SpaceX is poised to launch astronauts into space this spring. Here's how SpaceX and Boeing became NASA's best shot at resurrecting American spaceflight.

· SpaceX completed an explosive test on Sunday that showed its Crew Dragon capsule can whisk astronauts to safety in the event of a rocket failure.
· The test was part of NASA's Commercial Crew program, a partnership between the space agency and private companies to build spaceships that can ferry astronauts to and from the...