Terraforming (literally, “Earth-shaping”) of a planet, moon, or other body is the theoretical process of deliberately modifying its atmosphere, temperature, surface topography or ecology to be similar to the biosphere of Earth to make it habitable by Earth-like life. The term “terraforming” is sometimes used more generally as a synonym for planetary engineering, although some consider this more general usage an error. The concept of terraforming developed from both science fiction and actual science. The term was coined by Jack Williamson in a science-fiction story (Collision Orbit) published during 1942 in Astounding Science Fiction, but the concept may pre-date 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 usually considered 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 it. 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.