Cracks discovered in the shuttle's external fuel tank have sparked a lengthy investigation.
But in launch position, the orbiter is perched on the side of its 15-story external fuel tank and flanked by two solid rocket boosters.
But while batteries store all the chemicals needed for the necessary chemical reactions in a sealed package, fuel cells use an external fuel source and air from the atmosphere.
"Later, as Martin Marietta, it constructed the Viking Mars landers and the Magellan spacecraft to Venus and designed and produced the external fuel tank for the space shuttle."
Bathed in xenon spotlights, the white spaceship, attached to its twin solid rocket boosters and orange external fuel tank, crept 3.4 miles on the back of an enormous tractor called the Crawler.
Around two minutes after launch, the boosters on either side of the shuttle will fall away, followed shortly afterwards by the giant external fuel tank strapped to the spaceship.
But a fully reusable spacecraft proved too hard to build, which is why the shuttles carry a huge external fuel tank that is ditched into the ocean after each flight.
