Terraforming

Terraforming is the process of altering the environment and ecology of a planet, moon, or other body deliberately modifying its atmosphere, temperature, surface topography or ecology to be similar to those of Earth, in order to make it habitable by terrestrial organisms. It normally involves inserting plants and/or flora, introducing new minerals or substances, or other changes to fulfill the required goals for the planet in question. It can take up to several centuries to fully terraform an Earth-sized planet, or even slightly larger.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 terraforming. 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.Terraforming is a process of planetary engineering, specifically directed at enhancing the capacity of an extraterrestrial planetary environment to support life as we know it. The ultimate achievement in terraforming would be to create an open planetary biosphere emulating all the functions of the biosphere of the Earth, one that would be fully habitable for human beings.The Requirements for sustaining terrestrial life is an energy source, but the notion of planetary habitability implies that many other geophysical, geochemical, and astrophysical criteria must be met before the surface of an astronomical body is able to support life. Of particular interest is the set of factors that has sustained complex, multicellular animals in addition to simpler organisms on this planet. Research and theory in this regard is a component of planetary science and the emerging discipline of astrobiology.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."Once conditions become more suitable for life,the importation of microbial life could begin.As conditions approach that of Earth, plant life could also be brought in. This would accelerate the production of oxygen, which theoretically would make the planet eventually able to support animal and human life.

Definitions for candidate planets of varying degrees of human compatibility:
 * Habitable Planet (HP): A world with an environment sufficiently similar to the Earth as to allow comfortable and free human habitation.
 * Biocompatible Planet (BP): A planet possessing the necessary physical parameters for life to flourish on its surface. If initially lifeless, then such a world could host a biosphere of considerable complexity without the need for terraforming.
 * Easily Terraformable Planet (ETP): A planet that might be rendered biocompatible, or possibly habitable, and maintained so by modest planetary engineering techniques and with the limited resources of a starship or robot precursor mission.