NASA officials, however, said Wednesday that a broken drill bit was inadvertently lodged within one of Discoverys three powerful main engines during a routine overhaul. Concerned that the one-half inch bit could cause engine trouble during Discoverys climb into orbit, managers ordered the replacement of the $35 million power plant. The replacement work is expected to take 10 days to complete. That, however, likely will add at least three days to normal launch preparation work, and the mission could be delayed accordingly.

"The problem is you dont want to have foreign object debris floating around in an engine," said Kennedy Space Center spokesman Joel Wells. "We dont want a repeat of the situation we had on Columbias (July 23) flight." A gold-plated engine plug the size of a small nail shook loose during that launch, triggering a hydrogen fuel leak that could have led to a dangerous emergency landing. The plug shot through the engine about 3,000 mph, rupturing coolant lines inside its bell-shaped nozzle. A resulting liquid hydrogen leak caused the engine to shutdown a split-second early, leaving Columbia in an orbit seven miles lower than intended.

Wells said it is unlikely that the broken drill bit within Discoverys engine would cause serious trouble during the upcoming launch of the Hubble repair mission. Managers, nevertheless, decided to play it safe.

"It's very unlikely that this thing would cause us a problem on lift-off. It's very unlikely that it would cause us a problem in orbit," Wells said. "But we decided to err on the side of caution and replace the engine."

The engine work is one of two problems shuttle mission managers were wrangling with as they gear up for the Discovery launch. The second was the possibility that suspect thermal tiles might have been inadvertently installed on Discovery, Wells said. The tiles protect shuttles and their crews from searing temperatures the ships are exposed to during atmospheric reentry. Consequently, NASA managers ordered a "paper chase" and determined none of the suspect tiles are on Discovery.

Discovery is now scheduled to move from its hangar to KSCs 52-story Vehicle Assembly Building early Thursday. Once in the building, Discovery will be outfitted with a 15-story external tank and attached solid rocket boosters. The assembled shuttle is expected to be moved to the launch pad next Tuesday or Wednesday, where the engine replacement work will be done.