 Four volunteer NASA crew members on a mission to Mars, a mission that didn't leave Earth, left their craft this weekend after more than a year in the simulated Mars environment at Johnson Space Center in Houston. The four lived inside the 17,000-square-foot 3D-printed habitat, simulating a mission to Mars as NASA ponders whether humans would be able to visit that planet. Two additional missions are planned, and crews will continue simulated spacewalks and gather data on physical