The mobile launcher nears the base of Launch Pad 39B at NASA's Kennedy Space Center in Florida.

The mobile launcher, or ML, nears the base of Launch Pad 39B at NASA's Kennedy Space Center in Florida, away from the protection of the pad's three 600-foot-tall lightning masts. The ML is making the 4.2-mile trek back to the park site near the Vehicle Assembly Building.

Data on the ML collected from structural and functional engineering tests during its two-week stay on the pad will be used in the next phases of construction. The 355-foot-tall ML structure, which took about two years to construct, will be modified by NASA’s 21st Century Ground Systems Program to support NASA’s Space Launch System, the heavy-lift rocket that will launch astronauts into deep space on future exploration missions. For more information, visit http://www.nasa.gov/exploration/systems/sls. Photo credit: NASA/Cory Huston