For the past few years, NASA's been working on a powerful rocket called the Space Launch System meant for missions to the moon, Mars and other places far from our planet. While the rocket's not quite ready to take Bruce Willis to an asteroid, it's at least getting nearer to a launch pad -- the agency has just finalized a $2.8 billion deal with Boeing to start building its core stage and developing its avionics system. The deal went through recently after the core stage passed its critical design review, where 11 experts inspected 3,000 of its artifacts. A core stage, by the way, is the middle part of a rocket that makes up most of its body: the one Boeing's building for the SLS is a 200-foot tower that'll store cryogenic liquid hydrogen and liquid oxygen to fuel the engines.