communication with professionals on Earth. Current suggestions for Mars exploration and colonization are to select individuals who have passed psychological screenings. Psychosocial sessions for the return home are also suggested in order to reorient people to society. Terraforming Various works of fiction put forward the idea of terraforming Mars to allow a wide variety of life forms, including humans, to survive unaided on Mars' surface. Some ideas of possible technologies that may be able to contribute to the terraforming of Mars have been conjectured, but none would be able to bring the entire planet into the Earth-like habitat pictured in science fiction. Minimum size of a colony To be self-sustaining, a colony would have to be large enough to provide all the necessary living services. These include Ecosystem management: producing appropriate gases, controlling air composition pressure and temperature, collecting and producing water, growing food and processing organic wastes. Energy production: this includes extracting methane for vehicles and if photovoltaic cells are used to produce energy this would include the extraction and processing of silicates, to augment or replace any original equipment. Industry: extracting and processing appropriate ores, manufacturing tools and other objects; producing clothes, medicine, glass, ceramics, plastics. Building: even if the base is constructed before arrival, it will need frequent adaptation according to the evolution of the settlement as well as inevitable replacement. Social activities: this includes raising children and educating them, health care, preparing meals, cleaning, washing, organizing the work and making decisions. Time for sport, culture and entertainment can be minimized but not eliminated. As the number of individuals grows, both activities and objects can be shared between them. Growth also will offset the risks of collapse of the society, caused by sudden deaths, accidents, infertility or inbreeding. But this may not prevent mortal combat between different groups of individuals, or the loss of efficiency due to inappropriate social organization. By mathematical modelling of the time spent by people on these issues and by keeping things simple, Salotti concludes that the minimum number for a colony on Mars is 110. This is close to other studies of the genetic problems involved in the longer journey to Proxima Centauri b (6,000+ years). Transportation Interplanetary spaceflight Mars requires less energy per unit mass (delta V) to reach from Earth than any planet except Venus. Using a Hohmann transfer orbit, a trip to Mars requires approximately nine months in space.