Desert irrigation for agricultural development is an example of the first, while the life-support systems of lunar landing modules, or orbiting space stations, exemplify the second mode of survival. Recently, the President of the United States called for the establishment of bases for astronauts on the moon and Mars. The first human outposts in space will, of necessity, be of the second kind, even though some local resources may be exploited by their occupants. Human settlements on other planets can become fully and permanently independent of Earth only of these distant environments are transformed to provide Earth-like living conditions and a local agriculture. The realistic possibilities for this latter type of planetary engineering, carried out on a global scale, are assessed briefly in this essay.
Life is a planetary phenomenon, but Earth is the only living planet in the solar system.
All are intricately coupled with each other, and with land, oceans and air by the recycling of water, carbon, oxygen, nitrogen and other inorganic materials needed to maintain life. We are exotic products of a planetary engine originally set in motion, and continuously fuelled, by energy from the sun. On other planets, high and low extremes of atmospheric temperatures and pressures, lack of free oxygen and liquid water, high concentrations of toxic gases, and deadly radiation levels variously preclude the existence of life. Though presently barren, Mars, nonetheless, is a biocompatible planet. Indeed, it was the hope that organisms might be found on Mars that made life-detection the top priority for NASA's Viking missions in 1976.