Add Yahoo as a preferred source to see more of our stories on Google. The first human spaceflight to the moon in more than 50 years could be days away. But first, NASA's rocket that will propel a crew of four astronauts toward the moon has to ace a crucial ...

Elon Musk's dream of colonizing Mars is taking a backseat as SpaceX shifts its focus to building a self-sustaining city on the moon within a decade.