What is Terraforming
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.
Ethics of Terraforming
On the pro-terraforming side of the argument, there are those like Robert Zubrin and Richard L. S. Taylor who believe that it is humanity's moral obligation to make other worlds suitable for Terran life, as a continuation of the history of life transforming the environments around it on Earth. They also point out that Earth will eventually be destroyed as nature takes its course, so that humanity faces a very long-term choice between terraforming other worlds or allowing all Earth life to become extinct. Dr. Zubrin further argues that even if native microbes have arisen on Mars, for example, the fact that they have not progressed beyond the microbe stage by this point, halfway through the lifetime of the Sun, is a strong indicator that they never will; and that if microbial life exists on Mars, it is likely related to Earth life through a common origin on one of the two planets, which spread to the other as an example of panspermia. Since Mars life would then not be fundamentally unrelated to Earth life, it would not be unique, and competition with such life would not be fundamentally different from competing against microbes on Earth.
On the other hand, for those opposed to terraforming, the impact of the human species on otherwise untouched worlds and the possible interference with or elimination of alien life forms are good reasons to leave these other worlds in their natural states; this is an example of a strong biocentric view, or object-centered ethic. Critics claim this is a form of anti-humanism and they assert that rocks and bacteria can not have rights, nor should the discovery of alien life prevent terraforming from occurring. Since life on Earth will ultimately be destroyed by planetary impacts or the red giant phase of the Sun, all native species will perish if not allowed to move to other objects, though some have argued this may not be true.
See also: Terraforming
Source: Wikipedia (All text is available under the terms of the GNU Free Documentation License)