with a sidereal rotation period of 58.6 days, means it takes roughly 176 Earth days for the sun to return to the same place in the day or a solar day. Which means a single day on Mercury lasts as long as two of its years. However, with the ice on the poles of Mercury, there is a suggestion that this makes the planet one of interest for colonization, as it might be easier to live off of, mine, and have necessary or desired minerals and materials to make colonization worthwhile.

This slow day has led to the suggestion that a city could be developed that, set on wheels, could keep moving to stay ahead of the sun. Other colonization plans for Mercury have included tunnel complexes that mobile habitats could enter to shelter from Mercury's intense heat. Further, the intense sunlight could allow humans to develop a lens to concentrate sunlight at the planet's surface and harvest the energy for powering solar panels or smelting of materials for habitats and perhaps a road system.

A look at the surface of Europa.

Europa's surface is mostly ice, with evidence suggesting there is liquid water or slushy ice beneath the surface. Scientists, based on probes and images, believe the ice shell surface is 10 to 15 miles thick. So while Europa is only one-fourth the diameter of Earth, and its ocean may contain twice as much water as all of Earth's oceans combined. Life, as it is known on Earth, has three main requirements: liquid water, the appropriate chemical elements, and an energy source. Europa has two of the three, with the energy source challenging to confirm. But it does suggest that there could be life surviving under Europa's shell.

Artist rendering of the exploration of the sub-surface oceans of Europa.

Plans to colonize Europa began in 1997, with scientists planning to inhabit igloos and drill down into the Europan ice crust to explore the subsurface ocean believed to exist underneath. According to the plan, there are discussions of using artificial air pockets for human habitation or using inflatable structures on the surface, while subglacial ocean exploration could be done with specialized submarines. However, there are several challenges with the colonization of Europa, including the high level of radiation from Jupiter's radiation belt—about 10 times as strong as Earth's Van Allen Radiation belt, and a human could not serve the near 540 rem of radiation per day that Europa receives (500 rem is a lethal dose). Any long-term colonization would require significant radiation shielding.

Alternatively, in many of the plans for the colonization of Europa, are mentions of Jupiter's other moon Callisto, which has been suggested to be a target for study, especially as the moon Callisto is subject to less radiation than Europa.

A look at the surface of Titan.

Saturn's moon Titan has been considered for colonization, despite the -300 degrees Fahrenheit temperatures and its skies rain methane and ethane and fall into a hydrocarbon sea. Titan is one of the more Earth-like bodies in the solar system, with lakes of methane and ethane similar to bodies of water, and it rains methane on Titan, with an atmosphere of nitrogen 50 percent thicker than Earth's. Saturn's magnetosphere also provides Titan with shelter. Despite the cold, the thick atmosphere means residents would not require pressure suits, just warm clothing and respirators, and means housing could be made cheaper and from materials produced on the planet. This planet is the sole other place in the solar system known to have an earth-like cycle of liquids raining from clouds, flowing across its surface, filling lakes and seas, and evaporating back into the sky.

Residents of Titan would not have to stay inside all day; they would even be able to fly in the thick atmosphere and offer residents various recreational opportunities. Titan's terminal velocity is also a tenth of that found on Earth, meaning landing in the case of an emergency for someone flying through the atmosphere would be easy rather than if the same happened on Earth.

Colonizing Titan would be difficult, given that it has taken spacecraft seven years to reach Saturn. However, Titan does offer a dense atmosphere shielding the surface from radiation and would make any structural failures problematic, rather than catastrophic. And while the atmosphere offers various chances for users to engage in recreational activities, drilling into Titan's crust would also give residents a chance to access the liquid water in the moon's subsurface ocean, and they would be able to extract the different hydrocarbons on the planet.

Surface of Ceres.

Ceres has been considered an important location with important amenities that have led many to suggest the colonization of the dwarf planet would be beneficial, with others suggesting terraforming the planet could even be worthwhile. Colonizing Ceres could involve many of the methods used to establish colonies on the moon, Mercury, and the satellites of Jupiter and Saturn. This comes down to the basics of establishing settlements that are optimized for bodies with little to no atmosphere and are subject to extreme temperatures. Regolith, for example, could be mined from surrounding asteroids to then 3D print a base layer next to the ice and print out structures. Further, locally harvested regolith, ice, and other organic molecules has been suggested for use in soil for growing food.

For a potential colonist of Ceres, they would have to consider the distance between Earth and Ceres, which would make messages take thirty minutes between the two bodies. The gravity is less than Earth, with everything on the planet weighing forty times less. However, Ceres receives about ten times less sunlight than Earth and has no substantial atmosphere to protect colonists from radiation or micrometeoroids, meaning any potential habitat would need to be robust enough to protect the people inside from those threats. And the planet does not appear to have any natural structures that could be used as habitats.

One suggestion for colonizing Ceres has included the building of floating habitats around the planet, with it being described as a mega satellite, capable of accomodating around 50,000 people, supporting an artificial atmosphere, and generating an Earth-like gravity using a centrifugal force from its rotation. These floating habitats could then use space elevators to transfer raw materials from the planet to the orbiting habitats.

One such concept, developed by astrophysicist Pekka Janhunen of the Finnish Meteorological Institute in Helsinki, described his proposal, which included cylinders capable of producing their own gravity, with a 1 kilometer radius, and about 10 km in length, capable of completing a full rotation every 66 seconds to generate centrifugal force comparable to Earth-like gravity. A single-cylinder in his proposal would be built to comfortably hold about 57,000 people. This proposal is based on the O'Neill colony concept.

Artist rendering of an O'Neill cylinder.

Proposed by Gerard K. O'Neill in 1976, the O'Neill cylinder is a space colonization concept. The design for the cylinder came as a result of a task he set a group of physics and architectural students to invent large structures that could be used for long-term human habitation. The resulting design was two cylinders that rotate on bearings in opposite directions, each one around 32 km long and 8 km in diameter, with six stripes along its length. The cylinder was envisioned to support industrial processes and recreational facilities to be on a central axis where it would be effectively a zero-gravity zone. The cylinder would generate Earth-like gravity through the rotation of the cylinders, and with the given dimensions, the cylinder would need to rotate about twenty-eight times every hour to simulate that gravity.

Variation on an O'Neill cylinder.

Each cylinder would have land areas alternating with three windows, and mirrors that would open and close to form a day-night cycle inside. The counter-rotation of the two cylinders would make it easier to keep the cylinders aimed toward the sun. Further, the cylinder is designed to maintain a similar atmosphere and pressure similar to Earth, with a controlled ratio of gases and a pressure at half of that at sea level, which provides less need for gas and less requirement for thick walls. The other concern would be the possibility of collisions with man-made space debris and small meteoroids. A suggested solution has been radar systems around the outer skin of the cylinders to map the region around the habitat to locate possible dangers, despite the inevitably of collisions. With the pressure lower than on Earth, any collision with the cylinder would render repairs less of an emergency than if the pressures were higher.

Artist rendering of a variation of an O'Neill cylinder.

Since the initial inception of the concept, researchers have developed variations of the O'Neill cylinder, including spheres, cylinders, and a ring-shaped torus. Each design is intended to rotate and create a centrifugal force capable of mimicking gravity for the inhabitants. In general, O'Neill cylinders are generally designed to be permanent and self-sustaining structures, using solar power for electrical energy and growing crops using the windows and mirror structures to provide sunlight to the crops the cylinders grow.

While much of the plans developed by NASA for space colonization is focused on permanent colonies on the moon and Mars, O'Neill cylinders offer a different view of space colonization, in which the habitats could be used as floating checkpoints between colonized planets, they could be used to colonize near-Earth space to remove industry or agriculture from Earth's surface, or to move colonies to space, and to develop essentially floating colonies capable of supporting colonization efforts around a body. Meanwhile, the O'Neill cylinder is designed to gather resources from space, whether from asteroids, the moon, or Mars, and to otherwise avoid the effort of transporting materials and goods out of Earth's gravity well.