No evidence has emerged linking the work performed on Columbia during the 17-month refurbishment to the shuttle's breakup Feb 1. Columbia flew one successful mission after the overhaul was completed in 2001.
However, inspection and work records from that overhaul at the Boeing Co. plant where the shuttle was built in Palmdale, Calif., may hold clues.
Among the modifications to NASA's oldest shuttle were increased protection from space debris and enhanced heat protection for the leading edges of the wings.
According to NASA, the spacecraft's aluminum frame also was closely inspected for signs of fatigue or corrosion. It's not clear, and NASA officials could not immediately say, what was found and how much repair work took place.
The shuttle's first layer of protection, the fragile reinforced carbon tiles, also were closely inspected and repaired or replaced where necessary.
Disaster investigators have said they believe a hole or gash allowed superheated gases to penetrate Columbia as it entered Earth's atmosphere. They don't believe overheating detected in the left wing before the breakup could have been caused simply by the loss of tiles. Other possible causes include space debris or the impact of a piece of hard insulation that broke off the external tank shortly after launch.
During the 1999-2001 overhaul of Columbia, much of the emphasis was on wiring. In its last mission before the overhaul, July 1999, a worn wire caused a power fluctuation that led two engine controllers to shut down five seconds after launch. Backup controllers took over automatically and the flight was not affected.
About 95 percent of the shuttle's 235 miles of wire was inspected, including wires that connect to the sensors that eventually reported higher-than-normal temperatures just before the shuttle broke apart.
Technicians alqo(semoved 1,000 pounds of old wiring and equipment used to monitor Columbia's earliest flights 20 years ago.
During the overhaul, an independent board reviewing safety measures noted metal shavings in the shuttle, on walking platforms and wire bundles.
"These occurrences are considered potential sources of foreign object debris and could damage surrounding wire insulation or provide an electrical shorting path," the report said.
Henry McDonald, the report's lead author and former director of NASA Ames Research Center, declined to comment.
A Palmdale Boeing employee, who asked not to be identified, said there also were many "stumble-ons," or instances when technicians happened upon something needing repair. But he was unaware of any case in which a problem was not resolved.
In March 2002, Columbia's first launch after the work was done, NASA considered aborting the mission because of a problem with coolant lines. It was later determined the problem stemmed from debris left during the overhaul.
As has often been the case with the shuttle program, the Palmdale project was both behind schedule and over budget. It lasted 17 months instead of the expected nine and cost $145 million instead of $70 million.
Even after Columbia was shipped back to Kennedy Space Center in February 2001, it underwent several more months of work until the March 2002 launch.
Al Feinberg, a NASA spokesman, said wiring work was the reason for the delay and it was not unusual for work to be finished at Kennedy.
Dan Beck, a Boeing spokesman, wouldn't discuss details about why the job took so long.
"Because of our firm commitment on safety for our flight crews and the vehicles, we weren't going to be tied to any specific timetable to complete that work if there were still some outstanding areas that needed the attention of the maintenance and modification crew," he said.
Officials of United Space Alliance, NASA's prime shuttle contractor of which Boeing is a part-owner, did not respond to written questions.
NASA says shuttles receive major overhauls once every three years. Until 2001, that work took place at Palmdale, about 60 miles northeast of Los Angeles. NASA and the United Space Alliance announced after Columbia left Palmdale that all future overhauls would take place at Kennedy.