Despite a declaration from President Barack Obama that the moon is not a planned destination for American astronauts, senior NASA engineers have quietly begun reconsidering using it as a staging point for an eventual mission to Mars.William Gerstenmaier, the chief of human exploration for NASA, does not see the president's plan of a direct, 900-day mission to the red planet as achievable. Instead, Gerstenmaier believes large amounts of ice at the lunar poles may provide an important reservoir of oxygen and hydrogen fuel to propel rockets and spaceships across the 40 million miles of space to Mars."If propellant was available...