Crawler-transporters are gigantic machines used to move rockets and spacecraft from the Vehicle Assembly Building at NASA's Kennedy Space Center in Florida to Launch Complex 39. The largest vehicles ever built, Crawler-transport 1 and 2 were constructed in 1965 and used during the Apollo, Skylab, Apollo-Soyuz, and Space Shuttle programs. Traveling at 1 mph, a crawler carries a launch vehicle and its mobile launcher platform to the pad with a laser guidance system, lowers them onto the pad pedestals, and after launch re-lifts the mobile launcher and returns it. Each transporter rides on eight tracked tread belts containing 57 tread belt shoes, and one crawler contains 16 traction motors, two AC and two DC generators, and two control cabs for forward and backward movement. Its jacking, equalizing, and leveling system keeps the upper deck and pickup points level at all times, even on an incline, to prevent the payload from toppling. Since the last space shuttle launch in 2011, NASA's Ground System Development and Operations Program has been refurbishing the crawlers, strengthening CT-1 for commercially operated launch vehicles and modifying CT-2 to support NASA's Space Launch System and the Orion spacecraft, while upgrading the leveling system to raise carrying capacity from 12 million pounds to 18 million pounds.