NASA's crawler-transporters, two of the largest vehicles ever built, have carried NASA rockets and spacecraft to the launch pad for the last 50 years. They will continue their legacy as the "workhorses" of the nation's space program as part of the agency's journey to Mars.

The crawlers are being modified to carry NASA's Space Launch System (SLS) with the Orion spacecraft atop it and potential commercial vehicles to their pads to begin space exploration missions. Originally constructed in 1965 to support the agency's Apollo Program, they also supported the Skylab, Apollo-Soyuz Test Project and Space Shuttle Program, helping NASA push the boundaries of human space exploration farther into the solar system.

To celebrate its 50th year of supporting NASA missions, the upgraded and modified crawler-transporter 2, known as CT-2, rolled out of the Vehicle Assembly Building (VAB) on Feb. 18 and began the 4.2-mile trek to Launch Pad 39B at NASA's Kennedy Space Center in Florida. NASA, members of the media and guests paid tribute during a viewing and tour of the crawler at the pad Feb. 23.

But there's no time for these giant machines to rest on their laurels. The Ground Systems Development and Operations Program (GSDO) at Kennedy has been busy beefing up crawler-transporter 2 to be ready to support SLS and Orion. CT-1 will be available to carry a variety of other launch vehicles.

Using these vehicles, NASA will send astronauts farther than ever before, first to an asteroid, and onward to Mars. The modifications will enable the crawlers to continue supporting human spaceflight for another 20 years.

The crawlers were initially manufactured by the Marion Shovel Company in Marion, Ohio. Weighing in at more than 6 million pounds each, the larger parts that could be assembled and fit on a truck were shipped to Kennedy in 1964 where final assembly took place.

Each crawler is 131 feet long and 114 feet wide, with four pickup points spaced 90 feet apart on the upper deck that fit into four interface locations on the mobile launcher.

"It's important to maintain an equal amount of weight distributed on each of the crawler's trucks," said John Giles, crawler project deputy manager. "Through tests and calculations, the center of gravity is determined for the mobile launcher. The interface locations can be different according to the weight and location of the space vehicle on top."

The unique features of the giant vehicles include 16 traction motors, two alternating current generators, two direct current generators powered by diesel engines, and two control cabs each to drive the vehicles forward and backward. The jacking, equalizing and leveling (JEL) system, among other systems, are monitored and controlled from inside the crawler's control room. The JEL system keeps the upper deck and pick up points level at all times, even when traveling up steep inclines to the top of the pads, to prevent its rocket payload from toppling.

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NASA's Giant Crawlers Turn 50 Years Old