The plan envisions using the moon as a staging area for deeper space exploration. Missions launched from the moon would gain from its low gravity and mineral resources. The president is calling for a lunar base to be established within two decades. Robots would be sent to the moon by 2008, and astronauts would arrive there as early as 2015 "with the goal of living and working there for increasingly extended periods." A colony on the moon could be used to exploit mineral resources of the lunar surface, such as helium-3, an isotope that theoretically could be used for rocket fuel. There are suggestions that the moon has deposits of water near its poles. Water could be chemically split to obtain hydrogen and oxygen, a combination that could be used as a rocket propellant. The oxygen could be used for an atmosphere inside sealed shelters. Setting a permanent colony would require designing and building new heavy lift rockets, spacecraft to take people to lunar orbit and still other spacecraft to ferry them to and from the surface. It would also require developing new techniques of housing and supplying people for long periods of time in a vacuum environment where the temperatures can range from 250 degrees to minus 25 degrees. Astronauts last walked on the moon in 1972; in all, 12 men trod the lunar surface over a 3½-year period. Click here for more on lunar exploration.