From the introduction:
The twenty-first century is the first time in human history where all the nations in the world united globally, under the supervision of the United Nations, toward a common goal: explore space and beyond.
With the advance in telecommunication and transportation, humans are no longer content with the confinement of Earth. Soon after the discover of a space travel technique, the Dimensional Door, humans attempt to colonize nearby terrestrial (earthlike, consist chiefly of iron and rock) planets and satellites. These small planets include Mercury, Venus, and Mars.
Although Earth is the only planet that has enough oxygen surrounding it and enough water on its surface to support life as we know it, scientists are able to design generators, for each planet, to produce the oxygen and water. Sometimes, pumps are installed to bring up underground waters on these planets.