The crawler-transporters are a pair of tracked vehicles used to transport spacecraft from NASA’s Vehicle Assembly Building (VAB) along the Crawlerway to Launch Complex 39. They were originally used to transport the Saturn IB and Saturn V rockets during the Apollo, Skylab and Apollo–Soyuz programs. They are currently used to transport the Space Shuttle. It is planned for them to carry the upcoming Ares I and Ares V rockets for the Constellation program as well. The crawler-transporters carry vehicles on the Mobile Launcher Platform, and after each launch return to the pad to take the platform back to the VAB.