A NASA space shuttle has to withstand a lot. It must hitch a ride on one or more enormous rockets for a dangerous launch, maneuver in Earth's orbit and then suffer through the intense heat of atmospheric reentry. And, as a reusable vehicle, it has to arrive safely on land to repeat the whole process again.
With all this to consider, you can bet engineers won't want to make these ships get around by themselves when they don't have to. Before launch, the orbiter -- with its entire rig -- weighs a gargantuan 2,250 tons (2,041 metric tons) [source: Boeing]. Moving this weight requires a Shuttle Crawler Transporter -- the largest kind of tracked vehicle in existence, according to NASA. This huge platform must carry a space shuttle from the assembly building to the launch pad using a 5,000-horsepower motor. "Crawler" is an apt name, however, as it moves the precious cargo a mere 1.24 miles per hour (2 kilometers per hour) [source: NASA].
Although the Crawler may not technically tow that prelaunch weight, the shuttle orbiter must be towed when it lands after its long, rugged journey back into the atmosphere. A 256-horsepower diesel tractor attaches a tow bar to the nose gear of the 121-ton (110-metric ton) orbiter for a two-mile (3.2-kilometer) trek back to the Orbiter Processing Facility. This tractor tows at speeds of up to 5 miles per hour (8 kilometers per hour). It's 16 feet (4.9 meters) long, 8 feet (2.4 meters) wide and can even tow up to 410 tons (372 metric tons) if needed [source: Dumoulin]. It also tows yet another vehicle that hauls the orbiter, the shuttle carrier aircraft -- a plane that transports the shuttle around the country and the world.