In his 1997 report, Katnik noted that the 1997 mission, STS-87, was the first to use a new method of ``foaming'' the tanks, one designed to address NASA's goal of using environmentally friendly products. The shift came as the U.S. Environmental Protection Agency was ordering many industries to phase out the use of Freon, an aerosol propellant linked to ozone depletion and global warming.
As recently as last September, a retired engineering manager for Lockheed Martin, the contractor that assembles the tanks, told a conference in New Orleans that developing a new foam to meet environmental standards had ``been much more difficult than anticipated.''
The retired Lockheed engineer, who helped design the thermal protection system, said the switch from a foam based on Freon -- also known as CFC-11 -- has ``resulted in unanticipated program impacts, such as foam loss during flight.''
In fact, he noted, the hits to Columbia on that 1997 mission, the same one Katnik studied, forced NASA to replace nearly 11 times more damaged tiles than it had after a previous mission that had used Freon-based foam.
Lockheed spokesman Harry Wadsworth said Monday that the company was referring questions to NASA. ``I cannot talk about any past problems with foam or the history of foam,'' he said. ``We're not talking about the investigation.''
Despite the concerns, NASA has never described the foam problem as a potential catastrophic threat, but considered it enough of an issue to warrant a series of tests.
In 1999, the Southwest Research Institute, a non-profit laboratory in San Antonio, Texas, fired insulating foam fragments from a compressed-gas gun into thermal tiles and recorded the results with digital cameras. After the Columbia crash, NASA asked the institute not to release those results. The space agency also has tested the foam in wind tunnels and aboard a research jet.