Columbia has flown 15 times since and has had a succession of modifications. The lines are exposed to the vacuum of space and wide swings in temperature once the shuttle's payload bay doors are opened, an operation that occurs about 90 minutes after liftoff. The water that circulates in the lines cools a floodlight in Columbia's payload bay. Once the lines penetrate the bulkhead, the water joins moisture from other lines that cool more vital electronics aboard the shuttle associated with computing and navigation systems. Last year, as a light fixture aboard Discovery was replaced, shuttle workers noticed a bulge in a small metal plate that functions as a heat exchanger for the bulb.
Engineers believe frozen water in the coolant system caused the bulge.
The finding triggered a wider engineering analysis of similar plumbing on Columbia, Atlantis and Endeavour. The assessment prompted an examination of photographs made of Columbia earlier this week as its countdown got under way. The length of Columbia's flight, two days short of the longest ever by a shuttle, and the schedule of the payload bay's changing orientation to the sun while the spacecraft is aloft, elevated concerns among mission managers when the photos showed the lines were not insulated. Meanwhile, forecasters said there was a 90 percent chance of favorable weather conditions on Friday.