The terraforming (literally, "Earth-shaping") of a planet, moon, or other body is the hypothetical process of deliberately modifying its atmosphere, temperature, surface topography or ecology to be similar to those of Earth to make it habitable by humans.
The term is sometimes used more generally as a synonym for planetary engineering. The concept of terraforming developed from both science fiction and actual science. The term was probably invented by Jack Williamson in a science-fiction story ("Collision Orbit") published during 1942 in Astounding Science Fiction, but the actual concept pre-dates this work.
Based on experiences with Earth, the environment of a planet can be altered deliberately: however the feasibility of creating an unconstrained planetary biosphere that mimics Earth on another planet has yet to be verified. Mars is considered by many to be the most likely candidate for terraformation. Much study has been done concerning the possibility of heating the planet and altering its atmosphere, and NASA has even hosted debates on the subject. Several potential methods of altering the climate of Mars may fall within humanity's technological capabilities, but at present the economic resources required to do so are far beyond that which any government or society is willing to allocate to the purpose. The long timescales and practicality of terraforming are the subject of debate. Other unanswered questions relate to the ethics, logistics, economics, politics and methodology of altering the environment of an extraterrestrial world.
Carl Sagan, an astronomer and popularizer of science, proposed the planetary engineering of Venus in a 1961 article published in the journal Science titled, "The Planet Venus." Sagan imagined seeding the atmosphere of Venus with algae, which would remove carbon dioxide and reduce the greenhouse effect until surface temperatures dropped to "comfortable" levels. Three billion years ago, the Earth had a carbon dioxide atmosphere. Blue-green algae and water evaporation changed the earth's atmosphere into oxygen and nitrogen gas. Later discoveries about the conditions on Venus made this particular approach impossible since Venus has too much atmosphere to process and sequester. Even if atmospheric algae could thrive in the hostile and arid environment of Venus' upper atmosphere, any carbon that was fixed in organic form would be liberated as carbon dioxide again as soon as it fell into the hot lower regions.
Sagan also visualized making Mars habitable for human life in "Planetary Engineering on Mars," a 1973 article published in the journal Icarus. Three years later, NASA addressed the issue of planetary engineering officially in a study, but used the term planetary ecosynthesis instead. The study concluded that it was possible for Mars to support life and be made into a habitable planet. That same year 1976, one researcher, Joel Levine, organized the first conference session on terraforming, which at the time was called "Planetary Modeling."
In March 1979, NASA engineer and author James Oberg organized the "First Terraforming Colloquium," a special session on terraforming held at the Lunar and Planetary Science Conference in Houston. Oberg popularized the terraforming concepts discussed at the colloquium to the general public in his 1981 book, New Earths. Not until 1982 was the word terraforming used in the title of a published journal article. Planetologist Christopher McKay wrote "Terraforming Mars," a paper for the Journal of the British Interplanetary Society. The paper discussed the prospects of a self-regulating Martian biosphere, and McKay's use of the word has since become the preferred term. During 1984, James Lovelock and Michael Allaby published The Greening of Mars. Lovelock's book was one of the first to describe a novel method of warming Mars, where chlorofluorocarbons are added to the atmosphere. Motivated by Lovelock's book, biophysicist Robert Haynes worked behind the scenes to promote terraforming, and contributed the word ecopoiesis to its lexicon.
Beginning in 1985, Martyn J. Fogg began publishing several articles on terraforming. He also served as editor for a full issue on terraforming for the Journal of the British Interplanetary Society in 1991, and in 1995 published the book Terraforming: Engineering Planetary Environments. Fogg also maintains an active website called The Terraforming Information Pages.
Fogg used the following definitions for different aspects related to terraforming:
Fogg also devised definitions for candidate planets of varying degrees of human compatibility:
Fogg designates Mars as having been a biologically compatible planet in its youth, but not being in any of these three categories in its present state, since it could only be terraformed with relatively greater difficulty. Mars Society founder Robert Zubrin produced a plan for a Mars return mission called Mars Direct that would set up a permanent human presence on Mars and steer efforts towards eventual terraformation.
The principal reason given to pursue terraforming is the creation of an ecology to support a world suitable for habitation by humans. However, some researchers believe that space habitats will provide a more economical means for supporting space colonization. If research in nanotechnology and other advanced chemical processes continues apace, it may become feasible to terraform planets in centuries rather than millennia. On the other hand, it may become reasonable to modify humans so that they do not require an oxygen/nitrogen atmosphere in a 1 g gravity field to live comfortably. That would then reduce the need to terraform worlds, or at least the degree to which other worlds' environments would need to be altered.
In its astrobiology roadmap, NASA has defined the principal habitability criteria as "extended regions of liquid water, conditions favorable for the assembly of complex organic molecules, and energy sources to sustain metabolism."
The exact mechanism of this loss is still unclear, though several mechanisms have been proposed. The lack of a magnetosphere surrounding Mars may have allowed the solar wind to erode the atmosphere, the relatively low gravity of Mars helping to accelerate the loss of lighter gases to space. The lack of plate tectonics on Mars is another possibility, preventing the recycling of gases locked up in sediments back into the atmosphere.
The core of Mars, which is made mostly of iron, originally held up the magnetic field of Mars. However, once the core cooled down, the magnetic field weakened. The lack of magnetic field and geologic activity may both be a result of Mars' smaller size allowing its interior to cool more quickly than Earth's, though the details of such processes are still unrealized. Re-heating the core of Mars is considered an impractical solution; one only theoretically possible (but still impractical) method would be to hold some sort of giant 'magnifying glass' over the planet to melt it, and possibly re-liquefy the core. However, none of these processes are likely to be significant over the typical lifespan of most animal species, or even on the timescale of human civilization, and the slow loss of atmosphere could possibly be counteracted with ongoing low-level artificial terraforming activities.
Terraforming Mars would entail two major interlaced changes: building the atmosphere and heating it. A thicker atmosphere of greenhouse gases such as carbon dioxide would trap incoming solar radiation. Because the raised temperature would add greenhouse gases to the atmosphere, the two processes would augment each other. Fossil fuels are a possible way to increase the temperature on Mars.
Terraforming Venus requires two major changes; removing most of the planet's dense 9 MPa carbon dioxide atmosphere and reducing the planet's 500 °C (770 K) surface temperature. These goals are closely interrelated, since Venus' extreme temperature is due to the greenhouse effect caused by its dense atmosphere. Sequestering the atmospheric carbon would likely solve the temperature problem as well.
Other possible candidates for terraformation include Titan, Ganymede, Io, Callisto, Luna, and even the dwarf planet Ceres. Most, however, have too little mass and gravity to hold an atmosphere indefinitely (although it is possible, but not certain, that an atmosphere could remain for tens of thousands of years or be replenished as needed). In addition, aside from the Moon, most of these worlds are so far from the Sun that adding sufficient heat would be much more difficult than even Mars would be.
Also known as the "worldhouse" concept, or domes in smaller versions, paraterraforming involves the construction of a habitable enclosure on a planet which eventually grows to encompass most of the planet's usable area. The enclosure would consist of a transparent roof held one or more kilometers above the surface, pressurized with a breathable atmosphere, and anchored with tension towers and cables at regular intervals. Proponents claim worldhouses can be constructed with technology known since the 1960s. The Biosphere 2 project built a dome on Earth that contained a habitable environment. The project encountered difficulties in construction and operation.
Paraterraforming has several advantages over the traditional approach to terraforming. For example, it provides an immediate payback to investors; the worldhouse starts out small in area (a domed city for example), but those areas provide habitable space from the start. The paraterraforming approach also allows for a modular approach that can be tailored to the needs of the planet's population, growing only as fast and only in those areas where it is required. Finally, paraterraforming greatly reduces the amount of atmosphere that one would need to add to planets like Mars to provide Earth-like atmospheric pressures. By using a solid envelope in this manner, even bodies which would otherwise be unable to retain an atmosphere at all (such as asteroids) could be given a habitable environment. The environment under an artificial worldhouse roof would also likely be more amenable to artificial manipulation.
It has the disadvantage of requiring massive amounts of construction and maintenance activity. The extra cost might be off-set somewhat by automated manufacturing and repair mechanisms. A worldhouse might also be more susceptible to catastrophic failure if a major breach occurred, though this risk might be reduced by compartmentalization and other active safety precautions. Meteor strikes are a particular concern because without any external atmosphere they would reach the surface before burning up.
There is a philosophical debate within biology and ecology as to whether terraforming other worlds is an ethical endeavor. On the pro-terraforming side of the argument, there are those like Robert Zubrin, Martyn J. Fogg, and Richard L. S. Taylor who believe that it is humanity's moral obligation to make other worlds suitable for life, as a continuation of the history of life transforming the environments around it on Earth. They also point out that Earth would eventually be destroyed if nature takes its course, so that humanity faces a very long-term choice between terraforming other worlds or allowing all terrestrial life to become extinct. Terraforming totally barren planets, it is asserted, is not morally wrong as it does not affect any other life. Some more cautious thinkers believe terraforming would be an unethical interference in nature, and that given humanity's past treatment of the Earth, other planets may be better off without human interference. Still others strike a middle ground, such as Christopher McKay, who argues that terraforming is ethically sound only once we have completely assured that an alien planet does not harbor life of its own; but that if it does, while we should not try to reshape the planet to our own use, we should engineer the planet's environment to artificially nurture the alien life and help it thrive and co-evolve, or even co-exist with humans.
Some advocates of space colonization have argued that the same financial investment required to terraform Mars or Venus could produce a larger area of "land" if used to build space habitats instead. They argue that a civilization that knows how to live in space can survive anywhere in the solar system, whereas terraforming Mars will only help us to live in one place. Some view terraforming as planetary chauvinism.
Terraforming has also been explored on television and in feature films, most prominently and famously in the Star Trek universe. In the Star Trek movie The Wrath of Khan, the film's antagonist Khan steals the "Genesis device", a device developed to quickly terraform barren planets, and wields it as a weapon. A similar device exists in the animated feature film Titan A.E. which depicts the eponymous ship Titan, capable of creating a planet.
Also in the Star Trek television series, humans terraformed Mars in the 22nd century by redirecting comets towards Mars' north and south poles. In the episode "Terra Prime" the Starship Enterprise crew uses a comet to avoid being detected by the antagonist.
In Joss Whedon's short-lived hit television series Firefly, and its feature film sequel Serenity, giant "terraformers" (ships or factories designed to generate atmosphere and perform other functions of terraforming) were used to transform the ecosystems of dozens of planets and hundreds of moons across a huge solar system into human-livable environments.
It is shown in the movies Alien and Aliens. In the first film, the atmosphere of LV-426 is unbreathable and John Hurt's character must wear an environmental suit; 60 years later an atmospheric factory has been used to withdraw sulphur and replace it with oxygen; producing a stormy but breathable atmosphere.
In the anime Cowboy Bebop humanity has terraformed dozens of moons and planets after a hyperspace gate accident fractured the Moon, raining debris on Earth. Asteroids have also been colonized to sustain human life. Also, the manga and anime series Aria takes place on a terraformed Mars. In Dragon Ball Z movie 4, the evil "Lord Slug" terraforms the Earth to make it suitable for his soldiers and to kill humanity. As well as the video game Armored Core 2, which takes place on a newly terraformed Mars.
In Stargate SG1 episode "Scorched Earth" an alien ship xenoforms a planet recently inhabited by Enkarans with the help of humans. The movie Blade Runner alludes to the existence of "Off-World Colonies" which are advertised as having a more suitable living atmosphere than the polluted Earth: the story is based on the escape of individuals designed as labor forces for the new colonists. In the movie Total Recall an alien device is activated to transform the atmosphere of Mars.
In the series Red Dwarf, the crew have to rescue Rimmer from a planet terraformed to match his own disturbed sub-conscious, and in the episode Rimmerworld, Rimmer sets off two "Eco-Accelerator Rockets" which after six days and nights transforms the planet into a "lush and verdant" world.
In the movie Red Planet humanity has partially terraformed Mars by putting algae on the planet's surface. As a result, the crew that crash lands on Mars can breathe. Also, in the Halo series there exist multiple colony planets that were made suitable for human habitation through terraforming.
On the TV series Futurama (set 1000 years in the future), in the episode Mars University it is discovered that Mars in the year 3000 is habitable, and there is a university there. It is discussed that when Mars University was established, they planted "traditional college foliage" including trees and hemp and that soon after the whole planet was terraformed. There are also native Martians, who are revealed in the episode Where the Buggalo Roam. The Martians however, sold their land to a Chinese man named Sir Reginold Wong for a single bead (an enormous diamond)- a play on the sale of the isle of Manhattan.
On the 2008 series of Doctor Who ("Doctor Who (series 4)"), in the sixth episode "The Doctor's Daughter", the Doctor and his companions stumble across the "Source", a terraforming device in the shape of a globe with metallic rings built around it, which is the reason for the generation-long war between the human and Hath colonizations on the planet Messaline. The Doctor shatters this globe releasing the terraforming chemicals and thus beginning the terraforming process of the planet and 'declaring the war to be over'. In the end of this particular episode, prior to Jenny's reanimation, she exhales a cloud of golden-green mist resembling the terraforming gas contained within the Source from earlier.
David Gerrold's currently incomplete novel series The War Against The Chtorr takes a twist with the terraforming concept and has the Earth being invaded by unseen an alien species which is Chtoraforming the Earth to match their own world. The species from Chtorr are estimated to be a half a billion years older than those on Earth, and thus evolved to a higher level of competition and trickery. The Earth's species are steadily losing the battle as they are unable to compete. Humans are steadily losing the battle, as well, as the different species interfere with and overpower their best technologies which they are not designed to work against.
Terraforming played a role in the simulation computer game Sim Earth designed by Will Wright and published in 1990. Similarly, in the final phase of Wright's creature simulation computer game Spore contains a vast amount of terraforming, including the placement of animals, plants, and terrain features. This is done both through machinery and energy rays that your ship eventually possesses.
In Star Wars, terraforming exists. The Yuuzhan Vong from Star Wars novels often "Vongformed" planets to jungles to get rid of technology. There are also other instances of terraforming in the Star Wars universe.
The planet on which the most recent Turok game occurs is in the process of terraforming, which causes an increase in evolutionary activity, which creates dinosaurs.