Terraforming is a theoretical process of transforming a planet, moon, or other celestial body in order to make it suitable for human habitation. This transformation would typically involve changing the body's atmosphere, temperature, and ecology to suit human needs. The word "terraforming" literally means "Earth-forming"
Complete terraforming refers to the terraforming of the entire celestial body.
The terraformation process is specific to each candidate body, and depends on the existing conditions. For example, a planet more distant than Earth from the Sun is likely to be too cold to support terrestrial life, and would need to be warmed. Likewise, a planet too close to the Sun would be to be cooled.
Several methods have been proposed for altering the temperature of a candidate body, including the use of orbital mirrors, alteration of the body's albedo, importation of perfluorocarbons and chlorofluorocarbons, and asteroid impacts.
Another modification that would be commonly required to terraform candidate bodies is the addition or modification of an atmosphere. In the case of a planet with an already-dense atmosphere, such as Venus, some atmosphere would need to be removed, and the high levels of unbreathable gases (for example, hydrogen sulphate) would need to be filtered. New gases might need to be imported if they are not already present.
Another challenge in terraforming is the abundance, or lack of, water on the candidate body. Some terraforming candidates (such as Europa) have an excess of water, whereas others (such as Mars) are lacking. Water could be imported from water-rich celestial bodies (for example, Europa, or the dwarf planet Ceres). Likewise, unneeded water could be exported to water-poor bodies.
Paraterraforming refers to the terraforming of a part of a body.
This is usually achieved by the construction of a dome or some other structure containing a self-sufficient ecosystem suitable to human habitation. There have been several attempts at similar projects on Earth (for example, [Biosphere 2]), which have been, in general, somewhat successful, but extremely expensive to construct and maintain.
It should be noted, however, that the cost of paraterraformation would (in most cases) be substantially less than the cost of complete terraformation, and, for the near future, it may be the more overall feasible process. Paraterraformed bodies could also later be completely terraformed, using the locally habitable areas as bases of operation for the larger process.
The most popular candidate for terraforming is Mars, as Mars is the most Earth-like planet in our solar system. Mars would require warming and the addition of an atmosphere. As Mars' gravity is substantially less than Earth's, and it lacks a magnetosphere, it is possible that the atmosphere would have to be periodically replenished.
Another popular terraformation candidate, Venus would require cooling, the removal of large portions of its atmosphere, filtering of its harmful atmospheric gases (such as carbon dioxide), addition of oxygen, the and establishment of a magnetosphere to deflect incoming solar radiation.
During the 2009-10 mission, Daedalus, the Habitat deposited several pieces of terraforming equipment on Ganymede. Despite a malfunction in the equipment's initial deployment, which necessitated the Habitat's return to Ganymede, the systems seem to be functioning properly and have begun a complete terraformation. The process will require other visits over time to add new systems to adapt and work with the changing climate, as well as to replace or repair ageing or damaged equipment.