Before a rocket can blast off into space, it must travel 4.2 miles from the Vehicle Assembly Building to the launchpad in Cape Canaveral, Florida. That's where NASA's crawler-transporters come in. They're the largest self-powered vehicles on the planet, and they're designed to transport rockets to the launchpad. We got an inside look at how NASA's preparing and operating the crawler for the 16-million-pound mobile launcher and Space Launch System, NASA's largest and heaviest rocket to date.