Colonization or settlement of Mars is the theoretical human migration and long-term human establishment of Mars. The prospect has garnered interest from public space agencies and private corporations and has been extensively explored in science fiction writing, film, and art. Organizations have proposed plans for a human mission to Mars, the first step towards any colonization effort, but no person has set foot on the planet, and there have been no return missions. However, landers and rovers have successfully explored the planetary surface and delivered information about conditions on the ground. Mars' orbit is close to Earth's orbit and the asteroid belt. While Mars' day and general composition are similar to Earth, the planet is hostile to life. Mars has an unbreathable atmosphere, thin enough that its temperature on average fluctuates between , yet thick enough to cause planet-wide dust storms. The barren landscape on Mars is covered by fine dust and intense ionizing radiation. Mars has in-situ resources, such as underground water, Martian soil, and ore, which could be leveraged by colonists. Opportunities to generate electricity via wind, solar, and nuclear power using resources on Mars are poor. Justifications and motivations for colonizing Mars include curiosity, the potential for humans to provide more in-depth observational research than uncrewed rovers, an economic interest in its resources, and the possibility that the settlement of other planets could decrease the likelihood of human extinction. Difficulties and hazards include radiation exposure during a trip to Mars and on its surface, toxic soil, low gravity, the isolation that accompanies Mars' distance from Earth, a lack of water, and cold temperatures. Commitments to researching permanent settlement have been made by public space agencies—NASA, ESA, Roscosmos, ISRO, the CNSA, among others—and private organizations—SpaceX, Lockheed Martin, and Boeing. Mission concepts and timelines Since the 20th century, there have been several proposed human missions to Mars both by government agencies and private companies. Most of the human mission concepts as currently conceived by national governmental space programs would not be direct precursors to colonization. Programs such as those being tentatively planned by NASA, Roscosmos, and ESA are intended solely as exploration missions, with the establishment of a permanent base possible but not yet the main goal. Colonization requires the establishment of permanent habitats that have the potential for self-expansion and self-sustenance. Two early proposals for building habitats on Mars are the Mars Direct and the Semi-Direct concepts, advocated