Space Shuttle Discovery (search) began a slow trek back to the hangar Thursday to have a new and safer fuel tank installed, along with a heater to prevent a dangerous buildup of ice on its surface.

The spacecraft is being readied for liftoff in mid-July on the first shuttle flight since the Columbia disaster 2 1/2 years ago.

Discovery was already on the launch pad when NASA (search) concluded that ice that forms on the external tank when it is filled with super-cold fuel could break off during liftoff and prove as lethal as the chunk of foam insulation that doomed Shuttle Columbia (search).

Discovery was rolled away from the launch pad on a huge caterpillar-track platform, along a specially built road almost as wide as an eight-lane highway.

The journey, at a pace of less than 1 mph, had been expected to take six or seven hours, but the shuttle ground to a halt for 1 1/2 hours when a bearing in the vehicle carrying it overheated. The trip resumed at a slower pace after workers let the bearing cool down and packed it with grease.

The rollback of Discovery to the assembly building was postponed by two days so workers could inspect it for any landing gear cracks of the sort discovered on space shuttle Atlantis. But no cracks were found, NASA said.