On July 13, the Discovery launch will be NASA's first flight since the tragic breakup of Columbia upon re-entry in 2003 that killed its seven crew members.
The Discovery's 6.8 km trip from NASA's Vehicle Assembly Building to the launch complex began Wednesday morning at 1:58 a.m. EDT -- about two hours late because of technical glitches, which NASA identified as overheating bearings.
With its new external tank and twin solid rocket boosters, the gigantic unit weighs in at about 17.5 million pounds; moving on its own mobile launcher at a top speed of about 1.6 kilometres an hour.
NASA's website says July's flight will be NASA's 114th, and will be Discovery's 31st.
Discovery was already on its pad last month, when NASA realized it needed to go back to the assembly building for some more safety work. Crews refitted the shuttle with a new fuel tank with an extra heater, which should prevent a dangerous buildup of ice on its surface.
NASA concluded that ice that forms on the external tank could break off during liftoff and prove as lethal as the ill-fated launch of Columbia two years ago.
The launch window for Discovery is from July 13 to July 31.