NASA has a new Vision for Space Exploration: in the decades ahead, humans will land on Mars and explore the red planet. Brief visits will lead

to longer stays and, maybe one day, to colonies.

Above: The Moon, an alien world in Earth's backyard. Photo credit: International Space Station astronaut Leroy Chiao. 

First, though, we're returning to the Moon.

Why the Moon before Mars?

"The Moon is a natural first step," explains Philip Metzger, a physicist at NASA Kennedy Space Center. "It's nearby. We can practice living, working and doing science there before taking longer and riskier trips to Mars."