The US Air Force has two X-37B space planes, and since 2010 each of them has flown two missions into outer space. Those flights have ranged in length from 224 to 717 days. The X-37B, which is autonomous and looks something like a miniature version of NASA's space shuttle, launches on top of a rocket and orbits the Earth before returning and landing on a runway. For the first four missions, the Boeing-developed space plane has launched on top of an Atlas V rocket, the military's go-to vehicl