As part of its new Vision for Space Exploration, NASA will first be returning to the Moon before sending human explorers to Mars. Although Mars is a much more Earth like environment, with an atmosphere, similar length of day and large amounts of water, the Moon is going to be the agency's first target. Why? Mainly, it's much closer, so astronauts can practice working in an extreme environment close to home before making the much more difficult and riskier journey to Mars.