The space shuttle Discovery completed a nearly 10-hour 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 2½ years ago.
Discovery was already on the launch pad when NASA concluded that the 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 Columbia. As a result, the agency decided to bring the shuttle back to the 52-story Vehicle Assembly Building.
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 1½ 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.
NASA planned to begin work next week to remove the shuttle from its external fuel tank and solid rocket boosters. The orbiter will then be paired with another tank and booster set, which originally had been earmarked for Atlantis, the shuttle scheduled to fly after Discovery.
Atlantis' tank already had been partially prepared for the new heater. Discovery's tank will be outfitted with an additional heater as well, then used on a future mission.